Vacuum leak?

So I fired up the motor after the HG change without adjusting the carb (leaving it how it was tuned when the car was running good before). The car idled with 16 degrees of base timing previously.

This time, the car needed at least 20 degrees to idle. So I had to turn the screws in a full turn on both sides to open up the butterflys. The motor has 1/2 a point less compress. and more lift than it did, could this account for this? Could a vacuum leak account for this?

:search:

Also, I noticed the motor takes longer to settle back to idle speed after I rev it up.

How did you install the intake ? You saying less CR, did you use thicker head gaskets ?

I would say that since you had to open the throttle to get more air, that precludes any vacuum leak.

Bird, what do you mean by precludes?


Karsten, I've always had intake sealing problems with this car. So I installed the double thick gasket coating it entirely with Mopar silicone adhesive, both sides. Yes, the HG dropped my comp. 1/2 a point.

Intake gaskets come in several thicknesses. When I installed my intake I raised it to where all bolt holes line up perfectly, I placed steel shims at the front and rear walls, then measured what thickness gasket I needed. I came up with .125" thick gaskets, available from felpro/Summit (standard gaskets are .065"). I used RTV instead of the rubber strips. No leaks :thumbs:

If you needed to open her up, it's probably sealed pretty good. Check the outside with propane/starter fluid. Pull the PCV and breather/plug them, and throw a vacuum gauge on the dipstick tube, and look for vacuum.

big2bird said:

If you needed to open her up, it's probably sealed pretty good. Check the outside with propane/starter fluid. Pull the PCV and breather/plug them, and throw a vacuum gauge on the dipstick tube, and look for vacuum.

Click to expand...

Hell, carb cleaner and WD40 will change the idle enough to find a vac leak. What is your current Vac reading at idle?:bounce:

glenn64vette said:

big2bird said:

If you needed to open her up, it's probably sealed pretty good. Check the outside with propane/starter fluid. Pull the PCV and breather/plug them, and throw a vacuum gauge on the dipstick tube, and look for vacuum.

Click to expand...

Hell, carb cleaner and WD40 will change the idle enough to find a vac leak. What is your current Vac reading at idle?:bounce:

Click to expand...

4" which is not so unusual. The best I've ever been able to get with this motor is 6-7"

big2bird said:

If you needed to open her up, it's probably sealed pretty good. Check the outside with propane/starter fluid. Pull the PCV and breather/plug them, and throw a vacuum gauge on the dipstick tube, and look for vacuum.

Click to expand...

So if you was to take a propane torch and not light it of course,and run the tip around the usual places it would cause the idle rpm to rise?
I like that idea instead of the carb cleaner,its hard on the paint.

Those tests do nothing for me, when this manifold has leaked in the past, it leaks from the lifter valley side.

enkeivette said:

Those tests do nothing for me, when this manifold has leaked in the past, it leaks from the lifter valley side.

Click to expand...

Ok, that is new to me, please explain.

When idling, if you do the propane test, the motor should speed up as the gas goes through the leak point and into the combustion chamber...even if you only have 4" of vacuum. No?

enkeivette said:

Those tests do nothing for me, when this manifold has leaked in the past, it leaks from the lifter valley side.

Click to expand...

Then it would pull a vacuum at the dipstick hole when you seal up the engine. Correct?

I can get 12-13" vacuum on Ol' Red. And that cam has big bumps.

What do you get without the S/C turning? Could it be putting a little pressure in the carb/manifold at idle?

enkeivette said:

Those tests do nothing for me, when this manifold has leaked in the past, it leaks from the lifter valley side.

Click to expand...

When that has happened to ME in the past, it was always NO smoke from exhaust but the oil consumption was rather high, about a quart/thousand miles maybe a bit less in miles....

IF you have had those heads milled at all, or someone else did, the shops frequently forget to check the milling is in correct angle to the intake plane and if it's not...for some damn reason they take more off the intake side of the head, than the exhaust edge....so that opens up the bottom of the intake gaskets....what you need do is pull that damn thing yet again, and check them gaskets for leaking and OIL in the passage floors, and wet oil stained gaskets...easy to spot on the typical standard blue FelPros....

that shit drove me crazy for a while, then I took a Edel L98 base I had to a local shop and had .020 taken off the TOP edge of the intake, on both sides, making total angle change to be .040, that forced the tops of the intakes to be more open at top compared to the bottoms...so when bolted down, THAT cured the freeking oil use....drove me crazy for a couple months.....

depending on the angle of the dangle of course, there are various amounts of this stupidity that can drive us crazy on a SBC.....another reason I preferr the old school PONTIAC engine designs....lifter valley cover, and a spider legged intake....but never bother with it on my DPFI car 350 NOW, as no one makes a LT1 intake with a separate lifter valley cover, that is affordable that is.....

