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View Full Version : Throttle positioning screw?


subzali
11-13-2007, 08:00 AM
What is this thing for? I was adjusting my carb last night and was going well except when I got to this screw it's dumb. If I screw it in to the point where the spring is compressed to keep the screw where it's at, then it increases the idle above where I want it and I can even remove the idle positioning screw (base of carb) completely and it makes no difference. The problem is that it screws itself in so after a while I have to pop the hood and unscrew it way back so the spring is far from being compressed, and that's why it's able to screw itself in again - the spring isn't doing anything.

I guess my question is, can I just remove this stupid thing? What would it be hurting and am I losing any usefulness by doing so? I'm going to read the FSM tonight a little more maybe, but just thought I would get some input.

Red_Chili
11-13-2007, 08:57 AM
Maybe the spring is sacked? Dunno. There ain't no optional parts though.

Uncle Ben
11-13-2007, 09:52 AM
First off it has nothing to do with throttle position (you don't have EFI). The idle air bleed screw you are referring to sounds (as Bill said) like the spring is toast. The fact that screwing it all the way in "raises" you idle suggests you have a vacuum leak. In a sense thats all the Idle screws are, is a way to have a controlled vacuum leak around the throttle plates when they are closed.

subzali
11-13-2007, 10:12 AM
I'll probably need to take some pictures, I don't think I'm coming across clear. I was just frustrated with it this morning. It's called the Throttle Positioning screw from what I can tell. I'm getting a little over 13" of vacuum at the manifold at idle.

wesintl
11-13-2007, 10:28 AM
13 is kinda weak. X2 what UB says/

Btw is the needle steady or does it move at idle. If should be rock steady. IF it's fluttering you have a vac leak.

corsair23
11-13-2007, 01:42 PM
All the experts with their advice :thumb:

I would have just told you to put a dab of superglue or threadlock on the screw and be done with it :hill:

MDH33
11-13-2007, 02:20 PM
Are you sure you're not confusing this with the choke limiter screw? pics a good idea!

subzali
11-14-2007, 07:49 AM
Here is the carb I have: Apparently it's for 9/77 to 12/78 Federal spec. My truck is a 10/76, but this isn't causing the problem I don't think. I labeled the picture with what I think the screws are, even though it doesn't really match what the FSM has. I need to check my TP diaphragm, because the throttle positioner is always in the position in the pic, and I think during operation it is supposed to rotate out of the way. I think you guys are right - if I'm not getting enough vacuum (but suck me sideways if I can find where I'm leaking from) then the TP diaphragm won't work and my idle will be around 1200 rpm instead of kicking down to 650. Thoughts? Sound like I'm on the right track?

subzali
11-14-2007, 08:12 AM
Here are the pages from the FSM on tuning the carb; notice some of my confusion has come from the pictures being different :o :D

subzali
11-14-2007, 08:49 AM
After thinking about it a little bit...max. vacuum is at idle right? So at idle the TPS is not engaged on the throttle linkage, so when is it needed to keep the engine at 1200 rpm (per the FSM)?

Also, if I'm not getting enough vacuum at idle then maybe it's not enough to pull the TPS away from the throttle linkage.

I think I'm still having brake booster issues...I get a little "whoosh" when I push the brake pedal, I've just gotten used to it since all of the three boosters I've tried on this truck have done it, though I know it doesn't happen on my red truck or Gary's 60. Also I noticed the other night, when I turn the engine off there's a little hiss coming from my booster for a few seconds, and I know that if I'm rolling down the road and turn the engine off then after a few seconds I have no remaining assist in the brakes, which confirms to me that the booster is still indeed leaking. So if I get that fixed will that hopefully solve my vacuum problem, given there aren't others? Maybe that will fix my TPS problem too.

subzali
11-20-2007, 12:12 PM
I fixed the problem temporarily by connecting the diaphragm to manifold vacuum and plugging the hose that goes to the VSV. I'll have to check the VSV later and see what's going on. It was funny though, I brought the hose over to the diaphragm (it was already connected to the manifold), it went "whoosh" and the diaphragm pulled the lever back and the idle dropped to barely running (TPS was screwing up my carb adjustments so my idle speed screw was set too low). It was real quiet for once, but I brought the idle back up with the idle speed screw and now I'm happy :D. I'll have to readjust the carb, but at least I'm learning :D

wesintl
11-20-2007, 12:39 PM
nice matt, good job

Uncle Ben
11-20-2007, 06:22 PM
I fixed the problem temporarily by connecting the diaphragm to manifold vacuum and plugging the hose that goes to the VSV. I'll have to check the VSV later and see what's going on. It was funny though, I brought the hose over to the diaphragm (it was already connected to the manifold), it went "whoosh" and the diaphragm pulled the lever back and the idle dropped to barely running (TPS was screwing up my carb adjustments so my idle speed screw was set too low). It was real quiet for once, but I brought the idle back up with the idle speed screw and now I'm happy :D. I'll have to readjust the carb, but at least I'm learning :D

:thumb::thumb::cool:

subzali
10-17-2011, 02:04 PM
Bumpity bump to link some findings from THIS POST (http://risingsun4x4club.org/forum2/showpost.php?p=193706&postcount=62). Guess I don't need two separate threads on the same subject:

"My system doesn't work properly because my VSV is toast for some reason, but it looks to me like my CB/TP diaphragm operates like this:
-at idle there is vacuum acting on the diaphragm, so it pulls the lever for the CB, as you say keeping just a little bit of air running through the carb as soon as the engine starts pulling vacuum. It also pulls the fast idle screw up and away from the throttle linkage so the fast idle is not in effect unless the choke is pulled.

-at high speed (activated by the VSV), there is no vacuum pulling on the diaphragm, allowing the fast idle screw to contact the throttle linkage when the throttle is at rest. During deceleration, the throttle plate would normally be closed. According to my Emissions FSM, this causes an overly rich mixture. But because the fast idle screw is allowed to rest on the throttle linkage at high speeds (because no vacuum is acting on the diaphragm), it keeps the throttle plates open just a tad to even out the A/F ratio.

Again, I have not been able to verify these results yet because my VSV is toast, but it's what the FSM says, and when I manually apply vacuum to the diaphragm I can recreate the action."