calling all engine gurus

[Another Ahab]

If your oil level is high then you have gas contamination. (Or you have made a way to create oil.. . . .)

 

[T. Highway]

Just saw this, here are some ideas.

Poor idle due to a Rich Mixture: Engine rolls or gallops

a) carburetor idle is improperly adjusted. (have you had a vacuum gauge on the engine?)

b) carbon deposit in bore near idle port. (scraping or sanding will remove)

c) clogged air bleeds. (clean bleeds and passages with solvent)

d) metering hole in low-speed jet or idle orifice tube is oversize. (since you stated this ran fine before I would doubt this is the problem, but if it was partially plugged prior and now the obstruction has flushed itself out I would still check with the shank of a wire sized drill)

You did mention that it runs fine at higher RPM, please see below:

a) did you notice black smoke while revving the engine?

b) have you checked your fuel pressure at idle?

c) if you partially close the choke at idle will it run rougher or smooth out?

d) at a higher sustained RPM do you notice the RPMs rolling up and down?

Bert

 

[Squirt-Truck]

Any update?

 

[Another Ahab]

Pretty amazing thread, because packed in it here (very tight) is:

- About everything possible to know about Carburetors and their "habits"

Thread title should have a sub-heading "Carburetors" for a cross-reference to help the Searchers. Excellent wisdom right here. It's a gold mine.

 

[dougimes]

Update,

The float is brass

I checked the oil level, there is no indication of significant gas contamination. I can't really judge it by level, so if you cold tell by texture, they I can't tell for sure. The level was not high.

I checked the carb cover for flatness. I dragged it across a piece fine emery cloth and saw uniform scuffs on all mating surfaces. The carb itself is harder to check. I need another look at it with a more telling straight edge. I am going to try doubling up the gasket. I would think that leaks here would cause too lean, however.

I will check vacuum. 16 to 20 you say.

Carb passages are clean by now, I have been over it a few times. carbon cleaned off as well.


No black smoke when reving
No rolling or galloping when at speed
Closing choke partially causes rougher run, misses badly


Help me understand the function of these two components. 1. There is a smaller brass piston that is held down by vacuum. When down it lowers a taper pin into an orifice.


2. The larger piston that has a rubber cup, This I have always understood as the accelerator pump. On this carb it works off of loss of vacuum as was mentioned I believe.




another Ahab- can I add carbs to the topic?

 

[oboyjohn]

Greetings from Canada. Back in the mid '80's we replaced a damaged gasser engine out of one of our old 5 ton wreckers. The rebuilt engine was from Contenental and the tags said everything was new. Once installed, it ran good, but not like it was supposed to. We tried everything to make it run well and searched hi and low for the problem. We finally found the culprit. The Cam shaft had been replaced, but not the gear on the distributor shaft. It was worn just enough to run but not like it should. And under some conditions, like coming off throttle, it would be fine. But under load, it was running like garbage. My 2 cents gentlemen.

 

[Another Ahab]

Gold Mine

Greetings from Canada. Back in the mid '80's we replaced a damaged gasser engine out of one of our old 5 ton wreckers. The rebuilt engine was from Contenental and the tags said everything was new. Once installed, it ran good, but not like it was supposed to. We tried everything to make it run well and searched hi and low for the problem. We finally found the culprit. The Cam shaft had been replaced, but not the gear on the distributor shaft. It was worn just enough to run but not like it should. And under some conditions, like coming off throttle, it would be fine. But under load, it was running like garbage. My 2 cents gentlemen.

What's great about a good solid thread like this, is that when 50 different people kick-in their own hard-earned 2 cents:

- BINGO!!

- Right there, in your hand, all-of-a-sudden is one whole fat Dollar, you know?!

 

[Squirt-Truck]

The smaller piston is the power enrichment circuit, sometimes called the "power valve". Purpose is to add more fuel under lower vacuum such as during a heavy pull or accelerating. If this sticks the mixture will be to rich. Again, need the vacuum readings at idle.

The larger is the accelerator pump. On sudden throttle opening it moves adding a shot of fuel to enrich the mixture to attempt to eliminate stumble.

Have you checked timing? Excess timing (advance) will make for a poor idle. The engine will run will way more timing than it will idle with. (Need vacuum reading)

Last, item, have you checked the distributor cap for cracks?

 

[FMJ]

Greetings from Canada. Back in the mid '80's we replaced a damaged gasser engine out of one of our old 5 ton wreckers. The rebuilt engine was from Contenental and the tags said everything was new. Once installed, it ran good, but not like it was supposed to. We tried everything to make it run well and searched hi and low for the problem. We finally found the culprit. The Cam shaft had been replaced, but not the gear on the distributor shaft. It was worn just enough to run but not like it should. And under some conditions, like coming off throttle, it would be fine. But under load, it was running like garbage. My 2 cents gentlemen.

I was thinking the same thing, jumped time. How many miles on the motor?

 

[Squirt-Truck]

Any update. Status, closure, us nosy people want to know.

 

[frank8003]

If it has one then put a new distributor cap on it and rotor in it
and problem goes away if the little wire didn't burn off. If it has
that and wires to plugs then a HT wire cooked off. Common.
When really dark look in there for the light show.

 

[dougimes]

Sorry for the long delay, it took Thanksgiving to open a few days to work on things. youll notice I have my Christmas Decorations up, in an MV sort of way.

OK- Vacuum is 16-18, looks good.

I have noticed something that I didn't really see before- seems to run a little smoother at after it has been running for a minute, but before it is really up to operating temp.

