[black12rr]

Thanks ananth for the gyan , good to see you back after long time .

[Sutripta]

Originally Posted by ananthkamath

I'm not an expert but can speak from experience, having worked on pretty much every single popular carb out there ranging from the Mikuni VM and BS series to Keihin flatslide to the Solex and SU carbs found on Fiats and Dolphins, the Jikov and Pacco found on Jawas, racing carbs on karts and formula cars etc.
........
So like I said in the beginning, it takes a lot of time to learn carb tuning skills but its not hard. It just takes a lot of experimentation and discipline on your part (i.e. keep notes of every adjustment you make, and the results) to achieve anything useful. Take it to the mechanic if you don't want to put in the time!

Hi,
Before we get deeper into carb setup, would like to add a couple of questions to headers list.

From a carb point of view, what tis the difference between
a) cars and bikes
b) 2 strokes and 4 strokes.

Later on would like to have your (or anybody elses) views on how to set up the carb for smooth transition from low load part throttle operation to WOT.

Regards
Sutripta

[ananthkamath]

Originally Posted by Sutripta

Later on would like to have your (or anybody elses) views on how to set up the carb for smooth transition from low load part throttle operation to WOT.

Regards
Sutripta

I already explained it in my post. These are things that are best learnt observing someone doing it first hand. It cannot be explained very well IMO, but there may be someone better than me at putting it into words.

[Sutripta]

Hi,
Thanks for your lengthy post, but must say though you

Originally Posted by ananthkamath

already explained it in my post.

really couldn't find specific answers.

Have dabbled with a carb or two in my time, but that time is long gone!

Regards
Sutripta

[headers]

Originally Posted by Sutripta

Later on would like to have your (or anybody elses) views on how to set up the carb for smooth transition from low load part throttle operation to WOT.

Hi, what carb are we talking of here? And can you explain what exactly you mean by a smooth transition from low load part throttle to WOT?

Are you talking 10%, 30% to WOT? or am i missing something? Do you get a jerk in transition or is it just a dull moment

[Sutripta]

^^^
My experience (of setting up carbs) is more (make that most) with cars, not bikes.

We were working on the carbs (Solex, Weber, DellOrto. No Holleys etc) to get high end power. So a twin carb setup. I found it difficult to get a smooth transition from part throttle (say 25% throttle) to WOT if you stomped on the pedal. Flat spots and instability.

Easily circumvented by not snapping open the throttle, but that was a workaround, not a solution.

Regards
Sutripta

[ananthkamath]

Originally Posted by Sutripta

Hi,
Thanks for your lengthy post, but must say though you

really couldn't find specific answers.

There are literally hundreds of different types of carbs, and I wrote the procedure I have used successfully in the past to calibrate Mikuni VM motorcycle carbs. Which carb specifically are you facing problems with?

Let me repeat that bit about transition again:

Originally Posted by ananthkamath

Start with the needle in the highest position (i.e. lowest slot) and keep reducing until you get very crisp acceleration at between 1/2 and 3/4 throttle. You have to play with mixture screw (the one that's on the outside of the carb) adjustment at the same time as needle position adjustment, in order to get the crispest response between 1/8 and 1/3 throttle.

Next is the off-idle and idle adjustment which is again an iterative process between the mixture screw and idle speed screw. Generally the crispest throttle response for spirited driving is not obtained at the manufacturer recommended idle speed - they do it for emission and fuel economy reasons. You may have to go a little higher and when you are done, the vehicle will take off very crisply from off-idle and transition to 1/2 throttle in a very smooth manner.

Sometimes the pilot jet size needs to be finely adjusted if the engine response is not as crisp as desired. It takes a lot of experience to reach this stage unless the carb is from a completely different engine.

Originally Posted by black12rr

Thanks ananth for the gyan , good to see you back after long time .

Thanks man, I just check in once in a while and try to post something useful.

[Beast_of_Burden]

Ananth has summarized carb setup succintly, it pretty much mirrors my own experience tuning a VM28 for the LB500(long story). It is a rather tricky business and needs lots of patience and time. The exercise will sorely test your endurance and stamina and you will have a new found respect for your friendly neighbourhood mechanic.

That said, I initially tuned the BS29 which is a CV carb by varying the pilot jet and main jet sizes. Followed by the needle height. Only to confirm the stock setup was already pretty optimal - ended up with a pilot size of 17.5 over the stock 15 - thats about it. Fact is most carbs from the factory are setup to optimize driveability and reliability(dare I say efficiency) in real world conditions. Also remember that the carb is one component in the chain - intake filter, porting, valve sizes, liquid cooling, exhaust pipe, etc are all interdependant. IMO very hard to better the factory setup in terms of driveability. Tuning for racing is another story - less emphasis on efficiency and driveablity and certainly maximum emphasis on flat out performance.

