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Old 06-23-2007, 10:27 AM
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vacuum leaks

Hello. 1980 Corvette 350 L-82 automatic tranny. New rebuilt. Installed yesterday.

The idle surges/pulses in drive and park (800 -1300 back and forth). I assume it is a vacuum leak. I sprayed carb cleaner around the base of the carb - no change. I covered up the top of the carb with my hand (all suggestions from this site) and it seemed to smooth out. What's next? How do i find the leak, and what exactly am I looking for? Any known culprits to start with? Thanks!

JG700

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Old 06-23-2007, 12:05 PM
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there`s several ways. one, check the vacuum advance canister, make sure it`s not ruptured. since sprays are messy and it`s a new engine, use a mini hand held propane torch. Don`t light the torch of course, just turn it on, go around the base of the carb, the vacuum lines, etc. when the leak picks up the propane the idle should go up. Also check the modulator valve line going to the tranny, make sure it`s connected and there`s no breaks in the line. If you go over everything and find nothing, then yank the carb and look in the plenum, if there`s oil in the plenum, then it has a leak at the intake gaskets. If the leak is at the gaskets sometimes you can retorque them and fix the leak, but it most causes not. If they are indeed leaking, and if your intake is aluminum, then upon reinstallation don`t use standard fel pro gaskets, they won`t crush and they`ll leak, the standard fel pro`s are made for heavy cast iron intakes and not light aluminum. Fel pro makes a gasket for aluminum intakes although I don`t recall the part number. I usually just use the Mr. Gasket jobs, they`re made of paper and have a sealant ring around the ports.
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Old 06-23-2007, 12:10 PM
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I chased the same problem on a 76. I would unhook & plug all the vacuum hoses from the carb. If the problem goes away then hook up one at a time to find the leak.

If it does not help, then it could be a major internal vacuum leak in the carb, like the 76 did. That was a OEM quadrajet.
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Old 06-23-2007, 04:00 PM
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Doc here,

I do not advocate using the propane torch for leak testing..PERSONAL experience..IF you hit the "spot" and the Carb spits back and dies..you have a fire on the manifold From the heavy low lying propane Gas..until it burns off..IT DOES work, but you run that risk..

Spray is safer, and if you have a good paint job and the engine is warm, shouldn't effect anything.

Fatten up your Idle jet Screws about a 1/2 turn..sounds like it's starving at random..IF you have any time on the plugs..an examination should confirm this..(clean white electrodes).

Check the fuel filter, Both at the body, and inside the Fuel inlet nipple at the Q~Jet..make sure they are clean..

AT IDLE..look at the center of the Q~Jet..they will be a "'Hole" just rear of the air cleaner stud..look down there..there will be a "Y" structure in there, that is the top of the float..use a Very small screw driver or Q~Tip, push it all the way to the bottom and hold..Fuel will rise and start to spill over the top of the carb..let it go before it dies..Do this 3 or 4 times..This will clear any crap in the needle valve.

Make sure the Mechanical advance isn't hanging up, or one of the springs hasn't fallen off...

IF the fuel is old..put some (double ) Octane boost in the tank as well as some cleaner / Water displacer.

Do a Volume / Pressure test on the Fuel pump..Because it is new does not mean it can't be dying..


Check the power brake booster for leaks..push the pedal and listen..if it roughs out and hisses..you have a problem.

Check the PCV for proper operation not stuck or sticking) AND that it is proper for the engine.
Doc
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Last edited by docvette; 06-23-2007 at 04:05 PM.
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Old 06-23-2007, 06:31 PM
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Hoping not to be lambasted by other members, I will offer a way to check whether the intake gaskets are sealing or not. I was thinking about this the other day because of all the posts I see on this and other forums concerning internal vacuum leaks.

The best way to eliminate leaks in the first place is to make sure the mating surfaces of the heads/intake are parallel in both planes when assembling the motor like this:

Occasionally, you may experience a vacuum leak into the intake ports from the crankcase of the motor due to the intake manifold / cylinder head interface not being machined parallel. No amount of propane or carburetor cleaner will find such a leak. The best way is to insure the elimination of such a problem while building the motor.

