7T9Shark

I have done the following troubleshooting items and the motor still won't run at less than 1500 RPM unless the choke is held 3/4 closed. (fuel starvation?)
Motor has plenty of pep when the throttle is cracked. no bog, but it just won't idle.

* Set timing to 12 degrees.

* Plugged ALL possible vacuum ports, including trans modulator, power brake, headlight and cruise control.

* Took carb apart and blew out all passages (carb was working just fine before rebuild)

* Replaced distributor cap and rotor

* Very carefully set valve lash to 1/4 turn.

What have I overlooked?

QuickVet

i dont understand it but it still sounds like a vacume leak.

7T9Shark

I agree, is does appear to be a vacuum leak, but where?

The only place left for this motor to leak vacuum would be the intake manifold itself. If it had a bad gasket match, it should be sucking oil into the combustion chambers like mad. I even tried sweeping the manifold/head junction with a propane torch to see if the engine speed changed, but no results.

The manifold appears to have an excellent seal all around.

Smokehouse69

Check for vacuum leaks around the base of your carb by spraying carb cleaner around the base while you are holding the rpm's steady at as low as you can get it. (1500 rpm?) If the engine speeds up then your base gasket is leaking.
Also check around the where the intake manifold mates to the heads, it's unlikely, but air could be leaking in.

Perfomer

What type carb? I have had the carb. gasket crack and leak around the base. Did you plug the vacuum at the source or where they ran to? Cracked line or fitting on manifold or carb. Did you check the P.C. valve it could be stuck open. Good Hunting small things become the biggest BEARS.

7T9Shark

Smokehouse,

I tried that technique this afternoon with propane and there was no change. Maybe I'll try it tomorrow with starting fluid. The liquid might be better than the gaseous propane.

hudman

If you have not already done so, try the carb cleaner around the base. I just went through the same thing 3 months ago. Car idled fine before I spun 2 bearings, then after the rebuild, it would not idle at all unless I cranked it up to 1200 or more. Turned out I had the wrong carb base gasket. The auto parts store only knew about the one piece gasket, but my manifold on my car requires a 3 piece gasket. Good luck.

7T9Shark

OK, Hudman, I'm just about ready to try anything. I'll post the results tomorrow

tshort

What kind of carb are you running? I had the same thing happen to me after a rebuild and it turned out that the holley gasket that goes between the base-plate and the main body is missing one small port cuttout hole. Just make the hole or buy the gaskets from Barry Grant and it will be good.

Matt Gruber

to check for an internal vac leak, hook a vac gauge to the dipstick, after taping shut the holes in the valve covers
.
your description reminds me of the time i left off the PCV valve hose by mistake :p:


[Modified by Matt Gruber, 8:51 AM 6/22/2003]

7T9Shark

I am running a stock Q-jet carb.


I assume I would remove the PCV valve and tape all holes shut. That's one thing I haven't tried. Need to get a vacuum gauge someplace........

Jvette73

Its possible to have the carb gasket on upside down. This would cause a big vacuum leak. When you pull the carb off you should be able to see the impression in the gasket from the tq of the carb onto the intake. Look closely all around both sides of the gasket and you might find a place where the leak is by looking at the impression. There are also a number of different Q-jet gaskets. I dont think they are all interchangable.

MasterDave

Here's my take from experience. I'm gonna assume you have an aluminum intake manifold. If you used the stock metal impregnated intake gasket from the Fel-Pro kit there-in lies the issue. You should use an aftermarket paper type gasket, like from MrGasket or the such. Even then you could have problems they're very difficult to seal unless done correctly. The leaks would be on the underside of the manifold and wouldn't necessarily pull oil into the pistons. This is how I did it to get mine to seal. On clean surfaces use Permatex Ultra Seal. All surfaces, both sides, very thin layer. A 1/2" bead of seal on fwd and aft side instead of the rubber seals. Install manifold and prior to installing intake bolts put seal on the bottom 3-4 threads and use washers under the heads. Torque in sequence at 1/2 torque #'s and do this 3 times. Then go to max torque and do it again 3 times. Follow the sequence in the book exactley. You can put the rest of the stuff on now but do NOT start the engine for 24 hours. Silicone needs this time to set up fully. I haven't had a vacuum or oil leak in a year now since doing this. Hang in there, you will get it fixed. :cheers:

7T9Shark

Dave,

I have a stock iron manifold, and I didn't put any sealer on the intake attach bolts, nor a washers under the heads. (they didn't have any from day one)

How is it possible for a vacuum leak to propagate through the bolts??

Do I hear you right, that you also add Permatex around all intake sealing surfaces to augment the Felpro Blue sealant that comes on the gasket? I'm using a 1256 gasket.

This is what I got from your instructions.

1. Remove intake and re-install using a 1256 Felpro gasket with silicone sealant added to the bolt threads and washers under the heads.
(not sure about what else should be coated with sealer, except for the end seals)

2. Install new carb spacer gasket

I'm beginning to fade into a state of catatonic confusion. I'm willing to perform one more "experiment" before I turn it over to my local wrencher.
I'm gonna need some counseling soon.....

Al

Phlash

Shark,

You might try disconnecting you vacuum booster and plugging the port on the manifold. I also would check the carb gasket. Are you sure you are not a notch out of time. That too can give you similar results.

Good Luck :seeya

Phlash 72 BB Roadster

munday

I had the same problem with my fiat spider, it was the brake booster, it went bad. Check the vacuum line to the booster, it's something commonly looked over. Also if the booster is bad it won't be a line, but the booster it self. If I got under 1500 rpm in the spider it would die.

7T9Shark

I plugged EVERY POSSIBLE vacuum connection, including the brake booster.

The only two things left that I can think of to consider are

1. Leaking manifold gasket, or
2. Cam off one notch.

I think if the cam were off, it wouldn't be so peppy when I crack the throttle at higher rpms. Right?

MasterDave

If you're using an iron intake the gasket supplied with your kit will be fine. I only use silicone and washers to keep oil leaks in check and not gouge the aluminum manifolds. I believe you get a better torque reading also when using washers. Don't second guess yourself on the cam timing, I believe you got it right. Hang in there, we're all thinking for ya.........hey, how's about unhooking your carb linkage? Maybe when you took the carb off and reinstalled somehow the cam linkage got stuck/jammed and won't allow a good idle? :confused:

Hobbes Blk '80

Hmmm... This sounds like one case of terrible corvette missfortune.

the only practical solution I can see, is you pull that used lanky motor out of your beauty, put it in a crate and send it straight to my house.

You'll never have this blasted vacuum problem again.

In turn, you will then buy yourself a GM Crate 383 for 6K, and a hotcam kit to go with it... sounds good? great! :D

Wish I knew enough to offer you some advice...sounds like a real gremlin.

7T9Shark

I appreciate all the good thoughts and humor coming my way. Thanks Can't wait to see what it ends up being. I should be an expert by then.