[neil.jericho]

One of the best threads I've read on any forum. Keep the experiences coming in along with the majestic photos. Its a shame that the bikes that ought to do well in our conditions somehow dont clock deserving sales numbers.

Originally Posted by ringoism

7. (Ladakh 2010): Was kind of, eh, racing a friend who had ridden a Pulsar. He shot down a shortcut at a series of switchbacks, and I missed it, so was way behind. The next shortcut HE missed, and I shot down it, without really looking to see how steep and loose and deeply eroded it was. Front tire washed out, bike started going down, and I gracefully launched myself off the footpegs, running my legs in the air, landing on my feet and continuing on down ahead of the bike, till I could break my momentum.

Straight out the movies!

[Sutripta]

Hi,

The usual solution (above): Have a roadside mechanic put a loop of wire through it. As I mentioned in another thread, I don't like this at all. Too hard to know how much fuel you're actually getting and whether it's right for your bike. Even if it were right, it's too hard to consistently repeat in the future (varying wire sizes, shapes of the loop, position in orifice, etc), and moreover, it might allow some small quantity of fuel to leak past the threads, as the jet is cannot now be fully seated. Bad Jugad.

With biking in Ladakh being mainstream activity,I've always wondered why someone (manufacturers/ dealers/ technical people of the riding groups/ clubs ...) don't work out the carb settings for their respective bikes for various altitudes, and either publish them, or sell 'kits'. Like you I'm amazed and appalled when experienced bikers think that 'wiring up the jet' is the only solution. And think of it as great 'engineering'!

Why don't you do the needful, and monetise it?

Originally Posted by ringoism

So the bikes' stock jetting is causing them to miss at 7,500ft. Just imagine how badly it would run (if it would run at all) at 13,000 - or 18,000ft!!!

Impulse has a slide (ie non CV) carb?

For my bike, I took that wired jet (actually out of a Fiero), which was a much smaller number, removed the wire, and drilled out the orifice with a #118 (1.18mm) drill.

Not #56?

Regards
Sutripta

[ringoism]

Originally Posted by Sutripta

Why don't you do the needful, and monetise it?
Impulse has a slide (ie non CV) carb?
Not #56?

Regards
Sutripta

1. Don't think it hasn't crossed my mind... even the local mechanics here in Manali have no stock in alternative sized jets (couldn't find many even in Leh!), and seem largely clueless... and none of this is actually that difficult.
2. CV carbs do have slides - but they are vacuum-actuated rather than being direct cable-operated like on traditional motorcycle carbs.
3. #56 would be what exactly???

Thanks for your interest.
-Eric

[ringoism]

Originally Posted by ringoism

So I got thinking that if I didn't just go ahead with this despite any reservations, I faced a strong possibility of not having anything to ride this season - can't find them (Impulse) used up here, and I was completely weary of researching / looking for bikes.

Ooops... just heard about a local Manali Impulse (green) with asking price of Rs40,000. All said and done, that would've saved me about, well, 40,000!!! And I'd have felt a lot better about tearing into a 2-year old bike for modifications than this new one... Anyway.

The depreciation rate on these is bound to be horrible, having been discontinued. Still enjoy riding mine, though, and believe it's got potential.

Saw something interesting on the NGK (sparkplug) website today,which probably sheds further light on the trouble with misfire in these bikes at altitude (so basic, but so few seem to understand these things):

"The spark plug firing end temperature must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. This is called “Thermal Performance”, and is determined by the heat range selected.

Misfires:
A misfire occurs when the spark travels the path of least resistance instead of jumping across the gap. Misfires can be caused by the following:
Carbon fouling...(etc, etc)

"Carbon fouling occurs when the spark plug firing end does not reach the self-cleaning temperature of approximately 450°C (842°F). Carbon deposits will begin to burn off from the insulator nose when the self-cleaning temperature is reached. When the heat range is too cold for the engine speed, the firing end temperature will stay below 450°C and carbon deposits will accumulate on the insulator nose. This is called carbon fouling. When enough carbon accumulates, the spark will travel the path of least resistance over the insulator nose to the metal shell instead of jumping across the gap. This usually results in a misfire and further fouling. If the selected spark plug heat range is too cold, the spark plug may begin to foul when the engine speed is low or when operating in cold conditions with rich air-fuel mixtures.

