Bonneville America Forums Forums Technical Forums Performance Mods My latest carb test results!!

OK.
Thruxtons are now in without the shims and running great. Went right to the limit today and only stuttered at the end. I'm on 130's.
Is it necessary to go down to 128's or 125's?

Now the popping!!
I'm on 42's at present and can't get enough turns out to stop the popping. Should I now increase to 45's?

Thanks all,

Jon

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Should I now increase to 45's?

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I'd say yes. I myself even went to #48's to eliminate popping without too many turns on the air/fuel mixture screws.

Are you sure you have not got any airleaks on the exhaust? Check for black soot round the silencer/header joint - if so, popping will cure itself as the joint fills up with soot - if not, try 1 size up on pilots. I would also go 1 size down on mains.

Jan

Top end is now great. I put in 128's and that has really helped.

I put 45's and I am at 3 turns out and still popping!! I'll check the plugs tonight to see what colour they are now before I adjust any further.
I don't mind the popping as long as I'm not doing anything to the engine.

Jon

Have you disturbed the headers at all or have the exhaust headers been off to your knowledge? - you could have a leak on the header flange where it sits on the crush washer in the head. This is guaranteed to cause backfiring on the overrun. Might be an idea to see if the nuts are tight (subject to usual provisos about being careful to not overtorque them etc.)

Jan

Could you elaborate on what effect you got going from 130 to 128?

Thanks Jan,
But where are the headers??!!! Is it where the exhaust fits into the engine?
I know, stupid lame question! but I really don't know.
jon

yes its where it goes to the engine

Frank

Checked the plugs and they're looking black and sooty. Rich right? Could that have been from when the top end was rich.
I've cleaned them up. Turned out now to 3 1/2 turns and will wait until the weathers better or Monday before trying her again. Its just that its started to rain and shes just had her bath!!
checked the header and everything seems normal there.
Thanks for the ongoing help. Shes almost there, only slight popping now.

Either drop the mains down 2-3 steps (from 130s to 125-127, or try the TBS needles instead, they are richer than stock, but not as rich as the thruxtons.
Generally you should drop down in mains whenever you use these Thruxtons, or at least that has been my experience on 100% of the ones I have played with.

So would the richness be from the top end and not from the pilots.
I'm now running the thruxtons on 128's and 45's.
I did take the bike up to full throttle and it bogged very slightly. I wasn't sure whether that was because there was too much or too little fuel.
I think the plugs have answered that.
I still have the original 120's. Would they be too small? What does it feel like at the top end when they are too small.
Thanks all?
Jon

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What does it feel like at the top end when they are too small.

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That's the rub, rich or lean can give the same feeling. Too rich is usually accompanied by backfiring and even black smoke.
Try the 120s and see what difference they make. Easy enough at this point to swap. Besides, practice makes perfect.
The plugs are telling you that you are rich for sure at the high end. Unless of course, you ride around at 1/4 throttle all day.

The different needles can give drastic and different results on a lot of bikes. The carbs are that way, a lot of tolerance. If it's still too rich, toss in TBS needles and the 128 jets and try that. You will eventually hit on the right combination for your specific carbs.
Just do ONE thing at a time. That way, you know exactly what the one thing did. With more than one change, you don't know for sure what causes what.