M170....rough idle/missing/backfire, what next?

iwojimavet · Post by **iwojimavet** » Tue Aug 13, 2013 2:47 pm

My M170 is suffering from a bad case of backfiring. I had the carb and fuel pump rebuilt. It ran well for about 40 miles, but started missing, and now backfiring Progressively getting worse. I installed new plugs, no real help.
It misses (stalls/shakes) and backfires mostly when accelerating. Fuel line flow is good. So what's the logical progression from here?
Timing? Distributor? Is there an easy way to tell if the distributor is bad?
Help!!!!!

RHarris · Post by **RHarris** » Tue Aug 13, 2013 5:51 pm

Hello, my M38a1, 1953 had the same symptons and replacing the condensor and readjusting the points cured it. I also had replaced the carb, coil, and fuel pump without curing the problem. RH

iwojimavet · Post by **iwojimavet** » Tue Aug 13, 2013 8:40 pm

Thanks! Sounds logical. I'll give 'er a shot this weekend

Post by **wesk** » Tue Aug 13, 2013 9:17 pm

I would suggest a proper regimen of troubleshooting. Not throwing dozens of high dollar parts at it until it goes away. There's an excellent troubleshooting guide in the TM 9-8014. The norm for we trained mechanics is not to replace something until we prove it is defective.

From what you have described we can make some wild guesses but it would help to know exactly what you changed, when you changed it and why you changed it. It also helps to know in what sequence each item was changed.

iwojimavet · Post by **iwojimavet** » Wed Aug 14, 2013 9:01 am

Thanks Wes. The jeep sat idle in my dad's garage for about 10 years. The carb and fuel pump needed rebuilding, and the exhaust system was replaced.
That was last Fall. It ran OK for a couple of months before the missing/backfiring started. I replaced all the plugs last week, as I do not think they had been changed since the late 80s. I also put a simple fuel filter on the line, and had scoped the tank to make sure there wasn't any sediment anywhere. Fuel flowing well. Took the horn off the carb the other day...everything looks clean and the float is fine (not sticking). Unfortunately, someone "borrowed" my manual and I do not have another to track what to do next.

Post by **wesk** » Wed Aug 14, 2013 10:28 am

Just go to our downloads page and download the full set of needed manuals in PDF. On this web site no one is without the needed manual.

http://www.willysmjeeps.com/v2/modules. ... =Downloads

Post by **wesk** » Wed Aug 14, 2013 10:35 am

Spark plugs wear out from running use not calendar years service. In most cases most MV's are driven very little and the Plugs are almost always easily rejuvenated with a good cleaning and regap.

1 - You gauge the quality of fuel delivery with a measuring cup, a watch and a pressure gauge. Not the eyeball.

2 - You gauge fuel delivery to the cylinders by interpreting the color of your spark plug electrodes.

3 - If it runs smoothly until you accelerate, then spark plugs, ignition points and coil were not your problem.

Let us know the outcome of reviewing 1, 2 & 3.

artificer · Post by **artificer** » Wed Aug 14, 2013 12:40 pm

Bad spark plugs are identified by lugging the vehicle down in top gear then slowly accelerating. Kangaroo-ing [missing, coughing & backfiring] is often an indication of plugs breaking down under increasing compression pressures.

That is why spark plug cleaners have air pressure applied to the plug when testing to establish if the plug is good @ load.

Post by **wesk** » Wed Aug 14, 2013 1:00 pm

Bad in the context above can mean both; junk and need replacing, or just need to be cleaned and re-gapped. In either case a quick cleaning, re-gapping, and testing on the shop machine will tell you for sure what is needed . Since your plugs are brand new last week it should not be an issue. Of course new has never been a guaranteur of good!