M-37 not starting opinions please

[Joe Flo]

The next time you get it running and it dies out on you, check it for spark right away. You may have a bad coil or ignition module that fails when it gets hot. if you have a spark, then you may have mechanical issues as Charles mentioned or a bad carburetor. Check the compression while the engine is hot. You may see something different. Has the engine timing been checked?

Good luck and keep us posted.

Joe

[sixtyninegmc]

Gas and spark

You can always do a cranking flow test at the carb when it is hot (careful with gas and a hot engine), or pull the airhorn and visually check the fuel level. I have a clear filter and bowl on the carb so it was quite easy to diagnose lack of fuel in my FFPW. You can also check the resistance of the coil with an ommeter (don`t remember the sequence off-hand, would have to look it up) as a visible spark does not mean enough to fire the mixture. Also, when you mention electronic ignition, I have had several GM hei modules start failing due to heat-soaking, and they would almost always restart when they cooled off, so electronic won`t always rule out hot start issues.

Good luck, when mine didn`t want to start I was 5 miles down a logging trail with 3 1/2 cords of birch on, if that makes you feel better :-D

[Kaiser2boy]

Just a thought.
When it does run does it smoke and if so what color is the smoke?
Black = way too much gas
Blue= too much oil
White = water.

The hot coil idea sounds good too. I dont know if Radio Shack still sells it or not but they used to sell a spray that was ment to chill components suspected of having a thermal problem. You could wait until is craps out and then spray down the coil and wait a few seconds and try it again. If it starts you will have a better idea of the problem.
I had a 56 Olds that would run just fine but if I got stuck in traffic it would die and not start for love nor money. It turned out to be a "breathing " resistor in the coil line. When it got hot it opened. After a half hour or so it cooled and all was well,,,for a while. It was found by useing Circuit Chill spray.

[Charles Talbert]

plenty of gas in the tank. As I said, it will run for 15 minutes very well and then die. If I wait several hours, it will start right up again.

I have had the gas cap off the whole time to rule out vacuum lock and I blew out the air cooling line to the distributor also.

Tomorrow, I plan on hooking up a small fuel tank under the hood to gravity feed the carb and see what happens.

You have me wondering now about a possible bad coil, the symptoms you describe are leaning in that direction. Have you checked to see if there is a decent spark at the plugs just after it knocks off and refuses to crank? If there is none or if it appears weak, that's the next move you need to make. This is still of course not relevant to the low compression issues.

[jzimm]

OK.....

I checked for spark again right after it dies and it is a nice blue steady spark. I also rigged up a small gas tank and ran bypassing the fuel pump and it still died. I checked compression while hot and it was a consistant 75 psi.

I have no exhaust smoke (blue, white or black) when running and it runs great until it dies.

It starts great cold, but will not start when hot.

[Gordon Maney]

OK.....

I checked for spark again right after it dies and it is a nice blue steady spark. I also rigged up a small gas tank and ran bypassing the fuel pump and it still died. I checked compression while hot and it was a consistant 75 psi.

I have no exhaust smoke (blue, white or black) when running and it runs great until it dies.

It starts great cold, but will not start when hot.

When it dies, how does it die? Like you shut off a switch? Or does it struggle and stumble for a while as it is dying?

[jzimm]

It dies almost like I'm turning off the ignition switch. Little to no symptoms that it is about to die, but I can time it to about 15-20 min after start up and the engine is only about 160-70 degrees at this point. It will not start again for at least a few hours afterwords.

[Gordon Maney]

There has been a lot of discussion here about the coil.

Your spark test; how did you do that? Disconnect a plug wire from a plug and hold it near ground?

It may spark fine in the atmosphere, but the required voltage rises under compression. That higher voltage may be finding an alternative path through insulation as a result of the coil's internals changing shape from temperature rise.

Do you have another, known good, coil you can try?

[jzimm]

I pull a plug and gound it to the block to check spark.

I'm not far from Midwest Military. I will probably go over there and get another coil when I get a chance. The coil is only 2 years old, but I suppose it can still go bad.

