exhaust manifold backfiring
Thread Starter
Le Mans Master
Joined: Jun 2008
Posts: 5,284
Likes: 235
From: Rockledge FL
exhaust manifold backfiring
so i have been trying to track down a reason for a backfire and just cant do it.
it is in the exhaust. i know this for a fact because if i rev the engine, when i let go of the throttle it spits a flame out from the joint between the manifold and the exhaust pipe.
the pipes and manifold are very very rusty.
timing is set at 12 degree with the vacuum line unhooked.
i took off both valve covers and all 16 valves go up and down equally
i have a set of hedman headers and 2.5 system ready to go in once i can get the car running enough to make it to the lift.
is it possible that it is just the crappy exhaust seals pulling air and igniting unburned fuel?
it is in the exhaust. i know this for a fact because if i rev the engine, when i let go of the throttle it spits a flame out from the joint between the manifold and the exhaust pipe.
the pipes and manifold are very very rusty.
timing is set at 12 degree with the vacuum line unhooked.
i took off both valve covers and all 16 valves go up and down equally
i have a set of hedman headers and 2.5 system ready to go in once i can get the car running enough to make it to the lift.
is it possible that it is just the crappy exhaust seals pulling air and igniting unburned fuel?
Team Owner
Joined: Aug 2006
Posts: 24,125
Likes: 15
From: Columbia Missouri
Yep, if it is a light popping and not a backfire, your fuel mixture is too rich and bleeding fuel during decel. Also, if the exhaust manifold has a large crack or gasket leak, it could ignite a rich unburned mixture in the manifolds.
Le Mans Master
Joined: Apr 2004
Posts: 7,106
Likes: 38
From: Newark DE
as per Lars' on one of his previous replies on this subject:
"Popping out the exhaust on decel is almost always a lean condition that occurs when the throttle is slammed closed at elevated engine rpm. What happens is that the cylinder mixture goes momentarily lean, you get a lean misfire on a few cylinders, and this misfire dumps unburned air/fuel into the exhaust where it then eventually ignites (explodes). It is possible to have an over-rich mixture causing burning in the exhaust, but this over-rich mixture will seldom cause an actual misfire, so the air/fuel is burning as it's leaving the cylinder and not "exploding" in the exhaust. You'll see this type of rich condition on some road race cars where you see the flames coming out the exhaust on deceleration. But with the flame, there is no "popping" with the rich condition - the popping occurs with lean misfire.
Incorrect timing advance can contribute to the problem: A lean mixture burns slower than a rich mixture, so you have to have adequate ignition timing to fully burn the lean mixture. If you're lean with retarded timing, the popping is increased."
why not try backing out the idle mixture screws one or two turns to see if that stops or reduces the popping?
"Popping out the exhaust on decel is almost always a lean condition that occurs when the throttle is slammed closed at elevated engine rpm. What happens is that the cylinder mixture goes momentarily lean, you get a lean misfire on a few cylinders, and this misfire dumps unburned air/fuel into the exhaust where it then eventually ignites (explodes). It is possible to have an over-rich mixture causing burning in the exhaust, but this over-rich mixture will seldom cause an actual misfire, so the air/fuel is burning as it's leaving the cylinder and not "exploding" in the exhaust. You'll see this type of rich condition on some road race cars where you see the flames coming out the exhaust on deceleration. But with the flame, there is no "popping" with the rich condition - the popping occurs with lean misfire.
Incorrect timing advance can contribute to the problem: A lean mixture burns slower than a rich mixture, so you have to have adequate ignition timing to fully burn the lean mixture. If you're lean with retarded timing, the popping is increased."
why not try backing out the idle mixture screws one or two turns to see if that stops or reduces the popping?
