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Old 9th June 2012, 21:40   #46
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Default Re: How to tune your carb properly.

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Originally Posted by headers View Post
Which car and which carb are we talking here? Looks to me like a poor amalgam of Ignition, timing as well as carb setting. Can you share details? Pictures?
BMC/ HM B series, 1750 cc (very unusual!). 2x Solex M32PBIC

Quote:
Originally Posted by Sankar View Post
Sizes on Jet always doesn't mean the same thing. Some are numbered according to flow and some are numbered according to the hole size.
I think everyone knows that.
My suggestion: take any two 'identical' jets. Put these in a flow comparator and see. Esp after you have 'cleaned' it and poked it with foreign objects. Wooden toothpicks, or soft (electrical grade) copper wire shouldn't affect flows, right?

Quote:
Flatspots and sudden loss of power while whacking open the throttle is prolly due to a lean mixture idle and mid circuit. In carbs with accel pump which pumps fuel under such whacking open /stomping scenarios richening the accel pump circuit if possible would mitigate this issue to an extent. Using a carb too large for the engine (low flow velocity at the venturi and lower vacuum to pull in enough fuel) is one reason why this happens, if the carb is right and still this happens then its because idle to mid cricuit being very lean.
Also all well known. And add Headers observation of a iffy ignition setup.
And knowing all this didn't help then.
In a street engine, light load running was kept lean. At least in the pre cat pre emission days. Idea was not to lean it out so much that it caused problems.

Somethings I suppose remain baffling. Like sometimes, a change of spark plugs changes the character of an engine. When there is apparently nothing wrong with the old set. And which works fine on another vehicle. (This is more applicable to 2 stroke bikes than cars)

Regards
Sutripta

PS. @A_Kamath: Your post came in while I was composing mine.
AFAIK, till 4 cyls (in a 4 stroke), air flow is pulsed. Prorata does not work.

Last edited by Sutripta : 9th June 2012 at 21:46.
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Old 10th June 2012, 06:26   #47
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Default Re: How to tune your carb properly.

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Originally Posted by Sutripta View Post
Somethings I suppose remain baffling. Like sometimes, a change of spark plugs changes the character of an engine. When there is apparently nothing wrong with the old set. And which works fine on another vehicle. (This is more applicable to 2 stroke bikes than cars)

Sirji, This is basics : A change in spark plug will change the way the engine develops power - The length of the spark, the strength of the spark over that length, the pulsing frequency of the spark, the amount of spark etc..
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Old 10th June 2012, 07:54   #48
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Default Re: How to tune your carb properly.

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Originally Posted by Sutripta View Post
AFAIK, till 4 cyls (in a 4 stroke), air flow is pulsed. Prorata does not work.
Does not work? Hmmm... The formula is correct and will work with the factors I've provided, BUT is heavily dependent on the type of manifold and the type of carb (to reiterate my post on carb sizing).

Tip No 1: some carbs have variable venturi size.
Tip No 2: Its all in the manifold. You figure out how to get rid of pulsing in a 4 cylinder engine (actually someone already did way back and won many many many races during the Sholavaram era).

This is my last post on this subject. Thanks all for your patience and good luck with your experiments.
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Old 10th June 2012, 08:11   #49
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Default Re: How to tune your carb properly.

Quote:
Originally Posted by Sutripta View Post
I think everyone knows that.
My suggestion: take any two 'identical' jets. Put these in a flow comparator and see. Esp after you have 'cleaned' it and poked it with foreign objects. Wooden toothpicks, or soft (electrical grade) copper wire shouldn't affect flows, right?
Just made that point because:

Quote:
Originally Posted by Sutripta View Post
By insights, I mean something like this: Learned the hard way never to trust the sizes written on jets (at least in the Indian context). Beg, borrow, or steal a (or in my case, make your own) flow comparator/ calibrator, sort out your jets, and then get to work.
Didn't think the jet sizes numbering norms were known. And no there are people who still doesn't understand the numbering differences between diffrent types of jets.

Quote:
Originally Posted by Sutripta View Post
Also all well known. And add Headers observation of a iffy ignition setup.
And knowing all this didn't help then.
In a street engine, light load running was kept lean. At least in the pre cat pre emission days. Idea was not to lean it out so much that it caused problems.
Rich you mean?
Unelss the carb was way too big there shouldn't have been any problems with getting the snap open response good unless the jetting/timing/compression was not ideal for the setup. If the carb had an accel pump setup tuning that circuit would've helped.
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Old 10th June 2012, 18:35   #50
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Default Re: How to tune your carb properly.

I finally got around to tuning the carb on my 96' 500 and got a good result, thanks to this thread.

