engine dies after 3 min

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markkish79
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engine dies after 3 min

Post by markkish79 »

I have been working on this truck off and on for two months trying to figure this issue out, and I'm defeated. Please someone help. I have a 1955 dodge m43 ambulance CDN. I rebuilt everything except the engine because it only had 20,000 miles on her and ran great.

I took it to a parade and she ran great for 3 hours straight. Then the next day the engine cut out on me while driving. I pulled over, started it back up with no problem and she ran for about 3 min down the road and died again. Got it home and it does this all the time now. Checked the gas cap by taking it off/ tightened it on, and even cracked it open a bit, still dies.

Rebuilt the carb still doing it. Checked the coil and it has 25 volts coming out of it. It has clean fuel filter, cleaned out gas tank. It has an electronic fuel pump on it with a regulator and oil pressure switch connected to fuel pump, and these systems worked fine before this issue happened. So I don't think itsa full issue unless somehow the float on the carb isn't right.

The truck runs for 3 min each time and dies, but before she dies the engine revs up to about 2000 rpms then chokes out and dies. I worked on it today and now it won't even start anymore. Its like its not getting spark now. Tried starting fluid and nothing.

I am to the point where I actually thought about selling it, and I don't sell these trucks . Please anyone? Revs up then dies after 3 min. Thanks, Mark
U404S
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Re: engine dies after 3 min

Post by U404S »

That kind of thing is just confounding. I wish I had some mechanic training and could figure those things out easily.

However, despite being short-skilled, here is a story that might help: one of my Unimogs was doing the same thing recently. It started after I rebuilt the carburetor. Ran great the first couple days and then progressively did exactly what your truck did. I thought it was the float valve and I changed it, adjusted it and added and subtracted spacing washers. I didn't have a good enough tool to measure float level right so I just did the trial and error thing. None of that made a difference although that could be your problem in that the valve is stuck in the "just about closed off" position. Additionally, I used a mirror to make sure fuel was squirting down the throat of the carburetor when the accelerator pedal was pushed so I kind of felt everything was okay with the carburetor. I kept looking for other faults. Just couldn't figure it out.

Finally, I decided it might be a vacuum leak so I removed the two big asbestos carburetor gaskets and the fuel drip pan and gooped them up with high temperature liquid gasket material and put it all back together. Runs like a top now. My guess is that air was getting in between the carbureter, manifold, gasket combination and that the air leak problem got worse with every heat and cold cycle. If yours started again, maybe you could find the air leak by spraying some ether here and there to see if it revs at a certain location. That method never worked for me, but I have had some pretty scary flash-fires doing it when I didn't notice the ether had been puddling in places. I don't do that anymore.

This week's confounding carburetor problem is that my M37 starts and idles perfectly and even zooms up nicely when goosing the throttle, but is totally, totally weak and gutless as I try to get it moving. No power. I'm guessing the accelerator pump. I kind of feel like buying that StoveBolt Weber conversion or maybe the Vintage Power Wagon Weber conversion....wondering what the difference is.

Otherwise, I did that Weber conversion with my wife's 1978 Fiat Spider and it is like driving a new car now. I also think a lot about converting to propane with these carburetor based trucks. Supposedly propane is a lot less trouble if you leave the trucks sit. For me, the down side is I read that here in Florida you have to pay $200 per year for a state tax stamp to be allowed to fill up with propane at an alternative fuel station. That's a lot of money for a low-use vehicle. I guess the alternative is to use the little forklift tanks and fill them at the hardware store.
just me
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Re: engine dies after 3 min

Post by just me »

Use carb cleaner spray to find the vacuum leak. Not nearly as volitile and is obvious when it covers a leak. The sudden high idle screams vacuum leak. Don't neglect to spray the manifold to block and that carb base.
You didn't use a sealant on the gasket between the main body and air horn did you? There are two very tiny ports that would get blocked if you used goo and would affect bowl vent and idle fuel air emulsification.
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ez8
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Re: engine dies after 3 min

Post by ez8 »

Before you rebuilt the carb, had you been working on anything else? That's where I'd start.

If not, the engine needs spark, fuel, compression to run. If you've got spark and compression (since the engine was running before this), your fuel is off and the vacuum leak is low risk area to investigate (remember: air is fuel).

