[86squarebody]

Just replaced the top end on a 305 with stock cast heads and an aluminum intake, Everything works fine run it around town for a few days it is all good. Take it out on the hiway and run it a little hard and I get a vacuum leak. This is the second intake and have tried gaskets from Mr. Gasket and Fel-Pro. New PCV valve and two breathers on the valve covers passenger side all new. I need help, getting real good at installing an intake but didn't want to.

 

[55 Tony]

How can you tell you have a vacuum leak on the highway, like what symptoms?


Also, Fel Pro makes an extra thick (almost 1/8") gasket for BBC's. Don't know if they make one for yours?

 

[timothale]

sealing engines leaks

An old neighbor used to build circle track cars, He would hook up a long vacuum hose from his ranchero to the dipstick tube on his new engines on the stand . then use a stethoscope to listen for leaks. He would then use some gasket sealer and the vacuum would suck in the sealer and stop the leaks.

 

[86squarebody]

Intake vacuum leak

55 Tony I don't know about the vacuum leak on the hiway it is after running it hard that the leak starts like on the hiway, It happens after running at about 5-6000 RPMs for a minute or less.

 

[55 Tony]

Hmm ... that's all I got.

 

[55 Tony]

An old neighbor used to build circle track cars, He would hook up a long vacuum hose from his ranchero to the dipstick tube on his new engines on the stand . then use a stethoscope to listen for leaks. He would then use some gasket sealer and the vacuum would suck in the sealer and stop the leaks.

That sounds good, but when I had leaks they were always going to the crankcase.

 

[86squarebody]

Timothale,

That sounds like a great idea a buddy has a vacuum pump for AC that should work even better. Problem is I don't know what to do about it happening again. The gaskets seal good and everything like I said it only leaks after running at high RPM for a little bit.

 

[eric32]

I don't know if you re torque you intake or not after a few heat cycles but i always do as my new build I did back in the spring after a few heat cycles I went to re torque and some of the bolts were loose more then i was expecting and nothing bad or to worry about but loose enough that it was off about 5 ftlbs or so. I always torque mine at 25 ftlbs. I would also try and have the engine running and I use a propane torch unlit of course and let it spray around the intake if I ever looked for vacuum leaks or spray with carb cleaner and see if the idle would change.

The best thing is if you have a smoke machine and look for any smoke being sucked in around the intake flange.

 

[RWENUTS]

55 Tony I don't know about the vacuum leak on the hiway it is after running it hard that the leak starts like on the hiway, It happens after running at about 5-6000 RPMs for a minute or less.

That would be a strange vacuum leak.
In those running conditions your vacuum should be close to zero.
Do you hear something or see something?

 

[Greg T]

Use a straight edge on the intake gasket surfaces and check for warpage. My guess is that the intake it out more than .010"

 

[techinspector1]

Use a straight edge on the intake gasket surfaces and check for warpage. My guess is that the intake it out more than .010"

Agree with Greg and suspect that the block if not square.

 

[86squarebody]

Thanks for all the info guys I will post as to what I find once it is torn down. Should be soon.