vacuum loss

[jjones]

I have a 259 w/Edlebrock 500 cfm carb, port-matched heads with R3 valves, Harbit R2+ cam, T-bow electronic distributer, and TH350 transmission in my M truck. Mid 70s Corvette master cylinder and booster. When the truck is warmed up and stopped at a light or stop sign, it will suddenly lose vacuum and rev up to around 1400 RPM. Take it out of gear and it slows back down but is still low on vacuum. Otherwise holds around 17 inches. Its intermittent and driving me crazy. Truck went about 17,000 miles with no problems then started this. So far, I have replaced the booster (twice), check valves, modulator, all rubber lines, and installed a large vacuum canister. It does it with the PCV valve taken out of the system. The system will hold 21 inches of vacuum overnight.

Any ideas?

[PackardV8]

You've done a good job of troubleshooting so far. My suggestions:
1. Temporarily replace the vacuum gauge and hose.
2. Temporarily remove and plug the distributor vacuum advance port.
3. Replace the intake manifold gaskets. (Have you measured how much, if any, your heads may have been milled? This can cause intake manifold alignment problems, which leads to intake manifold gasket problems.

thnx, jack vines

[jjones]

Thanks Jack, I hadn't thought of the intake gaskets and have been wanting to block the exhaust crossover anyway so will give that a try. I have already tried plugging the advance and heater/ac controls. I have been building hot rods for 50 years and have never run into anything quite as aggravating.

[PackardV8]

Hi, Jeff,

While you are have the intake manifold off, replace the carb-to-manifold gasket. Carefully check to confirm the new gasket seals all the passages between the two. Over the fifty years since the AFB and the Stude intake were designed, there have been many variations culminating in the Edelbrock. Some of the gaskets made over the years are not compatible.

Also, carefully check your manifold hold-down clamps. These can split and lose their clamping ability. The split can close up when the bolt is loosened and thus be difficult to see.

thnx, jack vines

[studebaker-R2-4-me]

I had the same problem last year with little vacuum. I replaced the intake manifold gaskets with the fiber type. Before you even start thinking about replacing the intake manifold gasket, order 8 NEW intake manifold clamps from SI and keep the spares in your Stude junk drawer. As soon as you start torquing down the clamps to 26 lbs you will find that 3 of the 4 clamps will hair line split and not torque down to specs. I had my intake off last week and split 3 of the them before I reached 26 lbs.

Good luck

Allen

[jjones]

Thanks,

I have the fiber gaskets and thin stainless pieces to block off the exhaust crossover. I put new manifold clamps on the engine when I rebuilt it several years ago because all of the original ones were cracked, but will order new ones just in case.

Thanks again.

[jjones]

Took the manifold off and checked gaskets, checked manifold and carb mounting surfaces for warps, cracks, etc. Everything looked fine. Magnifluxed manifold for cracks, none found. Replaced manifold and carb gaskets with new fiber ones, added new clamps to the hold downs. Same problem, no better or worse. I will install a vacuum pump and do away with using engine vacuum for anything but advance and modulator. Does anybody out there know anything about the Ford F150 diesel electric vacuum pumps? $90.00 vs. $260.00 for aftermarket pumps at speed shops and online.

Thanks

[PackardV8]

Jeff, yer truck, yer money, yer decision. However, installing a vacuum pump is akin to killing flies with a sledge hammer. Why not first test your hypothesis that it will run perfectly with no vacuum off the intake. The truck will be drivable with all the vacuum ports disconnected and plugged.

