Shudders, Judders and Surges

1959 - 1978, M151, M151A1, M151A2, Technical questions and discussions, regarding anything related to the M151.
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Crux444
G-Staff Sergeant
G-Staff Sergeant
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Joined: Tue Mar 23, 2021 4:15 am
Location: Burlington, ON, Canada

Shudders, Judders and Surges

Post by Crux444 » Sun May 30, 2021 5:30 am

Two issues I have noticed with my M151A2 lately that I’m looking for some insight from the more mechanically-minded member here - I’m like Oddball in Kelly’s Heroes “Hey, baby. I just ride on ‘em, I don’t know what makes ‘em work.”

Issue #1 - Surging: The engine tends to surge somewhat at idle; not excessively so, just a gentle surge but certainly noticeable and it should just idle steadily. I have adjusted the idle screw on the Zenith carb, but the surging persists.

Issue #2 - Rear Differential Shuddering: I’ve noticed that when I am up to speed in 4th gear and let off the throttle, i tend to get a shudder in the rear end of the vehicle which stops when I reapply throttle. Is this an indication of a concern with the rear differential, or just par for the course on M151s?
Canadian Army (1988-2011)
Kandahar, Afghanistan 2006-2007

M151A2 CDN (1974)


Rickf
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Location: Pemberton, New Jersey

Re: Shudders, Judders and Surges

Post by Rickf » Sun May 30, 2021 5:55 am

When you adjust the idle mixture screw can you adjust it down to where it stumbles and also up to where it stumbles? Then you would aim for the point in between those two points for the best idle. I usually add a 1/4 turn to eliminate the usual flat spot at cruise. If that all sounds good then I would go into the distributor, remove the ignition plate and look at the weights under the plate and make sure they are moving freely and both springs are intact. This area tends to gum up from melted potting material from electronic ignitions and rust on others. Be aware that the two springs will be different sizes and one will be loose, this is normal. As long as they are not broken. Put a small amount of 3 n one oil on the weight pivots.

You can also check this operation with a timing light by revving the motor while watching the mark move but I find it is much better to look at the parts in person.

And on the rear you could have an axle u-joint going out, this can be very dangerous because if it breaks the flailing axle can take out the brake line and on a single stage brake system you will now have no brakes! And the parking brake will also not work because it is on the driveshaft and that stops the rear wheels and with no axle, no stop.
The other possibility is the bolts holding the differential to the body are loose.
1964 M151A1
1984 M1008
1967 M416
04/1952 M100
12/1952 M100- Departed
AN/TSQ-114A Trailblazer- Gone

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