I just put a yoshi comp full exhaust and K&N air filter over the weekend. I rejetted it, replaced the needle and adjusted the idle. It idles perfect now and runs soo much stronger than it did before. However it is backfiring now during deacceleration after reving to high rpm's. This doesn't seem to be right to me, but my buddy said that it wasn't hurting anything. Is that true? I hope I am not hurting the motor.

 

backfire and popping on deceleration is a sign of lean pilot jet, you can try turning out the idle mix screw a 1/4 turn at a time until it stops popping, but Hondas tend to be a little lean anyway, when I piped my son's 300ex, I put a one size bigger pilot jet in, starts and idles great, and no problems with backfires or such.

 

Yep, thats a good answer, but be aware an intake leak or exhaust leak will do this too. And some exhausts are so free flowing that, mild popping cant be avoided.
Heres some info from the mikuni carb website.
mikuni




4: Backfires in Exhaust

Note:

It is normal for many high performance exhaust systems to moderately backfire or pop when the throttle is closed from mid-to-high rpm. In fact, one should expect a well-tuned high performance engine to "pop" and "crackle" when the throttle is closed at high rpm.

The popping is a result of the air/fuel mixture becoming very lean when the throttle is closed and the engine is rotating well above idle speed. It is also necessary that the exhaust system have rather open mufflers.

Why This (normally) Happens:

1) When the throttle valve is in the idle position, fuel does not flow out of the main system (needle, needle jet, main jet). Fuel is only delivered to the engine by the pilot (idle) system.
2) The combined effect of the closed throttle and elevated engine rpm is to create a fairly strong vacuum in the intake manifold. This vacuum, in turn, causes a high air flow rate through the small gap formed by the throttle valve and carburetor throat.
3) Under these conditions the pilot (idle) system cannot deliver enough fuel to create a normal, combustible air/fuel ratio. The mixture becomes too lean to burn reliably in the combustion chamber. It gets sent into the exhaust system unburned and collects there.
4) When the odd firing of the lean mixture does occur, it is sent, still burning, into the exhaust system where it sometimes ignites the raw mixture that has collected ---- the exhaust then pops or backfires.

 

Hey guys, thanks a bunch. From what you both have said I am going to check the mix screw. If that doesn't help I will settel for it to just pop a little bit when it is reving down. It kinda sounds cool anyways. Would a lean or rich mixture cause the pipe to turn a bluish purple color?

 

You might want to try going one up on the pilot jet, if the mixture screw gets to 4 turns out from bottom, and it still pops. Ideal spot for the mixture screw is about 2 turns out, so if the stock one is about 4 turns out, the next bigger pilot jet will get the screw back to 2 turns out.
The pipes turn color from heat, either too lean, too much idling, or thin wall pipe, any or all will turn the pipe blue.

 

my pipe does the same thing at high speeds and letting off the gas and im pretty sure its jetted perfectly so i guess its normal. if a pipe is red/purple means its too hot and is lean so u would need to go up one jet.

 

What if the pipe is purple for only 4-6 inches out of the motor? Is it still too lean and would I still want to go up one jet? Main? Pilot? Both?

 

Do a quick plug check. From the sounds of it you need a bigger pilot. Check the plug after running 3/4 - full throttle and kill the engin quickly and pull in the clutch. Check the plug to see how it looks. If it is real white then you should increase the main as well.

 

Once the pipe has turned color, it wont turn back, it may fade but, its gonna be there for awhile. Anyway, the conditions that made it turn purple, could still exist or could be gone. Id focus on the drivability, and symptoms of lean and rich running. The motor is going to run its best, at a point that is almost too lean, but unless you keep on top of the situation, its a fine line between perfect, and problems. Its better to be on the rich side, of optimum, because the downside is increased fuel usage.
Heres a website about jetting, and at the bottom is a list of symptoms of whether the motor is running lean or rich, jetting