Quick Quadraject question - Internal Leak?

[Shugalou]

When I turn the idle adjust screws all the way in (to the right) I was expecting this would cause the truck to die but it didn't...I didn't notice any difference in idle. I did this after I adjusted the idle screw.

Wondering could this be an internal leak? Or stuck on the high idle/choke? I'm new to carbs and learning as I go. Most likely I will be rebuilding the carb just wondering what you experts have to say.

Thanks guys much appreciated!

[SkinnyG]

What is it idling at? If it's mostly idling off the transition ports, the mixture screws won't do much.

[Dead Parrot]

What is your timing set at? Ported or manifold vacuum? Read post 4 in this thread for explanation: http://67-72chevytrucks.com/vboard/s...d.php?t=560085

Poor timing does have an adverse effect on idle adjustments.

Is your idle speed proper?

What vacuum reading do you have at idle?

[Shugalou]

Skinny - It ides at around 900-1000 when I turned the idle screw out further it started to stumble. I'm not sure what transition ports are (new to carbs).

Parot thank you very much for providing the link - I'll have to check the link and report back. Pretty sure it' ported as it's off the front of the carb. Again I'll have to read some more to make sure I'm correct.

I haven't set the timing yet so it could very well need adjustment. I picked up a timing light but I haven't had a chance to learn it yet.

I'm not sure what the vacuum is, I don't have any sort of tool to measure this and no stores loan them out (I already tried). Maybe I'll see if I can find a cheap one at princess auto.

Thanks again for the help guys

[SkinnyG]

The Princess Auto vacuum gauges work fine. They're cheap. Also check Canadian Tire - they often have good pricing, especially sales.

Is your engine stock? Does it idle nice and smooth?

If so, I'd expect to see it pull about 18" of vacuum at idle. If it's way low, or all over the place, you may have other problems. The gauges usually come with diagnostics inside, otherwise Google is your friend.

Try to get the engine idling around 550 in drive (but not driving - have a lackey on the brake pedal), and play with the mixture screws.

Transition ports....

There is a wee hole below the throttle plate that fuel comes out at idle. The size of which you can change with the mixture screws (they are tapered, and fill the hole, making it smaller).

As you open the throttle further, you reduce the engine vacuum being applied to that hole, so more holes (or in this case, a vertical slot) are placed at about throttle plate height, to allow more fuel to enter the air stream.

These slots or ports ~transition~ from the idle circuit (the hole below the throttle plate) to the cruise circuit (uses venturi vacuum - that restriction near the top, way above the throttle plates).

If the idle speed is too high (the throttle plates are open too far), these transition slots are uncovered enough that extra fueling will be provided into the air stream. If the engine is running more off the transition ports than the idle ports, the idle mixture screws will do nothing.

SO....

Find out why it's idling so high, and fix that. Then play with the mixture screws.

And yeah - set your ignition timing so that you are about 36° full advance with the vacuum advance can disconnected. Princess auto has a cheap advance timing light that works fine and is surprisingly accurate.

Theoretically your base timing will end up being around 10° (might be 8°, might be 15°, it doesn't really matter that much), but you can only check base timing at about 550rpm so that no mechanical (centrifugal) advance is happening.

The fun part....

Base timing, mixture screws, and throttle setting are all related. Mucking with one is affected by the others. You kind of have to much with all of them.

I usually get the idle super low, and set the ignition timing first. Then balance the mixture screw and idle speed screw. More fuel makes it run faster, so you have to reset the idle lower.

[100%Chevy]

The choke has an idle screw of it's own.
It's under the choke housing on the passenger side of the carb down low at the end of the throttle shaft.
When adjusting the carb,always check this first,because if it's set too high,the other adjustments won't work.
Mike.

[Shugalou]

Skinny thank you for the detailed explanation The next time I'm in BC I'll have to buy you a pint!

I will be reading this post a few times to totally absorb the knowledge!

100% thanks for pointing out that other screw I wasn't aware of it...so I'll be trying to adjust this the next time I work on it.

Thanks again everyone without your help I would be at a mechanic spending mod money.

[Shugalou]

Oh and not it's totally stock - it has a Weiand intake, stock manifolds - not sure if it was cammed or not.

[Shugalou]

OK Skinny, finally got around to checking everything out.

I set the timing to about 8 BTC this was while in park and the RPMs at around 850-900

I measure vacuum at the front upper port on the car where it is currently connected and I got about 18 just as you said...I also tested a lower port on the drivers side and it was a touch higher - guessing this is manifold vacuum.

I was also able to get the engine to react to adjustment screws more action from the passenger side than the driver side.

I lowered the idle so its about 800 in P and 500-550 in D. It seems much more dialed in and I thought I had it all figured out when I noticed the RPMS would climb to 1,000 after driving around a while in Drive and no throttle...I was able to bring this down to 500 ish again with a few WOT pulls - so I'm thinking something is sticking?

Also it died on me in the driveway (thankfully) while in D, however I was able to start it right up and drive into the garage.

So I've made progress but it needs some more fine tuning, any suggestions or input is appreciated!

At this point I'm thinking a Carb rebuild might solve some of my problems.

[1-Ton]

Sounds to me like the throttle shaft is worn out. A worn out throttle shaft will cause a vacuum leak, thus accounting for the lack of idle change when adjusting the idle screws. A worn out throttle shaft can also cause the throttle primaries to get hung up, thus causing the sticking throttle.

[Shugalou]

Hey 1 Ton thanks for the bad news ha ha I wouldn't be surprised if indeed that is what is causing my problems.

I have the rebuild book for the Quadrajet I'm gonna have to get familiar with it very soon I think.

[Jaysonb]

The kits to put bushings in for the throttle shaft are pretty inexpensive. If there is a shop near you that rebuilds carbs, they should be able to put bushings in there for you. The shop near me charges about 15 or 20 to install bushings.

[jwilson645]

I've been studying up on everything Quadrajet over at www.cliffshighperformance.com . He seems to be the most knowledgeable Qjet guy out there. He also has all the parts to fix/rebuild them.

[Shugalou]

J I have the book and have been studying it some more. I'll have to check out the website to see about the parts. Thanks for the link.

Jayson, I tried calling the carb shop by me to see if they will do this instead of a full rebuild but looks like they took Friday off. I know the shop charges about 250 for a rebuild because I've called prior, I wonder if that money would be better put towards a brand new replacement?

[Shugalou]

Ha well I called the Carb shop and they told me 100 bucks to rebush the damn thing...or 225 to rebuild everything included replating and rebushing.