I have fiddled with it many times before, but have always ended up making the tuning worse and going to my mech to get it perfect again.
Recently, due to a bad timing on my 500 which was not at first diagnosed, the tuning was adjusted instead in which the air screw was increased(loosened).

Yesterday I had turned it almost one and half turns after which I checked the plug and it was almost white and the engine was pulling well, but could feel it was strained .This morning, I made it a bit tighter which was like loosening the air screw anti-clockwise 180 degress or half turn and it made the engine much smoother. Did a ride after that to check if it made a difference and I enjoyed the ride totally.

I have a doubt though, should the air screw be open at all or is it for added performance only.? Since on my 350 Electra which runs the same type of carb, (but the screws are on the opposite side compared to my 500), the air screw is locked shut and so is a third screw which sits between the carb and the engine. This screw actually had a tiny opening which had a tube connecting to the resonance box which the later bikes (post 2002) came out with. Is this for emission purpose only ?

I have attached pictures of both the carbs, the first is of the 500.
Attached Thumbnails
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How to tune your carb properly-img352.jpg  

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Old 10th June 2012, 19:41   #51
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Default Re: How to tune your carb properly.

Quote:
Originally Posted by ananthkamath View Post
Does not work? Hmmm... The formula is correct and will work with the factors I've provided, BUT is heavily dependent on the type of manifold and the type of carb (to reiterate my post on carb sizing).


Tip No 2: Its all in the manifold. You figure out how to get rid of pulsing in a 4 cylinder engine (actually someone already did way back and won many many many races during the Sholavaram era).
A plenum chamber based design is an option for V8s. But for an I4?
More to the point is what size does on choose when going from 1 carb feeding 4 cyls to 1 carb feeding 2 cyl ((1,2) & (3,4)) or ((1,4) & (2,3)), or 1 carb feeding 1 cyl. Using traditional manifolds.

Quote:
Tip No 1: some carbs have variable venturi size.
Relevance to current discussion - carb sizing for one barrel for 1 or 2 cylinders?

Quote:
Thanks all for your patience and good luck with your experiments.
Carbs for cars:- dead subject.


Quote:
Originally Posted by headers View Post
Sirji, This is basics : ....
I remain baffled.

Quote:
Originally Posted by Sankar View Post
Rich you mean?
No, lean. People never wanted to pay for petrol. Esp when you are just loitering.

Regards
Sutripta
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Old 10th June 2012, 20:15   #52
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Default Re: How to tune your carb properly.

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Originally Posted by tharian View Post
I have a doubt though, should the air screw be open at all or is it for added performance only.? Since on my 350 Electra which runs the same type of carb, (but the screws are on the opposite side compared to my 500), the air screw is locked shut and so is a third screw which sits between the carb and the engine. This screw actually had a tiny opening which had a tube connecting to the resonance box which the later bikes (post 2002) came out with. Is this for emission purpose only ?

I have attached pictures of both the carbs, the first is of the 500.
The Electra carb's screws aren't locked shut, its there airscrew and the idle screw both are accessible. The VM24 and VM28 on these two bikes are from the same Mikuni series as you might already know, but the screws are on the different sides.

Airscrew should be set as required its for optimum performance of the engine.

The other screw you mention is not part of the carburetor its an RE thing, they placed a piece of aluminium between the carb and the engine with a screw on it. Loosening this screw lets in more air. This is a trick to make the engine run leaner, letting in extra un-metred air into the engine after the carb. This leans out the mixture as cab doesn't see this airflow it doesn't add more fuel.

Hope that screw is well tightened and shut, if not the engine is being fed dirty air since there is no pipe connecting that thing to airbox.
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Old 10th June 2012, 21:52   #53
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Default Re: How to tune your carb properly.

Quote:
Originally Posted by Sankar View Post
The Electra carb's screws aren't locked shut, its there airscrew and the idle screw both are accessible. The VM24 and VM28 on these two bikes are from the same Mikuni series as you might already know, but the screws are on the different sides.

Airscrew should be set as required its for optimum performance of the engine.

The other screw you mention is not part of the carburetor its an RE thing, they placed a piece of aluminium between the carb and the engine with a screw on it. Loosening this screw lets in more air. This is a trick to make the engine run leaner, letting in extra un-metred air into the engine after the carb. This leans out the mixture as cab doesn't see this airflow it doesn't add more fuel.



Hope that screw is well tightened and shut, if not the engine is being fed dirty air since there is no pipe connecting that thing to airbox.

Yes, it is fully tightened, I dont think it was ever opened at all. So if it connect a tube and open it a bit, what difference will it make to my mileage/performance?

