M38 no start condition

[JeepdaddyRC]

Been on the sidelines with health issues. M38 has been sitting for 3.5 months.

My usual start procedure (per Scoutpilot from carb and fuel pump rebuild): Full choke until engine kicks once. Then choke in, add throttle to start. Reminds me of chainsaw start procedure.

I realize the manual says full choke to start, but Scoutpilot's recommended method has worked well previously.

Now it just cranks, with a few random kicks, but nothing close to a start. With a CJ, I can prime the carb with fuel poured down the throat. But I don't know the tricks for this original waterproof carb.

Since the fuel has drained back and evaporated out of the bowl, what tricks or workarounds are available?

[RonD2]

Hi Jeep Daddy,
It's been a while. Can't see your M38 from here --- refresh my memory.

Original 24v ignition system?
Batteries fully charged? All connections clean and tight?
Corn gas or ethanol-free gas? If corn, how old?
No mice nest obstructing the intake or exhaust (or chewing wires)?
How long did you crank it? (Could take awhile to fill everything)
Pull the crankcase dipstick --- oil level ok? No smell of gas?
Remove a spark plug wire --- check for bright blue spark?

You could feed some gas into the bowl on the carb through the bowl vent line fitting, or a small shot of starting fluid directly in the throat (disconnect the pipe to the air cleaner).

I think I'd try the Army manual start procedure first (full choke until it starts, 2 clicks on the throttle cable). But that's just me.

Next up is the troubleshooting steps in the Army TM9-8012 Manual.

Good luck!

[JeepdaddyRC]

Thanks RonD2 You've been a big help over the years. Much appreciated. Been on the sidelines and M38 sat for an extended period.

Answers to your good questions:
Original 24v ignition system, carb and fuel pump (rebuilt)
Batteries fully charged with battery tender. Good turnover.
87 octane ethanol gas. Scoutpilot indicated the carb rebuild is OK for ethanol.
No mice nest obstructing and no chewed wires.
Removed and cleaned spark plugs. Good spark.
Waited at least 30 seconds after each 10-second crank so starter would not overheat.

Well it took a long time. Try for 5 minutes, wait 30 minutes, Repeat over and over again. Finally kicked more frequently and finally started.

Unlike most, this M38 does not start with full choke. Not sure why. Had carb apart several times and the diaphragms are good with correct metering rod.

The only way I can get this started is with close to wide open throttle (with no choke).

Any ideas why that would be?

[RonD2]

Glad to hear you got it going.

As I understand internal combustion engines, "more choke" (full choke means air being choked off) is to get more fuel into the combustion chamber faster for ignition.

Wide open throttle is to get more air into the combustion chamber, typically used when the engine is "flooded" with too much gas.

Obviously, the intent for fast starting is to have the right air-fuel mix in the right place at the right time.

The M38 has kind of a long and restricted air pathway --- crossover tube to a military air filter capped by a screen.
The internal element is a media filled mesh sitting on top of a pint or two of motor oil.
Maybe not getting enough air?
Have you cleaned, inspected, and serviced the air cleaner lately?
I clean my filter element with a good soak in a bucket of diesel fuel followed by a blow dry.

Because you didn't prime the carb I can understand it taking a while to get fuel uphill to the carb.

Only other thing I can guess is making sure the timing is spot on (including point gap).

If you set timing and adjusted the carb air-fuel mix by the Army manual for 1950 gas you may be off.
The engine is now dependent on the gas you have in the tank today.
Might want to get a vacuum gauge for a precision tune up.
I believe ScoutPilot is a proponent and has the procedure on his site.
Not hard to do.

Do the carb air-fuel mix tune-up with the air filter fully hooked up.
Don't forget gas tank has to be vented.
And a clean PCV connected in the right flow direction.

Finding just the right combination of happy start process might take some experimenting......

Maybe try full choke until it fires once, then choke off with partial throttle until it starts?

Just a few thoughts.
Good luck!

[4x4M38]

My understanding of conventional gas engines and carburetors is the choke simply reduces the air into the carburetor. It has nothing to do with increasing fuel flow. It does increase the fuel to air ratio, but will not put more fuel in the carb.

The throttle/ foot pedal linkage does two things. Opens the butterfly allowing increased air flow into the carburetor while also increasing fuel flow at the same time.

When you flip the linkage on a Holley the butterflies open and the fuel increases at the same time.

The choke on the M38 works the same. I need to think about the throttle/ foot pedal linkage but it makes sense it works the same way.

What I do remember is the M38 fuel pump won’t do a very good job of putting fuel into a drained or starved fuel system. That’s where the manual pump on the dash would help.

The other thing is ethanol gas is infamous for drying out and leaving deposits on everything. How long since last run and humidity and temperature have a lot to do with that. Some folks have success using the fuel stabilizing additives but your mileage may vary.

Hope I have some of this right in my head.