Barometric Pressure / Altitude
Air (atmospheric) pressure and cylinder pressure decrease as altitude increases. As a result, spark plug tip temperature will also decrease.
Fouling can occur more easily if the air-fuel mixture is not adjusted to compensate for the altitude."


The stock Impulse plug is an NGK-CPR8EA9, with the "8" being the heat range. Being that the bike is marketed in India which is overall a rather hot climate much of the year, I suspect the "8" is a little cooler than what I need for running rich at high altitude in the cold Himalayas. Could this, along with the over-rich jetting, be the main source of the missing and hence low power? I want to find out.

Not much selection of plugs up here in Manali. Will have to source them from Karol Bagh next time I'm in Delhi. I figure one or two heat ranges higher should improve things. They do have UR5DC Bosch here locally, which according to one source equates to an NGK CR7HSA, which would be one number hotter (yes, NGK's numbers go lower as they get hotter)... could try it for the (hopefully) upcoming Chandra-Tal ride and see if, along with the jetting mods already done, things are any better.

Wouldn't it be something if all these bikes required to run well enough in Ladakh/Spiti would be a Rs70 sparkplug, a Rs20 main jet, and a little finesse with a flat file?

How many gave up on these bikes entirely, or went in for ZMR transplants, before any proper efforts were made?

Still think it'd run better with around 180cc (working on that plan)... but anyway.

Will update.

-Eric

[vishwaschettri]

Hi Eric,

I took my unicorn (same 150cc engine as your Impulse) quite a few times to leh all stock with pillion and luggage and it used to match bullets on the moore planes. I remember touching a ton on the moore plains more than once riding it rally style with both me and the pillion standing on the footpegs.

Unicorn came tuned pretty lean from the factory with a 115 mainjet for fuel efficiency. I had added an oiled K&N filter to make it lean further for the passes.

My SOP on the bike was

In Delhi - 118 main jet from Achiever without K&N (very good low end and low noise)
On Highway upto Manali - 125 main jet from Karizma with K&N (very good grunt and top end. Engine used to run cooler as well. Only issue was a flatspot at 80kmph - 6000 rpm which I was never able to solve)
Ahead of Manali - 115 stock jet with K&N and the bike ran fine with perfect plug chop.

My advice will be to try more with main jets and needle positioning before sliding on to different heat range plugs. They are a pain to find even at Delhi and IMHO not worth the effort.

cheers
Vishwas

p.s. I just did Manali - Chandratal - Manali a week back on bullets hired from Manali. If you plan to do it, than leave early from Manali, Water flow was quite strong and level quite high at glacier crossings before chattru by lunch.

[Sutripta]

Originally Posted by ringoism

2. CV carbs do have slides - but they are vacuum-actuated rather than being direct cable-operated like on traditional motorcycle carbs.

True, but in a CV, the slide is the controlled element, the controller (air metering) being handled by the butterfly valve. In the slide type, the slide is the air metering device.

From your description, and my own experiences (Eastern Himalayas, ~ 7- 8 K Ft) i'd say the Impulse has a slide type carb, not a CV. Would want confirmation.

3. #56 would be what exactly???

The letter/ number method of designating drill sizes.
I've never come across #nnn = n.nn mm in the smaller sizes, so am surprised! Do you remember which companies drill it was?

Have dabbled a bit with jet flow rates, but think it would be severely OT for this thread.

Regards
Sutripta

[Hrishi_111]

A feminine bike enthusiast's group from Pune, informally known as 'Bikernis' have something to say about 'Hero Impulse', the cheapest off-road bike available in India.

Here is a link to the video which was shared with me by one of the group members:

https://www.youtube.com/watch?v=FC8S...ature=youtu.be

It's quite an interesting video to see and I am personally amazed to see capabilities (off-road + city maneuverability) of the bike which is generally ignored by mass audience.