I'd really recommend against tearing things apart to look at them until things have been diagnosed. If you had a vacuum gage you could follow the procedures in the manual to diagnose if your vacuum is sufficient (page 141 of TM 8030). You can get a vacuum gage at Harbor Freight for about $15.00.
markkish79
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Re: engine dies after 3 min

Post by markkish79 »

Thanks for the quick reply. I haven't done anything to the truck since I put on the electric fuel pump and regulator, but the truck was running fine after that conversion. I have noticed it runs good while warming up then shuts down after about 5 min. It starts right back up easy but then only runs for about 1 min. Then revs up load and dies, then again starts up fine for another min. I have sprayed around looking for the vac leak, can't find anything. Still need to get a gauge to check that way. I noticed an old crack in the exhaust manifold where it meets the block on the far back bolt, but I don't think that will effect the vac. Plus it was there while the truck ran fine before. Compression is good, and I'm getting spark. Its something with the fuel air mix ...I'm this king the carb. I rebuilt the thing but I'm no expert on carbs. I rebuilt the one on my m38a1 and its doing fine. Is there something basic I would have missed on rebuilding the carb. Other then the obvious cleaning and swapping out parts inside with new? I think the signs for what's wrong is in the large rpm increase then choking out. I'm just not smart enough to figure it out yet..lol. well just haven't ran into this experience yet . Would the float on the carb doing anything like this if the measurements were not on point at 5/64?
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Re: engine dies after 3 min

Post by Marmalute »

I agree that the revving up before it dies is a clue here. I agree with much of what ez8 says except that air is NOT fuel!
When determining no running or poor running of gas engines it is better to look for the quality of the engine's spark, fuel, air, and compression. Each of those 4 items can generally be identified, analyzed, and often improved, once you know what you are looking at and looking for.
The reason I take exception with "remember: air is fuel" is that air and fuel need to be combined into a fairly narrow range to get combustion. For that reason it is often helpful to be able to identify where all fuel is coming from, as well as where all air is coming from. It is the precise mixing of air and fuel that happens at the carburetor that is often at issue for correct engine operation.
Did you "flat plate" all gasket sealing surfaces of the carb? It is not unusual to have carb surfaces 0.020-0.030" or more out of "flat" so that vacuum leaks can occur. I use a precision ground granite flat plate and take a sheet of 240 or 320 grit paper and a little cutting oil or wd40, and true up sealing surfaces as needed. This process can get overlooked but it can make a huge difference in a carb rebuild as well as in rebuilding so many other mechanical components where gaskets are used.
Finally, when you took apart the carb, how did the governor look? The governor should be in charge of engine speed changes. If it is working correctly, then it sounds to me that you have a good sized vacuum leak somewhere.
Good luck, let us know what you find!
markkish79
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Re: engine dies after 3 min

Post by markkish79 »

I don't think this matters but I just found some oil spraying out on the fire wall, and down the block between the block and head. I know this sounds crazy but when I spray starter fluid in the carb while running it starts to kill the engine and drops to about 300 rpms or so. :roll:
rickf
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Re: engine dies after 3 min

Post by rickf »

That is because you are flooding it, raising the fuel air ratio above 12:1 fuel to air ratio will make it stall out at idle. The perfect stoichiometric ratio is 15:1 but getting that and keeping it can be a problem with these old carburetor, you can get very close though so adding extra fuel to the incoming air upsets the ratio and therefore drops the RPM because it is flooding.
1953 M37
1964 M151A1
1967 M416
1984 M1008
4/1952 M100
12/1952 M100 gone
markkish79
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Re: engine dies after 3 min

Post by markkish79 »

Well I learned something new there, I didn't realize that would matter as dumb as that sounds now..lol. I thought starting fluid would always add some bang in there. So I took the carb apart today and checked the float again. No leaks and is at 5/64 from the top. Changed out the rubber tipped fuel shut off pin and replaced it with the new all brass one from the original kit I got. Then rechecked the float measurements again. Surprise....it did work. Still doing the same runs for about 30 sec now and revs up then dies. I'm thinking about replacing the carb with a M68 hand grenade and see if that works. Anyone in the market for a 1955 CDN M43? I'm almost certain its something with the carb because I put another rebuilt one from a friend of mine on there and it worked. It just ran a little rougher, but I think it just needed tweaked a little. It just seems rediculious to take the carb apart for a fourth time. Itspretty basic on the rebuild idk.
rickf
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Re: engine dies after 3 min

Post by rickf »

Have you checked the ignition coil resistance? With the exception of the reviving before it dies this is the classic symptoms of a coil going bad.
1953 M37
1964 M151A1
1967 M416
1984 M1008
4/1952 M100
12/1952 M100 gone
ez8
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Re: engine dies after 3 min

Post by ez8 »

Marmalute wrote:I agree with much of what ez8 says except that air is NOT fuel!
I suppose I should re-phrase that to say Air is a part of fuel. I think we agree other than the semantics.

