Loss of pover (and backfire?) during driving

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Cisco82
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Loss of pover (and backfire?) during driving

Unread post by Cisco82 » February 14th, 2021, 8:28 am

Hi fellows,
it was snowing here and so i decide to brimg my M151 to meet the snow againf after many years (see following pics)
20210214_120644.jpg
20210214_120554.jpg
Unlikely i noticed a problem driving it around my garden : it is sometimes show loss of power and it is hesitanting mainly when i open the throttle or it is under load , it seems it is going to turning off and then it start again with some noises (is it backfire right? even no flame from the exhaust pipe lol).

Just to give you the situation : spark plugs and spark cables are new, i ve just installed new electronic ignition (my points was worn so i decided to use what i have available at home ) and i set the advance and it perfectly works.

I checked the coil (it is an old USA made) : the resistance between external posts are 5,6 Ohm (so good between 5 and 6 Ohm) and the resistance between extarnal and central post is 9500 kOhm (i know it is below the range 11000-15000), so is it is possible it is the cause of this porblem??

I thought it could be a problem of carburator: i have an original Holley but i never open it since i have no gasket to rebuilt it and i never cleaned a carburator before so i was pretty scared about this job.

Advice is always wellcome.

Thanks

francesco
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Francesco

M151-A1 1969
Alfa Romeo AR-51 1952

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rickf
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Re: Loss of pover (and backfire?) during driving

Unread post by rickf » February 14th, 2021, 11:51 am

That kind of sounds like the symptoms of a bad coil but if it continues to run for a substantial time after that I would say it is not the coil. When a coil is going bad and it starts cutting out the next step is it shuts off. Then it will usually restart after sitting for a bit and run again but for a shorter time before repeating the whole sequence over again. Eventually it will not start at all. In this case it sounds more like it is running lean, or starving for fuel. Do you have a clogged fuel filter? Bad fuel pump? If you don't know the history of that Holley it could be a float level set too low. If it only does it at the same time as you step on the gas but runs fine at steady load then it is probably the accelerator pump in the carb. You say you set the the timing advance so I ill assume you set the base timing at the mark?
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

Cisco82
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Re: Loss of pover (and backfire?) during driving

Unread post by Cisco82 » February 14th, 2021, 12:08 pm

Thanks Rick,

no problem to restart the engine if it turn off , but to be honest it never turns off because, after the moment of hesitating, it restarts (with backfire) i suppose so , considering your reply the coil is not the problem even if the reistance from central and lateral post is out of tolerances.

Regarding to the holley i simply know it was used for several years by my father then about 20 years waiting on the car and now back to life but as i told i only cleaned the external side never opened it , so i was thinking it could be dirty inside... but i m afraid to open it.

Regarding fuel filter i use a new one (not original) and it is new and trasparent so i exclude it.

I have an A1 fuel pump ... it works but to be honest i dont know how to check it.

Regarding timing , at 550 rpm the strobo light shows the pulley point in corrispondence of the post , and when i open the throttle i see the point moving so i suppose it is ok since i have no reference rpm - advance for different rpm range.

I tried to run a bit more this afternoon and it seems to getting better , maybe it only needs to run a bit , however it sometimes stop at idele since it is a bit difficult to keep an idle so lo 500-550 rpm.

Thakns
Francesco

M151-A1 1969
Alfa Romeo AR-51 1952

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Re: Loss of pover (and backfire?) during driving

Unread post by m3a1 » February 14th, 2021, 12:53 pm

You might also want to take a peek down in your distributor to make sure the mechanical advance is moving freely.

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Re: Loss of pover (and backfire?) during driving

Unread post by rickf » February 14th, 2021, 2:59 pm

He said the timing was advancing with the light, I doubt it is gummed up. Francesco, Is the engine fully warmed up and do you have a 180 degree thermostat in it? If it is running cold that can cause issues. And if it is getting better than it might just be gum in the carburetor and the gasoline will clean it out. The A1 pump will work right or it will not work at all so do not worry about that. If you want to check it take the line off at the carburetor and hold it in a can and with the ignition switch off have someone push the starter button for a few seconds. The pump will run as long as the starter is engaged. You are looking for a strong steady stream of gas. If you do not have this then either the new filter is clogged and you can check that by taking the line off before the filter and doing the same test. Or the filter in the tank is clogged.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

Cisco82
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Re: Loss of pover (and backfire?) during driving

Unread post by Cisco82 » February 14th, 2021, 3:22 pm

Thanks Rick,

i think the pump it is ok , it works ... I think the problem is into carburator , the only part not cleaned , where could i find a guideline to overhouling (at least cleaning) holley carburator?

