The Stovebolt Site Forums The Shop Area Engine Shop Tuning Challenge - I'm out of ideas!

Link to video

1952 3100 with 1954 235 engine
Head - 3701887 GM7 --CON 12 -- G73
Block -- 0030392F54Y --CON -- K163

Engine was running fine until about a month ago. Would run great one day, horrible the next. Here is what I've done following suggestions from a previous post and reading other posts....

Rebuilt carb (actually have tried 2 - both rebuilt no change)
Timing is about 2 degrees before BB - that gave me the highest vacuum
New cap, rotor, condenser, wires, spark plugs, coil, battery, battery cable, wire to coil, wire to distributor
Both the old and the new plugs are black and sooty
Dwell reading 38
Vacuum gauge reading about 20
Engine compression after running engine about 2 min - 132,136,138,136,134,135
Adjust valves after engine running for 15-20 minutes
Truck is still 6 volt and has had both the starter and generator rebuilt in last 6 months.
Wires to distributor checked to be in proper place.
Distributor checked to make sure rotor was pointing at 1 at TDC
Distributor does turn / advance when given throttle
All gaps checked, then checked again, and again....

If you try to drive it it begins to cough, chug, and popcorn and power is gone. If you rev it and hold the RPM's to 1500-2000 - it shakes and feels like you are forcing it to run and will finally backfire out the tailpipe. I'm hunting for someone local to take it to - but would love any and all input!

You might also want to remove the breaker plate in the distributor (can be done while installed on truck) and confirm that the advance weights are free and not rusted solid or gummed up to the point of not being able to move, and that the pins for the springs are not loose and that the springs are still installed. What plugs are you running?

You've gone through all I could think to suggest. Hopefully someone with a lot more knowledge chimes in with a suggestion/solution. Only thing I can think of is carburetor adjustments.

I had a similar problem and finally the problem was very clear when the fuel pump failed. It’s a cheap and easy replacement if you haven’t already done it.

The problem could be 2 fold:
The BB on a 54 235 is 2 degrees ATDC. If your engine is stock I'd recommend to try resetting the timing to either the BB or at the UC (TDC) mark.
2 - 4 degrees doesn't sound like much but it is on an engine designed to idle at 2 degrees ATDC...

I had a similar problem with a modified 54 235 engine and I too tried to use the highest vacuum setting for the timing and it ended up being way too advanced. The engine would start and accelerate fine but would hit and miss at any steady rpm.
Try retarding your timing 1st.

2nd problem could be the power valve in the Roch B carb is sticking, that is what may be causing your over rich condition. It should move up/down easily in the carb. Make sure the check ball is installed under the spring on the power valve too.

Another possibilty is the gasket under the carb doesn't have the vacuum hole in it to allow vacuum to the power valve. On the bottom of the carb on the mounting flange you'll see a vacuum hole, this is for the power valve. At idle and slow speed driving the high vacuum signal keeps the power valve closed since additional fuel is not needed. If theres no vacuum the power valve will remain open causing excess fuel to feed into the venturi. The hole must be open through the gasket to the spacer which is cut/molded to allow vacuum to the carb.

Good luck
Dave

Ok, here is your problem, "All gaps checked" Ah no---you need to open those points up 38 dwell is about 8 too much, surprised others did not catch that.

I've tried 2 different set of plugs - Champion 841, AC Delco r43

"If you try to drive it it begins to cough, chug, and popcorn and power is gone. If you rev it and hold the RPM's to 1500-2000 - it shakes and feels like you are forcing it to run and will finally backfire out the tailpipe. I'm hunting for someone local to take it to - but would love any and all input!""


Dwell of the points is too high, regap the points and open them up a bit, shoot for 30 dwell no more than that

Mike, load it up and bring it to the Reunion in KC on the 7th.
We'll put the experts on it!

Don

Check your points, that gap is too large and I would guess they may be burned as well.

Your symptoms concur with a point issue.

With all due respect to you and your post, Dwell is too high meaning the points are gapped to narrow, he needs to open the points up, lets not send him mixed signals here.

i missed the dwell. For a 55-62 2403 delco distributor Dwell is 28-35 degrees. Not sure what it is for the pre 55 motors but I can check tonight. but , I agree, 38 is too much.

I can tell you my 261 ran like crap with a dwell of 34, sweet spot at 30, my experience only.
Excessive dwell closes the points too soon after opening, cutting off the magnetic field collapse before it delivers all its energy. I would imagine strength of the coil comes into play too here.

Dwell specifications by year and distributor number:
1110052: 35 degrees 1940 pass car
1110090: 38 " 1941-48 pass car
1112353: 34 " 1949 pass car
1112353: 34 " 1950 pass car
112362: 34 " 1951, JJ & JK engine
112363: 34 " 1951 2100 engine
1112362: 39 " 1952 2100 engine
1112363: 39 " 1952 2100 Powerglide engine
1112389: 38-45 1953 "conventional" engine
1112388" 38-45 1953 Powerglide engine

My 1940-54 Motor manual does not list truck distributors, but apparently dwell specifications between 34 and 39 degrees were pretty common.
Jerry.

Good info Jerry, but I wonder if distributor nominal cam angle changes between small cap and tall cap distributors, all your info looks like the small caps. It is pretty much impossible to set your points correctly ie .016-.019 gaps and obtain some of those huge dwell angles that you published on the tall distributor. This is my opinion only but I have spent some time playing with point gaps and dwell readings. I'm going to head out to the garage and start looking at the distributor lobes between the tall and short designs to see if they differ.

