VCCA Home Forums TECHNICAL - 1945 AND EARLIER 1933-1936 Engine cutting out when accelerating

When I accelerate hard in my 36, (such as merging in traffic), the engine begins to sputter and will actually cut off. Fortunately I had enough momentum to downshift to 2nd and pop the clutch and it will come back to life but will not get up to speed without repeating the sputtering / stall. Driving at steady speeds up to 35, there is no problem. Is this a fuel or ignition problem? The mechanic that was helping me with the restoration thought I needed a fuel filter in the line and installed one the size of a soda can before the fuel pump (without asking me first) Too much restriction if the fuel line?

I would guess the accelerator pump, but the complete shut down is confusing. Maybe look for an open in a wire in the distributor. When the vacuum advance moves it shorts out.

Is the accelerator pump in the carburetor working? Remove air cleaner and using a flash light look into the carb. throat when opening the throttle. You sould see a stream of gas spurting in. This can be done with the engine not running.
If spurt is weak could be caused by worn out acccelerator pump, partly plugged pump jet or one of the check valves in the acelerator pump circuit.

Thanks I will take a look when I get home.

The fact that the car will run a constant 35 MPH shows most things are right.

Try accelerating from a stop in first gear, and wind the engine up above the engine speed that you would merge traffic. Is there any cut-out at the high end? Continue accelerating in second the same way. Any cut-out? Basically checking to see if the fuel delivery system is weak.

My GUESS is distributor advance mechanism, but one test is worth 1000 guesses!

RE the accelerator pump: Have not seen this for awhile, but 40 years ago, used to see quite a few carbs on older cars/trucks WITH THE ACCELERATOR PUMP REMOVED!!! This was common practice during WWII when gas was rationed. And once the engine has begun accelerating, it could still be accelerated at full throttle. The lack of the accelerator pump should not cause the engine to stall, and certainly would not cause it not to come up to speed when started by popping the clutch in second gear.

Jon.

My Carburetor was rebuilt by a club member ( I can’t recall his name), from Texas about 5 yrs ago, so I don’t believe it would be the accelerator pump. The engine was just overhauled and I am generally driving it gently. It could well be the distributor so I will take it back to the rebuild guy.

Check to see if the distributor is advancing. On this car the whole cap wires and all rotates when gas is given. You can see it by looking down at the distributor and gunning it.The so called "pot" or vacuum advance may not be working. Also you can remove the small tube at the corroborator and suck on it. It should not suck air. If it does the diaphragm is broken.

You could be running out of gas each time you reach max rpm on a take off. Did you ever change out the diaphragm in the fuel pump? If it has a hole in it; it will be leaking into your crankcase. Check to see if your oil level is rising. That is a good sign you have a gas leak past the fuel pump into the crankcase.

Check your corroborator to see what position your float is. If it is to low you will run out of gas. Or if crud has gotten under the float it will hold it in the up position not allowing enough gas to be in the bowl. The entry valve will not allow enough gas to be in there for a fast acceleration. Or it may simply be set to low.

From what you are saying you are running out of gas for some reason. Because after a few seconds you can re-start.

The vacuum advance does not advance the timing. It retards the timing when the engine is under load and the vacuum supply is low. This is done to prevent pinging (pre-ignition). If the diaphragm in the vacuum advance is leaking it could cause a lean mixture and hesitation.

If this problem just suddenly happened it could be dirt in the carburetor pump circuit. When the pump jet is plugged up it become difficult to "step on the gas" as the pump cylinder is filled with gas and it causes hydraulic lock.

Check your timing.

Isn't that just a backwards way of describing vacuum advance?

On a 1960s car, there is no vac advance when the car is stopped, the ignition would be retarded.
On the car in question, when the car is stopped, if there is no vacuum, the engine timing would be retarded.

On a '60s car, at high vacuum cruise, the timing is advanced.
On this car, at high vac cruise, the timing is advanced.

On a '60s car, at high load, low vac, the timing goes to retarded.
On this car, at high load, low vac, the timing is retarded.

The only diff I see is the '60s car advances the breaker point plate only vs the entire dizzy head.

Doug

I think that I was backwards. When there is no vacuum on a 6 Cyl. engine the timing is retarded. This would be at idle and under a load. When at a steady speed with light load and high vacuum the advance advances the timing. This is true for most Chevrolets also EXCEPT for 1958 to the early 1960's with 8 Cyl. engines, the vacuum advance got it vacuum supply from one of the carburetor mounting studs that was hollow and connected directly to the intake passage.
Chevrolet said this was more efficient and the engine ran cooler at idle speed. This was changed in 1963 if I remember correctly as this set-up cause more polution and they went back to the old way.

So the '30s Chev gets its vacuum from ABOVE the throttle plates?

As far as 50/60s Chevs go, most get their vac connection to the base of the carburetor, and those see full manifold vacuum all the time (same as the carb stud port you mention), in the '70s(and MAYBE late '60s) they went to ported manifold vacuum where the vac port is slightly above the throttle plate, so idle doesn't burn as hot and you get less NOX emissions (but more HC, which is why they added the AIR pump, and AIR pump car would most likely have ported vacuum.).

Doug

Grandpa said it needed to fire as the piston was already going down so you did not have the crank jerked out of your hand! It was easier to start that a way. He also said that as the engine runs faster that the spark needs to ark sooner so it has a chance to explode the gas before it is to late. It takes some time to explode. Getting it to explode earlier and earlier as it goes faster and faster is the only way to make it run faster....of course more gas is exploding, making it a stronger explosion, too. Now, Grandpa was born in 1895 and you have to see his mentality was that of a mule headed farmer.

Well I can't explain it but it is no longer sputters and stops when I accelerate hard. The only thing I did was remove the big fuel filter my "friend" installed. I have driven it three times with no problems.

The "big" fuel filter was probably creating too much restriction, therefore you were running out of gas on hard acceleration.

Yes, selecting an appropriate fuel filter can be a little tricky. The physical size of the filter is not an indicator of performance. It just means it can hold more dirt until it is plugged.

The 2 factors you are looking for are flow rate (something like gallons per hour) and filter size (probably in microns). A smaller filter size rating means it will trap more particles but also probably has a lower flow rate. And that rate will decrease as the filter fills up with collected particles.

I use a Fram G2 on the tank outlet of my '37 Master coupe. It has a 5 micron rating so it will trap a lot of fine material. I like it also because it is a clear plastic housing and I can quickly see how full it is getting.

I do not understand why the filter would cause it. If you come up to a stop sign there should be enough gas in the bowl (even ifthe fuel line is disconnected0) to run the engine for another block.
If the filter is pluggrd or flow is weak the engine should cut-out on a long steady up-hill drive at at least 1/2 throttle.

Lack of fuel on high demand can be caused by:
Restriction at the inlet in the tank,
dirty fuel filter,
stiff pump diaphragm, or pump check valve problem
worn pump cam or rod
other fuel pump problem (bad pivot pin),
and least likely but possible,clogged fuel lines.
Doug

I know this was resolved last August but it brought to mind a similar problem I had some years ago with my ”˜33. The ”˜33 air cleaner has a felt pad underneath the top cover. If this pad looses its stiffness it can be sucked downwards cutting off air flow under hard acceleration. My learned colleagues provided all sorts of explanations but only when I ran the car without the air cleaner did I find the problem and fixed it with a piece of screen to prevent it from ever occurring again.