1955 - Stumble on Full Throttle acceleration

[KAD36]

While on the topic of "how'd driving go over the summer", heres another bug that happened occasionally.

On a full throttle accel from a rolling start (like around 15-20 mph), the engine will stumble and run out of power somewhere around 55-60 mph. I can hold it to the floor, and I can't accelerate any beyond that. If I throttle back and wait a bit, like maybe 15 seconds, and try it again at 60 and push it to the floor, all is well and it keeps on pulling right up to, well, about as fast as I dare to go on bias plys. When passing at full throttle, like from 45 to 70, its just fine. I also notice that when accelerating and turning, like out of a parking lot while making a right hand turn, the engine will drop right out, and I have to pump the gas to keep it going, then it catches back up. Especially noticable if I have to pull out quick and have it in Low, then at the shift from Low to Drive at 30 mph, the engine will cut out and stumble. Seems like the turn aggrevates the situation.

At first I thought vapor lock, but it happens when cool too, though less often. I use 93 octane, have the timing at 7.5 deg, and the plugs are nice and tan/gray with no soot or load up. Recent points, condensor, rotor cap etc. Dwell is at steady 30 deg. Both fuel filters clean. I also notice that the glass fuel bowl strainer isn't always full to the top. Sometimes it is, sometimes is it only 1/3 full. For the life of me, I don't know how it runs when that fuel bowl is 1/3 full, but this is just another obervation I'm passing on. Maybe it matters.

So, I'm thinking, fuel pump - is it delivering enough fuel, at enough pressure to keep up at W.O.T. or fuel bowl, maybe the floats are set too low and the engine drains the bowls it until the pump can catch back up - or what? I have no way to measure fuel pump pressure. I was thinking of doing the electric pump thing to get some pressure and prevent any possible vapor lock, but would rather focus on root cause and learn something here if I can.

Looking forward to your ideas....where do you think I should start?

thanks!

[Guest imported_Thriller]

This sounds somewhat similar to a problem I've had with my Wildcat (not entirely sure if it is gone now with a replacement tank). The tank venting wasn't working properly, so vacuum was developing in the fuel line. Ultimately, my tank pancaked when low on fuel this spring. That causes similar symptoms to a poor fuel pump.

[Jim Cannon]

I agree with the possibility of a plugged fuel tank vent. Or see if the gas cap is supposed to be vented and someone replaced it with an unvented cap.

Also check out the fuel line from tank to fuel pump. I don't know '53s as well as later years, but see if there is any flexible fuel line between tank and pump. Replace anything you are not sure of. Internal softening or collapse of this rubber line can choke off fuel flow, especially when softened from heat.

See if your carb has a neoprene tipped needle on the fuel inlet needle valve. If the tip is soft, replace it. The tip could be sticking in the seat.

[KAD36]

All good ideas.

The gas cap vents fine - not sure if there is a separate gas tank vent.

Flexible fuel line is new and sealed tight at both ends.

Needle valves are all brass.

Thanks for the replies....

[JohnD1956]

Since this is intermittent I doubt it is fuel supply. Also under the conditions listed it may seem like a turn sets it off but I suspect that is more how you accelerate coming out of a turn, than the turn itself.

This is a shot in the dark but I had a similar stumble in my 56. Mine was more consistent than yours however so the fix may not apply to your car.

What I found was that the carb was not tight to the intake manifold. I noticed this when I had it running for a few minutes and saw some condensation on the manifold just in front of the carb.

If not this then I think you have to look at the springs for the weights in your distributor. Perhaps one broke off? This is not too easy to get to in the 56. It required taking the plate for the points off, but I was able to do it while it was in the car.

Good luck

JD

[Jim Cannon]

Another great angle, JD! Vacuum leak! Too much air will act the same as not enough fuel.

My '63 ran very poorly when it warmed up (but not when cold) and I traced it back to a vacuum leak where the carb mounted to the intake manifold. I doubled the gasket and put the stainless steel plate back in there that some previous owner had removed and all was fixed.

My friend had a clear stumble on hard acceleration with his '72 'Vette and it eventually proved to be intake manifold not sealing properly on one head when the engine warmed up. I forget how he figured it out but it just about drove him crazy.

Could also be the carb accelerator pump. Even if the plunger is new, it might not be good. I think your carb also has a little ball check valve on that pump inlet. Crud in that spot could keep the ball from seating. Fuel could be flowing backward thru the inlet instead of being sprayed into the venturis...

[old-tank]

Ken

Others have offered good suggestions/possibiliies. Two other considerations: float sticking in up position; dirt in bowl causing intermittant plugging of one of the main jets especially if you have the Rochester carb.

Willie

[KAD36]

Thanks for the additional ideas.

