Technical Engine backfire

[Italino]

Hello everyone! I am doing this on my phone and can't find a thread on this topic although I'm sure there is one. Anyway I have a '65 500 F with a 123 electronic ignition. When it gets hot it will backfire while decending a hill with no accelerator, I have noticed that the original air tube between the engine cowling and the heater channel to doesn't fit well because of an aftermarket exhaust. Can anyone comment on this problem and solutions?

 

[the hobbler]

Hello everyone! I am doing this on my phone and can't find a thread on this topic although I'm sure there is one. Anyway I have a '65 500 F with a 123 electronic ignition. When it gets hot it will backfire while decending a hill with no accelerator, I have noticed that the original air tube between the engine cowling and the heater channel to doesn't fit well because of an aftermarket exhaust. Can anyone comment on this problem and solutions?

I have electronic ignition (simple'hall effect') on my tuned '126'engine, and it backfires on the over-run---check your timing, and when its cold, it might be worthwhile checking the tappet clearances.

 

[Goldnrust]

I don’t know about the 500 specifically, but popping and backfiring on a closed throttle is common on engines with carbs. The high vacuum drawn with the throttle closed at high revs can cause the mixture to be wrong, the resulting incomplete combustion means that left over in burnt fuel and air mixture can then hit the hot exhaust valve or even the hot exhausts itself, and cause the popping and banging out of the exhaust.

Some more complicated carbs have extra passages or vacuum operated valve that open to change the mixture on over run to stop the popping and banging.

If this has only started happening since fitting the 123 ignition, I would check the timing first. As suggested above, tappet clearances are also a sensible thing to look at, and I would be checking the idle mixture on the carb.

If none of these things are wrong, maybe just embrace the popping and banging! Haha. The boy racers get their modern fuel injected cars remapped to sound like they’ve got the same backfires on a closed throttle!

 

[fiat500]

I don’t know about the 500 specifically, but popping and backfiring on a closed throttle is common on engines with carbs. The high vacuum drawn with the throttle closed at high revs can cause the mixture to be wrong, the resulting incomplete combustion means that left over in burnt fuel and air mixture can then hit the hot exhaust valve or even the hot exhausts itself, and cause the popping and banging out of the exhaust.

Some more complicated carbs have extra passages or vacuum operated valve that open to change the mixture on over run to stop the popping and banging.

If this has only started happening since fitting the 123 ignition, I would check the timing first. As suggested above, tappet clearances are also a sensible thing to look at, and I would be checking the idle mixture on the carb.

If none of these things are wrong, maybe just embrace the popping and banging! Haha. The boy racers get their modern fuel injected cars remapped to sound like they’ve got the same backfires on a closed throttle!

I used to think the popping on over-run was normal until I changed engines from one with worn valve seats to one with good valves etc. I have nothing other than that anecdotal experience to go by, but it doesn't happen any more.

 

[Goldnrust]

I presume the worn valve seats would potentially mean the same sort of combustion leakage as having a tappet clearance that has closed up?

 

[Italino]

I have electronic ignition (simple'hall effect') on my tuned '126'engine, and it backfires on the over-run---check your timing, and when its cold, it might be worthwhile checking the tappet clearances.

Thanks for the info (to you and everyone else who replied). I was told when I bought the car that it had a 695 engine. I'm aware that this might not be true, but if it is, it might also be from a 126. It has a 28mm Weber on it. Perhaps the backfire happens on these engines? Is there any way to confirm the displacement without removing a head to measure the bore?

 

[the hobbler]

Thanks for the info (to you and everyone else who replied). I was told when I bought the car that it had a 695 engine. I'm aware that this might not be true, but if it is, it might also be from a 126. It has a 28mm Weber on it. Perhaps the backfire happens on these engines? Is there any way to confirm the displacement without removing a head to measure the bore?

The engine number will tell you what engine you have in your car. The number is on a small machined 'flat' by the fuel pump. If the number starts with a "126A1" it is a 652cc engine from a Mk2 126. These had a 28IMB carb as standard.

 

[maggellwin]

Hello everyone! I am doing this on my phone and can't find a thread on this topic although I'm sure there is one. Anyway I have a '65 500 F with a 123 electronic ignition. When it gets hot it will backfire while decending a hill with no accelerator, I have noticed that the original air tube between the engine cowling and the heater channel to doesn't fit well because of an aftermarket exhaust. Can anyone comment on this problem and solutions?

Hi, you have a lot of good advise already, from my experience back in the day, if the exhaust system is old and getting thin, it gets excessively hot and re ignites that little bit of fuel that gets through, just a thought

 

[RalphM]

Any air leaks in the exhaust system will lead to unburnt fuel burning and so popping and banging.

 

[lazylobster]

I really don't see this as a problem. I'm all for popping from my cars. But only on the overrun. Anything else such as tick over, accelaration is not correct. Carb needs a tune, tappets, air leak or other.