All those numbers are for 6 volt systems, and short cap distributors. It appears that there was a cam design change with the switch to 12 volts and the tall cap and rotor. I just wanted to make the point that "39 degrees is too much dwell" might not be an accurate statement. I'll do a little more research on the later model distributors- - - -I think I've got some later model specifications stashed away somewhere.
Jerry

Roger that Jerry, thanks for your input, OP has a 54 engine so I assume he has the tall cap.

OK- - - -found some more numbers- - - -

1112314 54-55 Corvette 38-45 dwell point gap .016"
1112388 54 standard trans 38-45 dwell point gap .016"
1112396 54 Powerglide 28-35 dwell point gap .019"
1112403 55-56 6 cyl. 26-33 dwell point gap .019"
1112403 57 6 cyl. 26-33 dwell point gap .019"

The 1112403 distributor with the same specifications is used through 1962. I also have centrifugal advance and vacuum advance specs. if anyone is interested. PM me for those.
Jerry

Awesome job with that info.

Right or wrong for the truck, it's easy enough to change the dwell and see what it likes. Same with timing, try the extremes and see what happens.

He needs an old timer with a oscilloscope to figure out what it's doing. Anybody know of one?

Just an update --
Started truck and adjusted timing to the BB. Checked a few miscellaneous items mentioned - carb power valve, fuel filter, gasket under carb, etc.. Started it again - still bad.

Distributor is 1112403-6L29 with a tall cap.

Only had a few minutes last night. I tried to adjust the dwell and I think a moved it too far since I can't get the truck to start now. Having a friend that is more experienced setting points to come by this weekend. Will experiment with different settings. Will post more information after that.

Thanks everyone for the help!

Don't get obsessive-compulsive about the dwell. Adjust the point gap to .018-.019 and drive on. Check the gap on more than one lobe- - - -you might have a worn one or two, or a wobbly distributor shaft that's letting the timing change between cylinders. A one-degree change in dwell due to a worn or wobbly distributor cam changes the timing at the spark plug two degrees.
Jerry

If I am reading the date code correctly:

6= 1956
L= November
29= 29th day

So November 29, 1956 for the 1957 model year.

Some reference material for making your adjustments

We seem to be caught up in assuming an ignition problem. Could be something else too. It goes rough under any kind of load, right? But runs OK with no load? So as cylinder pressures go up the fire goes out irrespective or RPM?

Did you check for water in the gas? How much alcohol is in the last tank of gas you bought. Alcohol attracts water and mixes with it. The engine can tolerate small amounts of alcohol/water mix but it can cause problems at higher concentrations.

Since it does its thing when cold also, I'll rule out detonation but I've had that problem twice in the last couple of years both times I suspect a lean condition from excess alcohol.

Another possibility is cross-firing either inside the distributor cap from moisture or induction between spark plug wires -- possible but unlikely; make sure the plug wires don't run parallel but rather cross as close to 90 deg as possible. Since it did this before you bought all the tune-up parts we can rule out infant mortality on those parts. But just for drill watch the engine run in the dark to check for arcing wires and also check the spring tension on the points. If it's low they can bounce causing some of what you're describing and I'm hearing on the video. I've seen new rotors short out to the distributor shaft so look for carbon tracking there. A bad condenser can cause similar problems and I've seen new ones fail more often than old ones. Coils usually act up when hot but can fail cold too.

Plugs are dirty? perhaps from the misfiring perhaps from carburetion. Clean them.

How about valve timing? If you have a fiber timing gear, as car engines did, it could have given up the ghost and slipped a tooth. If it goes completely or even any more than a couple of teeth the crank will wipe out a cam lobe and you'll have to pull the grill to change cams. Generally, if cam timing has slipped, the engine will run at higher RPM but won't pull a load below a couple thousand RPM. Usually the gears fail catastrophically but not always. You can change the gear without pulling the cam if you replace the gear with an aluminum one and heat it before you install it.

Did you shake the rotor to check for a worn distributor bushing, which will let the timing bounce all over the place? Maybe a broken spring in the mechanical advance under the point plate. How 'bout a bent distributor shaft?

Now I'm out of ideas

Best of luck. Proceed logically from what you know toward what you don't. Assume nothing. Leave no stone unturned.

Update --

Changed the plugs again just to cross everything off the list. Installed another new condenser and points. Set point gap .018. Checked measurement off 2 different lobes. Engine fired and then adjusted the timing again to very close to the BB. Did some slight adjustments to the carb idle / air fuel mixture. Idling smoother than ever - Truck sounds great. Just got back from a 20 mile road test and it performed extremely well.

Thanks everyone for the help - it's awesome having so many willing to help share their knowledge.

We often warn about just, "trowing parts" at problems, but when they are as inexpensive and easy to change as plugs, points and condensers, this "parts trowing rule" can be ignored.
Glad to hear that you're back on the road.

The only thing that stood out (well 2 things) black/sooty plugs say the ignition is too advanced. Another clue to too advanced; coughing out the carb, when pushing on the throttle (gas pedal).
Coughing out the exhaust (during acceleration) is often either retarded ignition, or a leak in the exhaust.
If it were me; I'd retard the ignition little by little, and pull full to near full choke -- chevys are notorious for running lean when cold/first starting up.

HTH

Chris