Popping/backfire on deceleration?

[theruppman]

This condition is common on my Black Widows. I have found that it occurs when I run it a little rich. The Dellorto carb on the Black Widow really brings the engine to life, especially when you really pour the fuel to it. The drawback is popping that occurs when you back off the throttle quickly due to the unused fuel still left in the exhaust. The same thing was normal with the old flathead Briggs engines in kart racing. You would always hear the popping/backfire when you suddenly let off the throttle and the butterfly or slide was closed for a split second. I feel sure that your experiencing this due to the increased fuel flow from re-jetting. Best way to test... try running it hard and, instead of getting off the throttle quickly, roll off the throttle slowly. If you don't get the backfire, you're probably just a little too rich and you can fix this by dropping down a size or two on the jetting. But, if you're not worried about pissing off a neighbor or two, it's not going to hurt your engine.

 

[Raskin]

Maybe .. also try not doing high revs and off throttle near the neighbors..i try to not do the hard runs until in area of no or few houses blocks away from them ..if you live in an area all built up just piss people off 15 blocks from your house :laugh: then cruise home :innocent:

 

[Ldogg29]

This condition is common on my Black Widows. I have found that it occurs when I run it a little rich. The Dellorto carb on the Black Widow really brings the engine to life, especially when you really pour the fuel to it. The drawback is popping that occurs when you back off the throttle quickly due to the unused fuel still left in the exhaust. The same thing was normal with the old flathead Briggs engines in kart racing. You would always hear the popping/backfire when you suddenly let off the throttle and the butterfly or slide was closed for a split second. I feel sure that your experiencing this due to the increased fuel flow from re-jetting. Best way to test... try running it hard and, instead of getting off the throttle quickly, roll off the throttle slowly. If you don't get the backfire, you're probably just a little too rich and you can fix this by dropping down a size or two on the jetting. But, if you're not worried about pissing off a neighbor or two, it's not going to hurt your engine.

Thanks [MENTION=4293]theruppman[/MENTION]. This is what I figured was happening and will drop from a 37 to a 35 jet and go from there. I did not upgrade the emulsion tube so I am running stock. Do you think I would worsen the issue of runnin rich by adding the emulsion or would this help? My intent was to add the emulsion and drop from a 37 to a 35. [MENTION=4293]theruppman[/MENTION]
[MENTION=54903]Raskin[/MENTION]- I like your thinkin'! lol

 

[BBQJOE]

I'm going with air leak.

 

[DeadPixel]

I might be wrong but think the emulsion fattens up the mid range more then the top end. I have not messed with clone carbs yet so cant tell yar for sure but I would toss the smaller jet in anyways before the emulsion tube and see if it even helps. When trying to trouble shoot problems try to make 1 change at a time if you can so you know for sure what fixed your problem.

 

[mikey9821]

This is generally because with high engine RPM and closed throttle, the carbureted is running on the idle circuit. As a result the engine is excessively lean at this point. The small amount of air/fuel is unable to combust in the chamber and is generally passed through to the exhaust system. A build up of air/fuel is generated in the exhaust pipe and when there is an occasional ignition of air/fuel in the combustion chamber, it is passed into the exhaust igniting the build up of air/fuel and you get a backfire. Everything sounds normal and good to me. The reason more modern and sophisticated engines don't do this is either one, they turn the spark off during deceleration, or more commonly turn the fuel off.

 

[SAS289]

Ldogg29... On my Coleman I used the 140 E tube and drilled to a .033 jet. Removed the air box and run with the stock muffler. Runs fine, decent throttle response. Lower altitude than you.

Did you ever down jet and swap out e tubes? What were the results?

 

[KB2ROCKET]

Crank engine over till it is just few past tdc on compression stroke. Now exhaust valve is closed and try blowing inot end of exhaust pipi. You should not be able to blow in it. That will eliminate the exhaust as the culprit.

And don't forget to post pictures of you doing this Lol

 

[ole4]

Good one!

 

[super nate]

popping can be caused by a air leak, over rich condition which could be caused by rich jetting or the idle position being too high.

 

[CLH]

Hope not off topic, but can running without an exhaust or silencer cause this too? Like just the port of the engine, no muffler or pipe or anything screwed into/onto it.

Just wondering because my Tecumseh backfires some. I shouldn't complain yet though. The reason is because although I installed a new OEM carb, I haven't adjusted or ran the engine much at all because the thing is so loud without the muffler on it. LOL The muffler I needed for this engine is being shipped to me as I speak.