Knocking/backfire through airbox....

[1990dtgl98]

Hey guys...I have a slight knock and an issue with a backfire. You can kinda see here:

I took the seat off and you can feel the backfire through the air filter, which would indicate a lean condition. However, I rejetted last year to try and solve this issue. This year I synced the carbs and did the valve clearances (all were tight and replaced). The knock seems to have gotten less frequent/almost eliminated, however, it backfires more. Not more frequently, but more pronounced. Now you can hear it clearly at idle and low speed, rather than barely over the exhaust. It's also much more frequent if I crack the throttle 10ish percent, as shown in the video.

Backfire though airbox usually means lean, but all the hoses and boots look good. Any ideas as to what could be causing it?

[1990dtgl98]

Anyone?

[Suzuki Stevo]

Did you increase or decrease the clearance on the intake valves? I have seen a "Lean Pop" type scenario when the valve lash on the intake valves is too small/tight   

https://youtu.be/j418UsmVXew

https://youtu.be/jOGrEoyVMRc

[lucas]

I had popping through the airbox when I had a dead cylinder.  Have you verified that both cylinders are firing?  Have you been able to ride the motorcycle on the street and was it lacking in horsepower?

[1990dtgl98]

All valves went down in shim size. All are now at around 06 to 08 in clearance.

Both sides come up to temp the same (using a infrared thermometer). The right side (brake lever side) seems to be about 15 to 20 degrees cooler though. Drives fine, but honestly when I bought it it needed a complete carb cleaning and I rejetted it so it didn't run great jn the first place.

Before it had a knock that was frequent. Now that's gone but this backfire is present. Maybe one of the carb boots. If I spray some carb cleaner around the carbs the idle doesn't change like it's sucking in through a leak.

I tried it again and it's almost significant enough to feel the backfire sitting on the bike. Not often, but forceful.

[bmf]

I learned recently that popping through the air box is called backfiring and popping through the exhaust is called afterfiring. Backfiring can be from a lean condition but also can be from timing being incorrect. Are you sure everything is tight there?

Do your plugs look lean?

[1990dtgl98]

Plugs look fine. But they're only in for less than  200 miles from last year.

I doubt the timing is off on a 12k mile bike, but could that solve this issue? Usually timing issues on cars present problems throughout the rpm range.

This bike mainly backfires, sputters and "hunts" for rpms at 5-10% throttle.

[1990dtgl98]

Bump.... Still looking for advice.

[phoenix02]

I also have a sound thru my carb after coming in from a long ride.  It idles poorly like before I changed valve shims this summer.  Mine is an occasional hiss and not a pop with throttle closed or just off idle especially when cold.  I pulled the valve cover off and the valve clearnance is still good and loose and cam timing looks good. I'm watching your thread to see if your diagnosis is similar to what mine is.

[mr72]

It idles poorly like before I changed valve shims this summer.  Mine is an occasional hiss and not a pop with throttle closed or just off idle especially when cold.

to the OP, I'd check the compression. May be carbon buildup on the valves causing poor compression.

Or a vacuum leak could be part of the problem. Anyway, I'd check that out.

phoenix02 the hissing sounds to me like a vacuum leak definitely.