So I thing we are where we were on page 1 I think, either missing because of ignition- I will change the cap and rotor. or running a trifle rich. The idle screw is almost all the way in (1/8 turn) for best performance.
Any ideas how to lean out the idle if I want to isolate this problem?

I checked timing with a light, 5btdc, the valves could be off a tooth I suppose. Would it run reasonable well with the valves off a tooth, or would I really notice something.- remember, it runs well at speed, and rolls/gallops/misses a little at idle.

 

[ranger.41]

Dougimes,
If your truck was running good, then you filled up. How is your fuel system? Clean inline filter? How is you fuel tank, any rust?
You did not mention rebuilding the carb. If you even suspect that you have contaminated fuel, you should check the carb again. If your going to go throught the trouble of removing the carb you may as well rebuild it The ignition tuneup will do it good too TM 9-1826A/TO 19-7CCA-6 Ordnance Maint Carburetors (Carter) . This is a very helpful booklet. It was put out by Portrayal Press.
Is you intake manifold manifold heat control valve set on warm weather?
.

 

[Gamagoat1]

What is your fuel pressure? These carbs are very picky about pressure. I Know a guy that thinks he's a mechanic and puts an electric fuel pump on everything. Filled his M37 engine full of fuel. They are designed for about 5lbs. of fuel pressure. More than that will actually open the fuel valve against float pressure and try to flood. At idle will run rich!! Check your fuel pressure.

 

[dougimes]

so here is the conclusion to what turned into a nice discussion

The problem that prompted all of this- The truck was running fine, and then developed a rough idle, not seemingly corresponding to anything, but came on rather suddenly. At speed and under load, it seemed about like normal.

Following the old rule, check ignition, before you start looking at carburation, check ignition and find out what you missed.

Checked,
Timing 4btdc
new points, condenser, Point Gap 20 thou
Plugs gap 15 thou
voltage at coil term while running (way of determining if ignition switch is bad, high restistance= low volts at coil pos term)
Checked coil by looking at spark while running- easily jumped 30thou

Checked to see if issue was related to a certain cyl. lifted 1 plug wire at a time and listened for no difference.
Did not change cap, but was about to.


None of these investigations fixed the problem, but together made the truck run slightly better


Now looked at fuel, try and testing each in succession

Tried different Gas
tightened carb to manifold
pulled carb, cleaned it
Checked fuel pressure 2-3 psi
Pulled carb, rebuilt it
Pulled carb, double checked all of the idle passages. cleaned better. Checked rebuild.
Checked bowl height

None of these fixed the problem

Put a different carb on , set bowl float to shut off fuel flow at 2mm (5/64) set VERY carefully.

Started it up, ran perfect.


Something went wrong with the original carb that persisted after several cleanings, a rebuild and very careful inspection. Could be a crack or leak inside, a passage that somehow changed, a faulty rebuild kit,


So there you have it.

 

[Storm 51]

Thanks, that is a handy little procedural check list to have.

Hope I never need to use it, but I suspect I probably will.

 

[dougimes]

I couldn't let it go. I studied the carb that doesn't work. I looked at the carb manual to see what could possibly make it run rich at idle. I came up with a theory- what if the vacuum passage for the accel pump that passes near the idle passage was drawing fuel from the idle passage into the vacuum passage past the bowl cover gasket.

dressed the top of the bowl with a file, carefully. checked for flatness on the bowl cover. installed a new gasket. thought for sure I was on to it.

Reinstalled the "bad" carb. Started it up, still ran rich at idle. I pulled the bowl cover off while the truck was running. The truck still kept running.

Does anyone have any idea on how gas is getting to the intake with the bowl cover off? The idle passage is completed by the cover pressing the gasket over the bow. I am mystified.

 

[gerrykan]

I'm not that familiar with the M37 Carburetor, but maybe the casting is flawed, allowing fuel to pass through a crack, or porous spot.

 

[T. Highway]

I couldn't let it go. I studied the carb that doesn't work. I looked at the carb manual to see what could possibly make it run rich at idle. I came up with a theory- what if the vacuum passage for the accel pump that passes near the idle passage was drawing fuel from the idle passage into the vacuum passage past the bowl cover gasket.

dressed the top of the bowl with a file, carefully. checked for flatness on the bowl cover. installed a new gasket. thought for sure I was on to it.

Reinstalled the "bad" carb. Started it up, still ran rich at idle. I pulled the bowl cover off while the truck was running. The truck still kept running.

Does anyone have any idea on how gas is getting to the intake with the bowl cover off? The idle passage is completed by the cover pressing the gasket over the bow. I am mystified.

The only other way that I can see fuel getting in and the truck still runs with the bowl cover off is:

Pump Circuit on ETW1 and B&B have similar circuits for this function.
TM9-1826A, Page 157, Figure 84 shows the circuit.

I would double check the discharge passage check ball and spring. (NOTE: small end of spring must be in the down position) I know this passage is tiny, but can you see if there is any peening off center from the old check ball, or an out of round new check ball? (Roll the check ball on a flat PC of glass to see if there is wobble or skipping)

I wouldn't completely rule out a crack in the casting, but all you did is fuel the truck up and the problem started. I would venture to guess something flushed out a gummy deposit in this pump circuit and exposed some wear in the carb body.

This is purely a theory based on staring at the TM for an hour while I was waiting for paint to dry.

Bert

 

[Fredo Corleone]

I would try a laser temp gun on each one of the runners on the exhaust manifold. This will tell you if you have a bad plug, wire or cap. Also the manifolds crack.