carb tunning help

crf50 2008 · Jun 16, 2010

ok guys installed my tb 88 racehead kit with 20mm carb pro circuit pipe and perfromance cdi.
standard main jet is 89 pilot is 35
so i started with 90 as my main jet 35 pilot and 1 1/2 turn on air mixture screw
this is what was suggested as a good set up in 50s brotherhood mag. so i thought ide start there.
bike runs good down low, idles good, revs out good when i roll the throttle on smoothly
but if i jam the throttle on wide open quickly motor dies and have to let off throttle to get engine back. ive tried to go up to a 92 main jet and turning the air mixture screw from 1 out to 2 and still have this dead spot.
any suggestions????
ive ordered a bigger pilot jet to try that??? hope that may fix it
bike has new plug, high octane bp fuel incase you ask
any help would be appreciated

sean01 · Jun 16, 2010

if it performs good from idle thru to 1/2 but goes funny after or just before then you need to adjust the needle height

thump*140 · Jun 16, 2010

you didnt mention what sorta carb your running, that might help too...
20mm, i'm guessing mikuni or something?

if it's dying when you crack the throttle quickly, i'd say it's a combination of slightly too big pilot jet, needle is set too high, and possibly the main is a tad on the fat side...
Have you tried running the bike on 91 octane fuel, or even E85??? These bikes arent exactly F1 engines running bulk compression, sometimes they cant actually burn the higher octane fuel properly... might be worth considering...

SGONE · Jun 16, 2010

crf50 2008 said:
ok guys installed my tb 88 racehead kit with 20mm carb pro circuit pipe and perfromance cdi.
standard main jet is 89 pilot is 35
so i started with 90 as my main jet 35 pilot and 1 1/2 turn on air mixture screw
this is what was suggested as a good set up in 50s brotherhood mag. so i thought ide start there.
bike runs good down low, idles good, revs out good when i roll the throttle on smoothly
but if i jam the throttle on wide open quickly motor dies and have to let off throttle to get engine back. ive tried to go up to a 92 main jet and turning the air mixture screw from 1 out to 2 and still have this dead spot.
any suggestions????
ive ordered a bigger pilot jet to try that??? hope that may fix it
bike has new plug, high octane bp fuel incase you ask
any help would be appreciated

im running 89main 35 pilot 1 1/2 turns on my tb 88cc runs very well i suspect abit on the rich side though try dropping the main to 87 and play with the needle clip

crf50 2008 · Jun 17, 2010

thanks guys i ve gone back to 90 jet put the clip in the middle and mixture screw just over 1, bout 1 & 1/4 and put in a new plug seems to be working alot better mite even try just normal unleaded. i think i was a bit rich. thanks for the quick responce. the tb 20mm carbs seems to be very tempremental if there not exactly right compared to other carbs ive had