Erratic Vaccuum readings
 

Checked everything: compression, valve adjustment, ignition, fuel everything works.

When I start, vacuum is good then about 20 seconds later It go's crazy shakey and low. Stop let sit 2 min's start again and same thing.

What is it?

Carb or EFI, crossed plug wire.

250 blower with 2 holley 850.

please explain crossed!

2 cylinders firing out of order, spark plug wires are on wrong cylinders, still idles, vacuum dances all over the place, power is down pulling out. just a guess.

If a plug wire was crossed the bad idle would consistent.

My idle starts good and then go's bad in 20 sconds. Would a crossed plug wire do that?

Is this a new build? Maybe a lifter (or several) adjusted too tight. When they pump up, the valves are hanging open. Or broken valve springs

Could be a leak. Spray brake kleen (non chlorinated ) around all vac ports (like the boost port for gauge) and see if revs jump up. Also try around base of blower and intake....

No probably not, any black smoke when it's doing this ?

Does the engine sound like its missing at idle and shaking? Or is it just revving up and down like blower surge?

Restricted exhaust?

Missing, for sure I can feel each header tube and the front two are cold and I have spark.

Checked headers, all OK.

Checked all valves and compression is good.

Wires are correct and swapped with other good motor.

Not much more to check Maybe the front carb is screwed up causing those cyls to either run lean or flood? seems to be a very odd set of symtoms. keep us posted

Float needs adjusted, carbs over flowing, just a thought.

Typically when a vacuum gauge is erratic, there is a problem in the valvetrain. Either a rocker is out of adjustment, a lifter has collapsed, pushrod out of the rocker, etc. Pull the valve covers and start looking. I would bet that you will find your problem.
Eddie

Swapped carbs with good motor, no change

Swapped carb with good motor, no change.

Compression is good and I checked and adjusted the valves.

Remember, it starts out with a good idle and go's bad.

I noticed a tiny drop of water on the spark plug when I pulled it out so I pulled the header and there was small amounts of water in each pipe (tea spoon). I'm thinking it was reverting water when the idle dropped down lower. So I swapped headers with the good motor and same bad idle. Could it be that there is a crack or blown head gasket that when the cylinder warms up the crack or leak opens and lets moisture in and shuts down the spark plug.

If you are certain you have eliminated leaking exhausts, then it sounds like a headgasket issue. unless you left water in the motor and it froze over the winter possibly cracking the heads and or the cylinder bores.

It seems really odd that just the front two are having issues.

could you post a video link of it happening from smooth startup to when it starts missing? You tube or something...

I'm in California. Do you really think that can happen?

Burnt valve that acts up when getting warm??

I have good compression. Would I still have good compression with a burnt valve?

http://www.earlycuda.org/tech/vacuum2.htm Head gasket

:eekdrop:

I dont know does it get be freezing, where you are at in cali? Water turning into ice will crack metal easier then you snapping a toothpick.

If you think its a water issue, just remove the water from the equation as I have recommended prior. You wont overheat it idleing for 3 mins.

:eekdrop:

Well I figured it out!! STUCK VALVE. Did a leak down test and found air coming out of the #2 exhaust port. The reversion was from the open valve that was sucking water and shut down my spark plug. So I pulled the head sprayed WD40 on it and it went right back in but seem a little tight. No salt water reversion it's from the hose.

Is this happening because it hasn't been started for four months or should I look for something else?

What should I do to fix this?

Might want to take the head to a machine shop and have it checked, guide clearence, bent valve, worn out springs, just a thought.

http://i1317.photobucket.com/albums/...psa18a0c09.jpg

Well I pulled the valve out and found pitting on the stem. Same valve bent last year so I had a valve job done and it ran fine the rest of the year. The mechanic didn't tell me why the valve bent, maybe it stuck first. Coincidence that it should happen to the same valve? What causes that? Is there something wrong with the head / guide?

What brand valves are you using, are they Inconel ?

That's what I was told.

Did you stick a magnet to it to see what it does, you might want to look at all of the valves.

What should the magnet do?

Nothing if it's inconel.

It's not magnetic.

:eekdrop:

Inconel doesn't rust.

I didn't notice any rust. I could put it in water and see what happens.