Quarter gas misfires

[RustOnWheels]

225 slant six, Carter BBS, 1965 Plymouth Valiant station automatic.

When it’s cold and I prep the choke (one full step on gas and then a little gas, then start), it starts alright, then put it in Drive and from then on, after a few seconds - if I have it at quarter gas - it will stumble and rumble. After about thirty seconds it will get better again. But… it will always stumble a little at quarter gas.

With stumble I mean misfires and vibration. It’s warm outside now and it still does it. After a long drive it will also misfire at idle, in any gear (also P).

Now there is one thing I noticed just this evening: when I turn on my headlights it seems like the stumble and misfires at quarter gas seem to be less, the engine seems to run a little better as well. At the same time I also noticed that the gauges slowly went up after turning on the lights; the value of the temp went to perfectly in the middle in stead of a little low, and the gas gauge went a little higher.

So I suspect that I need to do something with the electrical system (voltage regulator? Alternator?). But to get it running perfectly at quarter gas I do think I need to do something more. I’ve already measured the accelerator pump height as in the workshop manual and checked the Carter as good as possible (bought it as refurbished, I believe even here), but the misfire persists with the pedal at quarter (without movement) so it seems it’s not the accelerator pump per se.

What do you think, on the voltage and the carb situation?

 

[missing linc]

On the electrical front I would guess possibly a bad ground. The misfires I say accelerator pump is not working properly, causing it to go lean and create the stumble.

 

[AJ/FormS]

That right there, could be related to a whole bunch of stuff, in order of likelihood;
a lack of choke plate tension,
a bad vacuum-break adjustment,
a low fuel-level,
stale fuel,
a lean power-valve,
Mixture screws set too lean,
a tip-in sag; from some combination of too much Idle-Timing and the speed screw too far closed,
a faulty accelerator pump, or adjustment,
Maybe but I doubt it faulty compression.
maybe but I doubt it, retarded ignition timing,
maybe but I doubt it, tight valve lash.
lousy cam timing.
Chassis problems.
I could be/might be, convinced to take that wagon off your hands. lol.

 

[Rug_Trucker]

How much slop do you have in the throttle shaft?

 

[RustyRatRod]

Sounds normal till it warms up a little. This ain't EFI.

 

[Dave999]

1/2 hour disconnect re connect engine side connectors bulkhead then from ballast to the coil
clean up and re bolt engine earth strap.
check the battery connectors undo and re do the end of the battery negative to the chassis

your car is behaving like it has a very poor engine to body to battery earth if nothing electric can see what 0 V is you get this inconsistent behaviour

NL? do you live in a cold damp hot humid or beach front place that might promote corrosion of exposed electrical contacts

1/2 hour job and try it, all before you touch carb, choke, kickdown or timeing

Dave

 

[slantsixdan]

Carburetor operation and repair manuals and links to training movies and carb repair/modification threads are posted here for free download. Choke adjustment info is here. Find tune-up parts and technique suggestions in this post.

Odds are pretty good once you've gone through and done all the adjustments and consumable-parts replacement and paid some attention to the carb, things will be a lot better or perhaps fixed completely. I think the apparent headlamp connection is a phantom; don't spend too much time chasing your tail on it.

Beyond that: "vibration" is too general. This could be anything and anywhere. Could be a worn ball & trunnion front universal joint, could be faulty engine mounts, could be a carb problem, an ignition problem…

 

[Bewy]

It does sound electrical related because when you change the electrical load, the symptoms change with it.

I would try, in this order:
- make sure alt has a clean ground connection
- same with the volt reg. Also check the grd strap, body to engine.
- try another battery

 

[RustOnWheels]

Wow thanks all for the advice! I’ve been a Mopar guy since 2002 so I know some of the quirks of Mopars throughout the years (the bulkhead, the ammeter, the voltage regulator of the dash er et cetera) and I know of the issues with carbs being worn, the prolonged misfires at quarter gas didn’t strike me as the usual.

I’ll start with cleaning and checking all the ground wires and connections.

In the past the first thing I did with Mopars was wiring the lighting directly so the bulkhead disconnect and light switch wouldn’t burn down the car.

I’ve bought this car 2 years ago and haven’t done anything electrical (except installing a new ground wire and battery - Bosch S4 004).

I did swap out the bad Holley carb for a Carter (I love Carters more than Holleys anyway). I’m aware of the old Carters having issues of wear as well. I’ve checked the Carter as per tips from UTG, the slant six forum and the workshop manual. I also bought a NOS Carter choke pull-off as it has a different stroke compared to the Holley.

I also had the valves set and timing chain replaced (preventative) by a classic car garage (specialized in US cars only), and it has a new exhaust, and the car was in daily use by the previous owner in Sweden, pretty good taken care of technically. I suspect the car was a very clean example when it was imported from SoCal (don’t know when). Because it was a daily in Sweden it isn’t perfect anymore but still quite solid.

I’ll report back with the outcome and also the other tests.

 

[Dave999]

ah so you are in the netherlands
ok similar weather to the UK we have a bit of sun we have a bit of rain.
-10*C to 38*C depending on season but mostly damp/humid due to the prevailing wind coming off the sea

I have an australian car that lived in the hot NSW country climate, then lived by the beach in syndey for 2 years, never the calmest of waters near by, you could smell the salt in the air.

it came to Uk and lived in the cold climate
most of my connectors got a covering of green virdigris, once introduced to cold damp environment.
made all of my connections resistant, made my wires hot , my ignition switch connector went on fire, nothing worked very well...

hence my comment above, can speak from experience maybe my car was just subjected to a unique set of circumstances ??
....still 1960s/70s connectors corrode, just like the rest of the car....

Dave

 

[slantsixdan]

I’ve checked the Carter as per tips from UTG

Stop following that idiot's advice, and your life will get better.

 

[Killer6]

Sounds like possible carbon tracking crossfire at part throttle when the vacuum advance is in,...?