[JayPrashanth]

Originally Posted by Sutripta

True, but in a CV, the slide is the controlled element, the controller (air metering) being handled by the butterfly valve. In the slide type, the slide is the air metering device.

From your description, and my own experiences (Eastern Himalayas, ~ 7- 8 K Ft) i'd say the Impulse has a slide type carb, not a CV. Would want confirmation.


The letter/ number method of designating drill sizes.
I've never come across #nnn = n.nn mm in the smaller sizes, so am surprised! Do you remember which companies drill it was?

Have dabbled a bit with jet flow rates, but think it would be severely OT for this thread.

Regards
Sutripta

Impulse has a CV, and even CV carbs have slides. Only difference is the actuation.

Cheers,

Jay

[Sutripta]

Originally Posted by JayPrashanth

Impulse has a CV, and even CV carbs have slides. Only difference is the actuation.

And that is a huge, fundamental difference! CV carbs are less sensitive (not insensitive) to altitude changes.

As I said, i have ridden at 7-8 K ft with CV carbs, plains (factory) setup. Am sure AFR was not perfect, and a plug chop would surely throw up a sooty plug. But ridingwise, still quite rideable. Am surprised at Eric's bikes extreme sensitivity to altitude now that it is confirmed that it has a CV carb.

Regards
Sutripta

[JayPrashanth]

Originally Posted by Sutripta

And that is a huge, fundamental difference! CV carbs are less sensitive (not insensitive) to altitude changes.

As I said, i have ridden at 7-8 K ft with CV carbs, plains (factory) setup. Am sure AFR was not perfect, and a plug chop would surely throw up a sooty plug. But ridingwise, still quite rideable. Am surprised at Eric's bikes extreme sensitivity to altitude now that it is confirmed that it has a CV carb.

Regards
Sutripta

It's not Eric's Impulse alone, many riders who've done the western mountain circuit on the Impulse have had to resort to downjetting/wire-in-the-jet method to get by. It's even more surprising considering that Hero MotoCorp usually uses leaner jetting on its motorcycles for maximum fuel efficiency. Many Unicorns/CBZ X-Tremes that have done the same circuit don't seem to have major jetting issues. So, I feel that the Impulse is a one off case. Or perhaps, Eric is too used to the RTZ125's crisp response even in rarified atmospheric conditions.

Cheers,

Jay

[ringoism]

Originally Posted by Sutripta

True, but in a CV, the slide is the controlled element, the controller (air metering) being handled by the butterfly valve. In the slide type, the slide is the air metering device.

From your description, and my own experiences (Eastern Himalayas, ~ 7- 8 K Ft) i'd say the Impulse has a slide type carb, not a CV. Would want confirmation.


The letter/ number method of designating drill sizes.
I've never come across #nnn = n.nn mm in the smaller sizes, so am surprised! Do you remember which companies drill it was?

Regards
Sutripta

Impulse comes with a 28mm Keihin CV-type carb. Confirmed. As an aside, it's odd (to me) that the N. American CRF150F, which has the same engine and same bhp, uses only a 24mm Keihin carb (also CV).

As you say, the slide is "controlled" in a CV - on the other hand, it also is ultimately what does both the fuel and air metering, since it is only when it begins to lift (via vacuum levels informed by the butterfly valve) that any additional flow of either fuel (via the attached main jet needle) or air (via the slide itself) can pass through. In that sense, you could say that the only difference between a CV and a conventional slide carb is in WHAT is controlling the slide (vacuum, or a direct cable).

Practical corroborating observation: I've had the CV's vacuum slide get stuck closed on a few occasions. Butterfly could be wide open even, but no effect (beyond the equivalent maybe 1/4 throttle), since the slide wasn't moving, hence no additional air/fuel entering the engine. I could only do about 40kmph, till I got the slide unstuck, then everything was back to normal.

Anyway, we're probably talking semantics here - I think we both understand how it works.

