54 Merc surging at stop in gear.

[JT1930]

I’m at a loss right now with my 54 Merc 256 y block. Car has been great all summer and all of a sudden it starts like surging or rpm’s dip at a stop in drive. Automatic car.

Rebuilt teapot carb. Petronix in the distributor. New coil. 12v converted. Temp is good. I also put a 3 port fuel filter between carb and mech pump and ran a line back to the tank. It was vapor locking when I got the car. This fixed it and car ran with no issues for months as is.

Gave the car a little more idle air still same issue.

Thoughts.

[paul2748]

Long shot - but did you check trans fluid ? Remember to check level with engine running and trans in neutral or park

[JT1930]

She is a dripper. I’ll check since it’s been some time checking. Shifts beautifully though.

[40 Deluxe]

Verify that harmonic balancer ring has not slipped (leading to setting the timing wrong). Verify that there are no vacuum leaks-intake gaskets, carb gaskets and all hoses. Make sure the throttle shaft is not worn and sucking air. Set the the idle mixture and speed with a tachometer-no guessing! Adjust each screw to obtain the highest idle speed. Reset idle speed to spec. and readjust the mixture. Repeat as needed.
Distributor bushings good? Are the advance springs in place and vacuum chamber good? This is an all-vacuum advance distributor.

[Daves55Sedan]

Agree with all the above, but could be a couple other things causing it.
If float is stuck open at the exact right spot at idle, fuel could be on the verge of flooding the engine out.
Don't know about Merc's, but Fords with Fordomatic had an anti-stall dashpot on the carb to deter the accelerator lever from dropping to idle too fast to keep engine running when coming to a stop. If dashpot diaphragm is shot, the engine could stall.

[JT1930]

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Checked trans fluid. Good.
Never touched distributor so that’s ruled out.
I will try some carb cleaner around intake gaskets.


I’m thinking of getting rid of this load o matic distributor and tea pot carb. Looking threw paper word from previous owner. He put a brand new rebuilt carb on and issue was still there. Probably best to go with the 57 and up dizzy/oil pump and shaft and a Holley 2300 carb with intake for it.

[dmsfrr]

Is it possible the old diaphragm in the vacuum canister on the distributor has failed?
It controls the timing advance in the distributor.
The seal of the diaphragm can be easily tested by gently blowing into the tube to see if it holds pressure.

[scicala]

Originally Posted by JT1930

Checked trans fluid. Good.
Never touched distributor so that’s ruled out.
I will try some carb cleaner around intake gaskets.


I’m thinking of getting rid of this load o matic distributor and tea pot carb. Looking threw paper word from previous owner. He put a brand new rebuilt carb on and issue was still there. Probably best to go with the 57 and up dizzy/oil pump and shaft and a Holley 2300 carb with intake for it.

That's an awful lot of expense and searching compared to just checking the original carb and repairing or rebuilding.
It could be something as simple as the float starting to fill with fuel and causing a high fuel level.
I would be happy to look at you carb and repair or refurbish it. Coud at least tell if the problem is with the carb or not. I have 57 years experience with these carbs, along with all domestic carbs from about 1935 and up.

Sal

[JT1930]

Sal it’s just piece of mind getting rid of the teapot carb and vacuum advanced distributor. Plus it would probably run even better with 57 and up parts/setup.

But I will check the float I have a spare carb here is any parts are needed. This carb is newly refurbished in 2020 also.

[JT1930]

Originally Posted by dmsfrr

Is it possible the old diaphragm in the vacuum canister on the distributor has failed?
It controls the timing advance in the distributor.
The seal of the diaphragm can be easily tested by gently blowing into the tube to see if it holds pressure.

I’m checking that also. But at any other speed it is perfectly fine running.

[miker98038]

Any chance you were on the “edge” of lean, and now with lower ambient temperature and a tank of “summer” gas (higher Reid vapor pressure) it’s enough to cause the problem? I’ve seen off season fuel do odd things. More likely winter fuel in the spring, hot day, and vapor lock, but I guess it could go the other way.

[JT1930]

No. This started at the end of summer.

I did notice something tonight. As you know the fuel bowl is over the carb on a teapot carb. The two screws that hold it to the carb were somewhat loose and the short screw on front of carb gasket is blown out. When I moved the bowl side to side it would squirt fuel. Now thinking of it the whole air box assembly sits on that and it’s screwed to that bowl. Maybe at idle the motor would shake and cause more fuel to dump into motor. Taking carb off to tighten and put a little bit of blue loctite on threads.

[scicala]

Originally Posted by JT1930

No. This started at the end of summer.

I did notice something tonight. As you know the fuel bowl is over the carb on a teapot carb. The two screws that hold it to the carb were somewhat loose and the short screw on front of carb gasket is blown out. When I moved the bowl side to side it would squirt fuel. Now thinking of it the whole air box assembly sits on that and it’s screwed to that bowl. Maybe at idle the motor would shake and cause more fuel to dump into motor. Taking carb off to tighten and put a little bit of blue loctite on threads.

I've run into that issue before too. The idle fuel travels through where the front shorter screw goes though. and with the bowl wobbling, it trashes the small gasket there and lets air into the idel channel.
That's probaly why for '55 they put the housing under the bowl with the choke plates, to keep the bowl from getting loose from wobbling.
Also, you might want to replace that small gasket too if it got damaged.

Sal

[JT1930]

Sal yes. I have a new gasket for it. Going to give it a try.

[Daves55Sedan]

Originally Posted by dmsfrr

Is it possible the old diaphragm in the vacuum canister on the distributor has failed?
It controls the timing advance in the distributor.
The seal of the diaphragm can be easily tested by gently blowing into the tube to see if it holds pressure.

Let's hope the vacuum diaghram in the vac advance is not cracked. Replacements for 54 thru '56 V8 loadomatic dizzy vac advance cans haven't been available for a few years now.
I'm not so sure a busted vac advance diaphragm would cause his problem though. It definitely would be a problem attempting to obtain acceleration though.

[JT1930]

I check that and it held vacuum.

Let car get warm today. Sat in drive for a bit. No issues so far. Test drive when no snow is outside.