Quadrajet woes

zzebby · October 03, 2004, 01:04:18 AM

Need another set of wheels on the road and the 34 lost out to the 66 Chevelle. Closer to drivable.
Starts right up but won't idle below about 1500 - no tach, estimate only.
Before I get into the carb, it's a 396 engine, freshly rebuilt, .030 overbore, stock heads, four tube headers, muncie 4 speed. There is a very lumpy cam that is at least 20 years old. Don't know the specs, but as it's old timey, it's probably long duration with lots of overlap.
Anyway, I looked long and hard for an old quadrajet that was pre emmision style and finally found a 70 vintage (by the purist's number decoding). Rebuilt by one of those cheapo sweatshop carb rebuilders. Ran, but only with the throttle stop turned waaaaay in. Primary butterflies open about 1/8 inch. Played with the idle mixture screws but they had no effect, even screwed all the way in, no change.
Now I'm no carb expert, but found a manual on old qjets and read up on the idle circuit. Seems that gas is sucked up from behind the main jets to the top of the body then sideways to a down tube which goes into the idle mixture screw opening. Book said that there may be extra air introduced but I didn't see where. Well I traced this circuit thru with a fine wire and then blew it with compressed air and all seems clear. Checked the gaskets and they seem to be compressing normally so the case is not warped. The rebuild shop had epoxied over the soft plugs and it was a bit messy, but I have no flooding problem.
Kind of stumped so am asking the rrt if it's possible that the lumpy cam will not creat enough vacuum to suck fuel thru the idle circuit. I had a similar cam back "in the day" (latest buzz words) and yet still could idle under 1000. Any words of wisdom or experience out there

??
Hate to pull the cam at this point, especially if it's something else. Do have a later model quadrajet with lots of emmission extras...........could throw it on, but it's been in the corner more than a few years

Suggestions?

C9 · October 03, 2004, 09:44:23 AM

Give the other carb a shot.
If it runs differently - better I hope - then you'll know if the present carb is the problem.

As far as carbs sitting for a while, it doesn't seem to hurt them.
I had a pair of new Carter 500 cfm's that sat for 8 years.
Stuck em on the 462" Buick engine and they ran fine.
I have a smog Holley - bought new - that I drag out every couple of years and it works fine as well.

Stick a vacuum gauge on the engine and see what you get.
Big cams seem to idle 700-900 rpm and about 10" of vacuum.
(A stock cam in a well sealed engine idles 17-19")

I'm guessing you may have a vacuum leak and that's contributing to the overly high idle rpms.

If no vacuum leak, you may want to double check the cam timing and the ignition timing.

Stakebed · October 03, 2004, 10:11:57 AM

I'm far from a carb expert but if your throttle plates are open 1/8 inch you will not be running on the idle circuit. That would explain why the idle screws make no difference when you try adjusting them. I also would suspect a vacum leak or timing problem, if the other carb acts the same. Have you checked the base gasket against the bottm of the carb? I have run into carb that have slight variations on the base and the wrong gasket doesn't completly seal everything.

enjenjo · October 03, 2004, 10:22:41 AM

I think Stakebed is on it. If you check the bottom of the carb, I think you will find heat passages that are not being sealed by the gasket. Some of the old Quadrajets required a stainless steel plate and two gaskets to seal properly.

But it sounds like a vacuum leak to me too.

zzebby · October 03, 2004, 02:14:41 PM

Yes I have the stainless steel heat shield and a few gaskets under the carb. The extra gaskets were to get away from the heat as Enjenjo says, there is an exhaust passage there.
If I close the throttle plates to less than 1/8 inch then the engine stalls.
The vacuum leak is a good possibility. Will look for that. Recall that I should spray ether around and if the engine speeds up then that is the leak

??
Or spray something else??
Then try the other carb if I can block off all the vacuum ports.

Have also seen that egay has a lot of used Qjets and cheap. Will shop for an old one.
thanks

Ralph · October 03, 2004, 03:47:43 PM

QuoteI'm far from a carb expert but if your throttle plates are open 1/8 inch you will not be running on the idle circuit

Like he says. (The entire quote!)My quadrajet rebuild book (Haynes Rochester Carburetor Manual) says, if you run a big cam with low idle vacuum, to drill 3/32" holes in the primary throttle plates to bleed air through so you can close the primaries and not have the transfer slots exposed so much at idle. You'll regain control of the mixture screws.Hint:don't try to remove the throttle plates from their shaft. They're staked on and you'll bugger up the threads.Experience talking here. :oops: Just take the carb base off and drill them in place. About halfway between the shaft and the edge of the plate, on the transfer slot side. It worked for me.
Ralph
PS: Woohoo! finally a question I can help with! I'm so proud!

zzebby · October 04, 2004, 02:06:11 AM

Went to the garage to try out the smog carb and though..............try the old one again just for grins. Woweee for some reason it idled down and saw no gas coming thru the venturis. The idle mixture scews would change the sound and the idle speed. It works..............but why?

? I think the rrt was right. The pcv valve popped out when I was poking around and the grommet around it in the valve cover was like wood and split. Could that be the vacuum leak.
Ran out of gas so couldn't test it out very long.

Good idea from Ralph on drilling the throttle plates. Will do that if the problem comes back.

zzebby · October 04, 2004, 02:09:28 AM

Ralph. Are you right in Winnipeg ? I've got some tin out of Arborg and plan to go back there for more. You a scrounger??? Farmer?
fellow canuk

Ohio Blue Tip · October 04, 2004, 06:56:37 AM

Quote from: "zzebby"Yes I have the stainless steel heat shield and a few gaskets under the carb. The extra gaskets were to get away from the heat as Enjenjo says, there is an exhaust passage there.
If I close the throttle plates to less than 1/8 inch then the engine stalls.
The vacuum leak is a good possibility. Will look for that. Recall that I should spray ether around and if the engine speeds up then that is the leak ??
Or spray something else??
Then try the other carb if I can block off all the vacuum ports.

Have also seen that egay has a lot of used Qjets and cheap. Will shop for an old one.
thanks

I have had good luck finding vacuum leaks over the years using my LP torch. Just attach a hose to your propane torch and with the gas flowing (don't light it) move the hose around suspected vacuum leaks with the engine running. If there is a leak the engine will increase idle speed and you have found the leak. It's best to do this in a well ventilated area and be careful, but it works great.

Ralph · October 04, 2004, 02:11:53 PM

Quote from: "zzebby"Ralph. Are you right in Winnipeg ? I've got some tin out of Arborg and plan to go back there for more. You a scrounger??? Farmer?
fellow canuk

Yeah, I'm a Winnipegger. City boy, though.Just another fool wasting his time and money on old cars. What kinda tin you lookin' for? If you come back this way, look me up. We could do a few garage tours. lots of projects in process around here. We all hide in the garage from December to April to keep from freezing to death!
Ralph