Reproduction Parts for 1916-1964 Chevrolet Passenger Cars & 1918-1987 Chevrolet & GMC Trucks
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I’m looking for some advice on diagnosing an engine problem: 1937 pickup with a full oil pressure 235 engine (from about 1960). I’ve run the truck for limited mileage (ca. 100 miles) per year since 1996, and it has always performed well with routine maintenance. I took the truck to a show about 6 miles away and it ran fine, as always; on the way back home, however, it started lagging under acceleration, hesitating, sort of bucking—I had to feather the foot throttle up and down to get her home. My first thought was that some stray particle or crud got into the carburetor. I removed the carb (it’s a Rochester B with manual choke), ordered a kit, and did a rebuild. In addition, I drained my gas tank, blew it out, removed all gas lines and blew them out, checked and re-gapped all the plugs, reset the points, checked the dwell. The distributor and condenser are fairly new. The truck idles very well, but after all the above work, it still hesitates and bucks under acceleration, and threatens to stall unless I nurse the gas pedal. I suppose it’s possible that the accelerator pump in the carb is not functioning correctly, though the throttle seems to deliver gas when operated manually; and it seems odd that it would suddenly start malfunctioning on my trip home—not to mention the fact that it was replaced when I put in the new kit (though I’ve read that some kits contain subpar accelerator pumps).
I have not switched out the plug wires (they don’t have many miles on them and appear virtually new), nor have I inspected the valve train. It does seem like a fuel problem, but perhaps it’s electrical or valve related? Any other ideas? I’d appreciate hearing other theories—as I said, the engine has always performed well, and I’d like to get it back to that status.
Thanks in advance for your help.
Jim
Hi Jim
I notice there are a few things you didn't mention checking.
The fuel pump for one.
A weak pump may not keep up to the demand for more gas at anything above idle.
It does sound like the power circuit in the carburetor may not be delivering... did you have the power piston out when you had the carburetor apart?
Make sure it operates freely in it's bore and isn't sticking up and preventing the power valve from opening.
The spring should reliably push the piston back after pushing the piston against it.
Hope you find the problem! 🙂
Ole S Olson
Saskatoon, Sask, Canada
1946 DR 3/4 ton stake
1139 old site posts
@james-may @ole-olson Jim, I would also check the vacuum advance on your distributor. You should be able to suck on it to make it work if you are holding it in your hands. I don't know if you can do it, via a hose, with it still in place. A hand held vacuum pump would do the trick.
Mike
Many Miles of Happy Motoring
3469 Posts on Old VCCA Chat
I had a similar issue with my 1954 3/4 ton pickup truck. It has a 235 in it and would stumble badly on acceleration. I found that my vacuum advance was not working at all. The way I checked it was at engine idle I would manually accelerate the engine from the engine. If the distributor does not move, the vacuum advance is not working.
@ole-olson Thanks for the replies. Yes, I had the carb entirely apart, and interestingly, when I took it apart, I found that the power piston was stuck; I fixed that and thought that would do the trick, but apparently that wasn’t the problem. I may try to locate another working carb and switch it out to see if it runs any better. Mike, I hadn’t thought of the vacuum advance; I’ll try to check it out.
In reading your list of things checked, rebuilt etc, it appears that changing out the inline fuel filter was not mentioned. Does your truck not have one? If so and not already changed for new, that is something to consider doing.
Mike and David—just checked the vacuum advance using David’s method of accelerating from the engine—the advance seems to be working fine; the distributor responds quickly and moves on acceleration. So that is apparently not the problem…
Okay, next up...
Coil... unless I missed it, you haven't mentioned the coil.
Pressure in the cylinder at the point of ignition is dependent on throttle opening.
When you open the throttle, it increases the pressure of the air-fuel mixture the spark has to jump through.
That requires higher voltage.
A weak coil can have the voltage to fire at idle, but not enough to fire under open throttle pressures.
In addition, the higher RPMs of course shorten the time available for coil saturation and therefore reduce the peak voltage the coil is capable of producing.
That's why a weak coil will often start an engine, but give trouble at running RPMs.
I had this problem with a flathead ford in a grain truck many years ago, except in my case the coil was just weak enough to show itself as a stutter/miss and lack of power at higher speeds and loads.
PS
Could just be a bad condenser too, as it will weaken the spark (lower peak available voltage) due to it's effect on how the magnetic field collapses in the coil.
Ole S Olson
Saskatoon, Sask, Canada
1946 DR 3/4 ton stake
1139 old site posts
The coil was the culprit! It looks like the problem is finally solved. After Ole’s last suggestion, I bought a new coil and condenser and replaced them. Just took the truck out for a quick spin and I detected no real problems in acceleration, or bucking or hesitating at higher speeds. I think the condenser was probably fine—I had replaced it a couple years ago with a newly styled condenser that was designed by a Stovebolter on this and the Stovebolt forum; I put such condensers in all 5 of my old Chevrolets and the others are performing just fine. The coil was the original coil I had installed when I overhauled the engine way back in 1996 or 97. Even so, I wouldn’t have suspected it seeing that the truck started and idled just fine—It was Ole’s last post that set me on the right track—thanks very much Ole, and all who helped solve the problem; much appreciated! Now on to the next project…
Jim
I agree that your condenser is probably OK.
7046 old site posts
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1938 Master Business Coupe-Sold, now living in New Jersey
1953 210 Sedan
Ole S Olson
Saskatoon, Sask, Canada
1946 DR 3/4 ton stake
1139 old site posts