High Idle at 1000rpm, cannot get it lower. SBC 350, newly rebuild q-jet.

[cosmic_3000]

Hello ladies and gents

I've got a 78 Chevy K10 with a q-jet that has just been rebuild by a shop. While the carb was out getting rebuild, I've also replaced head gaskets/intake gaskets, replaced the distributor and fuel pump.

Now I got the carb back from the shop and have a high, sporatic idle that I can't seem to lower. There are 5 vacuum ports and 4 being used: PCV, spark advance, and brake booster on the carb, and an HVAC control on the intake. Carb number is 17058213.

If I remember the truck also had a high idle problem before the carb was sent out.

Here are my symptoms:
- Idle speed screw is backed all the way out to where it isn't touching the throttle arm
- High idle cam screw is backed all the way out
- High idle cam is in the down position resting on arm, choke all the way open
- Timing is 8 BTC with advance disconnected, vacuum at 1000rpm is 19-19.5 inches
- Took carb back off, verified primaries and secondaries are all the way closed.
- Idle mixture screws are 3 turns out. If I turn mixture screws in, the idle comes down a bit.
- Capped off all vacuum lines: 4 on the carb, 1 on the intake manifold. No change
- If I pull off the vacuum cap near the choke, the long tube, Idle goes up by about 50-100rpm.
- The primary nozzle are dry and not leaking at idle.
- Lifting up on the secondary metering rods causes the engine to bog down and nearly die, too rich I assume
- Spraying carb cleaner into the primaries makes the engine bog down and almost die.
- Spraying carb cleaner around the intake manifold, around the carb does not do much at all.
- Occasionally the engine will diesel one or two stroke when turning key off.
- Idle is sporatic, will hunt +/- 50-100rpm.

I'm kinda at a loss at the moment. My guess is high fuel pressure. I've got an AC Delco mech pump coming in tomorrow, hopefully that will fix it.

Thank You for any help.

[Cliff Ruggles]

Most likely getting too much air around the huge secondary throttle plates.  There are times when I'll spend half hour or so "indexing" them to prevent that issue.  Not uncommon at all to find a bent or twisted shaft holding one or both open too far as well.

Make sure the linkage coming back to the secondaries isn't adjusted incorrectly and holding them open just a tad.

To idle at 1000rpm's the air has to be coming from someplace and if it's not a leaking vacuum port on the carb, leak at the brake booster, trans modulator, carb to intake or intake to the heads it's going to be the throttle plates more times than not.....

PS:  who did the rebuild?.....

[cosmic_3000]

I think the secondary throttle shaft is leaking. I sprayed some brake cleaner right at either end and engine rpm went up by a little. More noticeably at the end with the little stop pin. I can move the shaft up and down a very tiny amount when I wiggle it.

The throttle plates look to me like they are sealing pretty good, but I'm no expert on this.

Here are some pictures: https://imgur.com/a/gZX3pIk

quadrajetpower.com did the rebuild.

Thank You

[Walleye4Days]

This may be anecdotal, but I had a similar issue on my 305sbc (Gen1 from 82' with the 2PC rear main. So, same engine family I think) and what it ended up being was a little too much slop around there throttle shaft on the secondary, allowing a little too much air to slip through.

As you know, these SBC's are like Hoover vacuum cleaners, and the slightest vacuum leak leads to having a hard time tracking it down because of how just a tiny little air slipping by impacts the idle heavily and does all kinds of other unexplainable things (hence, why the idle air screws do so much, with just tiny turns to balance the air/fuel mixture).

Anyways, I couldn't detect a leak around the throttle shafts, the base plate, the main body, nowhere at all. I had great accelerator pump streams, and I was at witts end. But, as a last ditch effort, I ended up getting Cliff's throttle shaft bushing kit for both throttle shafts. After reaming and pressing them in the secondary throttle shaft (I did that one first) it ended up eliminating all the high idle/idle issues I kept encountering!

So, even though I couldn't exactly "detect" the leak, it had to have definitely been there. The secondary throttle shaft, previous to doing the bushing, had just ever so slight amount of "front to back/up down" play in it (it was literally, barely noticeable, unless you REALLLLY tried to feel for it) and that was enough to cause my air/fuel mixture to be all wonky and let enough fuel/air slip by to cause high idle. I went ahead and did the primary throttle shaft too after that just for the sake of having it done.

Not sure if this helps, but it's what I experienced and it ended up fixing the issues I couldn't track down after making sure all else was correct. A lot of times, shops that "rebuild" don't take the time to throw the carb on engine and really get all the tuning done before sending it out the door - and most, unless they specialize in or do A LOT of Quadrajet work, don't even touch the throttle shaft clearances or give a second thought to bushings. Not saying that's what your shop did, but just mentioning that for future viewers who may run across this.

[Cliff Ruggles]

The factory tolerance on the secondary throttle shaft is about .008".  That may seen a little wide but they relied on correct indexing of the huge throttle plates the bores to effectively seal off the incoming air and minimize any leakage.

Sounds good in theory but I encounter quite a few untouched and a LOT of rebuilt units where the throttle plates are very poorly adjusted and allowing too much air past them.

I also see a LOT of carbs where the linkage that opens the secondaries is incorrectly adjusted and holding them open slightly.

I also see a LOT of folks using manifold vacuum to the advance and loosing complete control of idle speed as the engine they are working with just doesn't like, want, or need that much timing at idle speed.  Doesn't appear to be the case here but needed to mention it.

