I couldn’t find much info on any of the yota forums about de-smog/rebuild/tuning the 20R (1980 model) carburetor so I thought I would share my experiences from the last few months…

So this started off as a clutch job which led to ~ 9 months of work with carb stuff consuming way too much time. With the trans removed the EGR cooling plate and other AI stuff were more accessible so I thought a de-smog was in order. I bought all the block off plates from LCE and removed all vacuum lines except for the AAP, charcoal canister (routed directly to the intake so I could remove the VSV) and vacuum advance.

After everything was back together I took it for a test ride and everything was great… at least that is what I thought. After driving it for ~30 minutes, assuming everything was at operating temp even though the thermostat stabilized in 20 minutes, it started to hesitate under load. If I stopped and rev’d the engine or slowly accelerated everything was fine. Anything more than half throttle would cause the hesitation. At this point nothing was done to the carb but knowing there are a few temp dependent switches I started poking around the carb/intake.

Then with the air filter removed I noticed the primary barrel was nice and clean and the secondary was very dirty and overall black. So not know much about the function of the secondary I started scouring the FSM and found it is vacuum operated and should only open under full throttle. Then I found a youtube video about reversing the secondary diaphragm spring so it would open at half throttle making it more mechanically operated, not vacuum. I removed the carb to look at the secondary and to do the spring mod and found one of the vacuum ports on the bottom was clogged. After cleaning the port and reversing the spring I put it back together and it ran like a million bucks… Better than ever! It was a wonderful day! I ended up cruising around Charlotte for a few hours in disbelief the 20R has that much power. It’s awesome hearing the secondary open up at half throttle.

Then for the next two weeks I was on vacation so the truck sat without running. After coming back from vacation I attempted to drive it to work and the carb gremlin returned. Again, after 30 minutes of driving it would hesitate under load. I was very annoyed at this point and figured something else must be clogged and decided a complete carb rebuild was in order. Fast forward a few weeks and the carb was rebuilt and running awesome! There were a few issues with the rebuild kit but once they were figured out everything was good. The big one was the spacer plate gasket blocked one of the vacuum ports, which is the same port that was originally clogged causing the hesitation issue. I also dialed in the air/fuel mixture per the FSM and that seemed to smooth out the idle.

I also replaced the stock air box with a universal K&N. I think it looks pretty good and makes the engine bay look cleaner.

The future plan is to try a larger primary jet. Right now I have a 121 primary and 177 secondary. I read on an FJ forum bumping up the primary 1 size helps with the low end acceleration but who knows with this engine. They are cheap and easy enough to change so I don’t mind experimenting.

I hope this is helpful to someone...

 

I have a '79 20R in my RV that is acting badly and I'm hoping you might be able to offer a suggestion. It runs great for a few minutes until one of these "temp dependent switches" (I'm assuming)takes over and from that
point on will not allow the engine to rev down anywhere near idle speed. This is a rear pain when sitting at a stop light with the engine blasting away at high rpm. Any suggestions? Thanks for your post and pics. Your 20R looks great.

 

Thanks.
Checkout the high idle cam. It's temperature dependent but usually doesn't work and you would have the opposite affect. There's a coil that heats up and allows the cam to move past the throttle linkage, it on the backside of the carb. See pic

 

Great... since you suggest it usually doesn't work, this problem occurs quite frequently. Is the fix as simple
as disabling this mechanism, or does it need to be repaired or replaced/or might it have just flipped out of
position somehow. I realize there are limits to what you can assess here but I'd like to get your best guess.
I've taken this problem to the local Toyota dealer/they charged me $200 and had no idea what they were doing.
Then I took it to a mechanic who does call in radio shows and is considered and expert on these old carbs and he told me it wasn't worth the cost to try to figure this out. I was going to run a wire through the firewall so I could yank the throttle to pull it to back off the rpms when needed. So I'm really glad I've got a possibility of an intelligent approach to the problem here. And thanks so much for your time and for the great pic.

 

You can check it by looking at the cam after it warms up. The throttle rests against the cam when cold and then moves when warm. If it's not working it's very easy to remove (2 clips) and will not hurt anything. It's just there to help the engine warm up faster.

 

There can't be to many places outside this website where this knowledge of these carbs from a bygone era is still available. Thanks for the help Biketrials25. This '79 4 cylinder doesn't push this RV enough to do freeway speeds, but it gets me from Duluth to Superior Natl Forest and Sawbill Trail via the 2 lane roads just fine. Many thanks for your time and knowledge...