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So I spent the entire long weekend putting my M43 back together again (new exhaust manifold, down pipe, muffler, spark plugs, heat shield, fuel filter and rebuilt fuel pump).
I primed the pump and was pleased to see gas squirt into the filter and not leak from the pump. It fired up after some prolonged cranking (soon to be the topic of another post) and ran great for a few seconds. Then it just sputtered and died. Thinking that maybe the system wasn't fully primed, I used the priming lever again and the same thing happened. It will start, but not stay running. I know I have gas in the tank and spark so my next guess was that I messed something up rebuilding the fuel pump since it seems to just run on whatever fuel I prime into the system.
Any other thoughts? Fuel filter appears clean. Here are 2 images of my setup for reference. Thanks.

You're filling up the carb bowl with the priming lever and, when that's used up, the engine dies. My guess is that you've got an obstructed fuel line. Try blowing it out and see if it helps.

Thanks Lifer,
so wherever the clog may be the force of the priming level is enough to overcome it? I am assuming the clog is somewhere before the pump since all parts from pump to carb are new. Would you disconnect the fuel line at the tank and start there?

Try disconnecting it at the pump and then blowing air back into the tank. It's probably picking up something in there that blocks the pickup tube. When you prime it manually, you don't pump fast enough to suck up whatever's in there, but with the engine running, the pump can easily do it.

Thanks, I'll give that a try

First,
disconnect the f/p line to the carb. Crank the engine (pull plugs if you want to make it spin easier).

Is the f/p pumping? Yes - it is not a fuel supply problem to the carb [check carb for problems/vacuum leaks].

No -
Is the f/p drive excentric worn (lob flat)?

Yes - you need a new cam or weld and machine a new lobe onto cam or install an elect f/p

No -
Is the fuel supply to f/p from tank restricted? [check filter/kinks/rust clogged inlet] or hole in fuel line ruining suction.

No - you missed something and redo this checklist.
Yes - fix restriction and see if have fuel supply to carb. No? redo this list.

Thanks Drew,
I hope to find the time to run all these tests this weekend. What's odd is that the truck ran fine last August before the manifold cracked. It's been sitting since then. I had no idea how much gas was in the tank so I added 5 gallons and some MMO. Perhaps that stirred up some junk from the tank that's clogging the line. Since I replaced so many things at once, it's a bit harder to track down the real culprit.

monkeymissile wrote:So I spent the entire long weekend putting my M43 back together again (new exhaust manifold, down pipe, muffler, spark plugs, heat shield, fuel filter and rebuilt fuel pump).
I primed the pump and was pleased to see gas squirt into the filter and not leak from the pump. It fired up after some prolonged cranking (soon to be the topic of another post) and ran great for a few seconds. Then it just sputtered and died. Thinking that maybe the system wasn't fully primed, I used the priming lever again and the same thing happened. It will start, but not stay running. I know I have gas in the tank and spark so my next guess was that I messed something up rebuilding the fuel pump since it seems to just run on whatever fuel I prime into the system.
Any other thoughts? Fuel filter appears clean. Here are 2 images of my setup for reference. Thanks.

Check this out. I'll bet this is your problem; on the new diaphram used when you rebuilt your pump, the metal backup plate next to both sides of the flexible diaphram is too large to fit into the pump body cavity like it should. This keeps the diaphram from getting a full stroke when the pump lever is operated by the cam lobe. It will get enough stroke to pump when using the primer lever, however the stroke from the mechanical workings is slightly shorter than the primer stroke, therefore no pumping. Pull your pump back down far enough to measure the diameter of your diaphram back up plates & compare the measurement to the plate on the old diaphram. Odds are very good that the new diaphram plate diameter is around 1/8" wider than the old one, if so you've found your trouble. One supplier of pump rebuild kits has been supplying diaphrams like this lately, he has been made aware they will not work, but seems he has a hearing problem.

Ooooh! Another gem of wisdom for me to add to my notebook. Thanks, Charles! The possibility would not have occurred to me.

Check this out. I'll bet this is your problem ...

Charles: That exact thing happened to me. Drove me mad for an entire evening trying to figure it out. The 1/8" oversize measurement is spot on. Took me awhile to convince the supplier that the plate was too big, but I eventually got a replacement. I'm with you, I bet it's MM's issue too ... Same symptoms.

--Steve

Thanks Charles,
I will definitely check that out this weekend. I got the rebuild kit from Tom at Then+Now. I will call him today to see if he's aware of this problem. That makes sense since everything was working before and I do recall one of the diaphragms feeling a bit tight. I never would've thought to check the diameter of the backing plate. Learn something new every day!

Oh no.... I just had 2 pumps rebuilt by T&N.... we have the fuel tank pulled and can't test run. Do you think this is a problem with T&N?

More likely a problem with their suppliers that they may need to address. I'm sure that, when made aware of it, they'll fix the problem for their customers.

Hello Everyone, and I just installed a rebuild pump from BIg John Midwest, and had no such trouble, thank God. I did have a clogged gas line, and just went ahead and replaced it from tank fitting to pump. I also changed the in tank filter, and however, confirmed the new fuel pump performance by installing a pressure guage between the pump outlet and the carb.( permanently ) I now can readily observe the pump performance at 4.5 PSIG .

peter e mark wrote: I also changed the in tank filter, and however, confirmed the new fuel pump performance by installing a pressure guage between the pump outlet and the carb.( permanently ) I now can readily observe the pump performance at 4.5 PSIG .

That sounds nifty, can you post a pic of the gauge and setup?