L-134 fuel pump camshaft lobe

1950 - 1952, M38, questions, discussions, regarding anything related to the M38.
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tga
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L-134 fuel pump camshaft lobe

Post by tga » Thu Aug 09, 2018 11:36 pm

Anyone know the specs (specifically the lift) for the camshaft lobe that drives the fuel pump? Brief history on this M38 is that I just bought it and am trying to get it running after unknown years of sitting. Runs on gravity fed (no pressure other than atmospheric) fuel tank hung from windshield. I've tried the original military pump with the 9/16" spacer (pre and post rebuild) and a brand new civilian (glass bowl type) with no spacer and spaced with washers from 1/8" to 1/2". Both pumps work and pump when actuated by hand, but neither will pump when installed. I can see (when spaced with washers) and feel that the fuel pump arm is engaging the lobe and is moving when engine is turned over although movement is minimal. I put an indicator on the lobe and am reading .256" movement. I can not find any reference in the engine TM as to any specs on the camshaft lobes (fuel pump or valve), only specs on journal sizes. Obviously I am suspicious that the camshaft lobe is worn out but would like to be sure replacing the camshaft will correct the problem before I start pulling the engine. Anyone have an NOS camshaft laying around they could measure or know where I can find the specs? Thanks much.

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Re: L-134 fuel pump camshaft lobe

Post by 11ACRLT » Fri Aug 10, 2018 11:03 am

I don't have an NOS cam, but the one I checked came out of a running engine. I got .260 lift checked on bench center.
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Re: L-134 fuel pump camshaft lobe

Post by dpcd67 » Fri Aug 10, 2018 7:34 pm

My fuel pump thread two down from this one; last week I put a dial indicator on my cam lobe and got .260 of movement, so yours is fine. Turned out my fuel pump had an internal check valve installed backwards so it was basically stuck between sucking and blowing.
It works now, so you can stop worrying about your cam and focus elsewhere.
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Re: L-134 fuel pump camshaft lobe

Post by MrVisB » Fri Aug 10, 2018 9:22 pm

Went thru this on my father's 44GPW this last winter. Suspicious of the camshaft lobe.
Bottom line, it's never the camshaft lobe. It's always something else.
Apparently, the idea that that lobe would wear down enough to fail has become lore enough that we go there quickly, but I've never heard of an actual instance where that has occurred before other parts of the camshaft had worn to failure first.

Be real happy to hear about an actual instance where this happened.

Ended up with new pump, proper sealing around all connections, and LOTS of hand cranking the engine to finally prime the pump. Not a problem since.
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Re: L-134 fuel pump camshaft lobe

Post by Radtech » Sat Aug 11, 2018 6:12 am

There were 2 type operating arms used on the original M38 fuel pumps.Part number 5592295 and 5593764.The early part number arms wore down the cam lobes so they were changed to an arm angled to put less pressure on the lobe.


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Re: L-134 fuel pump camshaft lobe

Post by tga » Sat Aug 11, 2018 6:49 am

Thanks for the replies, sounds like the lobe is where it should be or at least close. I replaced all the fuel lines with new from the tank to the shut off valve where the fuel line comes thru the bracket at lower left firewall. From there I have rubber fuel hose going to the fuel pump and from the fuel pump to carb. I can remove fuel pump from block (leaving fuel lines attached) and pump actuating arm by hand and start and run engine. Fuel is primed and pumping by hand....when pump is then reattached to block....no fuel. Hence my suspicion of cam lobe. Since lobe is apparently not worn appreciably I'm at a loss as to why it won't pump. Guess I'll keep searching.

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Re: L-134 fuel pump camshaft lobe

Post by dpcd67 » Sat Aug 11, 2018 7:33 am

Send it to Then and Now and the M38 pump expert will tell you in 2 minutes. That is what I did last week. I thought it was the arm as it had a lot of slack in it; no, it was a check valve installed backwards.
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Re: L-134 fuel pump camshaft lobe

Post by MrVisB » Sat Aug 11, 2018 10:08 am

On the older pump, is the operating arm itself worn down? We did address that before replacing our pump. The arm was significantly worn, so had welder "build it up" back to match the arm of a new pump. Unfortunately, that didn't solve the issue on the old pump.
The old pump did seem to be functioning when operated by hand. Lots of fuel pushed out per "pump" so we weren't suspicious of a blown diaphragm, etc. Just could never get it to pump correctly anymore when installed.
(The old pump HAD worked during the Xmas parades, but after jeep parked inside for the off season, could not get fuel to the carb like 6 weeks later... Which is why we were at first thinking the diaphragm had dried and split. Still don't know what is the old pump problem.)
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Re: L-134 fuel pump camshaft lobe

Post by dpcd67 » Sat Aug 11, 2018 3:42 pm

Yes mine worked too when you activate the arm far more than the .260 inch that the cam moves it. As I learned, that means the pump needs work inside.
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Re: L-134 fuel pump camshaft lobe

Post by signsup » Thu Oct 11, 2018 4:10 pm

I can't add much to this thread other than the M38 I have in my shop came in for various items like wiper motors, hand brake stuck, etc. The owner wants me to check leaking fuel pump. Mentioned previous restorer had welded onto the fule pump arm and it runs the jeep, but leaks elsewhere. So welding onto the arm seems to be a shade tree repair for this issue.
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Re: L-134 fuel pump camshaft lobe

Post by Ben Dover » Thu Oct 11, 2018 4:41 pm

If there is sufficient wear on the Fuel Pump Lobe, it will be evident with grooves in it. If it is grooved, change it anyways as it will cause faster wear on the fuel pump rocker arm.
The Fuel Pump lobe tends to get overlooked by most.
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Re: L-134 fuel pump camshaft lobe

Post by artificer » Fri Oct 12, 2018 1:18 am

Who has ever seen a L/F 134 fuel pump cam lobe worn out?
Not on this front!
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Re: L-134 fuel pump camshaft lobe

Post by Ben Dover » Fri Oct 12, 2018 4:27 am

artificer wrote:
Fri Oct 12, 2018 1:18 am
Who has ever seen a L/F 134 fuel pump cam lobe worn out?
Not on this front!
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I have sold a few hundred new Jeep camshafts to customers over the past 7 decades, and have probably replaced nearly as many while reconditioning Jeep engines.
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