Popping on Deceleration - Another Newbie Question

TedD

New member
I cannot seem to eliminate the popping I get when decelerating into a corner coming off from a wide open throttle condition. From what I read so far, this seems to indicate a lean condition, is this correct? Engine is BSP with BSP2 cam, 8 degree flywheel key, blue restrictor plate, 39 high and 21 low jets running 87 octane pump gas. Track is 7/10 mile sprint track. The problem seems to noticeably worse since advancing the flywheel. Any recommendations on a fix for this or will I always have the "Deceleration Popping" to some degree?

Thanks,
Ted
 

TedD

New member
Forgot to mention the plug has a very slight gray\white burn to it. Thinking I might need to go up a number or two on jet size?
 

TedD

New member
Is there a recommended lash setting for my configuration or should I go .003 on both sides?
 

rab3rd

New member
I have read to go zero when the engine is cold or lash to 3 on the exhaust and 2 on the intake while hot (ran my engine and got it hot and then measured the lash and its was nearly 6 on both after having set them to 3 ex and 2 in) I lash mine to 1 on both cold and still get popping. That's with the 20 degree factory timing advancement and the same jetting as you have.
 

mike00b

New member
Yep. Richen it up. When you let off throttle, the engine is still turning hi rpm and sucking air in. The only source of fuel is from the idle circuit... I start with 24 in the stock clones I build.
 

rab3rd

New member
Yep. Richen it up. When you let off throttle, the engine is still turning hi rpm and sucking air in. The only source of fuel is from the idle circuit... I start with 24 in the stock clones I build.

Do you have any issues at idle with the jet that big? Such as fouling the plug? Remember there is a restrictor plate involved here too.
 

mikey56

New member
When you decelerate, you reduce the amount of throttle. This causes an overrun, that is, the motor’s rpm is turning faster than the fuel provided can support, so the motor begins to spool down. This causes a couple of things to happen. First, when you close the throttle, you are also closing the throttle plate. This reduces the air and fuel floating into the motor, and increases the vacuum. This results in less air and fuel in the cylinder during the power stroke, which results in a lower pressure in the combustion chamber. When you reduce pressure in this manner, the mix burns slower, which results in two things: Now you end up with non-burned fuel in the exhaust, and burning fuel being ejected into the exhaust, which results in a backfire. The bottom line is that backfiring is perfectly normal and should actually be expected. It is up to you if you really want to get rid of it. But it's not caused by a lean condition...... research deceleration popping on google....

of course, what do i know?
 

rab3rd

New member
Sounds to me like you know plenty... ;-)

cut and paste from another forum.


If you think about the mechanics of what is going on upon decel, it makes perfect sense:
You start out with an engine that is turning high RPM with a carb that is open, flowing freely. Next, you close that slide. This has the effect of creating a tremendous vaccuum in the intake manifold (CHOKE!). This vaccuum does curious things:
1. It pulls air and fuel from wherever it can--including the idle and low speed jetting in the carb.
2. To counteract the vaccuum, air will take the path of least resistance--including sucking exhaust gases back into the combustion chamber during valve overlap (the brief period of time that the exhaust and intake valves are open at the same time.

So what does this mean? Well, now you have a disproportionate rich mixture heading for the cylinder AND what little oxygen is drawn in is further diluted by gases that are coming back into the chamber from the exhaust valve. The result is poor, incomplete combustion! Depending on the temperature of the exhaust pipe and gases, reignition is possible once this mixture is introduced into a more oxygen rich environment. This produces the "pop."

Why does a lean running engine have a tendency to excessively pop? The exhaust gas temperature of a lean engine is substantially higher than one that is running on the rich side ( an EGT probe will show this). Therefore, the high EGT serves to heat the pipe more and the gases themselves become a much better source of re-ignition. By richening things up, you are cooling down the exhaust temp and therefore reducing potential re-ignition sources.

Any 4 stroke engine--even those that are flawlessly tuned will pop on decel. It is harmless, but can be annoying--granted. However, I would never alter the jetting of an otherwise performing motorcycle to get rid of it. Go by the seat of your pants and the color of your spark plug rather than the popping as an indication of the jetting of your scoot!
 

mikey56

New member
I take that back...a lean condition can result in popping, just as a rich condition can....Go by the the color of your spark plug rather than the popping as an indication of the tuning of your kart! It is normal. Annoying, but normal. Can be reduced, by trial and error, but normal.
 

mikey56

New member
true....but at least we get the information to TedD so that he knows he's ok and that it's nothing to worry about!!
 

rab3rd

New member
I know Ted, we race together. Pretty much started at the same time. I was a few weeks ahead of him in my first race vs his. Right now Im staying ahead of him, when I can keep the tires pointed in the right direction, but hes coming for me. LOL
 

mikey56

New member
that was my problem the last race of this season....keeping the tires pointed in the right direction and in contact with the asphalt!
 
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