Vacuum leak question - Pontiac GTO Forum
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post #1 of 8 (permalink) Old 02-13-2015, 08:51 PM Thread Starter
 
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Vacuum leak question

I had a vacuum leak at the intake manifold gasket. I replaced it and finished up today. When I went for a drive it started out ok. After about 5 minutes i had a loss of power so I drove home. I pulled in the garage and turned the key off. It kept running. Didn't even flinch. I turned it on then off again and same thing. I had to cover the carburetor to kill it.

Any ideas? Did I not get a good seal on the gasket?

Also. When I put in the new gasket it did not have an opening for the heat crossover. The packaging said it would have to be modified. So I cut an opening. Is this wrong? If I end up having to do it again should I block off the heat crossover?

I have a 400 from a 69 Firebird in my 67 GTO. Don't know why. It has a medium cam and edelbrock performer RPM intake manifold.

Thanks for any help.
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post #2 of 8 (permalink) Old 02-14-2015, 11:27 AM
 
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"After about 5 minutes i had a loss of power so I drove home."

Could be anything from a fuel problem to an ignition problem, or something done wrong with the intake gasket install at this point. Need more info.


"I pulled in the garage and turned the key off. It kept running. Didn't even flinch."

There may be something grounding out in the wiring and this may be why you had the loss of power as well. Could be a bad ignition/key switch. Assume the car was running well (other than the gasket leak) before you did the gasket change. It would seem something has changed after the gasket install. Make sure all your wires are installed correctly, none are broken/pinched, and all your grounds are in place.


"If I end up having to do it again should I block off the heat crossover?"

You want the heat crossover to work. You don't want it blocked on a street car.

Do some careful looking over the work you did and see if you can see anything suspicious or out of place before you start replacing another gasket which may or may not be part of the problem. Since the car will not shut off, I would be looking at electrical.
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post #3 of 8 (permalink) Old 02-15-2015, 01:23 PM Thread Starter
 
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Hey Jim. Thanks for taking the time to respond.

You were dead on about the electrical problem. Though the solution confuses me.

I went for another drive today. The only difference is I topped my tank. During my last drive I was at about a 1/4 tank. I had been testing my brakes to see if it stopped easier now that the vacuum leak was repaired. Maybe the sloshing of the tank gave me a little air in the lines or something causing the loss of power or sputtering.

When I returned today the same thing happened with it not turning off. I popped the hood and found that my MSD box still had the red light glowing. I looked for a bad wire and I found that the wire to the electric choke had come off. I had left the key in the off position, so as soon as I attached the electric choke wire. MSD box red light went off and the car stopped. Problem solved. Though, like I said I don't know why the electric choke wire would cause the car to stay on.

Anyway, Thanks again for your response.

Oh yeah, any idea why my car dies or almost dies at full braking. I imagine the gas in the bowls is pulled forward and cuts off flow to the engine. But is there a fix for it?
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post #4 of 8 (permalink) Old 02-15-2015, 02:42 PM
 
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run on or dieseling

KJK, sounds like Pontiac Jim helped you find that dieseling! dieseling is caused by two things.......

1) something glowing hot in the combustion chamber

or

2) too much fuel still entering the still hot chamber on shutdown

you have to find the cause of one or the other, in some rare cases you could have both. So maybe just maybe that short at the MSD box was glowing the plug hot, hot enough to fire off the residual mixture. You fix the short, you fix the problem.

conversley an overheating chamber, from say ...."Overheating", Ha!

or running too hot can cause the metal elements or crud to glow red hot in the chamber and fire the mixture, so it keeps dieseling.

when it runs out of fuel it will stop....or it runs out of heat source it will stop.

By shorting the choke on the MSD box you may have actually been the only guy in the known universe to be able to make both happen at once, the closed choke making the cylinder mix richer, the same short making the plug glow hotter!

At least that is my theory and I am sticking to it

who knows where the elecrical paths of that short went

PontiacJim is right about the heat crossover, I have done it both ways, racers may need it closed on the track, they may want a denser mixture. But with the heat crossover open your car will run 10 times better and smoother as it vaporizes the mixture easily and therefore is more efficient.

I have seen Geeteeohguy talk about that crossover as well and he is spot on.


Do you have a heat riser valve? It is in the passenger side exhaust manifold and works off a thermostat spring: cold it shuts and directs all of the heat back to one side of the exhaust......

through the crossover. If it sticks closed it can give you some problems, sticking open, not so much........... unless you live in Boston.

But then you have other things to worry about today..................., as you won't see the car until july!

If you do live in Boston (a great city!)..................

