M38 fording system vacuum leak?

[wesk]

RonD,
When I read Xamon's post I reviewed this entire post again and low and behold I saw this::

JeepdaddyRC
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Joined: Jan 10, 2020
Posts: 124

PostPosted: Sat Feb 18, 2023 3:30 pm Post subject: Reply with quote Edit/Delete this post Delete this post View IP address of poster
Update:
Removed fuel lines and cleaned. Tested fuel at carb inlet. Fuel clean.
Restarted Jeep. Same symptoms, hard to start, only starts with almost full throttle, will not idle. Runs better with a little choke
Again checked for vacuum leaks at all areas with carb cleaner, no change in rpms (however this was not done at idle, because Jeep will not idle).
Removed and disassembled carb again. Confirmed no foreign objects, blew out passages with carb cleaner, confirmed idle passages clear.
Reinstalled carb. Started Jeep. No joy. Same symptoms and same problems.
Sprayed carb cleaner around manifolds, carb base and vacuum fittings. No change in rpms.
Mystery continues. Possible vacuum leak since carb idled in Scoutpilot's shop. But cant locate it.

It is not Scout Pilot that I suspected may have inadvertantly re-assembled the carb wrong. He had already proved not so by test running it in a video for jeepdaddy.

[JeepdaddyRC]

The spring for the metering rod diaphragm is in the correct position.
Putting manifolds back on now. Will check carb low speed jet one more time for any blockage. Then install with NOS carb base gasket with diffuser.
Will report back. Fingers crossed.
Ron D2, fuel pump has not been rebuilt since my ownership. Only pressure reading I have was 4.1 psi at 1800 rpm’s. No other testing for flow yet.
Thanks everyone.

[JeepdaddyRC]

Update:
Thanks to Wes, RonD2, and all that helped. I think sharing final outcomes is important rather than leaving a thread hanging.
I removed and re-installed the manifolds with new gaskets.
Confirmed heat riser working correctly.
Confirmed valves adjusted correctly.
Went thru the carb again to confirm all idle passages open.
Confirmed gas to carb clean.
Replaced the Omix repro carb base gasket with NOS carb base gasket with diffuser.
Still needs 1/2 choke until fully warmed up.
Will now idle - but only when fully warm. Some random popping at tail pipe.
19.5 vacuum at idle with slight needle flutter.

[RonD2]

Glad to hear that JeepDaddy!
I think some choke until warmed up is fairly normal.....you never told us your location, altitude, or local temps....

If you had to pin the cause of the problem down, what would you say it was?

[JeepdaddyRC]

Wisconsin at 612 ft above sea level. Temp during test run was 30 degrees.
Not sure why I’m getting some popping at tail pipe. More testing required.
My previous experience with a YS carb on the M38A1 was choke to start then push in right away. With the WO carb we would sometimes maintain choke for awhile as engine warmed.

[wesk]

This would be a good time to get some test numbers.

1 - Fuel Pressure at idle and 1800 RPM.

2 - Fuel Flow volume.

3 - Leakdown test on all 4 cylinders.

4 - Dynamic timing.

[JeepdaddyRC]

Since the post considered a fording system vacuum leak, I thought it may be a problem with the PCV system.
Disassembled and cleaned PCV, confirmed correct orientation of parts.
During test run closed fording valve from crankcase to intake manifold. Vacuum increased from 19.5 to 20.0 psi at idle.
Is this what you would expect? Or a greater increase since you essentially take out the controlled vacuum leak of the PCV system.

[wesk]

That's a tough question considering it requires a gifted memory bank and plenty of vacuum gauge readings in recent enough history for comparisons. Unfortunately there are no published numbers specific to the M38A1 to refer to for expected change in Manifold vacuum with varying fording valve positions. You should see an increase in vacuum with any closing of any allowable leak sources. At idle I think .5" increase is a reasonable increase.

[JeepdaddyRC]

Removed vacuum gauge from intake manifold.
Unfortunately now it will only “idle” when warm with 1/2 choke.
Is it possible there is a vacuum leak at the fuel/vacuum pump?
There seems to be some oily substance (not gasoline) on the lower portion of the pump.

[RonD2]

Is it possible there is a vacuum leak at the fuel/vacuum pump? There seems to be some oily substance (not gasoline) on the lower portion of the pump.

