Backfiring on deceleration

[Downeaster]

Since I rebuilt the engine, my '79 (Pamco, stock advance, stock carbs, stock airboxes and elements) has been backfiring badly on deceleration, particularly on the right side.

When I rebuilt the engine I ran the carbs through a sonic cleaner and installed carb kits from Mikes (yeah, I know...) and recently put in new Genuine Mikuni floats as the left one sprung a leak. Float level is right on.

I've double checked the valve gap (all good, right exhaust was just a tad loose) and the cam chain tension, synched the carbs by checking for equal exhaust pressure, and backed the idle mixture screws out to 3 turns.

Starts good, idles good, plugs look good, just backfires a LOT on deceleration.

Gonna resynch the carbs with my home made manometer, but I'm relatively confident they're pretty close.

So, I guess my question is, What Next?

Should I scrap the Mikes jets and go back to the stockers? I didn't have a real reason to change, I did it "just because". AFAIK, they were the same jet sizes, but I didn't write it down so that could be part of the problem I guess.

Looking at three days of heavy rain so this is the perfect time to putter in the garage for a while. Got to put my new clutch and throttle cables from 650 Central on anyway.

 

[hooser]

Check exhaust connection at the head.. normally back fire deceleration is do to exhaust being loose leaks

 

[fredintoon]

That, or running lean at small throttle openings.
My own bike's popping and farting on overrun went away completely after a thorough carb cleaning showed a partially blocked slow running jet.

 

[5twins]

Three turns out on the mix screws is a lot for the '78-'79 carb set. The factory spec was 2.25 turns but I find many like 2 1/2 or 2 5/8. That 3 setting may be partly responsible for some of the popping, it's richer than normal. The MikesXS jets could also be contributing. They're repros so whether or not they are as precise as a real Mikuni jet is anyone's guess.

 

[jetmechmarty]

What kind if jets did you put in it? If they're not genuine Mikuni, they won't perform like Mikuni. Agree on lean with closed throttle or possible exhaust leak.

 

[Downeaster]

Thanks guys, I'll try the suggestions and get back to you.

Never thought to check the clamps on the headers, I bet that's it.

Hope so, it's the easiest fix...

 

[Downeaster]

FOUND IT!

Vacuum leak in the lines to the petcocks.

Due to clearance issues with the throttle linkage on the left side, I teed off from the vacuum port on the right carb to both petcocks.

Don't know if it was the line(s), the tee or one or both of the petcocks, but as soon as I disconnected it and capped off the port, idle and acceleration improved and the backfiring the problem went away.

Kewel.

Running the bike with the petcocks on Prime right now, but I'll do the conversion and get rid of the vacuum petcocks entirely.

Of course this was AFTER I'd pulled the carbs, pulled the jets, compared sizes (they were the same as the stock ones that came out) cleaned everything for the umpteenth time and dug out my yardstick manometer and resynched the carbs.

No biggie, I wanted to check the jets anyway, plus I found a leaking fuel filter cartridge and I installed the new throttle cable and clutch cable from Mercury while I was puttering.

 

[scabber]

Good to know you found the problem. The pesky petcocks.

 

[nh-jk]

pet cocks, are always fucking people over.....