Running lean under load only

Hi everyone. Long time lurker, first time posting.
I have had my M715 for a few years now yet rarely drive it. This summer I decided to do something about it. But each time I drive it the engine burns lean, starts to backfire, miss, and even dies. I'm sure this is more a general engine issue rather than specific to our trucks but since this is the only carb engine I own I am soliciting here.

Problem solving steps I've taken:
Electric fuel pump installed on frame near tank. Cleaned tank (professionally), rebuilt carb (professionally). I've verified that the electric fuel pump is supplying sufficiently, and now that I've had the carb rebuilt I am at a loss.
Engine runs fine, rev's fine in neutral, but within 2 blocks of driving... goes lean and dies.

I'm out of ideas on what to check?
Thanks for anyone's random thoughts.

Quote:

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Originally Posted by kurtz075

Hi everyone. Long time lurker, first time posting.
I have had my M715 for a few years now yet rarely drive it. This summer I decided to do something about it. But each time I drive it the engine burns lean, starts to backfire, miss, and even dies. I'm sure this is more a general engine issue rather than specific to our trucks but since this is the only carb engine I own I am soliciting here.

Problem solving steps I've taken:
Electric fuel pump installed on frame near tank. Cleaned tank (professionally), rebuilt carb (professionally). I've verified that the electric fuel pump is supplying sufficiently, and now that I've had the carb rebuilt I am at a loss.
Engine runs fine, rev's fine in neutral, but within 2 blocks of driving... goes lean and dies.

I'm out of ideas on what to check?
Thanks for anyone's random thoughts.

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Timing set to spec? When the fuel tank was cleaned, was the screen on the fuel pickup cleaned/replaced? Mine was completely packed off, so we removed it, and it sounds like your truck might be fuel starving under load.

Yes, certainly fuel starving.
The pickup screen did come clean. When I had it cleaned I was convinced it would need a new pickup solution, the varnish was so heavy it appeared to be rust, but after they boiled it it was amazingly clean.

I can run the fuel delivery system separate of the carb and it seems to be pushing adequate quantity. I haven't exactly taken a volume over time measurement (just ran it into a can to validate that a lot is coming out) so maybe I'm wrong. In my head I'm thinking that a 2 barrel carb won't be consuming much fuel but maybe that is in my perception that my knowledge is lacking?

What changes in the dynamic of the carb for operating under load vs idling/reving with no load? I can run/rev just fine in the driveway. It is most frustrating to drive it around the block to test, just to have it choke and fail, and have to tow/push it back to the house.

Is tank vent clear,try driving with cap loose or off.

Is it possible that the power valve is not functioning as designed? There i also the economizer.

The tank should be vented via a piping system which works its way back to the atmosphere portion of the air filter (speaking about he first generation air filter, 1967-68 trucks with no governor and a carburetor mounted air filter). But considering all of this, is it possible to feed fuel from a Jerry Can to the fuel pump? What I did before I got a new stainless steel tank was to use a Jerry Can in the can holder next to the driver door. That way, I knew i had adequate fuel available.

When I got my truck, there was no carburetor on it. The owner handed me a carburetor. I rebuilt it but the truck stumbled and never ran well. I got another carburetor from a Zoner and rebuilt it. It was a known military carburetor. Note: the jet numbers were different. I suspect it was actually a Chrysler slant-six version that I received with my truck.

Just a thought.

Well there are a few things to consider that I didn't before so many thanks!

I have a sticking solenoid on the starter so I have to sort that out first but I'm going to stay after it.