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why does it backfire?

R

ripper

Moped
I've noticed a backfire on deceleration . So I was wanting
To ask a few questions.

During tb sync could I have put a cap back on a nipple
incorrectly or cap torn?

When I plugged the throttle bodies with paper towles
Could i have messed up the action of a butterfly?

I have a small exhaust tic?

One of the sensors bad?

If you guys could help it would be great. Also is it a big
Deal or not? This will take me alot of time troubleshooting

Chuck

 
A backfire normally means one of 2 things.
Either there is a leak between the head and muffler which when you close the throttle, sucks fresh air into the muffler which cools the gases resulting in a pop.
Or,... (and this is a "proper" backfire) some gas has gotten into the exhaust and is getting ignited in there.

The first is pretty common if you've fitted an aftermarket muffler or if the exhaust has come loose somewhere.
The second would surprise me if it happened on a modern fuel-injected engine.
 
The PAIR system is usually to blame, as it lets fresh air into the exhaust ports. Adding air to the hot exhaust causes any unburned fuel to ignite, and this is what causes the backfire. It normally only happens if you have modified the exhaust.

You might also have a loose exhaust header, and this is a pretty common problem. If you tighten up the nuts that secure the header pipe to the head, it will probably fix it. They tend to loosen up over time (and as the bike breaks in),  I suspect because the gasket heats up and starts to get compressed.

 
Fred_Harmon_TX said:
The PAIR system is usually to blame, as it lets fresh air into the exhaust ports. Adding air to the hot exhaust causes any unburned fuel to ignite, and this is what causes the backfire. It normally only happens if you have modified the exhaust.

You might also have a loose exhaust header, and this is a pretty common problem. If you tighten up the nuts that secure the header pipe to the head, it will probably fix it. They tend to loosen up over time (and as the bike breaks in),  I suspect because the gasket heats up and starts to get compressed.
Click to expand...

Fred. Can you define "loose"? I tightened mine awhile back and it was pretty easy to turn them with a small socket wrench, but I didn't have a torque wrench. I later found that the recommended torque was something like 13 in/lb which seems really low.
 
wally_games said:
Fred_Harmon_TX said:
The PAIR system is usually to blame, as it lets fresh air into the exhaust ports. Adding air to the hot exhaust causes any unburned fuel to ignite, and this is what causes the backfire. It normally only happens if you have modified the exhaust.

You might also have a loose exhaust header, and this is a pretty common problem. If you tighten up the nuts that secure the header pipe to the head, it will probably fix it. They tend to loosen up over time (and as the bike breaks in),  I suspect because the gasket heats up and starts to get compressed.
Click to expand...

Fred. Can you define "loose"? I tightened mine awhile back and it was pretty easy to turn them with a small socket wrench, but I didn't have a torque wrench. I later found that the recommended torque was something like 13 in/lb which seems really low.
Click to expand...

IIRC it is actually 13 ft/lb.
 
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