I've had this truck since 1981 (or so) and it all that time, it has run well. It is basically the stock V8 setup, although it is a Mercury engine block. 6V ignition. Over the last few years, work and life has left little time for this truck, but I recently put some work into it to try to get it back in shape. I started with a carburetor overhaul/gasket set, new upper radiator hoses/pipes and new spark plug wires. After that, I was having issues with misfiring/dead cylinders. When I moved the plug from a dead cylinder to another cylinder position, the new cylinder position became the dead one, and the previous dead cylinder came back to life, so I assumed the plugs were bad and replaced them all. It ran OK for a few minutes but then I started getting more dead cylinders...different ones each time. I then replaced the coil and the condenser...no improvement. Also installed a brand new carburetor...no change. Took compression readings....most cylinders are about 110 to 130, but there were two low ones, 1 @ 90 and 1 @ 80. Not the best news but somehow, I don't think this is the real problem. I have not yet tried to correlate the compression readings with which cylinders are now dead...will try this weekend.
Cap and rotor appear to be in good shape. They are replacements that I installed maybe 10 or 15 years ago and there are not that many miles on them. The distributor is dry inside. I adjusted the points to spec dwell angle. It was not far off to begin with. Timing at idle is on the mark. Vacuum advance seems OK...I could see it advance when I worked the throttle.
Very common for valves to stick after sitting a long time. Try revving it up and drizzling some SeaFoam or Marvel Mystery Oil down the carb. (It will make LOTS of smoke!) Add some to the gas. See if the compression numbers pull up and the miss goes away. Obviously that does more good to the intakes valves than exhaust.
I'm with ALBUQ F-1 on this one. It sounds like you have sticky rings and valves. If you pour enough Marvel oil or automatic transmission fluid down the carburetor you will pump enough unburned oil through the cylinders to lubricate the exhaust valves too (Tons of smoke and you might foul out some plugs). I do this to all engines I get running that have set up for a long time.
I have also done what Ross and 38 coup suggest. And also suggest you do this.
Just note that it will loosen carbon in the combustion chambers and valve
stems. This can lead to a bent valve or worse. Apply Murphy's law here.
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Let me know if you think this would help even more: Crank for a minute or so with the ignition off, while drizzling in the MMO, then let it sit for a day or a few. My theory is the not firing the engine while introducing the MMO would allow more of it to accumulate around the exhaust valves (instead of being burnt off).
Suggest that you take the top off the carb and look inside.
If you see this then you probably have the same crud in your fuel tank. You will also need to clean out the fuel pump bowl, flush the lines and drain the fuel tank. Hopefully draining it will clean out most of the crud. If not some radiator shops will flush and clean them for you. I would also recommend adding a fuel filter between the pump and the carb.
Just my 2 cents. Good luck. Let us know what you find.
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Update: I've done the MMO thing....several times. With the engine running at normal idle speed, I slowly drizzled the MMO into the carb throat, usually until the engine gasped for breath and died. Then let it sit. Start it up again....repeat, and yep, lots of cool smoke!
Ran rough every time, i.e., same as it had been doing.
This past weekend, I used my timing light to check which cylinders were firing, or not. The front 4 (1, 2, 5 and 6) were all firing reliably, at idle and during throttle up. The rear 4 (3,4 7 and 8) were firing at idle, mostly (one was intermittent, forget which one), but each one died completely on throttle up.
Oddly, there seemed to be no correlation between which cylinders were having problems and my compression readings.
I cleaned and gapped the plugs in the 4 bad rear cylinders and, made sure the plugs fired by testing them on my snow blower engine (because it was there) and reinstalled them. The snowblower engine lit each plug with a good hot spark. Now all except # 3 seem to be working at idle and throttle up. The engine still rocks a bit at idle but not too bad.
So did the MMO thing again, let it die, and will try again.
Petemcl, that is one nasty looking carburetor. The first thing I did in resurrecting my truck was to overhaul the carburetor. Tore it completely down, cleaned with spray cleaner, blew all passages out with compressed air, and reassembled with a new overhaul kit from Dennis Carpenter. It wasn't too bad when I started, nowhere near as bad as the one in your picture...just some light brown deposits from evaporated fuel. I have an electric pump, with a regulator and a filter...my original fuel pump doesn't work, cam is worn or something. I've had the electric pump for years and for many miles with no problems. I was a bit puzzled by the new power valves that were included in the DC overhaul kit. Neither one matched exactly the one that was in the carb when I disassembled it. I tried the one that was the closest match, then put the original one back. Seemed to make no difference. In a fit of desperation, I bought and installed a brand new carb from DC, and this is what is on there now. It did not seem to make a difference.
So, some improvement, but still not a great situation.
I think I'd run it at fast idle without any MMO to make sure all of that is burned off, and not fouling the plugs. Then just drive it under load around the neighborhood and see if it clear up.
6v condensers can do strange things and are often bad right out of the box. Also, even tho it is new, it's worth checking the float level on the new carb. Not likely it was handled with kid gloves during shipping, may have gotten knocked out of whack.
What brand of plugs are you running? Autolite #216's seem to be more resistant to fpouling and other problems than Champions. Might be worth a try. It sounds like you've done everything obvious.
I've got the Champions. I did run it for a while before cleaning the plugs, but maybe not long enough. Now that it's running a little better, I'll try a short drive.
And I guess I ought to check the float level too.
One other thing I didn't mention: Hesitation on initial throttle advance is extreme. I've checked the obvious...the accelerator pump is putting out a healthy stream, so I don't think that is the problem. Could there be too much accelerator pump volume?
It almost sounds like you may have a vacuum leak. Have you checked the vacuum hose that goes to the wipers? The other thing that could give that stumble is timing. Are you setting it with the vacuum line from the carb disconnected and plugged? Does the advance work when the throttle is opened?
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