Thought I would give this board a try. I have a '41 coupe with original V8 engine. Let's start with low compression, but fairly even. New plugs, wires, rebuilt carb, new Stromberg electronic dist. Engine starts right up with a little choke. As I give it more gas and hold it steady the RPM's starts to wander up and down. When it is up it seems to run pretty well. Then the engine starts to run down missing as it goes. Missing badly until it comes to a stop. Hard to start, but it does. Give it more gas and the wandering continues then runs fine again. Really can not drive it in this condition. Maybe it is just the low compression. Any ideas ? Drive gears seem tight when moving the fan to rock back and forth. No teeth missing. Car was delivered to me. New to me project.
Sounds fuel related,,,,probably a weak pump,,,,,but,,it could be a vent problem . I really don’t think the compression is involved in your situation . Tommy
It has a new fuel tank and lines. Tried a Stromberg carb. Just the same. I agree , even if the compression is low I would think would run fairly even. Not much power, but smoother.
If the cam is timed correctly, and your ignition is good..... I'd try running a "fuel cell", like a 5 gallon can of gas with a cheap electric fuel pump. Then you know you have good fresh gas and plenty of volume. One of these 5psi parts store specials is fine.
If I'm not mistaken, incorrect cam timing can cause low compression on all cylinders. Worth checking if all cylinders are low but even, as wear rarely gives an even low for all cylinders.
The fact that it is fluctuating means that it is not low compression that is causing it. I would first check to see if your fuel pump is working properly. Just remove the line from the carb and place it in a bottle. Start the engine ( it will run for a while on what is in the bowl) and see if you get a good strong feed of fuel coming out of the line. Next I would consider some dirt might have entered the fuel system. Removing the top of the carb and looking in the bowl will tell you if there is some sediment . If so, some may have made it's way through into all of the orifices. That often causes fluctuations and then starvation to stall. That would mean a teardown of the carb and a cleaning job including the fuel lines. Also look for sediment in the fuel pump bowl as an indicator. If the fuel system checks out, then I would think about that distributor. I didn't know that stromberg made electronic distributors. Now I do. If it has a separate coil, I would look into whether it is the correct one and then change it out with a known good one. Hope you find it. Let us know if you do because I for one am interested in this one.
Start tracking a fuel problem from the beginning: 1) Remove gas cap 2) Pour gasoline of choice down its gullet 3) Leave gas cap off The reason I'm saying this is that it sounds like maybe you should have a vented cap, but instead have a NON-vented cap. 4) Check output of fuel pump
Lot of older posts around about that electronic distributor and issues with it. Might want to do a search to see the comments.
Timing...cam... Spark Fuel VOLUME Strange vacuum leak Those are about the only things I can think of that would do what you describe. Fuel volume would be my first suspect.
What is amazing is the car ran like this when I took it off of the trailer. After I fixed all of the brake issues I took it out for a drive. That was when I realize it needed help. As the first step to solve the problems I did a compression test. Low 40-50 #..... Thats when I started replacing everything. What confuses me is the up and down wandering and misfire. Acts like it is dropping cylinders, like its arcing in the cap, but it also has been swapped. It ran like this before everything has been replaced. If my problem was just low compression , I would think it would just struggle under power. When I drive it, it runs just like it does in the shop. Struggles to get going, then a few second of smooth power, then breaking up and misfire then back up again. The saga continues. I know it obviously needs a rebuild.
I don't think it is compression, but that is still the very first test I would make. If the compression is good: I was unaware of a Stromberg electronic distributor; but have had HUNDREDS of prospective customer with "carburetor problems" over the last decade that had installed electronics WITHOUT upgrading to an alternator (more stable voltage). Most solved their "carburetor problem" with a return to points and condenser, or upgrading to an alternator. Just saying! If you had exactly the same issue with points and condenser, then maybe give the distributor a temporary bye. Next test would be a fuel pressure gauge right at the carburetor. A few tests, and you will find the issue. Jon.
The erratic or intermittent nature of the problem points toward something that isn't a fixed condition. The compression is fixed, it isn't changing, so while it may be low, it isn't causing this erratic condition. I know nothing about Stromberg's electronic distributor, so it's hard to troubleshoot it from afar; but if it was a standard distributor it's possible a failing condensor could give intermittent issues like this. I like Jon's comments about low voltage, it would be easy to check the voltage and see if it's fluctuating at the same time when the poor running condition occurs. I also like the idea of checking fuel pressure, see if it drops when the condition occurs. More than 1 guy has found a shop rag in the gas tank that intermittently blocks off flow of fuel to the outlet of the tank.
The issues were there before the elec. ignition was installed. We tried a different carb with the new gas tank and new coil. 6V
What do the spark plugs look like after it goes thru the missing episode? Not really suggesting that it might be a bad spark plug or wire (though it could be), but wondering if any particular cylinder(s) are the ones doing the missing. If you have one or two plugs that look significantly different from the others, that might tell you it wasn't a fuel problem. Also, while you got decent compression while cranking the engine, is there any indication of a sticking valve due to backfiring or exhaust noise?
Does it run better if you hold the choke in different positions? 1/4 closed--1/2 closed--3/4 closed? That would tell if its fuel.
With 40-50lbs. compression. I would have stopped right there. You need to figure out why that's so low. All the new plugs, points and fuel in the world isn't going to correct that problem.
Lots of suggestions, but I go back to testing. As I posted earlier, it doesn't sound like compression, but that is the first test I would run. Why? Because if the compression is bad nothing else you do will help. In the FWIW category, the results of a compression test on a fresh engine that ran like you describe found a timing gear one tooth off. The fact that the issue existed prior to the installation of the electronic ignition does not absolve the ignition system. Would be nice to see a "parade" picture of the firing voltages from an oscilloscope. If the ignition is faulty, nothing you do to the fuel system will solve the problem. If the firing voltages check out when the condition exists; then the third test is fuel. I lean toward this, which is why I suggested the fuel pressure gauge; but the testing in order compression ignition fuel is because if the leading component is defective, then other items are moot. EDIT: Just read (missed it initially) where you did a compression test. With compression that low, figure out why, and correct the issue before you do anything else. Jon.
Man my worn out flat head Briggs&Stratton lawn mower has more that 50 psi compression. Your cam timing must be off for all cylinders to be 40-50psi. I think even without rings at all you'd get that much compression.
A flathead cam gear has 44 teeth. 'Cam timing off'? That would end up @8* plus, either before or after TDC. Think it'd start up and run? Distributer is driven by the camshaft. Is the distributer turned an inordinate amount advanced or retarded? (visually) Valid questions.
Wonder what you might learn by checking manifold vacuum while driving as well as at idle. And the cranking vacuum with the ignition disabled and the throttle plates completely closed, and any other manifold vacuum ports temporarily sealed.
I think that engine has the fiber cam gear. I’d be thinking to taking the front cover off and have a look. One that is way smarter that I am can maybe tell you about pulling a head, measuring valve lift vs crank position vs distributor/rotor position, etc I brought up the fiber gear because I placed one (still pressed on the cam) on my welding table and a few small chunks came off it.
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