Low Manifold Vacuum

"problem" performance continues in a rebuilt 2.8L GM ..... Engine now has 4000 miles on it and still has relatively low manifold vacuum. Vacuum is running 14-15 in hg. at idle and should be a few inches MORE vac. to adequately run all the vacuum controls: Brake booster, heater controls, dist. vacuum advance, *power piston* (2SE carb), etc.

I checked and checked for manifold leaks, reset the damn manifold gaskets three times.... etc. Im pretty damn sure the problem is the engine 'mechanicals' . Compression is 130 psig all cylinders Mechanical valve timing has been degreed, and verified OK. Ignition timing perfect - both by vacuum gauge (max rpm less 1/2" hg.) and by accurate degreeing + timing light. --- both methods equate to the same EXACT timing advance. NO leaking vacuum hoses or controls. If I remove a vac. hose from the manifold port the engine

**increases*** rpm --- which tells me I dont have a leak. Poor vacuum causes power piston in carb to lift 'early' (for over-rich mixture) - if I manually hold power piston / metering rod and remove a manifold vac hose = same rpm increase.

Engine was broken in with 20 weight straight weight non-detergent oil.

Im about ready to dump a half can of cleanser down the carb to scuff the cylinder walls in desparation and run the valve lifters with ZERO lash ......

Your sage advice would be appreciated. I need another 2-3 inches of vacuum to make all the vac. controls, brake booster and power piston-ed carb. to operate properly. Any advice of why this damn engine is running 'short' by about 2-3 inches hg. of vacuum and what to do about it would be VEEEEEEEERRRRRRRRRRYYYYYYYYY much appreciated.

;-)

Reply to
Rich Hampel
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Reply to
L.W.( ßill ) Hughes III
1st up, I'm not familiar with the 2.8, but is it possible you have a leak in your brake servo ? You say the problem continues in a new engine ?

Dave Milne, Scotland '91 Grand Wagoneer, '99 TJ

Reply to
Dave Milne

I would guess that your valves are a bit short on time if you don't have zero lash on them. My book calls for zero.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's

Rich Hampel wrote:

Reply to
Mike Romain

I've isolated EVERY exterior control and vacuum body including the brake booster and am pretty much convinced that its an 'engine' problem.

Im us> 1st up, I'm not familiar with the 2.8, but is it possible you have a leak in

Reply to
Rich Hampel

At cruise speeds and with the throttle plate essentially closed there isnt much difference of manifold vacuum than at idle -- just an inch or two. .... just low vacuum. Ive isolated and locked-out all the vacuum accessories one by one to prove to myself that these were not the cause.

Reply to
Rich Hampel

Yeah... my book has zero lash ... but is defined as frictional contact on the pushrods, then 3/4 turn on the friction nut.

I d> I would guess that your valves are a bit short on time if you don't have

Reply to
Rich Hampel

I can put in a less strong spring under the power piston in the carb; but, I'll still be operating the brake booster with insufficient vacuum.

It all comes back to (and no matter what the 'numbers' on the vac gage are) is that this engine just isnt getting proper vacuum in the intake manifold and Im stumped on how to increase the vacuum.

Its not the carb as I have set up the carb on a stationary engine pulling proper vacuum .... and when I move it to the Jeepster it runs like SH*T because of the low vacuum. Yeah I can re-jet and fiddle with different thickness metering rods .... but the real problem is LOW manifold vacuum !!!!!

Ive never been quite so 'stumped' like this before ... therefore the posting for HELP!

:-)

Reply to
Rich Hampel

haha...you beat me to it Bill!

We use compressors at work and were chasing a leak for a week (I can rhyme, what a time!). In the end it turned out the leak was in the regulator guage. Didn't soap there! Ah well!

Reply to
SteveBrady

All good ideas. Also, it is wise to look for exhaust restrictions, especially the convertor. This will cause lower vacuum as well. One other thing that I didn't see if it was posted, is the camshaft stock or aftermarket? An aftermarket cam with more duration will cause lower vacuum as well.

Chris

Reply to
c

You know, my instinct says that you might have a problem with one or more of the vac loads. We have heard of vac boosters on the power brakes failing and letting the vac pressure drop. I have to wonder if you are suffering for something along this line.

I am not sure, but I think the test for this is to unplug all vac loads in the system, and block them off. See what the vac is, then plug each in one at a time and see where the vac drops.

Reply to
Jeff Strickland

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