[Charles Talbert]

OK.....

I checked for spark again right after it dies and it is a nice blue steady spark. I also rigged up a small gas tank and ran bypassing the fuel pump and it still died. I checked compression while hot and it was a consistant 75 psi.

I have no exhaust smoke (blue, white or black) when running and it runs great until it dies.

It starts great cold, but will not start when hot.

I'll have to mention this once more, 75 PSI compression is not enough to run. I'm working on one that is skipping on #5 this morning. I checked spark and all related to it first off, no issues there. Pulled the plug a checked compression, 70 PSI, dead give away, all indications point to a valve issue, looking into that this afternoon. If you have checked all other issues and it sounds like you have done that thoroughly, it all seems to point back to the low compression issue based on the info you are posting here.

[Gordon Maney]

I'll have to mention this once more, 75 PSI compression is not enough to run. I'm working on one that is skipping on #5 this morning. I checked spark and all related to it first off, no issues there. Pulled the plug a checked compression, 70 PSI, dead give away, all indications point to a valve issue, looking into that this afternoon. If you have checked all other issues and it sounds like you have done that thoroughly, it all seems to point back to the low compression issue based on the info you are posting here.

I am uncertain about that. If the engine never started, I believe that would have more plausiblity. The fact that it runs and then quits, particularly in the manner described, suggest to me it is not a compression issue.

Your missing on cylinder 5 is understandable if there is a big variation in compression. Meaning, if the others are all 110 and one is 75, then yes.

Not sparring with you Charles, I just am not certain I can buy it under these circumstances. I have seen some engines that had — uniformly — very low compression. They ran very smoothly, and may have smoked, and certainly did not have a lot of power, but started easily and ran like sewing machines as long as you wanted to run them.

[Charles Talbert]

I am uncertain about that. If the engine never started, I believe that would have more plausiblity. The fact that it runs and then quits, particularly in the manner described, suggest to me it is not a compression issue.

Your missing on cylinder 5 is understandable if there is a big variation in compression. Meaning, if the others are all 110 and one is 75, then yes.

Not sparring with you Charles, I just am not certain I can buy it under these circumstances. I have seen some engines that had — uniformly — very low compression. They ran very smoothly, and may have smoked, and certainly did not have a lot of power, but started easily and ran like sewing machines as long as you wanted to run them.

I know what you mean, I've seen just what you say more than once also. The point is every engine seems to react differently. If the engine stops running for what ever reason, it is much more likely that it will not restart with low compression. There could absolutely be more than 1 issue in play. The theory I most often go with is this, if low compression is a known issue, for certain you will not have a good engine with that in play. I usually troubleshoot and fix the compression issue first, then look for minor issues later if need be. Most often the issue at hand is fixed during the rebuild process anyway, so why fight it when a rebuild is the only sure fix for all the ills anyway.

I've never been one to spend lots of time hoping to find a minor issue when the obvious hand writting is displayed plainly on the wall. More often than not, your time is better spent attending to the obvious. Even if you did find something you can do to get it to run somewhat, what have you really accomplished in doing so? Just my theory.

[Joe Flo]

As Charles said it seems to be a low compression issue. I'm wondering if you spray ether in the carb after it stalls if it would start. This would be telling enough (if it starts) that the compression is too low.

Joe

[Gordon Maney]

As Charles said it seems to be a low compression issue. I'm wondering if you spray ether in the carb after it stalls if it would start. This would be telling enough (if it starts) that the compression is too low.

Joe

....or that it was a fuel delivery issue.

[sixtyninegmc]

I`m with Gordon

Saying 75PSI is not enough to run is quite the blanket statement, especially when you consider how many engines run around 75 brand new. Low compression could certainly make for a harder restart after dying, but I can`t see it causing the sudden shut-down, either.
I would say try to find the procedure for checking the coil with an ommeter, check it that way, and then look into fuel. I would not look to fuel myself, because of the sudden dying, it sounds more like spark to me.