280C "cough" when shutting off.

[cefalu]

I am experiencing a strange mini backfire when I shut my '74 280C off. Right after the motor comes to a rest after shutting it off, it gives out a little cough through the carb.

I am using a Holley economaster, but it is brand new NOS. It did the exact same thing with the previous Holley too. In fact that's why I bough the new one because I thought it might be the carb. So I am thinking it's not the carb anymore. My timing is 10 deg BTDC. It's the M110 DOHC USA version engine.

Otherwise, the car runs perfect, as good as fuel injected. Perfect steady idle.

[Frank Reiner]

The original Solex four barrel was equipped with idle fuel shutoff solenoid valves; for good reason.

[cefalu]

Your post infers that the unique nature of the M110 requires a fuel shutoff for any carb fitted to it. What is so unique about the M110 that would require fuel shutoffs? The vast majority of pre emission motors fitted with carbs did not have fuel shutoffs and operated just fine.

Fuel shutoff solenoids emerged as a response to emission control devices such as lean jetting, air injection, EGR valves, and ATDC timing. None of which my M110 has.

I doubt the M110 has a design that is so unique it requires a fuel shutoff to function correctly. I asked my original question because I suspect that maybe something else is going on, like cam timing being off, which might explain it. Or maybe an explanation as to what the cough actually is.

Whatever it is it is related to the speed of the engine at shutoff. If the car is in (D) 900 RPM, then the idle is lower at shutoff and I don't get the cough. It's only when the car is in (N) 1200 RPM at shutoff do I get the cough.

[alabbasi]

Ove-run protection valves are pretty common on cars with emissions stuff and carbs. My 73 MGB had them also.

Have you checked your timing?

See here for more general information about the problem: Dieseling - Wikipedia, the free encyclopedia

[Frank Reiner]

While I am at a loss to identify the specific reason for the sensitivity, the M110 is indeed touchy about the shutoff of mixture. The FSM makes reference to afterunning as the reason for the presence of the valves in the section that deals with the testing and updating of the shutoff valves.

As a workaround for your specific case, allow me to suggest that you reduce the idle speed to 700-750 in N, which is the low side of the spec for the 73-74 engines w/ distributor #0 231 310 002. I would also suggest operating without the vacuum retard, and with timing set to 13-15 deg. BTDC @ 1500 RPM. The 75-76 distributor did not have any vacuum controls, and is the one that I prefer (#0 231 311 001).

[cefalu]

Well, I feel pretty stupid about this. But I figured it out. One of the header to downpipe round asbestos gaskets has been slowly dying, and creating an exhaust leak.

The cough I am hearing is actually coming from the failed exhaust gasket, not the carb. What is happening is the unburnt air/fuel left over after turning off the ignition is collecting in the exhaust system and then igniting due to the air entering through the failed gasket.

My 280C has that thermal reactor thing, which must be hot enough to cause the exhaust ignite once the air hits it.

Thanks all

[Idle]

Is it possible to discard the thermal reactor? I am only guessing, but is it possible to replace the exhaust manifold(s) and the downpipe to the exhaust pipe and be done with it?

Some states do not require emissions equipment on older cars and some do, so that is one inhibiting factor, but short of that: Anyone know if a simple manifold and pipe change is possible?

I only bring this up because someday the reactor will crack (it is a large ceramic piece under the stainless cover) and this will allow exhaust gasses to escape which will burn the paint off of your hood, so the elimination of the reactor would appear to be a positive thing.

If it is possible.

[cefalu]

It's on the list. I already have M110 49 state exhaust manifolds and down pipes I plan to install someday soon.....

But the access looks horrible on the thermal reactors. It looks like it's a knuckle buster too. just been putting it off.