Nevertheless, I recommend just duct taping a snow shovel to the front of your vehicle, drive south until someone says, "Hey, What is that thing?'......Stop.....Buy a House!

and get warm Then check the Heat Riser valve.
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post #5 of 8 (permalink) Old 02-15-2015, 08:40 PM
 
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"Oh yeah, any idea why my car dies or almost dies at full braking. I imagine the gas in the bowls is pulled forward and cuts off flow to the engine. But is there a fix for it?"

Hmmm. I don't think that is a problem with the carb/fuel, but it could be. Check these things first.

Is your choke fully open when your car reaches operating temperature? If partially closed, this might cause a problem.


Assuming you have power brakes? An inexpensive vacuum tester/gauge would be a good investment that can be a useful tool that may help.

1. Good clean air filter? If too dirty or too small, not enough air gets in to carb. I might try a couple good braking tests with the air cleaner off to see if there is any change.

2. What is your idle speed while in drive(factory is 600-650rpm's in drive)? Might it be a little too low?

3. Bigger than stock cam? This can lower your engine's vacuum and can create braking problems on cars with power brakes if it does not develop enough vacuum.

4. To me, this sounds more like a vacuum problem if you have power brakes. I would check all hoses/fittings coming from the carb to the power booster. Check the grommet where the check valve goes into the power booster and you may want to simply replace the check valve and grommet if its the original one anyway This is an example of what it should look something like http://www.summitracing.com/parts/rn...FQ8waQodo5MAdQ.

5. Now it could be something like your timing is retarded too much or you need a tune-up?

6. Lastly, it could be excessive fuel slosh caused by too high a float level or even a combination of idle screw mixtures/idle speed incorrectly set.

Again, no simple solution without a little investigation and testing to narrow the problem down by eliminating some of those things that could be the problem.
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post #6 of 8 (permalink) Old 02-16-2015, 06:10 PM
 
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PontiacJim laid it out......solid assessment

I would just build on that and say some carb engine combinations use an Anti-stall dash pot.......it is a vacumn like chamber affixed to the carb on driver side usually and it ensures the car does not stall under hard braking......

Auto trans usually and later models...70's.....I don't use one and many guys don't....

But if all else fails, check it out...sometimes called an anti diesel ink solenoid as it also prevents gas drip in the throttle by closing the throttle blades....

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post #7 of 8 (permalink) Old 02-17-2015, 12:52 PM
 
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Originally Posted by Lemans guy View Post
PontiacJim laid it out......solid assessment

I would just build on that and say some carb engine combinations use an Anti-stall dash pot.......it is a vacumn like chamber affixed to the carb on driver side usually and it ensures the car does not stall under hard braking......

Auto trans usually and later models...70's.....I don't use one and many guys don't....

But if all else fails, check it out...sometimes called an anti diesel ink solenoid as it also prevents gas drip in the throttle by closing the throttle blades....

You are right on, but I did not mention it as I too thought they came on later year cars. Cars with AC also had this feature as it would up the RPM when you turned on the AC so it would not stall the engine out at the lower idle RPM's.

The 1970's carbs had an electric activated solenoid and was as you described. It basically replaced the idle speed screw that you normally adjusted with a screw driver-which left your throttle blades just barely cracked open. When you turned off your ignition on a car with the electric solenoid, the solenoid plunger retracted and allowed the throttle blades to completely close off so no air (however slight) could be sucked down the carb and keep the engine "dieseling" on through the idle/fuel circuit of the carb. It would happen on other brand cars as well and I hated when that would happen. I would usually turn the ignition switch back on, drop the car in Drive if it was an automatic, then shut the car off to kill it. If a 4-speed, put in gear, shut off the car, and let out the clutch to kill it. At that time, I was unaware of the electric solenoid deal and just dealt with it in my way. It wasn't an all the time thing, just on those really hot days or when the engine was run up hot by hammering on it.

I plan on adding an aftermarket electric solenoid to my engine build for the same reasons the 1970's cars had them -to completely shut off the carb's throttle blades.
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post #8 of 8 (permalink) Old 02-18-2015, 02:59 PM
 
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Originally Posted by kjk990 View Post
Oh yeah, any idea why my car dies or almost dies at full braking. I imagine the gas in the bowls is pulled forward and cuts off flow to the engine. But is there a fix for it?
WhoSaidTyler on Youtube mentions this in one of his troubleshooting videos [ http://youtu.be/SOAkuJf9mmw?list=PLJ...Spgx-HvjgnJpoz ] It should start at the correct time, but if not jump to 11:46. Might not be THE answer, but its one that he addresses.
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