Re: drills - no markings / numbers on them (too small for that); these were passed down from my grandfather, so unsure of their origin. I'm simply measuring them with a digital caliper and seeing that the mm-readings (i.e., 0.99, 1.01, 1.18, etc) match up perfectly to the stampings on jets of known sizes. Jet size designations seem to have been internationally standardized in metric sizes from way back - I think American cars of the 60's-70's had jets designated the same way, long before the metric standard even arrived in N. America.

Being that jets are produced in India (a friend in Delhi had met one of the manufacturers a few years ago), I suspect that these small sizes would be available in mill supply shops around Ajmeri Gate, Old Delhi. Thanks for reminding me to check for them there next time.

To get back closer to an Impulse-specific topic, one local mechanic suggested I fit the Pulsar 150's carb, that it would work better at altitude. Not sure if this is based on conjecture / whim or actual experience. But Pulsar's CV is a Mikuni, and externally larger despite similar venturi sizing. Makes me wonder whether the Mikuni's are better at altitude-compensation for some reason.

What Jay is saying below (forgot to multi-quote) re: other 150HH's/Hondas seeming to do okay in the W. Himalayas and the Impulse's unique trouble there makes me wonder about the the restrictiveness of the intake / exhaust systems (which would be the only unique components vs. the other models) - according to the local Hero technician, it is the only bike among them that is catalytic-converter equipped.

Having said that, I did see a substantial loss of power and some missing on the Unicorn I took to Chandra-Tal (14,000+ft) a couple years ago, though not as severe as the Impulse's (missing only started near the top, as I remember, where the Impulse was seeing it at under 8,000ft) - and my carpenter friend tells me his CBZ-Extreme has the same trouble up on the passes, as does a local mechanic with a Hunk. Even the KB did run a lot better jetted-down. Lost power without it, BUT didn't have the actual missing that the Hero sees.

As suggested earlier, think I might do simple mods to the pipe (silencer) / intake tract first, then raise compression to at least 10:1 (using a milled second-hand head), optimize my sparkplug, and see how it all responds, before resorting to big-bore and potential associated complications - a less intrusive and more incremental approach, and one that besides any power increases will almost certainly raise fuel mileage (via increased volumetric efficiency) rather than lower it (via larger displacement).

-Eric

[RishikeshK]

Warning: This is a long post describing my experiences with the Impulse on Leh trip.

I bought an Impulse just for the Leh trip which I completed in June this year. Biggest reasons I went for an Impulse were awesome suspension, high ground clearance and comfortable upright seating position. The Impulse proved itself in all these aspects and worked flawlessly. I had no back-pain whatsoever in all of the 1300km journey, the bike was easy on maneuverability, there were absolutely no rattles when the trip completed. So full points on suspensions, build quality and maneuverability. I have no complaints about these aspects at all.

However, the incessant fluttering and lowered engine power were two ugly enemies that plagued the beautiful experience. I took the bike in the stock form to Leh. The bike started fluttering from an altitude as low as Sonamarg (~8k ft). It climbed Zojila all in 1st and 2nd gear mostly at 7/8K RPM fluttering all the way. It had real tough time climbing Namika La (3700m) and Fotu La (4108m). I had to constantly keep the engine buzzing at 7/8K RPM and make heavy use of bike's momentum to do the job. Thankfully bike's handling capabilities and awesome suspension helped me in this but I did miss on the scenery and was focused solely on climbing to the top. The bike managed respectable speeds only on down-slopes. Even in the straights, it wouldn't go past 60kmph easily.

I had to resort to the wire-in-the-carb mod when I reached Leh (big thanks to niks_devil666 for runtime guidance). I knew that the bike simply wouldn't climb Khardung La in the stock form. The mod got rid of fluttering and power did increase to some extent. I could comfortably climb Khardung La without any fluttering. I was mostly doing 6/7K RPM 2nd/3rd gear till south pullu and 5/6K RPM strictly 1st gear from thereon till Khardung La top (mostly because of extremely bad road conditions).

