I have a 1985 5.0L crusader and am experiencing backfires through carb. No problem at idle or low speed but when applying the gas the back fire occurs. The accelerator pump seems to work fine. The timing is to spec and the plugs were just replaced at the proper gap. I know the engine is tired but wondering if there are any tips on this problem?
Back Fire through carb
- Thread starter buckeye
- Start date
Sometimes a sticking float valve can be freed by hitting the carb with the handle of a plastic or wooden screwdriver. The fuel inlet fitting is close to the valve and is a thicker area of the carb body.
Also, check for an intake air leak around the base of the carb like Billk said.
Also, check for an intake air leak around the base of the carb like Billk said.
A lean mixture fires prematurely. Sometimes prematurely enough to fire back through the intake.
OP, please check your Distributor Cap and Rotor. They can be fouled with carbon and metal traces. But the specific check I have in mind is to make sure that the center contacts in Cap or Rotor have not lost springiness. The cap on our 5.7 used to have a solid center contact and the rotor incorporated a leaf spring to make contact. Newer parts seem to have reversed that. Solid contact on the Rotor with the Carbon in the Cap riding on a hidden Coil Spring. I've had two of those fail with a rusted spring, jamming the carbon contact so it doesn't stay in contact with the Rotor as it wears. It has shown itself as a miss under heavy load.
OP, please check your Distributor Cap and Rotor. They can be fouled with carbon and metal traces. But the specific check I have in mind is to make sure that the center contacts in Cap or Rotor have not lost springiness. The cap on our 5.7 used to have a solid center contact and the rotor incorporated a leaf spring to make contact. Newer parts seem to have reversed that. Solid contact on the Rotor with the Carbon in the Cap riding on a hidden Coil Spring. I've had two of those fail with a rusted spring, jamming the carbon contact so it doesn't stay in contact with the Rotor as it wears. It has shown itself as a miss under heavy load.
SLW,
Why is it that you think timing advance would be a problem I would consider timing very low on the cause.
As for stuck needle valve, what is it that makes you think it is a possible cause. What may be happening with a stuck float valve is the fuel level drains down faster than a stuck valve lets fuel back into the float bowl. This causes a lean condition. A lean condition is too much air in the fuel/air mixture. This mixture allows for a premature detonation in the combustion chamber.
Why is it that you think timing advance would be a problem I would consider timing very low on the cause.
As for stuck needle valve, what is it that makes you think it is a possible cause. What may be happening with a stuck float valve is the fuel level drains down faster than a stuck valve lets fuel back into the float bowl. This causes a lean condition. A lean condition is too much air in the fuel/air mixture. This mixture allows for a premature detonation in the combustion chamber.
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Timing would be my first thought. Even with static timing set timing weights often rust and limit timing advance..Check timing advance to insure it is advancing.
Charlie,
To clarify: I'm in a "student" role here. This is the first time in 16 years that I've had to deal with carbs on big inboards, so I'm trying to soak up as much as I can.
It was actually JimPend who suggested timing, but it makes sense to me because timing issues are so often the cause of backfiring through the exhaust. I would think timing could also explain a carb cough too, particularly in engines with a cam overlap...?
I was thinking the other way around with a stuck float valve. It stuck OPEN in my outboard this spring, so the bowl flooded. It didn't backfire, but with that much excess gas, I expected it to.
My question about the lean condition was to get an understanding of what being lean does. I could see how a cylinder could be SO lean that it doesn't spark-fire, so instead it spontaneously fires instead. (backfire) However, it sounds like the accepted explanation is that a lean mixture is more volatile and more likely to detonate BEFORE the spark... I guess I just had it reversed.
To clarify: I'm in a "student" role here. This is the first time in 16 years that I've had to deal with carbs on big inboards, so I'm trying to soak up as much as I can.
It was actually JimPend who suggested timing, but it makes sense to me because timing issues are so often the cause of backfiring through the exhaust. I would think timing could also explain a carb cough too, particularly in engines with a cam overlap...?
I was thinking the other way around with a stuck float valve. It stuck OPEN in my outboard this spring, so the bowl flooded. It didn't backfire, but with that much excess gas, I expected it to.
My question about the lean condition was to get an understanding of what being lean does. I could see how a cylinder could be SO lean that it doesn't spark-fire, so instead it spontaneously fires instead. (backfire) However, it sounds like the accepted explanation is that a lean mixture is more volatile and more likely to detonate BEFORE the spark... I guess I just had it reversed.
Timing NOT advancing would LESSEN the chance of backfiring thru the carb. Think about it, normally, the timing advances earlier and earlier as the RPM's go up. A stuck advance prevents the spark from advancing. It's firing late. To backfire thru the carb, it has to fire earlier and earlier to be able to make the combustion leak up the intake.
A lean condition makes the engine combustion chamber run hotter and hotter which makes piston and spark plug start to glow. This glowing is what prematurely combusts the fuel/air mixture.
An overly rich mixture won't burn all the fuel in the cylinder so unburned gas is escaping into the exhaust pipes. The hot exhaust from the next combustion will set off the unburned gas. An example is some knothead running own the street will momentarily turn his ignition off the back on and he'll get a very loud backfire out his exhaust pipes.
As far as stuck float valve, where is sticks will determine if it will run rich or lean. How the engine is running will give you a clue if the valve is sticking and where.
A lean condition makes the engine combustion chamber run hotter and hotter which makes piston and spark plug start to glow. This glowing is what prematurely combusts the fuel/air mixture.
An overly rich mixture won't burn all the fuel in the cylinder so unburned gas is escaping into the exhaust pipes. The hot exhaust from the next combustion will set off the unburned gas. An example is some knothead running own the street will momentarily turn his ignition off the back on and he'll get a very loud backfire out his exhaust pipes.
As far as stuck float valve, where is sticks will determine if it will run rich or lean. How the engine is running will give you a clue if the valve is sticking and where.
easttnboater
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Also, just because you have spark to the plugs does not mean that the plugs themselves are firing. I had a hard starting, backfiring 5.0 Mercruiser engine - 3 out of 8 less than one year old NGK plugs were not firing.
So, just because the plugs are new does not mean they are good.
So, just because the plugs are new does not mean they are good.
Thanks for all the notes. I will attempt to work on it this weekend. Couple of questions, if I rev. the engine while the boat is in the slip and the timing advances can I assume it will also advance when it is under full load?
Likewise, if I rev the engine in the slip would I expect it to backfire or would one think the enginer would need to be under load to backfire?
Likewise, if I rev the engine in the slip would I expect it to backfire or would one think the enginer would need to be under load to backfire?
I won't say it Cannot/Will Never backfire in Neutral/Low Load. I WILL say ours did NOT, Not until we were out and trying to accelerate. It rarely backfired in steady state cruising, although it did that a few times also. One time I changed Cap, Rotor and Wires. Another was Cap and Rotor and Plugs. This time it was Cap and Rotor only, Plugs recently changed on another project. Wires were replaced only on the first repair. NAPA premium set, now 10 years old. Notice that every repair for the Backfire and Missing involved Cap and Rotor.
Ha, like eastern said, brand new plugs can be bad. They can cause a backfire, too.
All you have to do to check if the mechanical advance is working is to manually turn the advance mechanism (weights). If it turns smoothly (advances) and returns, you're good.
All you have to do to check if the mechanical advance is working is to manually turn the advance mechanism (weights). If it turns smoothly (advances) and returns, you're good.