Howdy,

I just picked up this gem with 46k original miles. It's an RV, nothing special. It sat for about 5 years, it seems.

Fuel is fresh, spark is there, timing is set. I'm not that knowledgable on carburetors.

The truck will fire up with the air cleaner off, some starting fluid, and constant pulsing of the throttle. After 5 minutes or so it will idle on it's own. What's odd is that if I simply hold the throttle at 25% after it's running, it will rev down and die, same at any throttle position. I have to constantly pulse the throttle to keep it running.

It does this even when it is warmed up. I have been able to take it down the road, and when driving it seems to have that same pulsing lack of power, won't go over 20 MPH an it wants to die with more throttle.

From what I have read, this could be an "accelerator pump"? Is there anything else I should be looking for?

Internet is tryign to sell me an accelerator pump that is square shaped, with 4 screws. What is on the carb is round with a triangular screw pattern. Does anyone have the correct Toyota or Aisin part number?

Thanks in advance

 

No one remembers how these work? LOL

I rebuilt the carb, rerouted some hoses that appeared to be wrong per the diagram.

Now she starts and idles great.

Still losing power with throttle. Engine dies.

Is there a process of priming the accelerator pump? I cannot see fuel squirting out where it should be.

 

 

From your "thumbs-up" post 10 days later I assume you fixed the problem? Even if you did fix it, I'm going to post some info here on the 20R carb for anyone else that might come along & need it.

On the 20R carb there are 2 different accelerator pumps, a main pump and an auxiliary one. The round/triangular one is the auxiliary accelerator pump (also known as the AAP, see diagram) . That one only works on cold starts & warm up, as long as your thermo vacuum sensing valve (marked TVSV - see diagram below) is working correctly that is. It's a part of the original emissions system, which may or may not be working correctly after all these years. It's original purpose was to allow the carb to draw a little extra fuel when cold & while warming up to allow faster warm-ups & reduce emissions. It monitors coolant temp & control a valve inside to either let vacuum flow (cold) or shut off vacuum (hot).



I have my '78 20R "desmogged", meaning that I have removed all the old non-functioning emission stuff, including this valve. I used a threaded plug with sealant in the hole it was in to prevent coolant leaks, and capped off the vacuum port on the AAP housing.
You do not need the AAP to be working as originally intended for a 20R carb to work well. As long as you have a solid diaphragm in there that doesn't leak fuel you're good on that.

Process of priming the accelerator pump? There is a passage drilled between the fuel bowl & the accelerator pump bore. Fuel just runs thru that passage from the bowl to the bore. If it's seeming that your accel pump bore is "dry" & you cannot see fuel squirting out when operating the pump, I'd suspect that it's possible that the fuel level in the float bowl is too low for the fuel to go thru that passage, & "prime the pump" so to speak. Fuel level in the fuel bowl should look like this at the sight glass:



If you need to adjust the float to change the fuel bowl level, here is the procedure & specs, pages 4-20 & 4-21: http://www.retrojdm.com/ScanViewPage...Carburetor.jpg

I happen to know that too low of a fuel level in the bowl can give the symptoms you're describing. Proper fuel level in the bowl is critical.
Other than that it could be a fuel delivery problem (fuel filter (either in-tank "sock" or under-hood filter), fuel pump, fuel pump resistor, fuel pump relay).

I like to see/hear the upshot off what you have/had going on with this & what solutions you came up with posted here.