I was mostly very happy with the wire-in-the-carb mod since that was the sure-shot way out for a non-technical person like me. However, this mod didn't fix the problem altogether. I was severely low on power in the Moore Plains. The bike just wouldn't move faster than 60kmph. All the Bullets, an Avenger 220 and even a Pulsar 150 (all stock) in our group were having gala time speeding away to glory. They would slow down for me to catch up and zoom ahead instantly doing 100kmph. I had real frustrating time in this patch because of low bike power. (I did make up for that on really rough roads enroute Rohtang pass because of suspension/handling advantages ). I still feel that something is amiss in Impulse's engine tuning for higher altitudes. The stock Pulsar 150 didn't flutter ever and experience very low power loss even with much heavier rider (90kg in comparison with my 70kg). So the fun part in fast riding in mountains was definitely severed because of the engine tuning (even with wire mod).

I did meet Impulse owners at three different occasions during the trip.

1. First, I met an Impulse rider (from Delhi) at Zojila. His bike was facing exact same issues as my bike uphill Zojila. We couldn’t ride in sync with him and so don't know how he addressed the issue.
2. Second, I met one gentleman (from Hyderabad) in Leh who rode to Leh from Manali side on an Impulse. He said he got the air-filter removed at Manali itself and was mentioning that it solved pretty much all the power issues. He mentioned that the bike was very smooth and could do 100kmph with ease. He didn't seem to be bothered about the dirt getting into the carb (and the engine) though. He was ok to replace all the damaged parts upon completion of the ride.
3. Third, there was a group from Bangalore with 3 Impulse riders riding to Leh from Manali side. All three Impulses behaved differently. One Impulse could easily carry two-ups with wire mod, the second Impulse could carry single rider ok with the wire mod and the third one was having real tough time climbing even with the wire mod.

It is sad that the bike which is touted as a mountain bike (even the bike’s advertisement pamphlet shows a rider riding in Himalayas) fails on the engine tuning front so badly. It does need some deep analysis as is being done by ringoism. An after-market Leh-kit would be awesome for making this bike a complete Leh package.

I apologize for the long post but I thought my experience with Impulse would help the technical discussions happening here.

[Sutripta]

Originally Posted by ringoism

As you say, the slide is "controlled" in a CV - on the other hand, it also is ultimately what does both the fuel and air metering, since it is only when it begins to lift (via vacuum levels informed by the butterfly valve) that any additional flow of either fuel (via the attached main jet needle) or air (via the slide itself) can pass through. In that sense, you could say that the only difference between a CV and a conventional slide carb is in WHAT is controlling the slide (vacuum, or a direct cable).

Practical corroborating observation: I've had the CV's vacuum slide get stuck closed on a few occasions. Butterfly could be wide open even, but no effect (beyond the equivalent maybe 1/4 throttle), since the slide wasn't moving, hence no additional air/fuel entering the engine. I could only do about 40kmph, till I got the slide unstuck, then everything was back to normal.

Anyway, we're probably talking semantics here - I think we both understand how it works.

We might know, but other readers might not, so I think we should be accurate.
I would say that the slide in a CV does not meter air. Metering is done by the butterfly valve.
Nowadays (except on bikes) carburettors have gone the way of the Dodo, but at one time SUs and Stromberg CDs were quite popular. And any good explanation of their working would highlight this point.

Re: drills - no markings / numbers on them (too small for that); these were passed down from my grandfather, so unsure of their origin. I'm simply measuring them with a digital caliper

The correct way!

and seeing that the mm-readings (i.e., 0.99, 1.01, 1.18, etc) match up perfectly to the stampings on jets of known sizes. Jet size designations seem to have been internationally standardized in metric sizes from way back - I think American cars of the 60's-70's had jets designated the same way, long before the metric standard even arrived in N. America.

Dimension vs. flow? Weber? Or in the American context, Holley?

Being that jets are produced in India (a friend in Delhi had met one of the manufacturers a few years ago), I suspect that these small sizes would be available in mill supply shops around Ajmeri Gate, Old Delhi. Thanks for reminding me to check for them there next time.

