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34mm to 38mm...that big a difference?

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10K views 11 replies 6 participants last post by  TrailBurner  
#1 ·
I was running a stock PJ34 on my '87 R for a good while and got a wild hair to swap it out with my hybrid's PJ38. I've got a stock bore, fmf gold pipe, rad valve and cr gasket. No porting or head. Open air box with foam filter. When I had the 34 on, my jetting was 42 on the pilot and 155 on the main and it was a very safe rich mixture. I only switched out the carb and have to run a 48 pilot and 170 main. And in think it's still lean at those settings. I'm thinking it'll end up with a 50-175.

Does the carb size make that much of a difference on the jetting?
 
#3 ·
I just thought it was odd that it'd change 3-4 sizes on the pilot and 6-7 sizes on the main. Not that I'm complaining. I figured a few sizes maybe but it just seemed like an awful big jump. I've heard that small carbs help bottom end torque too. That isn't so from my little test. The 38mm pulls much harder off the bottom than the 34mm. The power seems to be much broader and a lot smoother on the big one as well.
 
#4 ·
Yes, it makes that big of a difference. Think about it for a sec. The same jet will go in a 34mm or a 38mm. It doesn't discriminate or change it's inner size just because of the carb change. However, you are now sucking much more air through than you were before. You have to change the jets to compensate for that additional air flow to get back to that same mixture you had on the smaller carb. Make sense?
 
#6 ·
Funny thing, Im running a 40 PWK, I'm running a 160 main.... most run 170-180 with a 310cc and 40 PWK, but if I go up I notice it in a negative way. What works for 1 quad may not work for another, too many variables between each persons R, meaning add on / aftermarket parts/ pipes/ gearing. Use other set ups as a guide, not as etched in stone it will work.
 
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#8 ·
I appreciate the advice and I guess I am on the right track then with the 38. I read an article a while back about Bill Balance I think, and how he had his bike set up and he was only running a 35mm on a 300cc bike for the best low end torque. But I can't seem to find any sign of that 'smaller carb theory' about how they do better down low and bigger carbs do better up high, when comparing the 34mm to the 38mm. The 38 still wins out there.
 
#10 ·
As a reference, in SoCal on our Race Engines we are running 175/180 Main, 55/58 Pilot, DGH 3rd clip, AS at 1 1/2 on 38/39MM carbs on a 250/265 CC engines; open air box, K&N or UNI, on Race Fuel, BDT WURKS pipe. Temp is around 50-80 F, Hum is 40/50%, and altitude is 500' to 1000'...we do tend to run on the Rich side.

Carlos
 
#11 ·
I'm thinking that is probably where I'd need to be. I did add my pre-filter cut out lid last night and it was running soooo sweet. Without the lid I probably do need to come up to the low 50's on the pilot and mid to high 170's on the main. But checking the plug after adding the lid cover, it had a nice carmel color to it with the BR9ES plug. I can't believe how beautiful it runs and has some serious low end grunt that I've not been able to get out of my 250's before. I'm guessing the higher cylinder pressure, RAD valve and bigger carb made that happen. I wasn't too thrilled with the pipe before but now I'm not sure I'd think about swapping out the FMF gold pipe. I do have another XC race this Saturday so I'll write up a report on how it ran compared to the last race. Stay tuned... (no pun inteneded) lol
 
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