[RonD2]

Your understanding is much better than mine Brian.

I seem to recall that until the engine starts, the throttle only opens the airway.
Except for the pump filling the float bowl, gas doesn't move much until the engine starts.

I'm still trying to digest this article on the theory of operation that I found: https://www.hemmings.com/stories/articl ... e-army-now

I'm sure Wes will be along shortly to straighten me out.

[JeepdaddyRC]

I’ll admit the M38 and A1 carbs are somewhat confusing to me. I’m used to a conventional mechanical accelerator pump that gives you a squirt of gas before starting,
Because of this, I’ve considered adding a cold weather primer kit to give a shot of fuel directly into the intake manifold.
This recent experience has me revisiting that idea.
Anyone add a dash mounted primer pump? Seems like a good a way to prime the engine prior to starting - instead of relying on the vacuum actuated accelerator pump?
Is my thinking reasonable?

[wesk]

Very good choice. I would look for a more modern version of the dash mounted primer pump. The originals are still around but they are expensive and offer very little in any available spare parts or seals. Some say you will be beating dead horse with this old issue. But that can be true if you don't start with a modern plan.

Note when you google hand pump dash primers a lot of $6-10 units pop up. These are for very small carb units used on lawn mowers and jet skis. Ideally you want a hand primer dash pump that will force feed about 6 to 8 Oz's of gas to your intake. So stay with an automobile sized unit. All the original army kit parts are still out there on the market for the plumbing. It is just the very odd seals that can't be found.

If your only problem is your fuel bowl dries up when setting for long periods then try just the hand pump lever type fuel pump which allows you to refill the bowl after long setting periods before you procedd with a normal start up.

Folks often wonder why the Army discontinued the dash hand pump's use. I believe it was because of poor maintenance which allowed them to deteriorate and leak a lot. Also many folks just could not get the feel for using the hand primer correctly resulting in both under priming and over priming causing battery abd starter issues so motor pools just took the culprit aways since they could not afford to just get rid of the operators.

Another good place to look for brand new prodution dash primers is in aircraft parts supplier's catalogs. Granted these are priced very high due to the inflation of the last 5 years or so but they can be found used at more reasonable prices and they use very common type "O" ring type seals that are easy to find.

https://www.google.com/search?q=C291502 ... nt=gws-wiz

The reason these are getting so expensive today is most MV's today have fuel injection which again makes priming a very simple task using the already in use components.

[4x4M38]

Thanks Guys,
That helps my recollection. Mechanical fuel pumps put gas in the carb while cranking. If I understand the vacuum fuel pump like the M38’s it won’t move fuel to the carb until sufficient vacuum is created. Does that sound right?

I know the Army removed the dash pumps, but in our common application with not daily driving our vehicles, it does seem like a decent fit.

Does anyone know if the guy selling the cups is still around?

[RonD2]

Assuming pump, carb, timing, etc, etc, is all tuned and in good working order --- it seems to me the problem is gas in the float bowl evaporating between long time starts?

I already have a fuel-vac pump with the priming lever, so guess I'll stick with squirting 2 ounces of gas directly into the float bowl via the vent fitting when necessary.

I use full choke until she coughs once or twice, then choke off, two clicks on the throttle knob and she fires up in short order.

Took a while to figure it out.

MWM sells the full Monte here: https://shop.midwestmilitary.com/produc ... mer-kit-2/
He might have maintenance parts as well.

[JeepdaddyRC]

Well don't I feel stupid. It's been so long since I've driven the M38, that I completely forgot about the hand pump on the fuel/vacuum pump. Scoutpilot rebuilt it so it should work.

RonD2's suggestion for filling the carb bowl thru the fitting also seems like a good work around after longer periods of sitting.

I like Wes's suggestion to use a modern dash primer. Reviewing used ones out of a Cessna. Would this mount like the original military version using copper compression fittings to the fuel line and 2 manifold jets?

[wesk]

Yes to all accept the fittings. I have all the service and parts data for those since I still maintain those on a day to day bases. Let me know what exactly you need. They use modern style round cross section "O"rings as well and my parts lists can cover all you need.

[RonD2]

If we're going to fit old aircraft parts to these old jeeps to get them going, why not a Coffman starter?
You know somebody will do it.

https://www.youtube.com/watch?v=IACjOvyx5hs

[wesk]

The hand primer in that illustration is somewhere between 1976 and 2020 age wise. Not as old as you think the M38 parts are which I have NOS hand primer still in the bag that are dated 1953.

The old Coffman cartridge starter systems used cordyte and other smoke generating explosives to start an engine and the smoke was not worth it.

[JeepdaddyRC]

Just a clarification Wes.
It will “accept” the military copper compression fittings sold by MWM?
Or, yes to all “except” the fittings?
If it does not accept the original military fittings and tubing, what does a Cessna use for fuel lines to the dash mounted primer?