In that case might I suggest you making yourself a flow comparator/ calibrator. (Believe me, if you don't want to emulate a dog chasing its own tail, you will need it).

To get back closer to an Impulse-specific topic, one local mechanic suggested I fit the Pulsar 150's carb, that it would work better at altitude. Not sure if this is based on conjecture / whim or actual experience. But Pulsar's CV is a Mikuni, and externally larger despite similar venturi sizing. Makes me wonder whether the Mikuni's are better at altitude-compensation for some reason.

Maybe someone could list out all the bikes, and their carbs, for reference.

I am now officially intrigued by the extreme loss of power aspect of the Impulse.

Regards
Sutripta

[ringoism]

Just wanted to thank everyone for their excellent contributions here - I suspect a solution cannot be too far off now:

Originally Posted by Sutripta

I would say that the slide in a CV does not meter air. Metering is done by the butterfly valve...at one time SUs...were quite popular. And any good explanation of their working would highlight this point.

In that case might I suggest you making yourself a flow comparator/ calibrator. (Believe me, if you don't want to emulate a dog chasing its own tail, you will need it).

I am now officially intrigued by the extreme loss of power aspect of the Impulse.

Indeed, many of us are intrigued (and also perplexed / frustrated / exasperated... and at worst, remorseful...).

Had to Google "flow comparator"; Could "jugaad" it probably, but otherwise looks pretty pricey vs. a handful of jets / drills, which have normally worked pretty straightforwardly / predictably / accurately enough for me in my past tuning experiences. The Indian-made jet sizes don't seem to be far off their markings, either. There are at least a few carb-component exporters in Karol Bagh, and I'm guessing those Rs10-20 jets end up in the $80 tuning kits you find in the motorcycle specialty houses abroad...

Been reading through various SU pages - thanks for that reminder. Was not wholly unfamiliar with these, having owned three vehicles ['71 Datsun 510 w/ the Japanese-market SSS carb kit, '66 Volvo PV544 Sport, and a 1969 MG Midget] that had twin-SU setups - though I confess I never touched them, as (surprisingly) they always performed flawlessly...

I've gotta be careful not to hi-jack my own thread here... but anyway, couldn't find anything explicitly defining air metering, but think you're probably right - the slide in a CV(SU) seems more about altering venturi size / maintaining constant velocity / vacuum than about controlling (metering) actual volume of air flow.

Only trouble I'm still having is this, I think from the Wikipedia page (as also stated by one bhpian and myself earlier): "The slide carburettor has the same piston and main needle as an SU carburettor, however the piston/needle position is directly actuated by a physical connection to the throttle cable rather than indirectly by venturi airflow as with an SU carburettor. This piston actuation difference is the significant distinction between a slide and an SU carburettor. The piston in a slide carburettor is controlled by the operator's demands rather than the demands of the engine."

So here's the thing: if CV's and slide carbs are essentially the same except for method of slide actuation, and it's the butterfly valve in a CV that meters the air, then does it follow that there is no air metering in a slide carb, since it has no butterfly valve???!!! Hmmmmmmm.........

Originally Posted by RishikeshK

The bike started fluttering from an altitude as low as Sonamarg (~8k ft). It climbed Zojila all in 1st and 2nd gear mostly at 7/8K RPM fluttering all the way. It had real tough time climbing Namika La (3700m) and Fotu La (4108m). I had to constantly keep the engine buzzing at 7/8K RPM and make heavy use of bike's momentum to do the job. Thankfully bike's handling capabilities and awesome suspension helped me in this but I did miss on the scenery and was focused solely on climbing to the top...

...I met one gentleman (from Hyderabad) in Leh who rode to Leh from Manali side on an Impulse. He said he got the air-filter removed at Manali itself and was mentioning that it solved pretty much all the power issues....

I apologize for the long post but I thought my experience with Impulse would help the technical discussions happening here.

