Engine issues after engine rebuild

1950 - 1952, M38, questions, discussions, regarding anything related to the M38.
Scott in NM
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Engine issues after engine rebuild

Post by Scott in NM » Tue Mar 30, 2021 3:11 pm

Having engine problems and trying to get things back to normal after engine rebuild. The following issues exist:
1. Fought like heck just to adjust things so it would start. Still has difficulty starting, must let it grind for 10-15 seconds, little to no choke.
2. Blows black smoke when it does start.
3. Throttle response is OK when just running the engine but no power when put in gear. Can't even engage clutch to any degree without bogging down.
4. The fuel/air mixture screw on the carburetor has no effect on engine performance. Currently set at 1 1/2 turns out from full in, can put it all the way in, take it almost completely out, no change on engine as the screw is being turned.

Things already checked and not in question:
1. Jeep is as it came from the factory to include underwater ventilation system. No modifications.
2. Engine, carburetor, and fuel pump all professionally rebuilt.
3. No timing marks anywhere. TDC determined by a borescope camera, so TDC is visually checked.
4. Oil pump installed and aligned per the manual.
5. Distributor installed per manual.
6. Points gapped to 0.020"
7. Spark plugs gapped and no abnormal deposits indicating improper burn.
8. Best vacuum is 15 PSI (we are at 7000'), but once warmed up is more like 10.

Things have been checked and rechecked over a 12 hour period. I'm out of airspeed, altitude, and ideas. Open to any ideas on the next things I can do or check but need more experienced minds. And if anyone is in the Albuquerque area and thinks they have an answer I'll bring the jeep to you if you would like. Any help greatly appreciated.

Scott


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Scoutpilot
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Re: Engine issues after engine rebuild

Post by Scoutpilot » Tue Mar 30, 2021 3:18 pm

Dig out your torque wrench and pick a spot to start. Don't stop until all have been checked.
'70-'71 'A' Batt., 377th FA, 101AB
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Re: Engine issues after engine rebuild

Post by jnissen » Tue Mar 30, 2021 3:48 pm

Black smoke indicates excess fuel. Verify there isn’t crap in the fuel lines after the rebuild is holding the float valve open. Good idea to check pressure on the fuel pump as well. Look down the carb when starting and look for fuel drizzling in.

Assuming you have a good ignition ( did you verify spark is bright and blue?) then it’s only timing and compression. If it’s been properly rebuilt let’s assume the compression is ok.


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Scott in NM
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Re: Engine issues after engine rebuild

Post by Scott in NM » Tue Mar 30, 2021 6:33 pm

Timing is as perfect as it can get.
Float valve is set properly and opens and closes as it should.
Spark is bright and blue.

I've spent roughly 16 hours the past two days trying to figure out what is wrong. I've reviewed the L134 Static Timing flowchart from the CJ3 site and all M38 manuals. The excessive gas, black smoke, and the fact that adjusting the fuel-air mixture doesn't do a thing should be clues but the conclusion is beyond me. I've checked everything but obviously overlooking something. HELP!

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Re: Engine issues after engine rebuild

Post by mdainsd » Tue Mar 30, 2021 6:35 pm

You can't look down the throat of an M38 carb. Does make things more difficult.
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Engine issues after engine rebuild

Post by jnissen » Tue Mar 30, 2021 7:46 pm

Well there must be excessive fuel coming from somewhere. Black smoke and inability to tune with mix screw says you have a bum carb or your choke is woefully adjusted. How did you verify the float height? Says you had them professionally rebuilt so that would indicate to me your hoping it was done correctly.

Only one way to verify and that is to remove the top and inspect for debris under the needle then verify and measure float height.

I’ve had metal shavings from a fuel line fitting find there way into the needle and seat which resulted in flooding with plenty of black smoke.

Do you have a timing light and was the engine verified with 5 degrees BTDC? A scope will verify TDC but you need to back that up with a painted timing mark on the pulley and the reference mark on the timing cover with a timing light to make sure.

Have you tested compression?

I’ll assume your ignition is OK based on your comments but question this as you had an earlier post saying it had worn bushings.

How old is your fuel? If older than a couple months and especially if it’s an ethanol blend you may be trying to burn water! Ethanol is terribly hydroscopic.
Jim Nissen
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Re: Engine issues after engine rebuild

Post by JMZ421 » Wed Mar 31, 2021 2:43 am

Scott,

I would first make sure your valves are adjusted to spec. After that, connect your vacuum gauge to the port near the intake and turn the distributor until you get the highest vacuum reading. Also, all the vacuum line fittings need to be tight on the M38, no leaks. Are the points adjusted correctly? Good luck, John

Scott in NM
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Re: Engine issues after engine rebuild

Post by Scott in NM » Wed Mar 31, 2021 5:03 am

Responses to all the inputs above:
1. You can look down the throat of the carb if you take the top off.
2. I've not tested compression but will do so today (only so much time to chase all the gremlins).
3. Float height just fine and works just fine. I can blow into the fuel inlet and verify the needle valve is not sticking as the float is raised and lowered.
4. Distributor bushings replaced, points reset, everything here tests OK.
5. Not checked valve spacing but the rebuilders had the specs. If you don't trust the pros to do the job right with the proper manuals, then there is no reason for their existence (whether carb, engine, fuel pump, etc). May as well strip the jeep down to nuts and bolts and do everything yourself if you can't trust others to do their job. I have to trust the pros as I have neither the tools, equipment, nor time to perform all the assembly and checks. With that being said, the jeep is worse off now than when I originally took it in, so something isn't right.
6. Fuel is fresh.

