Jetting on a 38mm Keihin carb

[oachs83]

I bought this 86 250r with the complete motor rebuilt and all new parts. Then the piston burned a hole right in the top. The guy claimed it to be a faulty aftermarket cdi and a BR8es plug. I was shown reciepts etc to prove the motor was rebuilt. Anyways I am switching cdi's with a 89 and want to know what I should be at jetting. I have not taken carb apart yet but just maybe he was running lean that caused the failure as well and want to check EVERYTHING over. Basically with a 38mm Keihin slide carb what should ball park jetting be? Here ar my specs

38mm Keihin PWK carb
68mm bore 0.80 over (262cc)
open air box foam filter
FMF pipe and silencer
1989 CDI, coil, and stator

Thanks in advance.

[IcutMetl]

Well I'm not the resident 250r guru here, but it sounds like to me that the previous owner's improper tuning was the cause of the hole in your piston. I think the earlier motors spec'd a B9xx spark plug, but if your jetting was on I don't think it would've caused the hole in your piston.

I had a very similar setup to yours before; an 88 stock jug, midrange ported, 67.5mm bore, FMF exhaust, open airbox with foam filter, Boyssen RAD valve, 38mm Keihin PJ carb, and 195-200psi compression on race gas mix. I mixed at 40:1 fuel/oil ratio and also used a B8ES plug. I'm speaking in the past tense here because I switched to a 38mm PWK carb and will have to re-tune a bit, I'm sure.

I had the best luck with a DGH needle on the 4th clip down, a 48 pilot jet, and a 175 main. Airscrew was 1.5-2 turns out. With that setup, it nearly always started within 2 kicks (cold or hot), would idle for ever, never fouled a plug, never loaded up, and burned a nice color on the spark plug; a very nice golden brown/tan.

Good luck!

[oachs83]

Thanks, I will use those numbers as a ballpark. I just want to know that when I see the pilot or main jet I have an idea of what is to lean for these machines.

[IcutMetl]

Hey no prob, it was frustrating as heck to me until I finally got close. I would say it'd get you pretty close, but don't count on it being where you'll end up. Search for tips on here; there's plenty of them. Also, go to www.duncanracing.com; there's a jetting section there that helped me quite a bit.

[oachs83]

May have just found the problem. I opened up the carb and there is a 152 main jet and a 48 pilot jet. Isn't 152 small? I mean I remember 152's being in a 125cc dirtbike on 34mm carbs. I have a 168, 172, and 178 lying around should I go with the 172? Should I ride for a little while, while breaking my motor in and watch the spark plug color? Thanks

[IcutMetl]

I think you should try starting out on the 178 first; dial in your other areas, then work on the 3/4-top end main jet; drop it back if you need to. Richer is always safer.