CJM
06-24-2010, 04:08 PM
Finally got the 416 down to my shop to tune it. I did take it for a test drive there and it seemed ok, took it for a ride at home and slight slight pop on decel (lean).
Im gonna yank the plug, but I think its fine. Before I had it done it was popping like CRAZY, now its just very slight and not all the time. Mostly its popping slightly when you come from a high rpm (say 3rd or better) and use the engine to brake (downshift) to say 2nd or 1st. Only complaint is WOW this suckers hot now, but the headers arent turning blue.. I mean I am running 11:1, 416, HC II and I have the spal fan pulling heated air basically into my legs...
But I found something interesting out in my recent google searches on this stuff..check this out..makes perfect sense actually. A car for example wont do this b/c of fuel injection-but a carbed one will and I seen./done it.
After spending the day on the dyno Saturday, my 450 popped and shot flames out 4 inches after 11,000 rpm runs. The O2 sensor indicated we were a tad rich on the pilot and needle. I attributed it to unburned gases in the exhaust igniting on decelleration and jetting wont help that. Most times a lean condition WILL be indicated by popping, but not always. TW
This is exactly correct! 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!
Im gonna yank the plug, but I think its fine. Before I had it done it was popping like CRAZY, now its just very slight and not all the time. Mostly its popping slightly when you come from a high rpm (say 3rd or better) and use the engine to brake (downshift) to say 2nd or 1st. Only complaint is WOW this suckers hot now, but the headers arent turning blue.. I mean I am running 11:1, 416, HC II and I have the spal fan pulling heated air basically into my legs...
But I found something interesting out in my recent google searches on this stuff..check this out..makes perfect sense actually. A car for example wont do this b/c of fuel injection-but a carbed one will and I seen./done it.
After spending the day on the dyno Saturday, my 450 popped and shot flames out 4 inches after 11,000 rpm runs. The O2 sensor indicated we were a tad rich on the pilot and needle. I attributed it to unburned gases in the exhaust igniting on decelleration and jetting wont help that. Most times a lean condition WILL be indicated by popping, but not always. TW
This is exactly correct! 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!