I've started from Square One at least a dozen times screwing around with each part per the manuals and all guidance. At this point, it will be an expensive lawn decoration for a long time to come as all this just pisses me off.

Scott in NM
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Re: Engine issues after engine rebuild

Post by Scott in NM » Wed Mar 31, 2021 5:21 am

And the part that bugs me most is that I get black smoke (excessive fuel) and can't lean out the mixture using the set screw. If I had another carb I would put that on and see if it makes a difference.

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Re: Engine issues after engine rebuild

Post by dpcd67 » Wed Mar 31, 2021 5:58 am

You said it; Do not trust the "Pros" if you mean any local auto shop; they are usually clueless about these old vehicles. Trusting others is a good way to get frustrated on old vehicles. I run into this all the time; these guys either spend tons of money, or just give up and sell the jeep. Hate to say this, but these things are not modern jump in and drive, systems. They require a lot of PMCS and also repair up to depot maintenance level.
Anyway, which Pro built your carb? If it was Scout, then, it will be right.
If not, send it to him. These carbs are vacuum operated and harder to work with than the mechanical ones.
What fuel pump do you have? Do not say electric. Those will overpower your float valve.
It has to be something that will seem obvious once you find it.
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Re: Engine issues after engine rebuild

Post by Adam » Wed Mar 31, 2021 11:09 am

Are all 4 cyls firing? firing order correct?, dist turns ccw with 1342, didnt hurt to ask.

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Re: Engine issues after engine rebuild

Post by Scoutpilot » Wed Mar 31, 2021 11:42 am

Are you trying to run 87 octane gas with 5º BTDC timing at 7000 Feet MSL? If so, you need to advance your timing to 6-7 degrees BTDC and lean the mixture.
'70-'71 'A' Batt., 377th FA, 101AB
(Slicks and LOH's)(Col's LOH Pilot)
'71-72 CHARLIE TROOP,
16TH AIR CAV, 1ST AVN BDE (AEROSCOUTS)
LOW LEVEL HELL.
'46 CJ2A
'47 CJ2A
'48 CJ2A
'48 B1PW126
'69 CJ5

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Re: Engine issues after engine rebuild

Post by sjalbert » Wed Mar 31, 2021 1:04 pm

Scoutpilot wrote:
Wed Mar 31, 2021 11:42 am
Are you trying to run 87 octane gas with 5º BTDC timing at 7000 Feet MSL? If so, you need to advance your timing to 6-7 degrees BTDC and lean the mixture.
Thanks Rick, you just answered a question I was going to ask. To wit what adjustments do you make when running at higher altitude.
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Scott in NM
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Re: Engine issues after engine rebuild

Post by Scott in NM » Wed Mar 31, 2021 2:18 pm

Good input all. Collective response:
1. Rick rebuilt the carb
2. All four cylinders are firing and in the correct order. It purrs very nicely at idle once it warms up and "clears its throat". As long as in neutral runs nicely at RPM but ANY load and it will stall, even on a level surface.
3. Used vacuum for final adjustment of timing due to the altitude.
4. Can't seem to lean the mixture, turning the screw has no effect throughout entire range of travel.
5. The fuel pump is the issued one. I've not tested output pressure after its rebuild, only so many minutes in a day. On the list.

Total bafflement. May just start all over (oil pump alignment, distributor alignment, points set, valve spacing, spark plug gap, idle mixture screw 1 1/2 turns out from full in, use of vacuum, etc etc) but I've already done this probably two dozen times the past two days. Need a break from this !@#$% and mentally recover. Maybe I just need to hold my tongue right with a rising moon and the sun in the seventh house.

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Re: Engine issues after engine rebuild

Post by Scoutpilot » Wed Mar 31, 2021 2:52 pm

1 1/2 turns out is the starting point for adjustment. You should try backing the idle stop screw out a turn or two. Then adjust the mixture.
'70-'71 'A' Batt., 377th FA, 101AB
(Slicks and LOH's)(Col's LOH Pilot)
'71-72 CHARLIE TROOP,
16TH AIR CAV, 1ST AVN BDE (AEROSCOUTS)
LOW LEVEL HELL.
'46 CJ2A
'47 CJ2A
'48 CJ2A
'48 B1PW126
'69 CJ5


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