M38a1 idle off throttle stall

RichJohnson · Post by **RichJohnson** » Sat Oct 30, 2010 5:45 pm

Ok so here is my problem.
M38a1 stock set up, except for a fuel filter right before the carb.
My problem is when I throttle up the engine, parked or driving, when I completely let go of the throttle and let it return to idle, the engine speed drops below idle speed, and either
1 stalls out
2 fires very slow almost stalling untill it slowly returns to idle speed

While driving if I flutter my gas pedal when I stop I can keep it alive and running indefineatly, other wise Im in danger of stalling.

I have gone all through the carb, its a midwest mil US kit from a couple years ago. All seems ok, I have blow ports clear with Cleaner on a straw, and reset my float, which may have been just a tad low. I also went around to all vacume fittings and sprayed starter fluid looking for a possible leak. No reving with the spray so I dont think there are any vacume leaks.

I am at a total loss as to what to do next. I was told by some old guys that it sounded like my float was too low and the carb was flooding the engine when I backed off the demand for fuel, which makes sense, but the float is now reset.

Post by **wesk** » Sat Oct 30, 2010 8:22 pm

Start with the easy things. Vent system. Bowl vent from top of carb bowl to the cross over air tube. A slow responding metering rod? Is your metering rod diaprahgm spring on the correct side of the diaphragm? The spring location in the carb TM9-1826A is incorrect.

RichJohnson · Post by **RichJohnson** » Sat Oct 30, 2010 10:58 pm

I assume by bowl vent you mean the external steel line from the carb to crossover, do you mean just to check and make sure its clear?

Youve got me on the metering rod. Is that the highspeed circut rod that is part of the diaphram assy?
The spring goes between the external housing cover and the diaphram right? (I dont think it could fit on the wrong side of the diaphram)
I will have to go look at the carb manual I have, see if its that TM# and look at the spring location on the exploded view. I just have had it apart so many times, I kinda know where it goes automatically.

Post by **wesk** » Sun Oct 31, 2010 12:24 am

Yes the vent from the bowl to the tube and the fittings at each end.

Yes the spring has been put on the wrong side using that drawing.

This is my corrected image. Note that the springs also differ in length and wire thickness between the accelerator pump and the metering rod diaphragms.

If you compare the original figure 60 in your TM you'll see that item BB was in the wrong place.

Another problem found sometimes is the direction the ram tube inside the crossover tube is facing. The one you fasten the bowl vent to must not have a 90 degree ram air tube that faces the carb. This can tend to suck on the bowel causing a negative pressure and reduced fuel flow.