Snap crackle and pop

[James England]

I have a question for the combined knowledge of the forum....

Why do some/many aftermarket exhaust systems crackle and pop on the overrun? On my XJR, the aftermarket carbon fibre exhaust cans are effectively straight through perforated pipes with glassfibre wadding surrounding them. There are 'dB killers' at the ends of the cans....circular inserts which can be removed for a louder sound and more exhaust flow. With or without these killers, the exhaust crackles and pops when the throttle is released. I don't mind it too much but I'm wondering why many systems do this????

[jetmechmarty]

Mine did that. It did that until I richened the mixture. I've always assumed snap, crackle, pop meant lean condition or an air leak.

[TopCatGr58]

Hey James,

Let me see if I can Decipher your phrasing for us YANKS! You are saying that the exhaust crackles and pops when the throttle is released...I'm thinking that you are saying when you are running the engine with the throttle cranked a bit and open....ie. 5k rpms, and then you let OFF of the throttle handle...releasing it so that the butterflies are closed.

The short answer is YES....most aftermarket pipes are more free flowing, and that leans things out in general. And even when you have jetted properly, shutting the throttle down like that really leans out the F/A mixture while the engine is still running at a faster rpm, and this causes them to pop on decel.

Techniques to reduce it are to increase the pilot jet settings, set the idle so that it's not quite so low...so that the butterflies don't slam completely closed/idle position.

T.c.

[James England]

Hey James,

Let me see if I can Decipher your phrasing for us YANKS! You are saying that the exhaust crackles and pops when the throttle is released...I'm thinking that you are saying when you are running the engine with the throttle cranked a bit and open....ie. 5k rpms, and then you let OFF of the throttle handle...releasing it so that the butterflies are closed.

The short answer is YES....most aftermarket pipes are more free flowing, and that leans things out in general. And even when you have jetted properly, shutting the throttle down like that really leans out the F/A mixture while the engine is still running at a faster rpm, and this causes them to pop on decel.

Techniques to reduce it are to increase the pilot jet settings, set the idle so that it's not quite so low...so that the butterflies don't slam completely closed/idle position.

T.c.

Yep, that's exactly what I mean! Were all becoming bilingual in English. So, when you say increase the pilot jet settings, do you mean actually change the pilot jets to richer ones? Or maybe richen the mixture with the air screws?

[James England]

Mine did that. It did that until I richened the mixture. I've always assumed snap, crackle, pop meant lean condition or an air leak.

How did you richen the mixture? Jet change or just adjusting the air screws??

Ideally, I'd like to reduce the pops a bit if possible......

[bikerphil]

Richening the low speed mixture will help a lot and probably make the decel popping go away altogether as long as there are no vacuum leaks. James, I understood you perfectly bro.

[Schming]

If in fact there are air screws, on your XJR carbs, instead of idle mixture screws, like the XS 1100 has, then you would turn them clockwise a quarter turn, to richen the mixture, and test ride it to see the results, and adjust accordingly.

[jetmechmarty]

How did you richen the mixture? Jet change or just adjusting the air screws??

Ideally, I'd like to reduce the pops a bit if possible......

I adjusted the air screws. My jetting is stock, or nearly so. I got rid of the snap, crackle, pop completely.

[James England]

If in fact there are air screws, on your XJR carbs, instead of idle mixture screws, like the XS 1100 has, then you would turn them clockwise a quarter turn, to richen the mixture, and test ride it to see the results, and adjust accordingly.

yes, mine has air screws. I'll certainly give that a try. .....

Thanks fellas.

[James England]

I adjusted the air screws. My jetting is stock, or nearly so. I got rid of the snap, crackle, pop completely.

Reassuring! Will give it a go today....

[D0wn5h1ft]

This happened a lot with my old '78 with a 4-1 exhaust on it. I downshift (hence the name) and engine brake A LOT driving in the city and this was an issue. With my '79 I just went ahead and stepped up one size on my pilots right out of the gate when I did my most recent carb rebuild to prevent having to deal with this. The consequence was, however, I had to set my idle mixture screws to only about 1 turn out compared to the normal 2 - 2 1/2 turns in order to get the right mixture. Keep in mind, though, that I am running straight 4-2 exhaust...

[James England]

The cure

Thanks for the suggestions re the popping. My XJR does have air screws, so I screwed them in a quarter turn and it reduced the popping a lot. I also added a bit of glassfibre wadding to the dB killers at the end of the silencers. They were a bit too loud for my taste and I put them on the bike primarily to reduce weight (a lot) and for the appearance. I was never fully comfortable with the noise level, to be honest, and the extra wadding, only a small amount, has reduced the noise and presumably decreased gas flow....I presume this has helped in crackle reduction. There isn't a noticeable decrease in performance although I suppose it must reduce it a bit. Given the torque and power of the bike, it's not much of a loss.

Thanks again everyone