Re: your stock performance description at altitude, I feel your pain. Was so much more pleasant those years ago on the KB100 (my first KB) running with extreme ease over Zoji-La, even with that very attractive young Ladakhi ex-nun riding pillion (very temporarily, mind you)... Alas, the Impulse really takes all the fun out of riding when you're just wondering if you'll make it to the top and abusing it so badly to finally do so. Anyway, you should definitely not be apologizing; What you shared of your own and others' experiences is extremely helpful.

Re: air filter removal, it's actually the first "fix" I'd heard, a couple years back right after the Impulse was introduced. A few guys riding through Manali on them had done it and felt it had a good effect. And the showroom gave me a K&N type filter at the time of purchase, too, saying it would help. Thing is, the stock filter element looks plenty large and it's honestly really hard for me to believe that it alone, even when fresh/clean, could mess things up so badly. I mean, how much difference in size / material type is that element from those in the other HH / Honda bikes, that are supposed to run fine? Anyway, sans filter, ruining a several thousand rupees piston / cylinder lining is pretty much guaranteed considering the dusty conditions along much of the route, and if you were going to do it more than once, it'd definitely work out cheaper to just to the 223cc ZMA conversion. Gives some good clues as to the main source of trouble, though.

Originally Posted by Sutripta

And that is a huge, fundamental difference! CV carbs are less sensitive (not insensitive) to altitude changes.
As I said, i have ridden at 7-8 K ft with CV carbs, plains (factory) setup. Am sure AFR was not perfect, and a plug chop would surely throw up a sooty plug. But ridingwise, still quite rideable. Am surprised at Eric's bikes extreme sensitivity to altitude now that it is confirmed that it has a CV carb.

Yes, I was surprised, too - these "less sensitive" CV-equipped bikes would normally not be so badly affected as the Impulse seems to be. Quite mysterious... But as Rishikesh mentioned above, the trouble didn't really start appearing till 8,000ft, so in your case (mostly below that) you couldn't really expect to observe it as severely as others have at twice and more above that height. In fairness, the Keihin-CV-equipped CL350 along with us was also missing ("fluttering") pretty badly at Thanglang-La, etc - where with the smaller main jet installed, I was actually able to run a lot faster uphill on the Impulse than he could.

Originally Posted by vishwaschettri

My advice will be to try more with main jets and needle positioning before sliding on to different heat range plugs. They are a pain to find even at Delhi and IMHO not worth the effort.

Thanks for your thoughts - indeed it may not make much difference, though NGK's tech pages got me seriously thinking it could...

I bought (a few years ago now) various heat ranges for the KB right on Karol Bagh main road... can't remember the shop name (have the card somewhere), but it's one of the first shops on the right as you walk from the (Hanuman temple) Metro station. Had a full range of NGK's on an open shelf behind the counter. Plugs cost more than jets, but still pretty affordable and a bit quicker / easier to change on the roadside! Assuming availability, I feel it's a pretty feasible thing to try, at least once. If I go hotter and it doesn't help, I've just got an extra plug for my spares kit.

Originally Posted by JayPrashanth

Many Unicorns/CBZ X-Tremes that have done the same circuit don't seem to have major jetting issues. So, I feel that the Impulse is a one off case.

And if true, THAT's the truly perplexing part. but it also might hold the key to the final solution, since everything but the pipe/silencer and airbox are virtually the same as the other 149cc Honda-origin bikes. The camshaft is likely a little different, too, but that's supposed to be a "high-torque" profile which creates more cylinder pressure and power at lower rpm anyway, which should do BETTER than the CBZ, etc, at altitude... So the pipe and airbox. Simple.

I got reading about the effects of altitude on dynamic (vs. static) compression ratio, and am realizing what a big difference having a little more cylinder pressure could make for power (not speaking of missing now). Easiest way of doing THAT would be to advance the camshaft timing a little - which I can do pretty easily with a slightly modified chain sprocket... These are old hot-rodder tricks, actually.

Mmmm... We've got all sorts of ideas here, and sooner or later, the beast has just GOTTA run right!!!