I'd also mention that using vacuum advance via manifold vacuum is a very miss-understood topic  with LOT of information about it on Forums and even the "guru's" and "experts" advocating using it don't have a clue as to what they are talking about.  The factory used "ported" vacuum to most of these engines post 1966 and it wasn't "for emissions" as the experts are claiming.  Many of those engines had tiny little cams in them and super efficient at idle speed already making upwards of 20" vacuum at 600-700rpm's with very low base timing settings.  Adding another 15-30 degrees timing to them at idle just will NOT work, and I'd add here that I get to spend COUNTLESS hours on the phone every week trying to explain this to novice engine builders and tuners who call up here with carb and distributor issues with their new engines.

Anyhow, as it relates to this thread I would remove the baseplate and spend some time with it making sure ALL the throttle plates are very well sealed when closed.  This may involved straightening a twisted or bent shaft.  The engine is getting WAY too much air from someplace and provided you have exhausted all efforts to find it anyplace else that's where I would look next.....Cliff

[cosmic_3000]

Hello everyone, just a quick update. I'm currently redoing the intake gaskets with Fel Pro 1256, and also bought a 3/8" secondary shaft bushing kit from Cliff. At the same time I'm gonna install a fuel pressure regulator so I can set it at 5psi.

Hopefully this will fix my issue! Will update once everything is complete.

[cosmic_3000]

Hello again.

So I redid the intake gaskets, installed a fuel pressure regulator set right at 5psi, and bushed the secondary shaft. I think bushing the secondary didn't do much at all, there is a little bit less slop but I can't imagine much air going past the shaft.

I did spend quite a bit of time trying to get the secondary's to seal as best as I can. I think I got them fairly good but I'm no expert.

So here's what it's doing now. I did manage to get the idle down a bit. Most times it will idle between 700 and 900, surging up and down periodically. Sometimes it will idle down to 300-500 and almost die, and then minutes later be back at 700. It's just all over the place.

Currently I'm at 10 degrees ported vacuum, 3 turns out on the mixture screws. Idle screw is near all the way out. The primary's are bone dry, so nothing coming from the nozzles. I did notice however that the bottom of my secondary air valves are completely wet with fuel.

I'm planning on removing the air valve doors and letting it idle. With no valve doors, the secondary rods should never lift off their seat.

Also, if I pull a manifold vacuum port rubber cap off, the idle increases. Does that mean I'm running rich?

I also tried taping the secondary's shut. That made the engine idle extremely rough and barely stayed running unless I gave it gas.

Interesting thing is, once I get off idle, the carb is super smooth, much better than before the rebuild. It pulls strong, never hesitates or bogs even at low rpm and wide open, and even when cold. Just idle is very frustrating right now and all over the place, seemingly random. Sometimes it's very high and surges, sometimes low, sometimes it's steady. I may try to see if I can get the secondaries to seal better, maybe swap the throttle plates. I may buy a second qjet just to troubleshoot and rule out carb problems. I've also thought about going with an Edelbrock carb, but I really really like to stick with this carb as it is original stock setup. I honestly like the design of the quadrajet.

Here's a video of it idling: https://www.youtube.com/watch?v=zz8zpe0KJKc&feature=youtu.be

[Cliff Ruggles]

"I did manage to get the idle down a bit. Most times it will idle between 700 and 900, surging up and down periodically. Sometimes it will idle down to 300-500 and almost die, and then minutes later be back at 700. It's just all over the place."

Check the distributor to see if the timing is moving up/down at idle speed, common problem and it makes idle tuning nearly impossible.

You mentioned that you "replaced it".  Unless the original distributor was worn at the shaft or weight pins I would have cleaned it up and put it back in place.

Where did the replacement distributor come from?  If it's commercially "remanufactured" or one of the Chinese units currently being sold it's probably part of the problem, not the solution.......Cliff

[cosmic_3000]

I've got a known good distributor coming from a friend, so I can check that out and see if that improves it.

I took the carb back off and spend about an hour adjusting the secondary throttle plates. I ended up swapping them between the two bores, and they seem to fit better now where they are. I can see light through it but the gap is so very tiny, I doubt even a hair could get through there. Less than a thousands.

So currently it idles about 750-800rpm with exactly 19 inches of vacuum, 10 degrees advanced, and 3 turns out on the idle mixture. Idle speed screw is about a quarter turn from fully close. It still idles very sporatic. One thing I noticed when the engine is hot and I shut it down, when I restart it, it wants to idle really low, like 300-500, until I tap the gas a few times.

Questions: Even though the engine is pulling 19 inches at 750 rpm, could there still be an vacuum leak? Also, since the carb is freshly rebuild, is it possible to reuse the gaskets if I were to take it apart?

My next plans are to look for an internal intake manifold leak but pulling the plugs, and by sticking a video boroscope down the intake and see if one of the intake runners is oilier than the rest. That should show if the intake is sucking in air and oil from inside the engine. Also a friend has a Demon 650 carb I may try and see if that idles smoother. Plus the distributor.

Thanks again.

[Cliff Ruggles]

Your symptoms are CLASSIC for a distributor issue.

IF you can't get it to slow down below 750rpm's at idle speed, then shut it off and it's idling WAY too low, the mechanical advance is the problem more times than not.

Rev up the engine and it will not slow back down below 750rpm's or so tells you than when you shut it down the weights overcame the spring tension (or the mechanism is sticking) and the timing is NOT retarding back to the same setting.......Cliff