Has anyone had any trouble with low vacuum on freshly rebuilt motors? I have a 70 455 freshly done with a comp cams cam for a 70 442 AT. It ran good during break in at 2200 rpm but doesn't make any vacuum at idle so it won't power the brake booster and stalls immediately when put in gear. The correct 257 carb has been rebuilt and a pertronix kit in the dist. has been installed. With only 4-5 inches of vacuum I decided to pull the front cover off and check the timing gear alignment and found it oriented correctly. Next I put a degree wheel on it and found the intake valve opened 10 degrees later than the card says it should. Has anyone had this happen to them or am I the only one?

 

Cam profile?
How did you install it? Straight up? Did you install the timing set or did the builder?
Verify no vacuum leaks.
Is TDC really TDC?
Verify timing is in the ballpark. Try advancing the dist while it's running to get the highest vac reading. Does it respond?
Report back.

 

I had that happen with the previous cam in my engine. I installed it dot-to-dot and it had no power. I got a degree wheel and checked it, found it was 8 degrees retarded. So now I always recommend degreeing the cam and not trusting the alignment marks are correct.

And as asked above, what is the distributor initial advance setting? More timing = higher vacuum.

 

I had it at a base timing of 12 degrees, no vacuum leaks. I advanced the timing at least a whole terminal on the cap and the reading went up to about 9 but still wouldn't run in gear. The builder installed the chain set and I check it and it was right on dot to dot. Fun 71 who's cam did you have?

 

The cam is 474" lift 308 duration comp cams version of the 70 442 automatic stock cam. I compared the specs to the factory cam and they are the same.

 

It was a Crower cam but I never checked to see if the cam was off, if it was the cam gear, or a combination of the two.

 

My experience with the last two Oldsmobile timing sets is that they are WAY OFF! One was a 3 key way, and the other was a 9 key way. With the 3 key, I ended up retarding a tooth, and then using the advanced key way to get it close. Using a degree wheel is no longer optional!

 

I have three gear/chain sets I've collected over the years and I was comparing what the builder put in and they are all visually the same so I'm putting the blame on the camshaft.

 

I would also be somewhat cautious about putting the blame on the timing chain. I recently installed a new Cloyes double row chain (no cam change) and found it to be spot on. I would think any reputable brand would be good but there are several cheapies out there. Who knows?

 

There are manufacturing tolerances with every part, so it is possible to get a cam and timing gear combination that each are just within spec, but put together add up to too much variation.

 

KISP keep it simple, please....
That low of a vacuum reading is indicative of a large vacuum/intake/carb leak or an extremely retarded cam. A systematic approach is needed before assigning root cause (8D/DMAIC/Shainin).

-Did you verify TDC is TDC? Check it again. Pay attention to cylinder #1s rockers and the dist rotor.
-Do you have the correct QJ carb base gasket? There are several.
-Do you have another carb to try?
-What type of intake gasket set did you use?
-Did the intake get machined match to the heads, (if the deck and heads were milled?).
-Block off all accessory ports on the carb, including the brake booster. Is the problem still there?

If it ran at 2200 rpms without balking/pinging/puking etc and it won't idle it's very likely a vac leak. Or that cam is WAY retarded. Again let's do KISS thing first.

 

I checked TDC with a piston stop and a degree wheel so that's correct. The rotor is facing number one on the cap and the rockers on number one are in the valves closed position. We had the carb that the car came in with and tried that with no change and the gasket is correct. The engine builder put the intake on and used a turkey pan. The block and heads were milled a bit and I don't know if the intake surfaces were matched though when I dropped the motor off we had a conversation about the milling formula on Oldsmobile motors. He said he was familiar with it though he used
RTV on the front and rear of the intake to block surfaces instead of the rubber strips so I don't know if he did it or not. I pinched off and removed vacuum lines at the carb and there is no change. We were to busy to get to it today but tomorrow when I go in I am going to advance it with the +4 keyway and degree it again and if that isn't enough to be close I will start moving the cam gear one tooth at a time and keep doing it until I get there. I will briefly start the car with a vacuum gauge on it and see what I get.

 

Sounds like a plan.
Sounds like you have the basics covered.
Be interested to see if the cam is that far retarded.
Just for ***** do a compression check.

 

I'll post what I come up with tomorrow night..

 

Well I found the problem with the motor today, the camshaft was 7 degrees retarded. So I used the advanced keyway on the lower sprocket and that brought it back to within a degree of the cam card. You can't trust anything to be as advertised these days, bunch of junk. So I bolted on the damper, looped the cooler lines, mounted the fuel pump, hooked up a vacuum gauge and timing light and fired it up. I only ran it for about thirty seconds and had 15" of vacuum so I'm pretty confident it's fixed. Tomorrow I'll put it together and tune it. I won't be in a rush to use Comp Cams again.

 

Wow that is a eye opener! I would never think a name brand timing set would be that far off.

 

Well I spoke too soon about thinking I had it fixed. Put the whole thing together and fired it up and initially had 12 or so inches of vacuum but it didn't run very long before the vacuum started dwindling and the car was dying. It definitely started easier but it's not right. If you hold the throttle up it runs real nice with decent vacuum but at that point your at 12-1500 rpm and timing at about 14-16 degrees. I am not sure what is going on with this thing but tomorrow I think I'm going to get it back on top dead center and see how accurate the balancer is. The engine builder put a new balancer on it during rebuild but who knows what I'll see. It ran better with me cranking the timing up to about 20-25 degrees but something is still wrong with it. Over the years I have built probably 20 or so 455s and never even heard of this issue never mind experience it.