:clobbered::devil:

OH, edit...obviously you can spray the hell out of everything with that open valley intake passage problem and never get jack diddly in reaction.....don't ask how I know this shit, beat my head on the wall over it......ain't been quite right since......

enkeivette said:

Those tests do nothing for me, when this manifold has leaked in the past, it leaks from the lifter valley side.

Click to expand...

If it leaks from the lifter valley side it would pull in oil. Then it should smoke. I have 8 inches of vacuum in my 400 with 254 effective duration at .050. I take it the 4 inches is in gear on your automatic?

Bird, yes you're right. But I don't have a hand held vacuum guage. That is a trick test and I'll see if I can borrow my friends guage to do it.

SmokinBBC, how would I shoot propane into the lifter valley while the motor is running? My intake has leaked at the bottom of the port in the past, not the top.

MrVette, the motor has always had this problem, even with new heads. But I'm not so sure that it is an intake manifold leak, that was just a theory.

Glenn and everyone, 4" at idle with a manual trans. I've been able to get 6-7" with some time tuning. With or without the supercharger, doesn't make a difference. I have a 110 LSA 239 245 duration with 6" rods and low compression (9:1) all of which rob my motor of vacuum. The theory I have right now is that the larger effective duration with the 1.65 rockers and the lower compression (thicker HG) is making my engine vacuum worse, creating a need for me to open the primaries, and since my primaries are not adjusted properly (20/1000) this is screwing up the tuning causing the motor to slowly drop back down to idle speed after revving. The rule of thumb as I remember is that you need at least 11:1 compression when you run a cam that has 240+ duration. I'm at 9:1 with a 110LSA! :bonkers:

I'm going to try some smaller idle air bleeds to see if I can get the vacuum up. I'll have some time to work on the car today, I'll let yall know if I find anything.

enkeivette said:

Bird, yes you're right. But I don't have a hand held vacuum guage. That is a trick test and I'll see if I can borrow my friends guage to do it.

SmokinBBC, how would I shoot propane into the lifter valley while the motor is running? My intake has leaked at the bottom of the port in the past, not the top.

MrVette, the motor has always had this problem, even with new heads. But I'm not so sure that it is an intake manifold leak, that was just a theory.

Glenn and everyone, 4" at idle with a manual trans. I've been able to get 6-7" with some time tuning. With or without the supercharger, doesn't make a difference. I have a 110 LSA 239 245 duration with 6" rods and low compression (9:1) all of which rob my motor of vacuum. The theory I have right now is that the larger effective duration with the 1.65 rockers and the lower compression (thicker HG) is making my engine vacuum worse, creating a need for me to open the primaries, and since my primaries are not adjusted properly (20/1000) this is screwing up the tuning causing the motor to slowly drop back down to idle speed after revving. The rule of thumb as I remember is that you need at least 11:1 compression when you run a cam that has 240+ duration. I'm at 9:1 with a 110LSA! :bonkers:

I'm going to try some smaller idle air bleeds to see if I can get the vacuum up. I'll have some time to work on the car today, I'll let yall know if I find anything.

Click to expand...

Ok, sounds like a good plan. Well it does if Zukie doesn't bring those girls, you asked for, from Germany.:amused:

enkeivette said:

Bird, yes you're right. But I don't have a hand held vacuum guage. That is a trick test and I'll see if I can borrow my friends guage to do it.

SmokinBBC, how would I shoot propane into the lifter valley while the motor is running? My intake has leaked at the bottom of the port in the past, not the top.

MrVette, the motor has always had this problem, even with new heads. But I'm not so sure that it is an intake manifold leak, that was just a theory.

Glenn and everyone, 4" at idle with a manual trans. I've been able to get 6-7" with some time tuning. With or without the supercharger, doesn't make a difference. I have a 110 LSA 239 245 duration with 6" rods and low compression (9:1) all of which rob my motor of vacuum. The theory I have right now is that the larger effective duration with the 1.65 rockers and the lower compression (thicker HG) is making my engine vacuum worse, creating a need for me to open the primaries, and since my primaries are not adjusted properly (20/1000) this is screwing up the tuning causing the motor to slowly drop back down to idle speed after revving. The rule of thumb as I remember is that you need at least 11:1 compression when you run a cam that has 240+ duration. I'm at 9:1 with a 110LSA! :bonkers:

I'm going to try some smaller idle air bleeds to see if I can get the vacuum up. I'll have some time to work on the car today, I'll let yall know if I find anything.

Click to expand...

Sorry...must have been havin brain cramp when reading this:clobbered: