Quadrajet woes.

[ochre70]

Need some help from the q-jet gurus. So here's the story, I scored a decent looking quadrajet from my brother-in-law the other day and finally got the chance to put it on. Predictably, the thing doesn't want to run right. For whatever reason, it wants to idle high. I've adjusted the idle screw all the way out w/o any luck. I'm suspecting a vacuum leak somewhere (I'm thinking idle mixture screws), but I'm not sure. Also made sure the fast idle screw wasn't interfering either. The reason I say it might be the mixture screws is because they feel loose when I have them adjusted to give me the most vacuum on the gauge. What am I missing? Put the Holley back on and it runs fine...

[Coley]

Ok....lets start with some simple stuff....first can you post up the number code stamped vertically on the drivers side of the carb just in behind the linkage. Probably starts with 704xxxx or 702xxxx. That will give us some insight on just what carb is currently on your truck, which has a bearing on the tuning, etc. (also...any quick pics available?)
Next, the idle can be affected by both the fast idle circuit (drivers side) and the cold start circuit (passenger side)...and there are adjustment screws for both, although the cold start circuit adjustment screw is hard to see.
Check this to see if it is turned too far in.
Lets start there.
Coley

[ochre70]

The carb is at home and I'm at work at the moment... But, I know that the number starts with 1705... From what I've read its a 76-77 q-jet from a 454 GM truck. Not 100% on that though. I can give you a definite number and post pics of it when I get home this afternoon.

[D.PASSMORE]

There is alot of informative videos on you-tube. Its helped me solve a few problems with my Q-Jets. Take a look at them so when the advice starts flowing, you know where to go for the issues or adjustments. Plus pictures are always helpful...

[Wrenchbender Ret]

If the throttle shaft is loose in the throttle body it will cause a fast idle even when the screws are backed off.

[imjeff]

What manifold?

[ochre70]

There is a little bit of play on the driver's side of the primaries... I tried spraying some carb cleaner in that area and couldn't really hear a difference. Might just need an over-haul including the bushings...? It is the stock manifold as far as I could tell. At least it appeared to be by the amount of carbon build up in and under the exhaust crossover...

[68c10airstream]

atHi ochre 70, as a past ase mechanic of 30 plus years and did my share of qjet rebuilds and had the kent moore factory tools (i purchased these for my own use), if the choke blade is standing straight up and not on the fast idle cam as noted above i suspect th the secondaries (back big barrels) are open just a hair. If this is the case and you remove the carb and drain the gas, hold the carb overhead and look up through the secondaries and see if you can see daylight around the secondary blades. If so, loosen the 2 screws per blade VERY SLIGHTLY MAYBE A QUARTER OF A TURN and get the blades to re-center and seal off. Push on the linkage to help it seal, then tighten the screws. Now if the choke is staying on it's a different situation. Living up north i fixed many chokes hanging on because of what is known as a cold intake manifold. To have the system work properly and get the best drivability and fuel economy i will describe the system as such; enclosed aircleaner with the pipe going to (usually) the ps exhaust manifold. The ps exhaust manifold should have factory tin around the manifold completely drawing air around the rear of the manifold going forward to the pipe to warm the air that is going to the aircleaner to prevent carb icing. Second part is the most important, typically gm either put a heat riser on the ps side, a restricted y pipe with the restriction on ps only (during the 75 model year briefly) or a vacuum diaphragm with exposed linkage to a specific heat riser designed for the linkage all bolted to the ps exhaust manifold bolts run by vacuum switching valve piggy back on top of the thermostat housing. This heatriser causes the hot exhaust gas to be forced across the middle of the intake manifold and almost all of the exhaust then went out the ds manifold. The purpose of this is when the fuel goes through the carb and into the floor of the intake it hit this warm area to help vaporize the fuel until the engine warms up completely and the heatriser relaxes and opens up. This system works great and this system was called EFE which stands for EARLE FUEL EVAPORATION. The test was to start the truck cold and put your finger on the side of the base of the carb on the ds and feel for warmth in1-2 minutes. The qjet gets a bad rap unjustly so, but it was my favorite carb. I didn't look at your truck pics to see what you are running, but i felt the info is usually left out. Usually as a truck ages all or part of these systems get taken off. I don't know if all of the parts can be sourced any more. The typical engine gets a aftermarket aluminum intake and an open aircleaner, but this doesn't work very good up north in the cold, it would be ok in arizona. I fixed many cold intakes on gm,s and mopars by removing the intake and chiseling hard carbon out of the heat cross over area. This info i felt was important during this winter of colder than norm temps, just trying to be helpful, not posting an attitude. Good luck, Brian Fuller

[geezer#99]

Good info Brian.
I second the throttle plate misalignment. It can happen on the primary side too. I had a quad that had one plate bent a touch making the idle high. I think a backfire had caused the bent plate. Gentle tapping with a small hammer and drift cured mine.
And a warning too all. Go gentle on those screws.

[ochre70]

Thanks y'all. Brian, I'll give those secondaries a look and see if something isn't right there. In the meantime, here are the pictures I promised earlier. Also, the carb number is 17056212. Just in case you can't make it out from the picture.

[ochre70]

I was also wondering if the q-jet needed to use one of those thicker mounting pads I've seen in some rebuild kits? Right now I've only got a standard thickness gasket for it.

[slikside]

Someone correct me if I am wrong, but I think that carb came off a Pontiac. Fuel inlet is straight in on the front. My stock q-jet on my '72 Chevy has a side inlet for the fuel supply.

[slikside]

And I'll second you on the rebuild. Dang few used q-jets will bolt up and run well on your (anyone's) engine. These are great carbs, but they are finicky. It needs to be built to fit your application. If it came from a very diffferent application than yours, it still might not be right with a basic rebuild. So, if it did come off a Pontiac, It will be different.

[mmiddle]

For your OP...The choke is probably stuck on causing the high idle.

Q-jets came on 3.8 v/6's all the way to 500 caddy's. That number was for a Chevy truck 454 1976-77. It can be recalibrated to work fine and your engine and yes Q-jets are very finicky. But you will never beat the general street performance of one. I love them and hate them

[ochre70]

mmiddle, I checked the choke. It works like a charm. It stands straight up once the engine gets hot. I also got the 76-77 truck with a 454 when I looked it up. I've read that Cliff is the best guy to get a rebuild kit from... But, he is quite pricey. Are there any other options out there besides getting a kit from a parts house?

[chopnchaneled]

www.nationalcarburetors.com in jacksonville fla.is where i got mine, they will rebuild yours or take it as a core if not damaged.

[GASoline71]

Originally Posted by slikside

Someone correct me if I am wrong, but I think that carb came off a Pontiac. Fuel inlet is straight in on the front. My stock q-jet on my '72 Chevy has a side inlet for the fuel supply.

A lot of the BOP Q-jets had the front inlets, and I'm pretty sure all the Q-jets on Chevy 454's had the front inlet. That carb should be 850cfm.

Gary

[ochre70]

GASoline, that was my thought (hope) exactly. Although, I suppose it really doesn't matter on these q-jets. Since they will only flow what the engine will pull, right? At least that's what I'm banking on. Otherwise I'd have to change some internal stuff, right?

[imjeff]

Vacuum leaks are very common with the stock intakes that have the exhaust crossover under the front of the carb. The stock setup is gasket-stainless gasket- gasket. The clamp pressure has to be dead on for them to seal for any length of time. I've found that a 1" phenolic spacer holds good clamp pressure on the manifold gasket setup. Otherwise it erodes and you get a vacuum leak at the front of your carb.

[Gumby]

So many minor details to over look when picking used, and a $20 rebuild kit is not a cure all, spend a few bucks and get one thats been gone over proper, only way to make sure.
http://www.nationalcarburetors.com/