Rochester Guru?

[bkreutz]

Wow, what a puzzle. Your vacuum readings are obviously too low, considering what you've already checked the only other things I can think of is a restricted exhaust (not likely by the looks of your plugs, but it can't hurt to check, the idea is that if you can't exhale, you can't inhale) Another thing to check is valve timing, it's possible that the valve timing has jumped which could result in low vacuum. Other than these two things, I'd start re-checking things again. There's something amiss, just got to find it.

[jonnyc]

The exhaust system was sitting unused for about 5 years along with the rest of the truck, might be something there. Any way to check that? Block the end and see if there's any change?
When you say a change in valve timing, are you saying that the chain might have slipped? That's not a job I want to do again any time soon.

[bkreutz]

The exhaust system was sitting unused for about 5 years along with the rest of the truck, might be something there. Any way to check that? Block the end and see if there's any change? You should feel sufficient pressure on your hand covering the tailpipe. (not very scientific but will show a problem)
When you say a change in valve timing, are you saying that the chain might have slipped? (more accurately jumped a tooth or two) That's not a job I want to do again any time soon.

Is this something you've already done?, how recently? Considering the time you've spent with this it might be worthwhile to make sure that it's still in proper time.

[jonnyc]

The exhaust check....never.
Timing chain....2 years or so before the truck got garaged over an ignition problem (and laziness).

However, it starts quick and shuts off quick, so the timing doesn't seem to be off. That's always been my rough-indicator-rule-of-thumb for timing.

[bkreutz]

Valve timing and ignition timing are entirely different things. I've seen engines with jumped valve timing start normally. May not be your problem but if it was me I wouldn't assume anything at this point.

[milhouse]

Does your master cylinder have a brake booster? Perhaps the booster is bad and the vacuum leak is there? You should be able to easily check that by removing the line to see if there's any change in the engine idle, or seeing if there is any difference in pedal firmness when the engine is running or not.

[jonnyc]

I disconnect the booster when I attach my vac gauge, so that isn't an issue.

bkreutz, I guess I have learned something new. Is there a way you know of to check or adjust valve timing without ripping apart the whole front end? I hear "timing" and I automatically think of ignition timing.

[bmohan55]

An old trick and old mechanic showed me is to hold a dollar bill in front of the exhaust pipe. It should constantly be blowing away from the pipe, if it occasionally gets sucked back towards the pipe it indicates that a valve is sticking open or a vacume leak. YMMV!!!

[bkreutz]

I disconnect the booster when I attach my vac gauge, so that isn't an issue.

bkreutz, I guess I have learned something new. Is there a way you know of to check or adjust valve timing without ripping apart the whole front end? I hear "timing" and I automatically think of ignition timing.

Not that I know of on a LR engine. (I do know how to do it on a Porsche flat 6 though, but not much help in this situation )

[printjunky]

I'm still with the vacuum leak crowd, a bit. But if you've sprayed starter fluid all around any possible ingress points and gotten no rev, whatsoever, then you're right, you plugged 'em all. But I would re-triple check this simple thing before tearing the front cover off. It's just that about 50% of the time, I think I've entirely eliminated the simple stuff, moved on to the major things, and in the end find out I in fact did miss something when I was checking the simple stuff. (I can sometimes be too quickly resort to "The mechanical sky-is-falling!") This is why I now have significantly robust and redundant battery/engine/chassis grounds - too many times thinking I had good ground, quintuple-checking and meter testing the grounds, only to find that it was still a simple ground problem, and there was no need to have removed and disassemble/tested my starter (alternator/relay, etc).

The only other times I've has any similar issues that wasn't a vacuum leak, was when I had a tiny (TINY) piece of something stuck on my needle valve in the Rochester. It only came off when rubbed with a q-tip (in other words, it didn't just flush away). Fuel was just dumping into the intake.

And once I had a funky acting intermittent mechanical (original) fuel pump that caused some fine-starting, bad running conditions.

And last summer, right before I had a catastrophic failure as the top timing gear locker disintegrated and ruined the cam. Before the failure, the cam was not being held in position, so my valve and ignition timing was being affected. Not to say you have this exact same problem, but to corroborate other's proposed valve timing diagnosis.

But none of these three examples was quite as exact a match to your symptoms as the vaccuum leaks I've had.

[bkreutz]

I agree, I'm still thinking vacuum too. But after all this time and messing around, I wouldn't rule anything out. Taking the front of the motor apart is a real pain, but there has to be an answer. Maybe have a second set of eyes go through the obvious stuff again.

[Lance]

One thing you might try before getting in the motor is to make a metal plate and install it between the exhaust manifold and the intake. Basiclly isolating the two manifolds. It worked for me.