Bonneville America Forums Forums Technical Forums General Tech BANG! POW! WHAM!

So, every once in a while I get a loud backfire either through the pipes or through the carbs. I'm not really sure which. It only happens when I've been idling and the moment I crack the throttle even a little... BANG. Doesn't happen all the time, but it did happen twice today. This is NOT popping on deceleration - I don't have that at all.

it is likely (particularly where you have NO poping on decel) that your idle set up is way to rich...a little poping on decel is proper on a well tuned vacume slide carb...no poping at all indicates that you are running rich enough to allow for the much higher air flow and no fule flow, caused by a high rev, no throtle, condition, it is probable that it is a carb back fire...but not a sure thing

I had the same backfiring issue after idle at red lights. I corrected it by turning air mixture screws in slightly.

If you are cracking the throttle another possible cause is you are opening and closing it very fast and the slide doesn't have time to react. I crack mine as I down shift..pull in clutch , crack , shift, I feel it brings the slower parts up to speed during down shifting making it smoother. Every once in a blue moon I get a puff back through the carb if I crack it and I believe it is because the slide didn't have a chance to move.

Doesn't matter how quickly I crack the throttle. And the bike is never moving when it happens. It may happen if I'm just starting off, usually after it's been idling for a while.

don't have to be moving for it to happen when it happens to me it is usually at a stop or approaching one. Not saying that is whats going on with your bike was just putting it out there.

That's cool. Just trying to narrow it down.

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So, every once in a while I get a loud backfire either through the pipes or through the carbs. I'm not really sure which

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Critical information there John. Need to know which it is.

I'll try and isolate it.

>I corrected it by turning air mixture screws in slightly.

My hunch is John is on the right track here.
Collection in exhaust @ idle.
Fires when you give it some air.
I'd guess that it's even possible to create a backfire through a carb this way because of a wasted spark cycle during valve overlap.

I'll give that a shot.

Does the 270 engine work on the "wasted spark" principal?

>Does the 270 engine work on the "wasted spark" principal?

Yup.

If you can narrow it down a bit, it will be easier for you to sort out. If it's from the pipes, it could be a very rich idle mixture or weak spark allowing raw fuel to build up in the exhaust system and being light off when you open the throttle.

If it's from a carburettor, the mixture is a bit lean. This could be the idle mixture adjustment, a clogged transition jet (the small holes in the bottom of the carb throat just above the throttle plate), a slide stuck partly open, an air leak or too small pilot jets.

I get it through the carbs and my pilot is slightly rich. I mean slightly too my insulator of my plug is brown but I get just a little black on the outer part by the threads. I run 48 pilot soon to be a 45 then readjust the screws. Like I said above it just once in awhile for me.

I rode to work today and I swear I heard it pop through both ends at different times.

I have days like that myself sometimes!!

Me too but it's never just a pop.