Ace17

Hello!

I've been having some sticking throttle issues, and I was hoping at least a few of you could chime in and maybe give me some creative advice. A little about my vehicle:

I've got a totally rebuilt 1988 GMC Sierra with the following mods: New 350 vortec engine, 454 TB, xr258hr cam, EBL flash, GMPP vortec TBI intake, 1.6 roller rockers, dual cat back exhaust, headers, etc.). I tuned it with the EBL flash system(which is awesome!), drove it across the country from Missouri, and am now based in California(and it actually passed smog &#128513. The tune runs very well, however, I'm having some issue with the throttle.

I've been having sticky throttle issues for a while now, and since it's only been getting worse, I decide to put new bushings in for the throttle shaft. After doing it, i noticed absolutely no play in the throttle shaft(which is good), but with the engine running, if I do the IAC adjustment where it's in the range of 5-10 steps, the throttle blades get stuck closed(with the engine running). Extremely difficult to drive. But when I open the throttle blades to the point where it feels smooth, it idles somewhere around 900-1100 rpms, but it's a pleasant drive, and it actually feels like I've rebuilt the TB, lol. Here's my theory:

I think it's doing this because the 454 TB is normally on a 454 engine, which requires more air to run. So where the throttle blades are at idle on a 454 engine might allow it to idle at 650 rpms(which is my target), but on a 350, which requires less air, will idle higher, since it's letting in more air than it would normally get. I basically have to close the blades more than the "factory setting" with the idle stop screw, which makes the throttle stick.

I've checked for vacuum leaks and haven't found any so far(it's got new TB gaskets, and all vacuum connections are secure).

I could just put a 350 TB on there(as this might fix my high idle problem and be easier to drive, since I can open the throttle blades more), but my system is tuned for a 454 TBI, so I'd kind of have to start the tune from scratch. That, and I'd end up losing quite a bit of power probably.

So it's either adjust it to idle correctly, and look like a maniac driving down the street, or adjust the throttle stop screw to the point where it's easy to drive, and have a weird high idle...

Any ideas are appreciated - thanks!

dmccain

iTrader: (2)

Get your TBI bored to 46mm . Perfect on a SBC

Ace17

Thanks for the response.

I forgot to mention, but in the initial tuning stages(years ago), I actually got my 350 TB bored to 48mm, along with my TB spacer, and intake, and it basically did the same thing, if not worse...but maybe 46mm is the sweet spot? I wonder what kind of HP gains I would lose if I would go to a smaller TB.

That's something I'll keep in mind though - thanks!

Tom 400 CFI

I don't think that theory is right, but maybe. I bored my CFI TB's to 2.03" which is slightly larger than the 454 TB's and it didn't have that issue. I'd resolve this in one, or both, of two ways...both ways that I resoved the same issue on my own bored TB's:
1. Unscrew the TB plates slightly so they're retained to the throttle shaft, but loose/movable on the shaft. Back off the Throttle stop screw so that it doesn't limit the closing of the plates. let 'em close as far as they'll fit, in the bore. The bore will "center" the plates on the shaft. Tighten the screws that affix the plates to the shaft.

2. With the throttle stop screw still backed off, open and close the throttle from FULLY closed (where they're probably binding in the bores), open them, close, open until they leave a shiney mark on the bore, where they're hitting. Using fine sand paper (maybe 200 grit? ish?) sand those areas to relieve them, and sand the shapr, bottom side edges of the plates. Do this until the sticking starts to feel more like a "hard stop" point, than a sticking/binding point. Then adjust your throttle stop to get your proper minimum air, re-set the IAC.

Ace17

Wow - awesome suggestion!! I'm gonna try this out tomorrow and I'll report back.

Thanks for the detailed response!

DynoDave43

iTrader: (11)

I picked up a used V6/small block TBI unit that had been bored to 46mm, and it binds in the bores like this. Never mounted it, but you can feel it when you work the throttle linkage, and actually hear it "chirp".

I had planned on sanding it, but I'll add this centering tip as well. I assume a little blue LocTite for those screw would be wise?

Ace17

Yeah I used blue loctite on mine(which was probably a good idea, since I now have to loosen them again )

Tom 400 CFI

That "little chirp" is what I experienced as well, with my binding TB's after I bored 'em. Yep. It's just that the plate isn't fitting the bore, perfectly and a little hand work will fix it.


I agree. Definitely.