By locked shut, I meant the air screw is in the fully tight/closed position on my 350. I should have worded it better.
However, I checked again and it was half a turn open from fully closed position. If the air screw is in fully closed position, I understand the engine is running rich?
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Old 11th June 2012, 11:47   #54
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Default Re: How to tune your carb properly.

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Originally Posted by tharian View Post
Yes, it is fully tightened, I dont think it was ever opened at all. So if it connect a tube and open it a bit, what difference will it make to my mileage/performance?
You will get a slightly better fuel economy but the engine might run hotter.
Leaning out a wee bit could produce more power, but its risky power. It has to be done by trial and error and set it just right otherwise there is a risk of engine damage by over heating.

I'd say don't bother with that manifold screw unless you're looking for some more mileage. It was put there so that the CI350 could meet the emission norms and provide more FE.

Quote:
However, I checked again and it was half a turn open from fully closed position. If the air screw is in fully closed position, I understand the engine is running rich?
Technically yes. But to see how rich it has to be checked and adjusted.
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Old 12th June 2012, 11:36   #55
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Default Re: How to tune your carb properly.

Hi Guys,


I have a Bajaj Avenger 200 CC, 2009 Model.

Would changing/Upgrading the carb help in performance improvement? And would it hit the FE?
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Old 10th July 2012, 20:44   #56
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Default Re: How to tune your carb properly.

Hi experts, My Royal Enfield standard uce 350 runs on bs29 carbs and was running well till the onset of the monsoon when the bike started to ping on morning rides at normal city speeds (40 to 50 kph). In the evening, ie on the return ride from the office, there is no pinging. My bike had it fuel screw turned out 3 full turns and was running slightly rich. I took the machine for a tune up and has ended up with 5 full turns of the fuel screw to achieve the previous smoothness without the engine pinging. What could have happened all of a sudden? A clogged pilot jet ? Vaccuum leak? Change in climate ? Please help me out with this as I am for sure about to drop my bikes FE which was 42 kmpl with 3 turns on the fuel screw. With the fuel screw at 3 turns out, my bike used to start on the first kick without choke in the morning, but it took some time to warm up. There was no overheating but for the pinging that I took it for the tune up.
Thank you all in advance,
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Old 12th July 2012, 10:01   #57
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Default Re: How to tune your carb properly.

Hi everyone, got the problem solved. It was due to exhaust leakage in between the bend pipe and the muffler.
regards adrian
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Old 23rd July 2012, 09:25   #58
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Default Re: How to tune your carb properly.

Carb gurus: Need an advice. I took possession of a Suzuki fiero recently and would like to perform a carb tune. From this post i understand that the carb has a fuel screw and not an air one. What is the best tune for max mileage? I have done tunes on my splendor but it used to be air screw and turning the screw used to return some engine feed back in terms of RPM. I used to do unlock 1 1/2 turns from lock but this doesn't seem to work for fiero. Please advice.
Thanks in advance.
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Old 24th July 2012, 09:39   #59
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Default Re: How to tune your carb properly.

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Originally Posted by ameaa View Post
Carb gurus: Need an advice. I took possession of a Suzuki fiero recently and would like to perform a carb tune. From this post i understand that the carb has a fuel screw and not an air one. What is the best tune for max mileage? I have done tunes on my splendor but it used to be air screw and turning the screw used to return some engine feed back in terms of RPM. I used to do unlock 1 1/2 turns from lock but this doesn't seem to work for fiero. Please advice.
Thanks in advance.
Hi ameaa, try the following link. A tacho would be handy if the fiero hasn't got one.

How To Tune Your Motorcycle For Performance and Fuel Efficiency

And by the way, the fiero being a four stroke bike has a fuel screw instead of an air screw and I think the engine rpm begins to rise somewhere after 2 full turns. Any way all the best with your tuning
regards adrian

Last edited by adrian : 24th July 2012 at 09:49.
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Old 25th April 2013, 14:44   #60
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Default Re: How to tune your carb properly.

Bullet Gurus need your help here..

Faced a lot of issues on my Previous bullet and got a new BULLET 350 UCE about a month back. I felt the bike was running very lean so I just checked the plugs to confirm the same. The current setting was 2 full turns (360) plus a half turn (180). Since the service manual also suggests 31 full turn so I just put the current setting to about 3.25 full turns. I'm attaching the pic of the spark plug in previous setting. Please comment whether the colour also suggests the same?? Was I right in fiddling with the setting and making it rich or shall I revert back to the previous one??

P.S. The engine is running on a freer flowing exhaust (genuine short bottle exhaust procured from RE showroom) and struggles to touch 100kmph whereas my previous uce bull touched 120
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