Hi,

I have a stock 1972 400, Thermoquad, no modifications, i.e. stock cam, distributor, dual exhaust, etc. Initial timing is at 14 degrees and at 2800rpm with the vacuum advance disconnected, 38 degrees.

At idle I have 4 to 5 inches of vacuum. I've checked for vacuum leaks through the following:

1. Removed all the vacuum lines to the carb, blocking off the ports, no difference.

2. Removed the vacuum line from the intake to the brake booster and blocked it off, no difference.

3. Changed to a spare carb, no difference.

4. Used a portable propane torch with a tube and checked all along the carb base including the throttle shafts, intake manifold along the heads and underneath above the valley pan, along all the vacuum lines and line to the booster, no difference.

5. The breather on the passenger side valve cover has vapours slowly coming out and the PCV valve is working fine.

6. At idle with a digital timing light, the rpms hover around 750. If I start to cover the carb with my hands, the rpms increase to between 825 and 850.

I'm thinking I have an intake manifold gasket leak between ports and that's why I can't physically locate the leak. Also, I don't hear a massive vacuum leak comparable to when I pull off the PCV tube from the front of the carb.

Is there any way to diagnose a port to port leak between the intake and heads to verify that's the true problem? Would anyone have any another suggestions I can try before taking the intake off?

Other than the low vacuum at idle, the car runs fine.

Assuming the problem is the intake gaskets, which intake gasket sets are the best to use?

Thanks in advance,

Mike.

I'd get a parts house Felpro metal valley pan gasket. the (383/400) one is different than a 440 one so specify which to them. dont use the 4 paper gaskets if it comes with them. I'd use permatex #2 in the toothpast like tube SPARINGLY/EVENLY on both sides of each of the 8 port edges on the metal valley pan gasket and a light coat on the intake bolts also.

Good advice and cheap to do.

Has the motor always done this or is this a recent development?

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5. The breather on the passenger side valve cover has vapours slowly coming out and the PCV valve is working fine.

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If the PCV valve is working fine on the driver's side valve cover, the breather on the passenger side valve cover should be sucking air IN not pouring vapors out unless the pcv isn't working right or you have piston blowby.

Hi,

Thanks for the advice so far.

DaytonaTurbo, I really can't say that I've noticed the vapours comings from the breather before but I'm thinking the PCV system is hampered by the lack of vacuum and can pull only so much. I figured once I have the vacuum problem resolved, the PCV system will improve and the vapours should stop coming out.

Thanks again everyone,

Mike.

Quote:

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Has the motor always done this or is this a recent development?

Quote:

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5. The breather on the passenger side valve cover has vapours slowly coming out and the PCV valve is working fine.

---

If the PCV valve is working fine on the driver's side valve cover, the breather on the passenger side valve cover should be sucking air IN not pouring vapors out unless the pcv isn't working right or you have piston blowby.

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Another angle to look at is that if you are slightly on the lean side (not too lean) but just on the lean side at idle or even a lot of times with the right mix the engine may have a tendency to pick up speed with a slight choking, up to a certain point before it loads up and kills out. On some of out old mechanical choke cars we would use the choke to get a faster idle at times if we were using it for something requiring a faster idle.