Carb Cough and Oil pressure

1952 - 1968, M38A1, M170, questions, discussions, regarding anything related to the M38a1 series.
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delta31
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Carb Cough and Oil pressure

Post by delta31 » Tue Mar 15, 2022 7:39 am

Hi
This is my first post on this site though I have used the site quite a bit for research.
1954 M38A1 new to me
Civilian Engine F 134/ converted to 12 volt
I think the engine serial number starts with 4J.

Rebuilt the carb with a Mikes Carb kit. Runs ok but will backfire thru the carb when accelerating. Timing set at 5 degrees BTDC.
Compression at about 100 psi. is this likely an intake valve problem? Can I diagnose this with out removing the head?

Cold start oil pressure about 35-40 psi. After a long warm up and drive, idle pressure barely readable like just above zero.
When driving about 20 psi. Using Rotella T4 15w-40 and bottle of zinc additive. Did the F engines come with the timing gear sprayer
that had the overly large orifice? Is this likely main or cam bearing wear?

Thanks in advance for any help.


KiwiMB
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Re: Carb Cough and Oil pressure

Post by KiwiMB » Wed Mar 23, 2022 11:42 pm

Hi,
so the backfire on acceleration is more likely to be an accelerator pump issue, what carburetor do you have fitted?

The oil pressure is a little low, do you know the history of the engine? I have installed a Melling oil pump on my engine and it runs at 45psi all day on the road, and 30psi at hot idle. I have however rebuilt the engine with reground crankshaft and new cam bearings.

I would be ok with 15psi hot idle but would want 40psi hot at 40mph. Your oil choice is fine, no issue there.

Cheers

Ian
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1953 M38A1 Nekaf
1998 Land Rover Wolf 90

parker007
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Re: Carb Cough and Oil pressure

Post by parker007 » Thu Mar 24, 2022 12:09 am

If less than 20 psi at high idle your engine need to be torn apart to see why. either bad oil pump not likely, or bad bearings. change oil pump first then look at crank.

KiwiMB
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Re: Carb Cough and Oil pressure

Post by KiwiMB » Thu Mar 24, 2022 12:34 am

Before you leap into tearing things apart, check that your oil pressure gauge is accurate. Best plan of attack is to find a mechanical gauge and an appropriate hose to tap into the oil gallery where your current gauge is connected.

Go for a drive to ensure your comparing apples with apples.

If the result is the same as, or close to your existing gauge, then start looking further.

It is most likely worn crankshaft bearings. Cam bearings are a possibility but one would replace these if doing an engine overhaul.
Cheers
Ian J
New Zealand
1941 Willys MB
1953 M38A1 Nekaf
1998 Land Rover Wolf 90

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Scoutpilot
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Re: Carb Cough and Oil pressure

Post by Scoutpilot » Thu Mar 24, 2022 12:57 am

Five degrees is too late in the compression stroke for, I'm assuming, 87 octane fuel. The motor is loading up with unburned fuel that is then burning when an intake valve opens.
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