View Full Version : Question for the Carb gurus

03-31-2011, 10:19 PM
I'm rebuilding a 20R carb and it appears it had been opened up previously. I noticed 2 springs missing when comparing to the FSM diagrams. Just wondering if anyone knows if these missing springs will have a serious effect on performance, or if I should just run it without them. (the truck was sort of running before the carb came off.)

In the pic below, there is a red arrow pointing to the diaphragm that is missing the spring.


This is from the FSM, arrow shows the missing spring:

The other that was missing was between the check ball and the stopper for the accelerator pump. In most carbs, this one has either a tiny spring or a weight. Red arrow is pointing to the stopper/plug. Check ball and nozzle are under this:


The rebuild kit of course came with new diaphragms and check balls, but no springs.

If anyone has a spare 20R parts carb lying around, I would be interested. Thanks! :beer:

04-01-2011, 07:48 AM
I would wager that the missing parts are not specific to the 20R and you could source from another "parts carb". I dont' recall if I have a 22R "parts" carb and will have to check when I get home. I do know that I have a 22R "spare" carb and you are welcome to grab the parts off of it till a time when you can replace them. I walked the yard on Monday and there were no 20R trucks in there. Usually the 20R rigs sit in there for a long time and no one pulls the carbs.

04-01-2011, 08:28 AM
Is that diaphram with the missing spring described as the "AAP" in your fsm? If so I'm betting the spring from a 22r carb may possibly work...

04-01-2011, 08:31 AM
The first one appears to me to be an AAP, Auxiliary Accelerator Pump. I believe the spring causes extra fuel to be supplied when the vacuum drops under hard acceleration. I would say you would miss out on a little get-up-and-go if you do not have that spring in place.

For the other one, if there is no spring or counterweight holding the check ball down, I would think you could possibly get excess unmetered fuel passing through the accelerator pump circuit. This could cause a rich running condition and poor fuel consumption.

That's my :twocents:

04-01-2011, 08:52 AM
Thanks guys. I think your diagnosis is correct. this truck was running poorly and getting terrible mileage, so this was probably contributing.

I don't recall the 22R having that AAP diaphragm, but maybe some years cross over. Maybe I'll stop by Yoda Jim's this weekend and grab another 20R carb. Always good to have spares.


04-01-2011, 09:55 AM
T...I don't recall the 22R having that AAP diaphragm...

Yes it does have the AAP, at least in 82 it does:thumb:.

04-01-2011, 02:29 PM
Hello friendly, trail runnin comrades! Martin I have a 20R parts carb that your welcome to. Also I have seen several, that were pulled and put into a bin, at the YODA YARD. When my choke pull off was leaking I found a "Niehoff" brand at checker. I still have the box with part # if you need it. Dean

04-01-2011, 02:52 PM
Thanks Dean! I'll let you know if I need anything after hunting at Jim's tomorrow. :thumb:

04-08-2011, 10:14 PM
I made it to Jim's yard today and found a spare carb. I was amazed at the number of slight changes from year to year. Wasn't cheap either.


I got the parts carb opened up and it had all the missing parts I needed. The rebuilt unit is ready to be bolted back on. I'll post up how it runs this weekend. I still have 20+ feet of vacuum tubing to replace and double check.