Talk carbs to me!

[arancara]

Sorry if this is in the wrong section!

Basically I started my bike off with a PZ26 carb with acceleration pump, which is what it came with on the Lexmoto XTRS (KS-125-24). I then upgraded to the big PZ30 with no pump, things were a lot smoother, plus better and after some tuning I set it up fine with the stock pilot and a 110 main jet. I have now rebuilt my bike with a 156cc big bore kit followed by +1mm lift cams.

My first assumption was to jet up, now that there's a bigger displacement its going to need more fuel, now it barely starts and I'm guessing its flooding out maybe??? I have no idea what's going on. Then I started researching carbs a bit more and I would like your information and experiences.

What I've basically been told is with a bigger displacement kit the engine sucks more from whatever jet you have and increases its flow, so really I should of left the stock jet or merely started to drop jet sizes! Yet I only found two sources with this information from Mr Google as no descent answers were coming up. Is this correct?

What are the general rules of thumb when jetting for the following as Mr google is no help and I'm daft and new to carbs in general:
1) When increases the carb do you typically jet up or down as a rule of thumb?
2) when fitting higher lift cams do you jet up or down as a rule of thumb?
3) when increasing the displacement do you typically jet up or down as a rule of thumb?

I would really appreciate some input as I've said mr google could not answer these questions and just gave me generic guides to tuning a carb not what to do when I increase carb sizes etc... I really look forward to you knowledge and input on carbs!

[cheesy]

First.

http://www.csgnetwork.com/cfmcalc.html

Second.

http://www.smellofdeath.com/lloydy/jetting.htm

or

http://www.mikuni.com/pdf/vmmanual.pdf

Plus

http://www.dansmc.com/carbs2.htm


Getting the carbs right on a modded engine can, and will, be a PITA at times. Experiment, experiment, experiment.

I got crazy a number of years ago and modded the 215ci six banger in my 1952 Ford F2. Shaved head, hi-lift rockers, dual point ignition, hothothot Mallory coil, Clifford dual exhaust header, and an Offenhauser dual carb intake with a pair of Holley 1904 single throats. The jetting on that took a couple weeks to sort out. :)
The guys at the speed shop were no help. I don't think they even knew there were other engines besides SB Chevys out there. They just kept telling me to "go fatter, go fatter". All I did was piss fuel:hmm:
It finally dawned on me that I hadn't increased the CFM of the engine, just gave the engine a second hole to suck the air in. I dropped back to the stock jets, and ended up just going up one size.:yay:

I know the above doesn't help your case much. Just letting you know that many of us have been there.

[Weldangrind]

A 110 main is too big for 156cc, IMHO. I'm feeding a 200cc with a 30mm Mikuni and a 110 main, and that's with a free-flowing muffler and a pod filter. Try a 100 main and decide if it helps.

You don't necessarily increase jet sizes due to a larger carb or cam, but you might. I know, that's not a very good answer. You would definitely need to increase the main jet if you increased displacement, given the same carb.

The bottom line is that there is a stoichiometrically ideal air / fuel ratio of 14.7:1, and that's what you need to strive for. If you do something to increase airflow (free-flowing exhaust, pod filter, larger displacement), you need to increase the main jet size to compensate.

Make sense?

The good news is that there is only one main jet, one slow jet, one pilot mixture screw and one jet needle clip. It won't take much experimentation to get it right, and jets are cheap.