Lack of Power Under Load

[xd2200]

I have been battling this problem for over a year now. It first manifested itself during July 4th parade last year. When going down the road I have no power. I have trouble starting off and it will only reach about 15MPH. When I rev the engine in the yard it sounds fine. I have installed new plugs. Also, I have checked the compression, fuel pump pressure, vacuum, points, condenser coil, distributor cap and plug wires. I was thinking that the gas had gone bad but I drained and re-filled and no improvement. I had the carb rebuilt by Treadwell - no change. I removed the oil pan and cylinder head and did a visual inspection - no issues seen. This lack of power seems to get worse as the engine warms up. My M37 is all stock except the steel fuel line has been replaced withe rubber and I installed a small gas filter just before the carb. Both of these mods were made several years ago. I would also note that the plugs get coated with black soot. Any thoughts on how to remedy this malady would be greatly appreciated.

[ashyers]

xd220,
Did the problem appear instantly at the parade or did it come on gradually over a period of time?
Is the engine running smooth and lacking power or does it missfire?
Will it pull to the governor with no load?
What was the compression?
What's the vacuum at idle and on the governor?
Once it's warm does the exhaust stink of fuel?

This may help in the diagnosis .

Andy

[xd2200]

Did the problem appear instantly at the parade or did it come on gradually over a period of time? Came on instantly
Is the engine running smooth and lacking power or does it missfire? Not smooth but not really a misfire
Will it pull to the governor with no load? Yes
What was the compression? As I recall all cylinders were around 90psi
What's the vacuum at idle and on the governor? When you say "on the governor" do you mean at high RPM?
Once it's warm does the exhaust stink of fuel? No

One other symptom comes to mind. When attempting to idle will stall out unless I pull the throttle out a little.

THANKS

[ashyers]

xd220,
As far as the vacuum questions, get a reading at idle then when the truck is run up the governed setting with no load.

You compression is not spectacular, but assuming it's even across the cylinders it's workable.

Though I doubt this is you power problem, you may have a vacuum leak that's affecting the idle. Grab some carb. cleaner and have a search for the leak by spraying possible leakage paths.

As far as the power issue the fact that the problem came on instantly makes me think ignition, as do the sooty plugs. They may be caused by a weak spark. My quickie method of checking spark with the factory ignition cables is to pull the eraser head off a pencil and drop it dull side down into the plug. Your ignition should have no problem sending a nice spark at least 1/4" from the lead to the tip of the pencil. If it's not capable of this you're going to have issues (Ask me how I know...). If the spark proves weak on all cylinders I'd try swapping the condenser. I've had issues with new ones, as have some of my friends with older points ignitions. I've learned not to toss the old ones, they have saved my butt on a few occasions.

I'm curious to hear what you find.

Good Luck!

Andy

[xd2200]

Andy - THANKS for your responses.

I like your idea of a weak spark - I will try the pencil trick. Is there any way to check the coil as well?

John

[Kaegi]

I second the spark check. and new condesners are junk. always buy NOS or an Accel brand performance condenser. Also check Dwell with dwell meter. the low cost investment that when used results in optimum performance and point life.
90 psi is getting to time for rebuild level. so oil additives might help bring that number back up.

[ashyers]

John,
See if the spark is weak. Once you have an idea of where you're at in that department you can start looking at the individual stuff. The pencil test is cheap and easy. It will help to get you pointed in the right direction without spending a bunch of cash and shotgunning parts. I spent a bunch of time chasing an intermittent miss twice on my truck and it can be frustrating to say the least. At least you don't have an intermittent problem !

Andy

[xd2200]

I performed the pencil test and the spark is weak.
I guess this narrows search down to condenser, points, cap and coil?
I had previously installed new condenser, points and cap but got no improvement.
Is there any way to test the coil!

[just me]

An old automotive analyzer like the Sears Penske unit could test your coil and condenser. (And a ton of other stuff)
I still have 2 of them in the shop.
An oscilloscope is the absolute best for diagnosing ign troubles. I no longer have my Sun unit, but I can look at the signals two at a time with my handheld scope..can see everything happening on both primary and secondary sides.

[ashyers]

John,
Being "cost conscious" (cheap?!) I save old parts, hopefully you do too. Assuming you don't have any diagnostic equipment, I'd swap the condenser and give things a try. You could check primary and secondary coil resistance of the coil while your at it and verify the point gap. I usually pluck the dizzy to do this, it's faster and easier.

Does your truck still have the 24v waterproof ignition system? Though this system is pretty slick, it does make diagnosis a pain without some adapters.

Andy

[xd2200]

Andy,

Yes, I still have original 24v waterproof system. Doing any testing/repair with this system is a challenge.

I had previously replaced the condenser, points, rotor & cap with no resultant change.

I found an old thread (2013) on this forum that discusses coil testing. It shows that the primary resistance should be 6 ohms and the secondary 14,000-20,000 ohms. I tested my coil and two other takeouts that I had on my shelve. All three coils had 6.2 ohms on the primary. The three secondary readings were; 10,780, 10,220, and 11,550 ohms. Not sure what to conclude from these readings?

I also found on another forum a reference to the filter at the bottom of the distributor (i.e., where the 24v feed enters.) It suggests that a bad filter could result in poor coil performance. I think that I will take a look at this when I return next week from vacation (MVPA Convention in York PA.)

[NAM VET]

I really admire those of you who can trouble-shoot auto electrical systems. I have never been able to understand how an electron can go to some light or switch, and then with a common ground, find its way back precisely to where it needs to travel to complete a circuit. We just finished painting a rec room and a hall, and I have to study carefully how to put switch plates back on, and not drop the tiny screws down forever into a floor vent. Now, to get back on that ladder and paint the Bath room.

NV

[vit16]

Hello XD! how did you solve the issue?
I had a blown condencer because it was 12v and I connected everything to 24v. It had a dedicate 12v installation.
I am reading 100 v with my multimeter at the batteries so I will diagnose it better.

Nan Vet, its as simple as the water going ´from a higher to a lower place!!! hehehe

[QCIM37CDN]

Hello XD! how did you solve the issue?
I had a blown condencer because it was 12v and I connected everything to 24v. It had a dedicate 12v installation.
I am reading 100 v with my multimeter at the batteries so I will diagnose it better.

Nan Vet, its as simple as the water going ´from a higher to a lower place!!! hehehe

Condensers aren't voltage specific, the 6 volt Power Wagon uses the same one as the 24V M37. Almost guarantee the OP's problem is the coil. M series coils will continue to run for a long time after they fail, they will even start right up and often produce great spark when cold, then as soon as you hit the road, the coil warms up, and the truck has no power. Not enough spark is produced to burn all the fuel under load, so the plugs foul. Intermittent miss could be from the filter where the power wire goes into the distributor.

[PoW]

This^^

PoW