Detonation while engine returns to idle

[izzni]

Well, at least I think it is detonation. Basically I'm cruising along, maybe 1/4 to 1/2 throttle and I left of the gas, and bring the sled to a stop at a normal rate, maybe with some brake, maybe not. Fairly consistently, as the motor is spinning down, between 4000ish and 3500ish RPM I hear a "drrrrr DOOM da DOOM DOOM drrrrrr". The "drrrrrrrr" is the normal engine noise, gradually getting lower in pitch as the RPM gets lower, and the "DOOM da DOOM DOOM" is what I think is the detonation. Note that the number of "DOOMs" and "das" is fairly random, but usually less than say 4 or 5 "DOOMs" max.

My question is what am I hearing? Is it actually detonation? And my followup question is what should I do about it, if anything. I have a thread about my jetting being screwed up due to a cylinder change, but this problem here only happens when the throttle is closed and the needle and main jets should be playing no part.

 

[Hardass]

Well i'm thinking after the 5 doom's and a da there should be a dee and then a da do. Is this making any sence? Really det sounds more like a rough throated mmm i'm not sure how to make the sound but i could not resist sorry man. Maybe some else can help.

 

[izzni]

Someone suggested that it was unburnt fuel in the exhaust pipe exploding, perhaps they were right?