1. Disconnect and plug every vacuum port. Drive the truck all day. Any sign of the malady? If not, go to Step 2.
2. Re-connect only one vacuum port per day. Drive the truck long enough for the vacuum leak to reappear if it is going to.
3. When the malady does reappear, you'll have isolated the problem component.

thnx, jack vines

[jjones]

I agree with you , Jack, I am just at my wits end. I have driven it with everything but the booster plugged, indicating the booster is the problem. I have replaced it twice now (2nd time on warrenty) with the same results. The engine itself seems to drop vacuum intermittently. I talked to one of the old-time racers today at a Memorial Day parade--he suggested a tight valve or two and asked if I was running oil with ZDDP. Its never had anyting in it but Valvoline 20-50 racing with zinc so that shouldn't be the problem. He talked about all kinds of problems with flathead Fords--like lifters wearing strange and changing valve lift as they turned in their bores, etc. He is in his 90s and has been racing Fords since the days of the Model T, and is still sharp as a tack. I will run the valves next weekend and see what happens. If it is that simple, I am going to feel really stupid.

Thanks for your help

Jeff

[Mike Van Veghten]

Don't know what to tell you Jack hasn't already...but one or two valves being tight....#1. isn't gonnal loose you that much vacuum...and 2. it would idle poorly.
If in fact the engine is idling rough, then it possible could be part of the problem.
When was the last time the valves were checked?

I've found in my daily driver 259...I need to adjust the exhaust rockers three or four times to one intake adjustment.

Mike

[bige]

The loss of vacuum AND coinciding INCREASE in idle speed is a real mystery to me. It seems that the loss of vacuum should be accompanied by a decrease in idle speed unless of course you were to blip the throttle then a momentary loss could be explained. And, if that weren't confusing enough when you take it out of gear the idle slows instead of increasing as it normally would when the car is no longer fighting the converter.

Vacuum leaks just don't seem to be the answer. Rarely are they intermittent.

I can't help but think there's something going on in the carb. Too strong a spring on the metering rods or mismatched one strong one weak that allows the rod to lift at idle loading the engine a little dropping rpm and vacuum but not enough to kill it. Then the engine starts to absorb the extra fuel raising the rpm. Take it out of gear and rpm should raise at least momentarily maybe just enough to pull the rod down where it belongs stabilizing idle some.

If a rod is stuck up or not properly centered in the jet you might have done all your initial settings, idle speed, timing mixture etc. with a maladjusted carb. Then depending on ambient temps, length of time idling etc. you could get some odd things happening.

I know I'm digging deep but that increase in rpm is the fly in the ointment. Somehow there's some fuel delivery coming into play because I can't help but think you need more than just air to cause a jump of 500-700 rpm.

I hope you figure it out and post the resolution.

ErnieR

[leyrret]

It maybe be the line to gauge or gauge. Have you tried plugging it off at engine.

[tinkerptw]

Check the throttle plates being properly secure in the throttle shaft. Also check throttle shaft having proper fit in carb.

[jjones]

The valves haven't been set for around 6,000 miles but it idles fine, is not noisy, and is quite peppy so I suspect they are ok. I do have a good Carter 620 AFB here and will try that to see if it makes a difference. The carb is a fairly new Edelbrock 500 cfm, with the factory jets and the metering rods one step (spring) below factory, so should be ok, but you never know. I can cause the same increase in idle speed by removing either the PCV or booster hose, and just assumed that to be normal. Is it? I have everything plugged but the booster, and tried to run it yesterday with the booster plugged but that was a bit too exciting with almost no brakes.

Thanks all, and if I manage to figure it out, I will certainly post the solution.

Jeff

[jjones]

Well, I have done all of the stuff mentioned above. Adjusted the valves--no change, exchanged the carb with another good 500 cfm Edlebrock borrowed from a friend--no change, ran the compression--135 to 140 psi in all 8 cylinders. I gave up and installed a vacuum pump for the brake booster only. That fixed the speeding up problem but the engine still drops vacuum intermittently at idle with the automatic transmission in gear. Does it with everything plugged including the gauge. With the booster hooked up it would drop to around 2-3 inches. Now it drops to around 10 inches--not enough to speed it up. Take it out of gear and it will gradually rise to its normal 17 inchs.

I have no idea what is causing this but at least it is safe to drive now. One thing I have noticed is that it usually happens after hard acceleration. I do know how to fix that, but having had a heavy right foot for at almost 50 years, that just ain't gonna happen.

Thanks for all your help

Jeff