g741.org

It seems to be a common problem; once my engine is good and hot it stalls when idling, such as at a stop light, and won't start again until it has cooled off for 30 or 40 minutes.

About 2 years ago the entire wiring harness was replaced (VWM), carb rebuilt, fuel pump rebuilt, new plugs and wires, new fuel filter, and everything in the distributor was replaced. It ran fine for over 18 months (though that was only a few hundred miles) before the stalling problem started.

What would have changed?

I run 87 octane ethanol free with lead substitute and Seafoam.

To try to fix the stalling I've rerouted the fuel lines away from the exhaust manifold, replaced the coil, made sure the distributor vent lines were clear, confirmed that I have a heat shield around the fuel pump, replaced the fuel filter (twice) though they seemed clean with clear gas, cleaned the spark plugs (though they looked good), adjusted the timing, turned up the idle a little, and cleaned out the air cleaner.

The only remaining things I can think to do is more carburetor work and add an electric fuel pump and a pressure regulator.

Any other ideas?

Thanks

Go with the Electric pump. Your vapor lock will dissappear

I had a similar issue. The M would run for a while and just cut out like it had no gas. Like you said 30 to 40 minute and it would start up again. Just for S & Gs check your fuel cap and make sure it's not set for fording. That pulls a vacuum and keeps fuel from getting to the carb. That was my issue. hope you luck out too.

majorhitt wrote:I had a similar issue. The M would run for a while and just cut out like it had no gas. Like you said 30 to 40 minute and it would start up again. Just for S & Gs check your fuel cap and make sure it's not set for fording. That pulls a vacuum and keeps fuel from getting to the carb. That was my issue. hope you luck out too.

Majorhitt, could you explain what you mean? I'm currently trouble shooting a vacuum issue myself, although it doesn't seem as extreme as the OP


-T

Look at you gas cap. It has a valve in it. It will say, open --- closed. When the vehicle is to go fording, the valve on the inside of the gas cap has to be set to closed, after fording it has to be set to open. If not the motor will only run a short while, it will pull a vacuum in the fuel lines and starve the vehicle of gas. It's not much of a valve and you may overlook it, in the center of the gas cap you'll see a small rivet that holds a small piece of metal twist turn this small metal item you'll see how it rides over a small hump, that will set the valve to open or closed. This will have nothing to do with the vacuum the motor has. I was looking for a picture of the gas cap but didn't have one.

T,

What Major is talking about is a mechanical vent inside the gas cap. There were a number of different gas caps used on M-Series vehicles, and the one Major is talking about was used on M35s I believe. As far as I can tell it wasn't the original cap that came on M37s, but many people use them. That being said, I'm not real familiar with the later B1's, so maybe this cap did come on those....I'm just not sure....but ORD for the first series M37s only lists the non-vented cap. This cap has the 5 spokes or star on the top.....as in the first pic.

The cap Major is talking about is like the one in the second pic. Notice it says "Open-Normal" and "Closed". There were other similar caps as well, but you get the gist. On the inside of this cap is a metal clip (sort of looks like an hourglass pinned in the center) that you rotate to either the vented or unvented position. If you have it set in the unvented position the tank pressure will become vacuum as fuel is sucked from the tank and air cannot replace it. At some point the vacuum becomes too great for the fuel pump to suck fuel........and you stall.


NonVented1 by w30bob, on Flickr


Vented2 by w30bob, on Flickr

Hope that helps,

regards,
bob

Also check your ignition condenser. When they start to go bad they will work when they are cold and quit working when they get hot.

Steve

T,

I couldn't find a pic of the underside of the vented gas cap, but I just checked Ebay and sure as shit one just got listed. Here's a pic of what the metal clip looks like on the underside of the cap.

regards,
bob


gas cap by w30bob, on Flickr

Thanks for jumping in Bob. I didn't have a photo. The cap on my M37 may not be the original. You gave a much better description than I did. I was a bit in outer space. I hope the photos and description helped. the other reply about the over heating issue is something to look into.
Ed

Bob, I'm still in the outer space mode a bit here. I didn't think about it until after I posted the last reply. My M37 when I got it had most of the fording parts on it, Air cleaner and pull out lever with valves. I think the cap was on there, can't recall if I did swap it out with a better one. The fording pipes were gone.

On my '52 truck, I have the vented cap in Bob's second picture. Wouldn't the vent line from the tank to the intake prevent a suction problem if the cap was in the fording position?

majorhitt wrote:Bob, I'm still in the outer space mode a bit here. I didn't think about it until after I posted the last reply. My M37 when I got it had most of the fording parts on it, Air cleaner and pull out lever with valves. I think the cap was on there, can't recall if I did swap it out with a better one. The fording pipes were gone.

One thing comes to mind when I read this, did you leave the distributor vent lines in place? If not you could of cooked the coil and or condenser.

ZGjethro wrote:On my '52 truck, I have the vented cap in Bob's second picture. Wouldn't the vent line from the tank to the intake prevent a suction problem if the cap was in the fording position?

The vent line from the intake elbow to the tank would cure this but all too often the vent line is clogged with rust and debris rendering it inoperable. It was my understanding that the vent to the tank was not a true vacuum line, it was used to equalize pressure in the tank when the cap it rotated to the fording position. The fitting in the intake faced upstream to the air flow to promote a positive pressure to the tank.

Bert

Thank you for the information! My gas cap also looks like the 2nd, although it says "NON-VENTED" Stamped into it.

I have been running with it closed, and without a vent line hooked up. These are probably part of my issue. So much good knowledge on this forum, awesome!


-T

One thing comes to mind when I read this, did you leave the distributor vent lines in place? If not you could of cooked the coil and or condenser.

Back when I was having the stalling issue, At first I thought I was out of gas, putting in five gallons didn't help. Reading other postings with this problem,the vent line possibility was the first one I saw. So I looked at the vent lines as being the culprit, I checked the tubing and the connections. Then I looked at the coil as the culprit, at that time I had several coils ,and in another postings on how to check the Ohm value I checked all the coils and installed the best candidate.Then after pulling what little hair out I have, I came across the posting about the gas cap fording valve. When I removed the gas cap and it was set for fording I preceded to do a verbal recital of every obscene word and phrase I could remember since forth grade. I even called myself a few choice names which my wife has written down for future reference. Oh yes, that did solve my stalling issue. I do hope you stalling issue is found and corrected. We're here to help. Ed

It makes sense that a problem arises when the vented gas cap is placed in the non-venting position, but whasn't the orginal '5 spoke' gas cap non-vented? Why didn't it cause the problem?

Is there any way to test if an older gas cap might not be venting even when it's in the open position?