283 Normal Vacuum Level?

[Mike B.]

What is an expected normal vacuum range at idle? The engine is a 1961 283, factory basic cam grind, hydraulic lifters, 245 hp, dual quad equipped with about 150 miles since total rebuild. The engine is timed to specs actually leaning a bit towards advanced, but the best vacuum I can achieve by fiddling with the carb front air/idle screws is around 15". It runs great but I expected something in the 18" range. The vacuum gauge needle is steady so I don't suspect a leaking valve although maybe they are a smidge too tight without enough lash. Any ideas or feedback is appreciated.

Thanks.

[Duke W.]

Re: 283 Normal Vacuum Level?

That seems a little low, which could indicate more overlap than the OE cam, however, the lack of vacuum advance on 245s lowers manifold vacuum. Also, idle speed is critical - the lower the idle speed, the less manifold vacuum, so at what idle speed are you taking readings, and is it in neutral or PG in Drive?

Do you have a vacuum at idle speed reading prior to the engine rebuild?

Though the original cam is no longer available, the sucessor cams including 3896929 and 14088839 should yield about the same vacuum - 17-18"@500 in neutral with manual trans, and about 15"@450 in Drive with PG on engine's with full time vacuum advance.

You talk about "lash", but hydraulic lifters should have none. They should be "preloaded" 3/4 turn down from zero lash for normal operation, but additional limiting speed can be obtained if they are preloaded only 1/8 to 1/4 turn. So how were the lifters preloaded.

Also, what is the manufacturer and part number of the "factory basic cam grind"? The answer could be there.

Leaky valves or rings can be confirmed with a leakdown or compression test.

Duke

[Mike B.]

Re: 283 Normal Vacuum Level?

Duke,

Thanks for the response. The cam is a GM "929". The car is manual so the readings are in neutral at about 600 rpm although the vacuum does not increase much at slightly higher revs. The distributor is a correct 891 but I had swapped an aftermarket Mallory in and out to diagnose another problem and it made no difference (I rotated both through a reasonable retard/advance arc and could not get the vacuum any higher).

I don't have previous vacuum readings as the engine was an jumble of incorrect parts and an agressive aftermarket cam. It has been rebuilt to as close to stock as possible.

I don't suspect leaky valves or rings as compression test after break in had consistent results (I recall around 130#), plus there is absolutely no smoking or oil consumption. The plugs are burning clean with a slight brown tan electrode.

I preloaded the lifters from new on an engine stand using the spin-the-pushrod-until-slight-resistance method at respective piston TDC. And yes, preload is a better word than lash for hydraulic lifters but that escaped my word box at that moment so thanks. Regardless, I have a sneaky suspicion that is where my problem lies - they may be a hair too tight. Do you suggest that I reset the valves now that the lifters are fully loaded using your method? There is certainly no rocker noise indicating sloppy fit so there might be some gain and little to lose by backing off the nuts a 1/4 turn.

But maybe I am barking up a wrong tree. Since there is no normal vacuum port on the 2x4 aluminum manifold, I am forced to take readings off of the rear carb base plate port. This is above the throttle butterfly and I wonder if that can cause erroneous readings, especially at idle.

I really appreciate your expert advice and ideas.

Thanks, Mike

[Duke W.]

Re: 283 Normal Vacuum Level?

"I preloaded the lifters from new on an engine stand using the spin-the-pushrod-until-slight-resistance method at respective piston TDC."

Once the pushrod/rocker won't wiggle, then you are at about zero lash and you preload 3/4 turn from there. Is that what you did?

The 929 cam should provide about the same idle vacuum as the original cam since they both have about the same overlap.

I think your car has vacuum w/s wipers, so tapping into that vacuum source should give you true manifold vacuum. IIRC '62s have a hollow carb. mounting stud to power the wipers, but I don't know about your configuration.

130 psi is low compression, but if they were all consistent, then it's probably okay.

Duke

[Mike B.]

Re: 283 Normal Vacuum Level?

Yup, I preloaded 3/4 turn at zero lash - just like the engine rebuild manual directed. But I am going to look again. Something tells me that I may have gotten overzealous as I passed through and "double checked" a few that may have already been tightened on a previous rotation. My guts are pointing me there.

The wiper motor is electric. I know about that hollow stud which is not correct or necessary on the non-vacuum advance distributor, but it is a great idea for the sake of tuning. Then again, it might not work as I don't know if the aluminum manifold stud hole is drilled through into the plenum like the cast iron version.

And the compression might have been higher - I can't remember. I was just confirming consistency and didn't bother to write it down. Maybe it was more like 140#.

I am going to review the valve preloads tonight and will report back tomorrow.

Thanks again,
Mike

[William C.]

Re: 283 Normal Vacuum Level?

Mike, what do your windshield washers pick up vacuum from? They should be plumbed to a direct manifold vacuum source, for sure if they work!

[Mike B.]

Re: 283 Normal Vacuum Level?

Bill,

Unless I am missing something, my wipers are electrically, not vacuum operated. The car is a 1961. I am 99% sure that it should be electric. The motor assembly looks to be right. But who uses the wipers anyway? I'm afraid that those under dash cables will whipsaw through some wires!

Mike

[Mike B.]

Re: 283 Normal Vacuum Level?

Post Script: I checked the valve clearances by backing off then resetting every rocker 3/4 turn after the pushrod friction point. If anything some were a bit lose, not too tight as I suspected. It made no difference - vacuum is still around 15-16" at idle. I tried moving the distributor while running and that made little difference until it was severely retarted wherein vacuum dropped significantly. I also fiddled with the four carb air adjustment screws and as it should be, the highest vacuum was found when it ran the best. But that did little to help.

I reset the timing and took it for a road test. The car runs out absolutely perfectly at all revs and gears. It will quickly pull back to idle with no wandering or desire to stall. Am I missing something? Maybe there is a slight leak somewere. It idles well but not perfectly in that there is a bit of roughness which I thought was somewhat normal, especially with dual quads. But I can't get the idle speed below around 650 rpm which now that I think of it may be due to an intake leak (the idle speed screws are backed all the way out). I will try a propane leak test around the intake mounting points tomorrow night.

Or, I am beginning to wonder if my vacuum gauge is out of calibration. It is about 30 years old. I think I'll put one on my Hershey list for this weekend.

As usual, all ideas are appreciated.

Thanks,
Mike