Here's how I set up the intake manifold/cylinder head interface to prevent vacuum leaks from the crankcase to the head ports.... Measure the thickness of a new intake manifold gasket. Get flat washers or shims that will measure that thickness. With the manifold off and the mating surface on the cylinder heads de-greased, put a dab of RTV on the washers/shims and stick them on each corner bolt hole on the cylinder heads. Let the RTV set up. Stuff paper towels into the ports to keep debris out. Make up 16 pea-sized balls of modeling clay. De-grease the intake manifold at the ports. Place the balls of clay on the top and bottom of each port of the manifold, squishing them down well so they stay in place. You want them to be thicker than the shims/washers that are RTV'd to the heads. With your fingers, coat a little oil on the heads where the clay will meet the heads to keep it from sticking to the heads. Now carefully place the manifold into place on the heads and use bolts on the four corners to just snug the manifold down until you feel resistance against the shims/washers. Remove the manifold carefully and measure the thickness of the clay at all 16 positions with the depth function end of your 6" dial caliper. You'll know pretty quickly if the manifold/head interface is square. Record the measurements on the manifold with a permanent marker like a Sharpie. The widest measurement will be the standard to which you will want your machinist to cut the other positions on the manifold to make it square with the heads, thusly sealing up the motor.

Now, with the motor already assembled, I was trying to envision some way to check the gasket seal. Here's what I came up with:

What if you made a steel or aluminum plate to cover the carb mounting hole in the intake. Drill one 0.453" hole in the plate to accept a tubeless tire valve. Drill another hole and tap it to accept a 0-15 psi pressure gauge. Back off all intake rockers. Use a bicycle tire pump and attempt to slightly pressurize the manifold plenum/head ports. If you couldn't get any pressure built up to stabilize the gauge, wouldn't that indicate that there is lack of seal somewhere?

Like I said, please don't flame me for this, I'm just trying to come up with a workable solution for Joe Scratchrubber who may not even have shop air at his garage.
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Old 06-23-2007, 07:05 PM
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Quote:
Originally Posted by techinspector1

Now, with the motor already assembled, I was trying to envision some way to check the gasket seal. Here's what I came up with:

What if you made a steel or aluminum plate to cover the carb mounting hole in the intake. Drill one 0.453" hole in the plate to accept a tubeless tire valve. Drill another hole and tap it to accept a 0-15 psi pressure gauge. Back off all intake rockers. Use a bicycle tire pump and attempt to slightly pressurize the manifold plenum/head ports. If you couldn't get any pressure built up to stabilize the gauge, wouldn't that indicate that there is lack of seal somewhere?

Like I said, please don't flame me for this, I'm just trying to come up with a workable solution for Joe Scratchrubber who may not even have shop air at his garage.

Doc here,

KWEL!

I think that's an inventive way to check internal leaks on an assembled Engine! Never would have occurred to me..

On my post I was referring to External leaks only , like carb to manifold, EGR to Manifold, Hoses, Bad Throttle-body sealing to the bowls, and the like..not internal leaks..

That other system you use is like "Plasti~gauge" For a Manifold another good check..When reassembling the engine.

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Old 06-23-2007, 07:53 PM
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Thanks Doc, a thumbs-up from you means a lot to me. I value your knowledge and opinions on this board.
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Old 06-25-2007, 10:53 AM
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Quote:
Originally Posted by techinspector1
Now, with the motor already assembled, I was trying to envision some way to check the gasket seal. Here's what I came up with:

What if you made a steel or aluminum plate to cover the carb mounting hole in the intake. Drill one 0.453" hole in the plate to accept a tubeless tire valve. Drill another hole and tap it to accept a 0-15 psi pressure gauge. Back off all intake rockers. Use a bicycle tire pump and attempt to slightly pressurize the manifold plenum/head ports. If you couldn't get any pressure built up to stabilize the gauge, wouldn't that indicate that there is lack of seal somewhere?
Great idea , wish I'd thought of it, don't forget to securely block all vacuum ports on the manifold.
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Old 06-25-2007, 12:41 PM
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Good idea. You need to market it. Become the next E-bay entrepreneur.
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Old 06-25-2007, 06:38 PM
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I just usually blow cigarette smoke in a vacume source downstream of the throttle plate with the oil cap off and the breather and pcv plugged, it may take a while to get enough smoke,but it will make any little leak show up,such as the throttle shaft.It also will come out around the shaft of some egr's.
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