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Hello all, hoping to find some help diagnosing the problem with my ETW1 carb.

BLUF: Possible air leak or foreign material in the high speed circuit of my carb based on stumbling of the engine. I need some help determining the next step before tearing things apart.

I took the truck for it's longest excursion to date, almost 40 miles. About 35 miles in the truck started stumbling. Idle was fine, but when I gave it throttle, it would begin to stumble.
Pulled over, turned off ignition, started back up no problem, but still stumbling. My buddy towed me to his place and I left the engine running during the tow, pulled the choke out farther thinking I was getting an air leak and it smoothed out to where it was running almost like it should. I could rev up no problems while in neutral. It should be noted that earlier in the day after changing the coolant I ran the truck for about 20 minutes to get gas and drop something off at my neighbor's house and had no issues.

We get to my buddy's house and start spraying starter fluid around the carb to try to find an air leak (the engine has gone back to stumbling at this point) and other than around the mix adjustment screw come up with nothing. Three weeks ago I put in a stainless steel gas tank from VPW, left in the original fuel lines (installed by VPW in the early 90's?), but put in place two fuel filters - one just after the tank, and one just before the fuel pump. Fuel pump is brand new from VPW, the gas in its bowl looks clear with no obvious foreign material.

Two weeks ago during some 40 degree F weather I drove about 17 miles up to an Honor Flight event with no problems at all. Truck was strong the entire trip although I do have to run with the choke slightly pulled out when I get up to operating temperature (this tells me there IS an air leak somewhere which may or may not be related to the current problem). Because this weekend was in the 60's I'm thinking that the engine compartment may have warmed up more than the last trip causing new parts to expand farther than before which caused an air leak. The other possibility is foreign material in the carb from using my old fuel lines, but because I haven't had this problem until just now (and have run the truck here and there many times since installing the new tank) and the gas in the fuel pump is crystal clear it seems unlikely.

I'm hauling the truck back to home base this evening and will start trying to diagnose the problem more thoroughly, but I want to avoid tearing things apart before I've really diagnosed the problem. Hopefully one of you carburetor masters can help me along the path.

Definitely sounds like a vacuum leak somewhere but you are not doing yourself or the fuel pump any favors trying to pull gas through two filters. That pump, assuming you are using the stock pump, was designed to pull freely from the tank through nothing more than a sock. You could be sucking air in around the pump diaphragm and have a resultant low float level. It is best to run the sock in the tank and any filters above the fuel pump. The exception to this rule is if you are running a gear type electric pump but that would be rare on a M-37 and would require a regulator for the pressure. rotary vane p[umps will pass fine dirt with little harm to the pump and if the pump is lower than the fuel level then a prefilter can be run on them. If you really feel strongly about filtering the gas before the mechanical pump then one filter is more than enough.

I've found the problem, but I'll go through the steps I used to diagnose it in case that helps someone else in the future.

1. I used a vacuum gauge to confirm my suspicion of a vacuum leak. Basically, a "good" engine should be pulling between 17 and 22 inches of mercury (inHg) at sea level. You hook this thing up to your intake manifold, start the engine and let it warm up to operating temperature before you take any readings. I have vacuum gauge from Harbor Freight. It works and that's all you really need.

You can see from my picture below that my engine is on the low side of "good" but the needle was drifting between 15 at a low and about 18 at a high so I've definitely got some kind of carburetor problem and my ignition is probably a little retarded.


2. Having a better idea of where to start, I whipped out this contraption a buddy of mine made to help him find vacuum leaks in boost systems of Subarus. I think we used it on a Porsche 944 Turbo one time, too. The way this works is you force air through the engine and spray water and dish soap at any potential air leak spots and look for bubbles. It was made out of "found" materials, but I suppose you could build one for a reasonable price pretty easily. I used 5 psi because you don't need much and you don't want to risk damaging your engine.


3. Through this method I found the culprit quickly. I was pleased that it's not the intake manifold because I really didn't want to have to replace it.


4. This little bugger looked fine and the vent line snugged up so the "wrench-feel" was that it was biting, but you can see the threads are busted.


I imagine that the tail on the end of this thing is shaped that way to create a vacuum as air passes by it, thus drawing air out of the distributor venting system. I'll have to make a new tail for it out of brass and solder it all together. I tried heating up this piece to salvage the tail, but it must be brazed or otherwise bonded together.

Anyway, this was my air leak. For now. I'll repair it this weekend and see how it runs.

ez8,
I'm sorry to rain on your parade, but that dizzy vent us upstream of the throttle. Though the fitting may be leaking you will not see manifold vacuum there and it won't effect your readings. Check your PCV fitting though, I did have one of those split at the brass adapter and create a leak that was a bear to find. Good luck on your search, I suspect you'll sort it out this weekend .

Andy

Andy, responses like yours are why I post here. It's a good check against my own enthusiasm. You're right, it's upstream of the throttle so it wouldn't matter. I'll need to fix it to help the vent circuit anyway, but it's not going to solve my problem. I'll check out that PCV valve, though. Thanks for the tip!

I went out and did a check again on bubbles south of the throttle and the only thing I came up with the was mix screw itself was leaking air like crazy. As a temporary counter-measure I put some thread tape around it, but I'm still getting some stumbling / misfires at high rpm under load. I'm thinking that my timing needs to be adjusted so I'll do that this coming week and see how she goes.

If adjusting the timing doesn't cure the problem, your ignitor might still be the cause. If the bushings and/or shaft in the distributor housing are worn, the shaft will wobble as it rotates, affecting the timing and dwell. There's a procedure in one of the manuals (I think it's the Autolite TM 9-1825B) for checking the bushing to shaft clearance. There are other wear points in the ignitor (advance weight pivots, etc.) but I'd start with the shaft bushings, as lubrication of the top one was often neglected.

I just replaced the bushings in one of these ignitors, and it doesn't take much bushing slop to exceed the spec.

Also replace your condenser for the points, that will give that very symptom. As will pitted or badly adjusted points. Be sure to set the points before setting the timing.

I finally had an afternoon to work on the truck and I believe the final issue had to do with timing.

After reading about the Pertronix MV-161 electronic ignition upgrade, it's something that I've wanted to do since I got the truck and I figured if I was going to play with the ignitor, I might as well go whole hog and replace the innards at the same time (I had previously replaced the capacitor and points). I installed the new, electronic ignition system, retimed, and have had no more burps, gurgles, or farts. Full power through all gears and I'm happy.

I will say that if you have thought about installing the MV-161 unit in your truck, it is so choice. If you have the means I highly recommend picking one up.