Classic cars forum & vehicle restoration.

[Rick]

Once the Dodge (car) is fully warmed up, it's possible to make it backfire on the over-run when, say, slowing for traffic lights. Occasionally it also emits a hearty bang when switching off - is this likely to be un-burnt fuel in the exhaust?

Timing springs to mind as a cause for backfiring, but what else might cause it?

thanks, RJ
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[ukdave2002]

Hi Rick

Do you know if the backfire exits the exhaust or carb? Both can sound like a rifle going off!

Dave

[Rick]

[Peter_L]

It could be an inlet manifold leak. I had it happen to me once on a 2000 Mk5 Cortina. Would happen when I switched the engine off. I seem to remember I found a temporary fix, by opening the throttle as I switched off the engine.

[Farmer John]

Rick, it is running lean on overrun. If adjusting the idle mixture to the most rich you are comfortable with does not cure it then the field is wide open. You will have lots of vacuum, so a very slight leak at idle becomes significant on overrun.
Anywhere from the throttle shaft to the valve guides (including all of the idling system) as Peter found.
The backfire when turning off can have other causes as well, particularly valves not sealing.
So mixture first then throttle shaft as all of the other faults will manifest themselves in other ways as well.

John

[Rick]

Interesting, thanks, I'll take a look at the plugs today to see how they're looking. There's no inlet manifold as such, the carb bolts direct to the side of the block. I replaced the carb:block gasket not long ago as the original one was in a mess, but I'll have another look at that too.

I do have a few doubts about the vacuum fuel supply, that could cause it to run lean also if it's not 100%.

RJ
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[ukdave2002]

It's worth checking all the HT components; as the fuel mixture is leaner when you ease up, the ignition voltage has to be higher to fire a lean mixture. If there is any weakness in the leads, plugs, cap etc, the coil energy may find an easier way to earth; sometimes firing a plug that may be in a cylinder during an induction stroke (no compression on an induction stroke so far easier for this plug to fire rather than the correct one).

I used have this problem when I ran a car on LPG; plugs would only last 8000 miles, leads about 25k and coils about 50k, the lean running puts more demand on the HT, and on overrun when it was really weak I would get the misfire when something was beginning to break down. All ways through the induction system, closing the plug gap slightly can help as a temporary fix.

If the mixture is lean and the HT voltage can't rise high enough to fire it, the unburned fuel will get into the exhaust and pop with the heat.

With either of these two scenario's just making the mixture richer may mask another problem; air leak or HT fault.

Dave

[Uncle Alec]

A leak in the exhaust system is a good cause of backfires. Such unburnt fuel as exists in the exhaust gases re-ignites when able to draw in oxygen from the outside.

[Rick]

[Salopian]

All good suggestions and heres a very long shot from me - valve timing slightly out. Not very likely but its been known and I suspect you don"t know the history of the engine.
You might try having a word with one of the Dodge owners in the VSCC with your problems. Not sure how helpful the main Dodge trialler is but you could find out?.
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Jonathan Butler
Alvis SD 12/50 1928 MG TD 1950

[Rick]

Good point JB, I might look them up sometime.

I noticed today that the mixture was quite lean, looking at the plugs, so I've richened it up a bit. I've also had another go at the manifold and come up with perhaps a better way of making it seal, the problem is that the four ports on the manifold don't line up 100% with the recessed ports on the block, which has made getting a gas-tight seal something of a challenge to date. The next test run should indicate if there's been a general improvement or not, richening up the mixture somewhat seems to have helped, but not eradicated, the issues, while the manifold appears to be sealing a lot better now - but will it last!?!??

RJ
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[ukdave2002]

These things can often be a combination of minor faults; when I used to run an Omega on LPG, with the first sign of a mis or backfire I'd pull into the next services and enrich the mixture on the LPG carb, that would be good for a few thousand miles, if I'd not bothered to fixed the problem , the mis/backfire would return and I'd whip the plugs out and close the gap a tad, this would give me another 1000 miles and perfermance would suffer ; so then I would really have to fix the fault (allways plugs,coil pack or leads) used to get some odd looks in motorway services as I whipped plugs out whilst suited and booted!

Dave

[stuchamp]

My first thought is that you need to advance the timing.

[Jim.Walker]

ALL my money is on Uncle Alec's suggestion.
Contrary to the weak mixture theory. A rich mixture will pass into the exhaust partly unburnt for lack of sufficient oxygen. Fresh oxygen leaking in often causes a BANG!
Jim.
_________________
Quote from my late Dad:- You only need a woman and a car and you have all the problems you
are ever likely to want". Computers had not been invented then!

[Jim.Walker]

ALL my money is on Uncle Alec's suggestion.
Contrary to the weak mixture theory. A rich mixture will pass into the exhaust partly unburnt for lack of sufficient oxygen. Fresh oxygen leaking in often causes a BANG!
Jim.
_________________
Quote from my late Dad:- You only need a woman and a car and you have all the problems you
are ever likely to want". Computers had not been invented then!