Regarding to termostat , how can i understand if it is 180 deg? I think it is standard one
Francesco

M151-A1 1969
Alfa Romeo AR-51 1952

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Re: Loss of pover (and backfire?) during driving

Unread post by rickf » February 14th, 2021, 5:05 pm

If you can get your hands on a laser thermometer just keep checking the upper thermostat housing and the top of the radiator where the hose goes in and see what the temps are when the thermostat opens. It probably will not be exactly 180 but a 180 will be close to that. The stock one was a 160 which is way to cold and harkened back to the 40's-50's when that was a common thermostat for those engines. I don't know if you remember those engines but they always had a milky goo on the underneath of the oil cap. That was water condensation that never cooked off because it was too cold. The hotter an engine runs the more efficient it is. That is why modern engine usually run at around 220 degrees. You could run a 190 thermostat in the winter and have no problems. You could actually run a 190 in the summer but if I try to explain thermodynamics and why that would work I would get flamed for weeks. That is a subject for another thread.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

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Re: Loss of pover (and backfire?) during driving

Unread post by muttguru » February 15th, 2021, 7:06 am

Cisco82 wrote:
February 14th, 2021, 12:08 pm

Regarding fuel filter i use a new one (not original) and it is new and transparent so i exclude it.
.

Cisco...
try this. Disconnect and remove that fuel filter and re-connect the fuel pipe. Then take the vehicle for a test run.
Cheap, small and poor-quality in-line filters cause more problems than having no filter at all.
Ken
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Ken

Always wanted - Details and pictures of M416 Trailer data plates & M151 data plates & body-tags for my research. Thanks!

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Re: Loss of pover (and backfire?) during driving

Unread post by rickf » February 15th, 2021, 9:54 am

Ken HATES inline gas filters. :lol: :lol: :lol: But he did get one point right, the cheap ones can cause problems. That is why I said to check the flow at the carb after the filter and if it was low to check it again before the filter. This will determine if the filter is the problem.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

Cisco82
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Re: Loss of pover (and backfire?) during driving

Unread post by Cisco82 » February 19th, 2021, 12:09 pm

Hi guys
today i drove my mutt and it is getting better, but sometime when I suddenly open the throttle it seems to seat and backfire from carburator.

Should I check the vacuum? Or what type of test would you suggest?

I aslo noticed it stays at idle but it looses bits.. could it help for a diagnosis?

Note: i made a vacuum test and it read low values with the needle vibrating high speed at idle, maybe a leak or a crack into intake manifold?
Francesco

M151-A1 1969
Alfa Romeo AR-51 1952

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Re: Loss of pover (and backfire?) during driving

Unread post by rickf » February 19th, 2021, 5:39 pm

Vacuum gauge vibrating is usually a sign of a bad valve, Or need for valve adjustment. Opening the throttle fast and getting a bog and a backfire is a bad accelerator pump. BUT! If it is running too cold that will cause those symptoms also. You can check for vacuum leaks around the intake manifold and carb by raising the idle to a fast idle and then spraying carb cleaner around the intake manifold where it meets the head, be sure to get under the manifold also since this is where they usually leak. You are listening for any change in the way it runs. If you hear a change you can try to pinpoint the spot that caused it by giving short bursts of spray in small locations. Be sure the hose to the air cleaner is on and tight or you will get false readings. If you do not detect any leaks and the engine is running at full temperature then you will need to do a leak down test to see if a valve is leaking or a piston ring is stuck.
If you do find the intake gasket is leaking you will need to replace it but be sure to pay close attention to the torque specifications when you tighten the bolts back up since they are not as tight as you may think and that causes a lot of problems if made too tight. One being it smashes the exhaust manifold and it will always leak.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

Cisco82
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Re: Loss of pover (and backfire?) during driving

Unread post by Cisco82 » March 14th, 2021, 3:30 am

Guys,

problems solved :

1 - installed a new intake manifold, the old one had a crack into the valve hole so it wal leaking. the vacuum needle stil drifting a bit but the value is good

2 - holley carburator cleaned and seal changed (i found the procedure into TM manual '68 if someone needs it) , it was dirty inside after 30 years still in garage and the gasket was worn

3 - the backfire cause was the pulgs ... so simple ... i used a Magneti Marelli plugs (easly available in Italy) used for military vehicles they seem to have the same spec but now im sure they are verly cold compared to M151 one. I bought a set of adaptors and used champion RJ6C now it run so sooth :)
Francesco

M151-A1 1969
Alfa Romeo AR-51 1952

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