If I ride up high towards the end of this month, I'll put a hotter plug (if nothing else the locally-available Bosch, which is at least one range hotter), and the K&N-type open-element filter right on the carb and see what happens. Five minutes and fifty rupees investment. If that doesn't do it, will do a custom low-restriction rear section for the pipe/silencer (can). If I'm still struggling, will install and re-tune the 27mm Mikuni CV I've got sitting around (plenty big enough for a 150, but 1mm smaller bore than stock), and maybe do the custom cam gear, I guess in that order. None of this is going to cost much. If these don't work, then I'll be totally stumped and will wish I'd simply prayed for divine wisdom a lot earlier. Come to think of it, I think I should do that right now...

But I quite enjoy this interactive real-world (re)engineering - vs. the desk jobs.

(or should I more humbly call this tinkering?).

Thanks again to you all,
-Eric

[Sutripta]

Originally Posted by ringoism

Had to Google "flow comparator"; Could "jugaad" it probably, but otherwise looks pretty pricey vs. a handful of jets / drills, which have normally worked pretty straightforwardly / predictably / accurately enough for me in my past tuning experiences.

Mine was also selfmade. Not costly (except that I had to get two special odd size taps made)
Guess you're lucky. Or I was unlucky!

The Indian-made jet sizes don't seem to be far off their markings, either.

You do know how to trim jets?

There are at least a few carb-component exporters in Karol Bagh, and I'm guessing those Rs10-20 jets end up in the $80 tuning kits you find in the motorcycle specialty houses abroad...

Jets can be turned out very cheaply by the thousands on a capstan/ automat. Or abroad on a Swiss lathe. The cost in a jet comes from its calibration, not the cost of the base jet. (much like the difference in cost between a normal weight, and a weight with a test certificate.)

The different flow of otherwise 'identical' jets becomes very apparent if one works with multibarrel carbs. When plug chops don't throw up identical plugs.

I'm talking of cars with fixed venturis. In the SU, fuel metering was controlled by the metering needle profile. The 'jet' was just a brass tube.

Anyway, your call!

Been reading through various SU pages - thanks for that reminder. Was not wholly unfamiliar with these, having owned three vehicles ['71 Datsun 510 w/ the Japanese-market SSS carb kit, '66 Volvo PV544 Sport, and a 1969 MG Midget] that had twin-SU setups - though I confess I never touched them, as (surprisingly) they always performed flawlessly...

Very simple carbs. And the metering needle moving in the 'jet' kept it clean. But the SU also had the reputation of being a 'gross polluter'!

Only trouble I'm still having is this, I think from the Wikipedia page (as also stated by one bhpian and myself earlier): "The slide carburettor has the same piston and main needle as an SU carburettor, however the piston/needle position is directly actuated by a physical connection to the throttle cable rather than indirectly by venturi airflow as with an SU carburettor.

Calling the piston to be the same is wrong, and is the source of subsequent confusion.
So

So here's the thing: if CV's and slide carbs are essentially the same except for method of slide actuation, and it's the butterfly valve in a CV that meters the air, then does it follow that there is no air metering in a slide carb, since it has no butterfly valve???!!! Hmmmmmmm.........

the slides are essentially not the same! As I said before,

Originally Posted by Sutripta

In the slide type, the slide is the air metering device.

But as Rishikesh mentioned above, the trouble didn't really start appearing till 8,000ft, so in your case (mostly below that) you couldn't really expect to observe it as severely as others have at twice and more above that height.

I thought you said you sere having problems even in Manali ~ 7000 ft.

Easiest way of doing THAT would be to advance the camshaft timing a little - which I can do pretty easily with a slightly modified chain sprocket... These are old hot-rodder tricks, actually.

On a single cam engine, the valve overlap remains the same. Whether the engine runs better, worse, or about the same depends on too many factors. Can't be predicted. Truth only after instrumented testing! (On Marutis, one tooth out, and the engine would run badly!)

More later, if you're keen.

Regards
Sutripta