Personally I loved the feel of the VM28 over the BS29 on the LB500. Very very different.

[ananthkamath]

Originally Posted by Sutripta

^^^
My experience (of setting up carbs) is more (make that most) with cars, not bikes.

We were working on the carbs (Solex, Weber, DellOrto. No Holleys etc) to get high end power. So a twin carb setup. I found it difficult to get a smooth transition from part throttle (say 25% throttle) to WOT if you stomped on the pedal. Flat spots and instability.

Easily circumvented by not snapping open the throttle, but that was a workaround, not a solution.

Regards
Sutripta

This may or may not be a jetting issue. Its pretty easy to over carb an engine. With a bigger-than-necessary carb, the reason for poor running is not enough vacuum (more vacuum = more pressure drop across the venturi = easier to jet), so you have to be conservative with carb sizing.

A very old formula for carb sizing that I have had much success with is:

D = K*sqrt(C*N)

D = carb venturi size in mm
C = engine displacement in liters
N = peak power RPM

The engine displacement should be per venturi, so if you have a single DCOE on a 4 cylinder engine, C = half the total displacement.

K is related to the volumetric efficiency of the engine, and also the type of carb. For a street two stroke like a RX135 or KB125 I have found K = 0.7 to work very well for a round slide carb- any bigger doesnt net too much for a mildly ported street engine. Older piston-ported jawas etc. more like 0.6. For an old Fiat 1100 race engine that had a single DCOE (these have two venturis) on a very trick manifold and made peak power around 6500 RPM, K = 0.75 seemed to work well. Street tuned modern engines like Esteem G13B etc. like around the same number 0.7 to 0.8.

On the other end of the scale, the 125cc Rotax engine in my kart (a very modern reed valve two stroke, makes peak power at 12500 rpm) runs extremely well at K = 0.85.

All this being said, with large twin carbs on a high revving engine, you wont get good throttle response when you're revving it with no load. The question is, at what point does this start to become a problem when you are actually driving?

[headers]

Found some interesting explanations that makes understanding Carbs better..

[voodoochild]

Originally Posted by headers

Found some interesting explanations that makes understanding Carbs better..

Headers, there's a very interesting article on carb tuning in the Gordon Jennings book on Two-Stroke Tuning also. Should be applicable anywhere, considering the basic carb remains the same for cars and bikes. A copy of that book is freely available on the internet.

Cheers,
Rahul

[Sutripta]

Originally Posted by ananthkamath

This may or may not be a jetting issue. Its pretty easy to over carb an engine. With a bigger-than-necessary carb, the reason for poor running is not enough vacuum (more vacuum = more pressure drop across the venturi = easier to jet), so you have to be conservative with carb sizing.

Overcarbing (and too much 'porting') are very well known mistakes of youthful exuberance. First thing not only to be checked, but also dialed back. In this case, I can say that overcarbing was not was not the case.

Was asking if someone had any insights to share. Not a problem solving answer. Esp considering that I've not had that vehicle/ carbs for close to two decades now.

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.

The engine displacement should be per venturi, so if you have a single DCOE on a 4 cylinder engine, C = half the total displacement.

You sure of this? The halving I mean.

The question is, at what point does this start to become a problem when you are actually driving?

For me, it was more a matter of why can I not get it right, rather than a half second hiccup/ flat spot being a problem which made the car undrivable.

Regards
Sutripta

[headers]

Originally Posted by Sutripta

For me, it was more a matter of why can I not get it right, rather than a half second hiccup/ flat spot being a problem which made the car undrivable.

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?

[Sankar]

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. Eg HEX jets and Round Jets. Jet numbering between Hex Main Jets and Round Main Jets that may fit the same carb (for eg Mikuni TM/VM/BS) means different things. So if you need to cross reference jet sizes don't go by the numbers alone.. measure the hole too its good enough to get it closer. Use a toothpick or large safety pin to get a size approximation if you don't have the right tools.

Then...4 stroke carbs need better fuel atomisation compared to 2 strokes since intake velocities are higher in 4 stroke engines, this is to reduce fuel shear. So.. for eg a VM carb that may be used in a 2 stroke bike and a 4 stroke bike both have different emulsion tubes. But... this is not always a limitation and sometimes either work decently well on the other and that depends on the carb and how well its been tuned.

Another point... always get the ignition timing spot on before starting to tune the carb.

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.

[ananthkamath]

Originally Posted by Sutripta

You sure of this? The halving I mean.

Pretty damn sure, but it depends on the manifold. I'll let you figure out why, since you seem to have walked the talk.