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problem after warmed up...


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My '71 521 stalls after I drive it for a while and is warmed up. I press the gas pedal and just starts to die...very dangerous! I was thinking it was my fuel filter being clogged...I changed it a few times...then it would run fine for me to get home...but, I think it started running fine cause it had time to cool down as I installed the new filter. (: Any ideas on this? thanks much!

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Lets start with this.

Do a tune up. These trucks recommended a basic tune up every 3000 miles, including adjusting the brakes. check point gap, timing, adjust the carb, check the plugs, and wires.

Every 12,000 miles, in addition, adjust the valves, check a whole bunch of other stuff.


Here is a handy picture of the inside of a 521 gas tank.


The pipe you see hanging down is the fuel pick up. notice it has no filter, and the crud in the tank, if it does not go into the pipe, can plug the end of it. If you stop the truck, and the fuel pump quits drawing a suction on the fuel pipe, the crud drops away from the pipe, and the truck runs again.


You should have about 2.5 to 3.5 PSI fuel pressure.

The fuel pump should flow about 1 liter of fuel in one minute, at 1000 engine RPM.


Does your truck have the evaporative emission controls on the gas tank? These trucks are easily identified by an extra cover in the bed, ahead of the left rear wheel well. These trucks vent the gas tank back through the air filter, through a valve or two. If you suspect the vent system is clogged on these trucks, remove the gas cap immediately after the engine stalls, or runs poorly, and see if there is a vacuum in the gas tank.

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basic tuneup is the foundation of goog running. You don't need to replace a single part, just adjust and inspect.


For the stalling problem, since it improves for a while once you change the filter then it is likely you have some crud inside the carburetor floating around near the jets. Just remove the float bowl cover and let the dirty fuel run out (cold engine). Be careful not to loose the little spacer when you take the cover off. Once the fuel comes out, put the cover back on and drive it.

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Cold engine runs fine... warm engine runs poor off idle:


When cold, the choke is on and motor running rich. If the motor is using more gas when cold, how can a clogged filter or poor fuel delivery to the carb be the problem?


The real problem is after warm up when the choke is off and the motor has to run on only the gas through the primary jet. (idle has it's own fuel circuit and adjustable 'jet' so it will idle fine) I would look at the primary jet function, possibly a small puddle of water in the float chamber or sediment. Perhaps a small blockage in the jet itself. There could be dried/gelled gas in the primary air bleeds or the jet if this has been sitting.


I don't know which is harder getting the damn Hitachi (I assume a stock carb???) off to get at the external plugs that allow access to the jets or making a short 12mm spanner just so you can remove them with the carb on the motor. I've done both but you can remove the jets and clean them. I used carb cleaner with a spray straw to blast out the passages too. You will need a stubby screwdriver with the shoulders ground down to get up in the hole and unscrew the jet. By all means remove the front glass cover and the float assy and clean the chamber out.

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Thanks much for all the replies and helping me figure this out. Sorry, I forgot to mention I have a new weber 32/36...not the stock Hitachi.

I also restored the gas tank...so, there is no sediment in there. New fuel pump too. I will check out the jets on the weber.

Yeah, the clogged filter or clogged anything isn't making sense to me since it runs just fine for the first 5-10? minutes of driving. Thanks again!

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Check the fuel screen in the weber, everyone can think I am crazy, but when a engine is running there is a constant flow going into the carb bowl, if there is any crap in that carb screen area, it gets caught in the screen until the screen is plugged, then the engine dies, when there is no flow into the bowl as the engine is not running, sometimes the crap falls off the screen and settles to the bottom, then when you start the engine, it can run again till the screen plugs again. The same thing can happen to the pickup tube in the fuel tank, rust chips plug the pickup tube, when the engine dies, the pump quits pulling, the gas in the tube back flows clearing the end of the tube, and then it will run again till the rust plugs the tube again. Call me crazy, but both of these things have happened to me.

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Good point wayno. I had a similar problem when hitting a large bump the motor would run like crap until I shut it off and count to ten then fine. It was a flake of something in the primary jet that would get sucked up and block it. Shut off and it would settle. Cleaned it out and was fine.

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