At a guess, my newly modifed TB seems to have bushings that are too tight. It cranks and idles fine. Bring it off idle, and it sometimes returns to idle when you let off abruptly. But if you do anything other than snap it closed, it stops at about 800-1000 rpms. I can close it by hand and it goes right down where it should, around 650.

Question, what do you do in a situation like this? I tossed a weak return spring on there, but it only seems to help a little. Is there an appropriate lubricant to use there that will not gum up? Never had this problem before with carbs or my very limited experience with TBI...

 

This is most likely a mis-alignment issue. The throttle blades are actually spherical in shape at the edge and fit very tightly to the bores. If the centerline of the throttle shaft is off for any reason they hang up in the bores and stick open.

 

Thanks. I was thinking down that path already but I'm headed out for a 12 hour off-road trip tomorrow and no time to pull it down. For now, I put a carb return spring on it and raised the min air a smidge to keep them off the wall as much as possible without raising the idle too high.

 

i had this same problem when i installed my tbi spacer. and i just bought a carb return spring. and that fixed the problem.

 

Well, I went through several things tonight, and it's doing better. Close fine with the engine off, but still wants to hold open just a bit with the engine running. I think it's just vacume? Closes to the stop bolt ever time with the engine off, even letting off slowly. But crank it, and suddenly it stops closing around 900 to 1000 rpm and you have to close it by hand to get down to 700 or so. Gotta set min-air again (A+B and IAC unplugged bit) after all these changes, but it's going the right direction... Maybe I've just got it closing too far now that it's closing smooth now and opening it back up with the min air adjustment will fix it all… I hope…

 

The vacuum exerts alot of force on the throttle blades and forces them down. If the base has worn into an egg shape around the throttle shaft, the blades will bind up in their bores. The force from the TV cable, throttle cable, and throttle return spring are what make it wear off-center.

 

It's got new bushings (reman and bored out TB) and no side to side play that I can feel. Seems to be fine and works smooth and easy when the engine's not running. Can't imagine it being any better. I just did a new chip for the 20 psi FP change, and recurved the IAC vs Temp to set 10 counts when warmed up. Then did the min air/idle adjust with A+B and unplugged, cranks and idles perfectly, but if you ease off on the throttle, or anything other than let it snap shut, it will stop closing at about 1000 rpms give or take. Gentle pressure will make it finish closing, but it won't close all the way until you help it or shut it off. No cruise canister, throttle cable works fine (not binding) running or not, and TV cable is not the culprit.

There has got to be some way to fix this... Someone mentioned fighting to set the preload on the return springs. Far as I can tell, you can't really set that on this TB, can you? I'm at a loss...

 

Forgot that it was rebushed as stated above. It should return on its own with the stock springs. Its likely a misalignment issue causing binding like I stated before. One or more of the bushings is slightly off center.

 

But wouldn't that be noticeable without vacume under the plates? This thing is totally smooth with no resistance at all when not running. And when I looked at the plates, or rather looked through the gaps at a light, I can't imagine a more even gap in closing to the point no light was visible. Is there a plate adjustment procedure that I should follow? I would hate to think I would have to install and remove it for every adjustment since it works fine until the engine is running.

 

Probably its on center if the gaps appeared even.

The vacuum might be enough to push it over the edge, or add additional friction due to the extra force on the throttle blades, or some other cause, like too much clearance between the throttle shaft and bushings. If its due to excess friction, which sounds like the probable cause, then a stiffer throttle return spring may help. This will also accelerate the off center portion of the wear, though, and increase pedal effort, which will eventually lead you back to square one.

This seems to be the usual problem with the larger bore units. With a 2" tbi, there will be some 60 - 70 lbs of force on the throttle shaft at idle, which will increase pedal effort, and even make small clearance issues a big problem. Your certainly not alone. I have this problem, and other people who have gotten the larger units have similar issues. The solution would be some high tech ceramic lubeless high load capacity bearings, but I dont think they make anything like that yet for our application.

There is also no adjustment procedure for the throttle blades.

 

Darn, I was afraid it would wind up here. Hmmm, maybe time to put the stock TB back on and see if I can sell this one to someone who really needs the little bit of extra flow...

 

I wouldnt give up on it too quickly. How did the carb return spring work?

 

Oh I'll take a whack or two more at it, I'm just starting to lean that way, particularly since I really don't need it to achieve my goals.

The carb return spring was partially successful earlier, but there were other contributing problems that have since been corrected along with quite a few other changes. It is now working so smooth with engine off, I really thought it would work without it. Unfortunately, as you see, I was wrong. So, I put it back on last night while cleaning up my shop (so it wouldn't get lost), but haven't even tried it again since, been working on my truggy all day today.

Anyway, thanks for all the help. I'll post up if I make any other progress.

 

I have the same problem, I don't understand why the wear would make a difference with a new throttle shaft and bearings. I was under the assumption that this fixed the problem.

Let me know what you come up with. I was considering a carb return spring, but the pedal effort is bad enough already.

 

That project got moved to the back burner, but I never did get it to work right. I could find no way to make it close except a carb spring, but as you suggest, it makes the pedal pressure unacceptably high. And as also suggested, it will increase bushing wear. I'll probably just put the stock TBI back on when I get back to it. It wasn't worn bad enough to be a problem, and it worked a heck of a lot better than this "upgrade"...