cb400f Shortened exhaust / popping sound on throttle release ?

[timeconsuming]

Hey guys

So my bike had a mac 4 -1 exhaust and I recently switched up the muffler to a shorty that I figured was a bit more open, so I was hoping the back pressure wouldn't change too much, however now whenever I release the throttle I hear some popping sounds coming out of the exhaust. Any idea what that signifies?

Air/fuel mixture need to be adjusted for the change in the exhaust length?

My assumption is that it's running rich, because though it usually requires choke to start the bike and have it idle while warming up, if I try to pull away into second gear with the choke still on (halfway), it gets bogged pretty quickly and I have to open the choke back up to continue on. Also checking the spark plugs shows good health (tan tip coloration).

Related: My carbs are definitely in need of some cleaning, and tend to leak frequently from the overflow valves on the bottom, so I'm thinking I'll start by pulling them off, cleaning them up and inspecting things. But any other thoughts are welcome!

 

[Rich Ard]

Hey guys

So my bike had a mac 4 -1 exhaust and I recently switched up the muffler to a shorty that I figured was a bit more open, so I was hoping the back pressure wouldn't change too much, however now whenever I release the throttle I hear some popping sounds coming out of the exhaust. Any idea what that signifies?

Air/fuel mixture need to be adjusted for the change in the exhaust length?

My assumption is that it's running rich, because though it usually requires choke to start the bike and have it idle while warming up, if I try to pull away into second gear with the choke still on (halfway), it gets bogged pretty quickly and I have to open the choke back up to continue on. Also checking the spark plugs shows good health (tan tip coloration).

Related: My carbs are definitely in need of some cleaning, and tend to leak frequently from the overflow valves on the bottom, so I'm thinking I'll start by pulling them off, cleaning them up and inspecting things. But any other thoughts are welcome!

I expect you'll find that once your carbs aren't putting extra gas into the equation the problem will go away. Check float height after you clean 'em up 'till they're spotless. Bet further that if you go on short rides rather than longer faster ones your plugs will be black.

 

[AlphaDogChoppers]

Decel popping is normal when you have more open exhaust. Decel popping is also a LEAN condition, not a rich condition. It's caused by the high vacuum condition of above idle RPM with closed throttle. The mixture is too lean, therefore there is incomplete combustion, and unburned air/fuel mixture goes into the exhaust where it sometimes will ignite. If you had a rich condition, there would be no oxygen in the exhaust for the unburned fuel to ignite.

Don't make any conclusions about your fuel mixture based on your fuel enrichener being engaged. It's ALWAYS going to be rich when the enrichener is engaged, fer fuck sake!

You can sometimes tune out the decel popping by adjusting the idle air screw to get the idle mixture a little richer. Sometimes you can't and you just live with it. It's harmless. Sometimes you may have to go up one size on your pilot jet.

Mufflers mask the decel popping, and when you open it up, you start hearing it more. Usually, it's there all along, you just don't hear it as much.

 

[Rich Ard]

Decel popping is also a LEAN condition, not a rich condition.

OOPS.

You should still check your float height and make sure they aren't taking on gas.

 

[AlphaDogChoppers]

I forgot to mention that leaky exhaust header gaskets can cause a lot of popping.

 

[timeconsuming]

Decel popping is normal when you have more open exhaust. Decel popping is also a LEAN condition, not a rich condition. It's caused by the high vacuum condition of above idle RPM with closed throttle. The mixture is too lean, therefore there is incomplete combustion, and unburned air/fuel mixture goes into the exhaust where it sometimes will ignite. If you had a rich condition, there would be no oxygen in the exhaust for the unburned fuel to ignite.

Don't make any conclusions about your fuel mixture based on your fuel enrichener being engaged. It's ALWAYS going to be rich when the enrichener is engaged, fer fuck sake!

You can sometimes tune out the decel popping by adjusting the idle air screw to get the idle mixture a little richer. Sometimes you can't and you just live with it. It's harmless. Sometimes you may have to go up one size on your pilot jet.

Mufflers mask the decel popping, and when you open it up, you start hearing it more. Usually, it's there all along, you just don't hear it as much.

Makes sense! Thanks for the educational logic, sorry I'm still learning. I'll look into adjusting the idle air screw (different than the idle speed screw, yeah?) and see if I can't just reduce the popping a bit. The popping itself doesn't bother me too much, just wanted to make sure it wasn't doing terrible things to my engine (though running lean is a concern).

Thanks guys.

 

[AlphaDogChoppers]

(though running lean is a concern).

Decel popping is not the same as "running lean." It's when your throttle is closed, and the engine is just coasting. The mixture returns to normal once the engine RPM is down near normal.

Again, decel popping is annoying, but harmless.