low vacuum problem?

[PeteF]

looking for comments on sudden low idle vacuum. 351W rebuilt by Jasper Engines in 2001, now with 41,000 miles. hyd flat tappet cam giving 365 hp dynoed when new. Holley 750, vac secondaries. I am second owner and engine has always pulled 13 in-Hg at idle over the three years I've owned it. Seems low to me, but I thought it was due to the cam, slight lope at idle. Last few months I notice the engine doesn't feel as strong, doesn't scare me at full throttle like it used to. Vacuum now reading a steady 9 in-Hg at idle. Google advice says causes could be 1) timing (checks good, 10 deg initial, 34 total) 2) late valve timing (can a double roller chain jump a tooth? hard to believe) 3) low compression (checked it today, about 130 on all cylinders cold) 4) intake manifold leak ( squirted fluid all around intake, no change, then removed carb, sprayed gaskets with Permatex red silicone spray, reassembled, checked all hoses, PCV, etc. Still 9 in-Hg after all that. Car runs good, but not scary good, and why the drop from 13 to 9 ! What am I missing......do any of you run that low a vacuum ? thanks for any comments

[Gaz64]

Have you some specs of the engine?

Capacity, cam timing, compression, ring thicknesses etc.

I'd run a little more initial timing, in the "teens" and limit your total to 34.

I've had V8s with big cams idle at 4 - 6 inches.

[vettestr]

I can think of several reasons for a low manifold vac reading. Here are a couple of things to do or check in search of problem. I doubt timing has jumped a tooth but timing could be set wrong or wrong point dwell or bad electronics in MSD box.

A= Advance initial timing until maximum manifold vac. is achieved and then check with timing gun to verify dampener has not spun on hub. B= Bad or stuck PCV valve will drop manifold vac reading. C= Adjust carb idle screws to set air/fuel for highest vac reading and remember you should be able to kill engine by closing either side or you have a vac leak. A pyrometer will help identify a lean cylinder. Take a reading about an inch from the head/header flange on each tube. Hope this rings a bell for you.

[PeteF]

I don't have the cam specs, but the first owner told me the compression is 9.5:1. I checked vacuum with and without PCV, no significant difference. Also adjusted idle trim screws to maximize vacuum. I'll try advancing the timing as you suggest, to see how high a vacuum I can achieve. The pyrometer is a good idea also, I have one, but I think it maxes out at 250 degrees or so. The ignition is a standard Mallory Unilite with vacuum advance, but I checked vacuum under the carb with and without the advance connected. Thanks for the help so far. I feel like I have an intake leak somewhere, but this has me stumped, since I didn't make any changes and that's a lot of vacuum to lose.

[olddog]

It is possible for the intake gasket to leak on the underneath side and still be sealed on top side.

Has it started using any oil?

When the carb was off, was there any signs of oil in the intake? If it is a single plane you should be able to see into the center cylinder intake ports. That is the best place to look for oil.

It wouldn't hurt to run a torque wrench over the intake bolts and see it they are loose. If any tighten up, see if it changes the vacuum.

If you replace the intake gaskets make sure they are metal reinforced. Victor something is excellent gaskets. I forget the spelling, but it is something like Victor Reinze.

[PeteF]

Yeah, it's possible that the underside of the intake is leaking, and I thought of that, but wanted to check everything else first. The engine doesn't use any oil, and there was no obvious oil in the (dual plane) intake under the carb. The plugs were a little dark and damp though. I'll check the intake torque next. Thanks

[vettestr]

Any chance you found the loss of vac. since your last post? I know you needed to verify timing still so maybe... A pyrometer is available from Harbor Freight for around 15 bucks on the lower end of prices. I bought one and it tracks with my higher quality and priced unit just limited on options and bling. The pyrometer should show a temp difference on the lean cylinder/s if manifold or gasket is leaking.

Were you able to kill the engine at idle by closing the idle mixture screws? Close one side at a time but either side should stop the engine from idle.

[PeteF]

Yes, I could kill the engine by closing the idle mixture screws. I'm going to check the torque on the intake bolts next. I also notice that the vacuum is higher right after starting the engine from cold, getting around 14 in-Hg, but dropping to 9 when fully warmed up. I also get zero vacuum during cranking, supposed to get 4 - 5 at least. Thanks for the pyro tip. I won't have time to work on this until next week, Vintage races car show at Limerock tomorrow. And, I can compare notes with other guys there too.......

[PeteF]

Update: I checked the intake manifold bolts, and they were indeed under torqued. All around 15 ft-lb, should be 25. So, in sequence, I brought them all up to 20, then up to 25, and was feeling pretty good at that point. But, after start and warmup, still 9 - 10 in-Hg at idle. I then advanced the initial timing to the max 34 degrees, to see if vacuum would increase, but 10 in-Hg is all she will pull. My next step is to my local trusted mechanic, to see if he has any ideas. Will also look into the pyrometer idea mentioned here. I guess it's possible that the intake gaskets are compromised and just retorquing may not be enough........or there is some other leak source I haven't found.

[vettestr]

Well Pete you have checked all I would look at. The pyrometer may identify hot single of pair of cylinders but who knows. With the intake as loose as you found her I am betting the gaskets are gone. Let us know what you find.

[PeteF]

UPDATE: low vacuum problem solved. My trusted local mechanic ran a smoke leak test and found a bad vac advance diaphragm. He then disconnected the vac advance and retuned the engine to run on centrifugal advance only (until I can replace the vac canister). He said he increased the torque on the intake manifold bolts slightly, and raised the carb float levels slightly, and set the initial timing at 12 degrees. Then adjusted the idle mixture screws. The engine now idles smoothly at 15 inches of vacuum and runs great. I am amazed and baffled, because I could never get more than 13 in-Hg vacuum, and I tried increasing the timing with the advance disconnected, and got no improvement in vacuum. I don't yet understand how increasing the float levels could have helped the vacuum, but that's the only thing he did beyond what I had already tried. He told me that he raced Boss 302s in the 70's, so he knows how to tune Fords. He also recommended running the vac advance to the ported/timed vac port above the throttle plates. I have always believed that manifold vacuum was the best source for pre-emission engines. He said for best performance you want advance to come with rpm, and to make sure the total timing does not exceed about 38 - 40 degrees. (If you Google this subject, you get mixed reviews of ported vs manifold vacuum advice). I may just leave it centrifugal only, and give up some mileage efficiency. I had a dual point no vacuum distributor on my '66 Shelby years ago, and it ran fine. I'll see what kind of mileage I get, and decide from there. Thanks for all the help and comments. Maybe this experience will help someone else someday.