Moving on,
markkish79 wrote: ? I'm almost certain its something with the carb because I put another rebuilt one from a friend of mine on there and it worked. It just ran a little rougher, but I think it just needed tweaked a little.
When you say "it worked" what do you mean? The engine didn't sputter and die quickly as previous? You were able to drive the truck in normal conditions (under load, stop, start, sustained operation)?

When you rebuild the carb each time, are your gaskets intact? Are you sure all of your jets are seated fully? Are your mix and idle adjustments set at the correct settings? Is your float valve clean and can it seat correctly? Are all superfluous parts removed from the carb (e.g. I once had a carb on a lawnmower that came to me with an extra spring installed on the float valve).

We're getting close, I believe.
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Re: engine dies after 3 min

Post by ZGjethro »

just me wrote:Use carb cleaner spray to find the vacuum leak. Not nearly as volitile and is obvious when it covers a leak. The sudden high idle screams vacuum leak. Don't neglect to spray the manifold to block and that carb base.
You didn't use a sealant on the gasket between the main body and air horn did you? There are two very tiny ports that would get blocked if you used goo and would affect bowl vent and idle fuel air emulsification.
I made up an air intake with a flange which bolts to the carb instead of using the air horn with its three 90 degree bends. It is a straight pipe with a K&N filter on it. Would I have covered up these ports? The truck runs great
just me
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Re: engine dies after 3 min

Post by just me »

I'm talking the carb body gasket. Between the center section and the top section. Not the air horn to intake elbow. The air very small ports in the air horn that will get plugged if you use any sealants on that gasket. The groove in the center of the top of the center section casting is where the fuel and air emulsify on the way to the bottom of the carb to the idle ports. There are at least 5 calibrated holes, two restrictions and two jets fed through that passage.
When you 'cleaned' the carb, did you remove the aluminum plugs? Did you poke any wires down any ports or jets?
"It may be ugly, but at least it is slow!"
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Re: engine dies after 3 min

Post by just me »

Watch this old video and then, watch it again. You will learn most of what you need to know about yours. The TM for Carter Carbs is also VERY handy for finding troubles. These aren't complicated but do require careful rebuilding.
https://m.youtube.com/watch?v=DTk1OJe5PFc
"It may be ugly, but at least it is slow!"
markkish79
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Re: engine dies after 3 min

Post by markkish79 »

I'm still beating my head against this truck. I'm going to the end of my rope hear. This is the fifth brake down and rebuilding and cleaning of this carb and yet its still not working. I did not put silicon on any of the gaskets. They are in good condition each time I take it apart so I re use them. I have taken off all plugs and ran a needle threw them. All parts inside are new except the float and it is clean and no leaks. The old carb started doing this before I touched it so I don't think it has to do with the way I'm rebuilding it, because its displaying the same actions of running for 1 min revving up then cutting engine off. Then starts right back up. I did a vacuum check today and it was a steady 15 psi. What is the standard pressure I should be getting, anyone know? I have another rebuild kit coming in tomorrow for a different carb. I'm going to see if that one fixes it. I do have the old fuel pump on there with all the lines still ran into the manifold and one line going into the firewall under the dash. The wipers are not hooked up yet and I haven't messed with that line under the dash yet . The fuel line is disconnected though and the fuel line goes from the tank, to a larger in line filter, to the 24v electric fuel pump, to a smaller fuel filter, then the fuel regulator, and finally into the carb. Would have the old fuel pump still attached do anything like this? The truck has ran good for a month with the old pump on there before this started. I just didn't know if the diaphragms inside the old pump would have dried up and broke and caused some sort of vac issue, but like I said I'm getting 15psi out of the top of the intake manifold. I'm going to check out this video about the carb rebuild real quick. Thanks for sending that my way btw.
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