Hi JeepDaddy,
Yes I think leaks are certainly possible in and around the fuel-vacuum pump.
"Oily substance" can only be crankcase oil. If fluid can leak out, air can get in?

Way back in this thread you said the fuel pump has not been rebuilt since your ownership --- and so far you haven't finished the textbook test for volume (obviously still needs doing).

I looked at the threads in this post (again) and collected the questions I asked about your pump --- but cannot see that you answered:

If the diaphragm is blown I think if you pull the crankcase dipstick can you smell gasoline in the crankcase?

Might be crud in the fuel pump?
(have to dismount and disassemble it to see)

Is it mounted with the correct 5/8-inch spacer to the block?

When it was installed, was the diaphragm preloaded as described in the manual?

I also seem to recall that the Army issued a modification order for pumps to drill an oil drain hole inside the body. Will have to look for it.

I thought this was solved!

[RonD2]

These old posts might be useful:
http://willysmjeeps.com/v2/modules.php? ... +fuel+pump

Fuel pump modification:
http://willysmjeeps.com/v2/modules.php? ... pump+drill

Fuel pump pre-loading the diaphragm:
http://willysmjeeps.com/v2/modules.php? ... eload+pump

[JeepdaddyRC]

Thank you for the helpful info and your patience.
I was ready to send the fuel pump to Scoutpilot.
He requested a FaceTime of engine running because it requires 1/2 choke to idle.
He led me thru these steps:
1. Turn idle stop screw counterclockwise 1/2 turn (slowing rpm).
2. Turn mixture screw counterclockwise 1/2 turn (enriching idle mixture).
We repeated both steps 3 more times while watching the vacuum gauge.
After 1.5 turns on each screw we achieved idle at 19.5 psi vacuum. Engine continued to idle while slowly pushing choke in.
The mixture screw is now 3 turns out. It will idle but has a stumble off idle with choke off.
No hesitation and good throttle response with choke at 1/2.
Perhaps enriching the idle mixture helps tune out the extra air, but I still think vacuum leak. Stumped indeed!

[RonD2]

Ok, nice confirmation with Scout Pilot that the carb is good to go, but I'm getting confused.

You asked about possible vacuum leak in-around the fuel pump, but haven't done anything to investigate or eliminate it?

Is it possible there is a vacuum leak at the fuel/vacuum pump? There seems to be some oily substance (not gasoline) on the lower portion of the pump.

Hi JeepDaddy,
Yes I think leaks are certainly possible in and around the fuel-vacuum pump.
"Oily substance" can only be crankcase oil. If fluid can leak out, air can get in?

Way back in this thread you said the fuel pump has not been rebuilt since your ownership --- and so far you haven't finished the textbook test for volume (obviously still needs doing).

I looked at the threads in this post (again) and collected the questions I asked about your pump --- but cannot see that you answered:

If the diaphragm is blown I think if you pull the crankcase dipstick can you smell gasoline in the crankcase?

Might be crud in the fuel pump?
(have to dismount and disassemble it to see)

Is it mounted with the correct 5/8-inch spacer to the block?

When it was installed, was the diaphragm preloaded as described in the manual?

I also seem to recall that the Army issued a modification order for pumps to drill an oil drain hole inside the body. Will have to look for it.

I thought this was solved!

[JeepdaddyRC]

Good question. The vacuum gauge was at the intake manifold port where the line to fuel/vacuum pump would go. So for ScoutPilot’s face time this essentially takes out any possible leak at the lines, fittings or vacuum pump. I will now remove vacuum gauge and reattach the lines to the vacuum pump. We’ll see if that changes things.

[RonD2]

So for ScoutPilot’s face time this essentially takes out any possible leak at the lines, fittings or vacuum pump.

Does it really?
You said you got vacuum gauge reading of 19.5 with Scout Pilot's adjustment advice.
His video of my YS-637S rebuild (and yours also I thought) showed about 23, which I believe is considered optimal or normal.

What could explain the difference?

Your description of how you have your gauge hooked up isn't clear to me.
Why not hook it up with the pump in the circuit? (might take a couple of extra fittings)

Yes, vacuum might be the problem, but so might fuel be.
Fuel and vacuum pump it is.

You still haven't answered the fuel pump questions, asked twice now.

Good luck!