stumble and backfire under load- bad coil? Ideas?

1959 - 1978, M151, M151A1, M151A2, Technical questions and discussions, regarding anything related to the M151.
Cargovoyage
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stumble and backfire under load- bad coil? Ideas?

Post by Cargovoyage » Mon Oct 08, 2018 10:41 am

Hey guys
So i had the carb on my M151A1 rebuilt 6 months ago. Has started and run great since (even without any choke). After not having driven it for a month or so, I took it out for a spin. Started up perfectly fine. Idles fine. Accelerates fine. But under load at speed it starts missing, hesitating, and backfiring. I put fresh gas in it and I thought that helped, but it came back. Is this symptomatic of a bad coil? I know I have an extra at home but they are all the shiny Chinese examples. Any thoughts?
Thanks

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Re: stumble and backfire under load- bad coil? Ideas?

Post by Rickf » Mon Oct 08, 2018 12:11 pm

Is it backfiring through the carb or out the exhaust?
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

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Re: stumble and backfire under load- bad coil? Ideas?

Post by Cargovoyage » Mon Oct 08, 2018 12:18 pm

Exhaust backfire

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Re: stumble and backfire under load- bad coil? Ideas?

Post by Rickf » Mon Oct 08, 2018 12:29 pm

That is a sign of ignition failure, could be a coil or ignition module if you have electronic ignition. Coil is most suspect. It usually will not do it until it gets hot and will get bad enough that it will shut down, then ten minutes later it will start and run fine but for a little less time. This will repeat with the pattern of less running time on each shot. Check the resistance between the primary posts on the coil and also between one of the primary posts and the secondary. If it is a government issue coil it should read around 5-6 ohms between the primaries and around 11,000-15,000 between the primary and secondary. Chinese coils are higher on the primary readings.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

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Re: stumble and backfire under load- bad coil? Ideas?

Post by Cargovoyage » Mon Oct 08, 2018 12:59 pm

Thank you. I'll check those coil resistances ASAP. If its bad- is there any way to buy OEM or NOS coils?


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Re: stumble and backfire under load- bad coil? Ideas?

Post by Rickf » Mon Oct 08, 2018 3:50 pm

Only if you are in the right place at the right time and have deep pockets! You can PM Fil Bonica on here and ask him, he had some at one time, don't know if he still does.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

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Re: stumble and backfire under load- bad coil? Ideas?

Post by W. Winget » Mon Oct 08, 2018 5:47 pm

Somewhere on the internet, I saw some use another commercial coil to replace the MUTTs coil by taking it apart and soldering on leads to swap the heads out. It was blue and white plastic in the photos, but I post it as a potential alternative. 3D printing an adapter plate might be another possibility, but have to find a short 24V coil to fit in there.
It may have been Argentine or Iran where the guy was, they usually have to resort to out of the box thinking to keep things running.
V/R W Winget
Last edited by W. Winget on Thu Jun 27, 2019 5:27 am, edited 1 time in total.
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Re: stumble and backfire under load- bad coil? Ideas?

Post by Rickf » Mon Oct 08, 2018 6:32 pm

It would be great if you could dig that info up Will, I would bet a ton that if there was some competition for the cheap Chinese junk and people stopped buying them then they would for sure actually make some good dependable coils and sell them cheap.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

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Re: stumble and backfire under load- bad coil? Ideas?

Post by Cargovoyage » Tue Oct 23, 2018 9:08 am

So it turns out my coil was an original and not a reproduction. I had a spare shiny Chinese one which I switched out just to see if it helped. It did not. Same problem. It was almost like it was getting starved of fuel at higher speeds/ under load. I switched out the inline fuel filter and that seemed to help. I think that may have been the issue but I didn't have time for extensive testing. Would a clogged filter make sense?

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Re: stumble and backfire under load- bad coil? Ideas?

Post by Rickf » Tue Oct 23, 2018 10:02 am

A clogged filter would absolutely make sense and is usually the first thing checked since it is easy and cheap. Just about everyone runs an aftermarket clear filter inline before the carburetor since the original filter was not all that great to begin with. The plastic inline filters are cheap and very easy to change.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

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Re: stumble and backfire under load- bad coil? Ideas?

Post by artificer » Tue Oct 23, 2018 11:20 am

Symptoms 'breakdown under load' is possibly indicative of spark plugs.
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Re: stumble and backfire under load- bad coil? Ideas?

Post by W. Winget » Tue Oct 23, 2018 11:32 am

More than likely there is water in the bottom of the carb bowl which gets sucked in as you get her up to speed. If the ethanol fuel sits long enough it will drop out the water and form a gummy crap (death nell for MUTT Carbs) if you can pull the carb and shake it out, shoot some cleaner into it's inlet and shake that back out, etc you may not have to take it fully apart. Possibly pull off the accelerator pump and spring and shake the bowl clean from that hole, but I seem to recall a flapper valve in there, not seeing it apart in my mind as I used to.
I always wanted to drill and tap a drain valve on the bowl to dump the fuel after parking so a few weeks later she would pump up and run fine.
V/R W Winget
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Re: stumble and backfire under load- bad coil? Ideas?

Post by Ben Dover » Tue Oct 23, 2018 12:15 pm

This is also a symptom of sparkplug wire failure on other M-Series vehicles such as M-38A1, M-37, M-35, etc.
What sometimes happens, is that the nuts get overtightened on the sparkplugs causing the barrel of the sparkplug to roll inward with a wire edge that grips the red insulation which causes the inner insulation to split, or pull apart when removing the cable. Best to tighten the nuts finger tight or "just snug" unless you are fording.
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Re: stumble and backfire under load- bad coil? Ideas?

Post by Rickf » Tue Oct 23, 2018 1:04 pm

W. Winget wrote:
Tue Oct 23, 2018 11:32 am
More than likely there is water in the bottom of the carb bowl which gets sucked in as you get her up to speed. If the ethanol fuel sits long enough it will drop out the water and form a gummy crap (death nell for MUTT Carbs) if you can pull the carb and shake it out, shoot some cleaner into it's inlet and shake that back out, etc you may not have to take it fully apart. Possibly pull off the accelerator pump and spring and shake the bowl clean from that hole, but I seem to recall a flapper valve in there, not seeing it apart in my mind as I used to.
I always wanted to drill and tap a drain valve on the bowl to dump the fuel after parking so a few weeks later she would pump up and run fine.
V/R W Winget
There is a drain plug on the bottom of the M151 carb bowl, you can get to it with a short wrench and the engine cold so you don't roast yourself on the exhaust pipe.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

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Re: stumble and backfire under load- bad coil? Ideas?

Post by W. Winget » Tue Oct 23, 2018 1:15 pm

Thank Rick, forgetting about the drain tells I need to get back into my MUTT...too many other projects been taking priority.
V/R W Winget
Looking for 1918 Standard B 'Liberty' truck parts

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