I checked the intake to carb mounting - I have the Carter WCFB, and it was always difficult to get it to seal up good on the intake - seems the two just don't match up with enough flange on the carb and machined edge on the manifold to make a good seal. I did use some sealer this time though, and I checked for any condensation and didn't see any. I also sprayed some carb cleaner around the base and saw no change in idle. Plus, it pulls about 17.5 inches of vacuume at idle, which has been pretty consistent, so I think I'm good there.

The dist springs are also a good idea. I pulled the dist out this afternoon and have it on the bench - I'll take a look at it when I have more time to get under the plate. House duty (fall yard cleanup) called.

The other good idea was the check ball and dirt in the bowl. My gut is just telling me this is something to do with the carb or the fuel pump.

Can someone give me any ideas why it wouldn't be the fuel pump flow/pressure or the fuel level in the bowl? And is the glass bowl on the AC fuel filter being only 1/3 full a tip? Is that normal? Why wouldn't it always be full if I had good fuel pump pressure, assuming the flow was properly regulated?

And I could have sworn I saw a post on the board of another person that had a Carter carb and a surging problem when accelerating around a corner - or maybe it was too late at night and I was imagining it.....

[JohnD1956]

Would the 55 have the stainless steel plate between the carb and the manifold when using a Carter carb? Seems to me it should have it.

I put an in line fuel filter on my 56 right before the Carb inlet. It is set up in the part of the line with the vertical rise. My filter is opaque and you can see the gas level through it. Sometimes there is more fuel in there than others but no matter, I do not have that stumble and my car runs good. Even on hot summer days.

I can't explain why the filter does not fill, but it does not seem to matter.

JD

[carbking]

Several possibilities on the acceleration problem:

Some WCFB carbs have different primary and secondary floats, with the only difference being the position of the arm (height) where it is soldered to the pontoon. The secondary floats were normally lowered. If this is true with your carburetor, and the floats were inadvertantly interchanged, the fuel level in the primary bowl would be low.

Possible pinhole in metal line from tank high enough so it doesn't leak fuel, but will suck air under heavy load, thus causing the carburetor to run low on fuel.

Some fuel tanks have a "sock" at the fuel pickup. These can clog.

Possibly too small an orifice in the primary fuel valve, or the fuel level is set too low in the primary side of the bowl.

Is this condition consistant and repeatable? Is it possible that it occurs only when the tank is less than half full, and inertial force is causing the fuel on hard acceleration to uncover the fuel pickup in the tank, causing the carburetor to run low? Can you duplicate the condition when the fuel tank is full?

If the manifold and the carburetor are the same year, make, and model; and the CORRECT mounting gasket is used, then there is sufficient surface to effect a seal WITHOUT resorting to gunk.

The turn problem probably means the carburetor has never had the factory "patch" for this issue installed. The vacuum passage in both the bowl and airhorm were to be milled slightly oversize for a specified distance, and a brass sleeve inserted. This sealed the vacuum passage against fuel slosh and a subsequent rich stall when effecting a hard corner. There are service bulletins on this factory patch.

Jon.

[KAD36]

Well, I went through the dist today and found nothing amiss. The counterweights and springs were free, and the vac advance works and holds vacuume. I obvilusly couldn't measure the mechanical advance, but everything appears in order.

JD - whats this stainless steel plate? My car never had it and I've been the only one to tamper with the carb and manifold. Is this something I need?

Carbking - The tech bulletins for 55 and have no mention of the "fix" for an over-rich condition. Do you or anyone have a copy of the bulletin, and how would I know if my carb had the fix?

I'm thinking now of swapping in a new fuel pump (which I have on the shelf and is an easy try) and if thats not it, I'll go into the carb.

Regarding repeatability, the stumble on acceleration on a corner is very consistent, especially if its a hard accel, like starting in Low to pull into traffic from a standing start with the wheels turned. At the end of the turn and as I straighten out and get around 15 mph, the engine runs out of power, and I need to pump the gas to keep it going until it recovers after the shift into drive and the rpms drop. If I pull out "normally", no problem.

The stumble on full throttle is more prevalent when on a hot day, although I've had it happen on a cool day, it just seems I can get it to happen on a hot day more repeatably. Again, not always, but often enough to get my attention. The engine has to run up through the rpm band before it starts dropping out. Its not a floor it and stumble/sputter, its a floor it, engine pulls strong for awhile, then starts to run out of power. I then have to back off, pump the gas a bit to keep it going until it recovers.

Does that help explain the situation a little better?

Thanks for your replies and ideas.

[Gene Brink]

Sounds like fuel starvation. Betting the fuel pump is not delivering enough fuel (or a line is pinching off for some reason reducing the flow). Good luck!

[JohnD1956]

Here's another wild shot.

If you have the stumble coming out of a right hand turn, but not a left hand turn, then I'd check your motor mounts. I'm thinking elevation of the engine off of the right side mount may be causing the throttle linkage to bind.

OR

verify that the valves over the secondary butterfly's operate freely and that the choke is unloading properly.

JD