backfires during deceleration

[stewart90]

I just got my 1974 back on the road after a carb bench sync and new bars and switches my problem is I have a loud constant backfire when I let go of the throttle and start to lose speed, what can cause this? I just bought the motorcycle a couple months ago and haven't really had it on the road much due to a bunch of small minor issues so I'm not sure if it did this before the sync and new jetting . Could it be since up sizing the main to the next step up and adjusting the mix screw ?

 

[mjpchief]

Could be lean. Try turning mixture screws out 1/2 turn and try it.

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[rahndo]

How old are the carb manifolds/boots? May have been damage/torn pulling the carbs off.

 

[stewart90]

I'll try the turning it out . The boots didn't look bad at all I tested by spraying starting fluid around the boots while it was running and don't have any leaks rahndo, I think that's what your getting at right ? Sorry about guessing

 

[deadchef]

you haven't told us what jets and/or mods you've done to the bike. backfire and/or popping on deceleration is a typical sign of a lean condition.

 

[RFLIVES]

Most likely lean / leaky boots. plugs left off of vacuum nipples cause this ask me how I know...

 

[weaselbeak]

It's a typical sign of a leaky exhaust. Check or just replace your seals at the engine, they are cheap. Then make sure the muffler connections are good.

 

[stewart90]

Um it had pod filters on it straight through exhaust on it with mufflers that look like cherry bombs ( they're not though) no baffles . Went up from 125 mains to 127 . Mix screw backed out 2.5 if I remember right to get a nice throttle response . I tested the boots when i first put the carbs back on, theyre good. That's all that I can think of as far as mods go that would affect anything that'd cause back fires . Like i said i cruised it around for 20 mins or so today and ran really good. I'll double check the exhaust tomorrow. Thanks for the info guys

 

[peanut]

as Weasel says usually a sign of a leak on the exhaust system somewhere.

 

[weekendrider]

stewart90,
Look at the chart 5twins made for the carb guide.
If you have the 74 carbs(can't tell anything but they are 38's from your pics in the Show your xs thread) you are way past the adjustment on the mix screw.
You might consider ditching those pods, and looking again at the jetting sizes for the '74 38's. Then read the jetting for mods section.
With all the bandages on the exhaust how you going to see/feel an exhaust leak?
Unless the slash cuts are welded on, the clamping area could be suspect for a leak but how are you going to know?

 

[stewart90]

Bandages ha I like that . It's previous owner stuff but you are right about not being able to tell might not be a bad idea on unwrapping them and taking a look, and I'll defiantly look at the carb guide again and see if I can get an idea . I will start with taking the exhaust off at the motor and looking at that seal . Weekend rider I thought I was past the adjustment I've always heard somewhere between 1-5 to 1 3/4 turn out . I am a newbie to all this tho. With all the great advice and information on this forum I'm sure I'll figure it out someday

 

[weekendrider]

Without looking at the chart I think the adjustment should be around 1 turn out +/- 1/2.
It isn't much I know. The chart will tell you for sure.
I'm not trying to be snobbish, or secretive. We are all virgins in the beginning. And I still refer to the chart cause my gray matter has long past it's prime.

 

[stewart90]

I didn't think you were being snobbish at all . All is good

 

[luckynumber0]

Is a slight exhaust leak (at the muffler clamping area and not at the headers) harmful to the engine?

 

[weaselbeak]

No, but it can cause the popping.

 

[stewart90]

I put new spark plugs in and the popping went away. That was easy