It's not often I find a car problem I can't solve but this one beats me.

400ci roller cam speed demon etc.etc. built right but not over built. 435hp on paper.

Recently checking for spark rattle after some tuning. Laid into it in high gear with the AC on a couple of time with no rattle, tried one more time and it jumped in to passing gear and I immediatly let off. It didn't over rev. Came to a stop sign and it would hardly run. Checked initial and vacuum timing no change.

Since then I have changed the carb twice, tried several combination of MSD ignition parts. And searched for vacuum leaks. All ignition parts are now new and carb rebuilt.

A couple of times after fooling with the ignition it ran pretty much OK. Parked it and now it won't keep running under 2,000rpm. Kind of surges, revs if you stomp on it.

compression is 170 in all 8. Leakdown test on a couple of cylinders = 0. (I have zero gap rings). Top dead center seams to be right on and only 5 degrees or less of slack in the rotor bug. All the valves seem to be going up and down as they should be.

Plugs are white. Air/fuel gauge claims dead lean but I don't really trust it. Could be just incomplete combustion.

Seems like a "huge" vacuum leak but I disconnected everything I can think of.

Any thoughts would be appreciated.
Beartracks

 

A/C runs off vacuum lines and switches, I believe. Mebbe a leak there?

 

I wouldn't think the A/C's vacuum would be enough to do that. But what about the brake booster?

 

Tried the booster, actually disconnected about everything to test. It would have to be a big leak to cause this much problem.

Just ran another compression test and have 170 on no. one.

I keep trying to blame it on the timing chain but ignition timing didn't change and I don't think I could get that much compression if it jumped.

 

I also flushed out the fuel system, changed the oil and used a magnet to look for metal and cut open the filter. Nothing there.

If it's not the chain I keep comming back to the ignition maybe somekind of interference. I took the belt off the alternator and no change.

I have vacuum in the crankcase PVC system.

 

First and foremost I'd connect a vacuum gauge and see what it reads.

One can get a smooth idle with ignition set as low as 4 or as high as 20 (with no vacuum advance), so your surging may indeed be from a lean fuel mixture which your AFR gauge is indicating.

I'd richen the mixture to see if the idle becomes smoother, and to see if the AFR gauge indicates a richer mix.

I'd also check the the fuel delivery is not being hampered. (E.G. Dirty filter.)

When you say ''spark rattle'' do you mean ''pinging'' ?

 

It won't idle at all. Have to keep it up to 2,000 to keep it running. I've been getting good vacuum readings when it does idle. Good fuel level in the bowls. Yes pinging.

 

Thank you for clarifying spark rattle beartracks.

Oops, sorry, you did originally say it won't idle, so a vacuum gauge wouldn't help just yet !

Did the engine run great since first firing it up after the rebuild, or did it run questionably since the rebuild ?

What made you start checking for spark rattle ?

A lean mixture can cause pinging.

Did you build the engine yourself, or did a shop rebuild it ?

You parked it. Was it parked long enough for the gas to go stale ?

By good vacuum readings, when it did idle, were you getting a steady needle at say 18" or higher vacuum ?

Was your cranking pressures of 170 obtained with the butterflies fully opened ? (8 * 170 =very good )

 

WOW! I don't know, could it be a bad seal between intake and head or intake and carb flange? Could be down in the valley and not found with carb cleaner. that would make it lean. I have actually seen holley carbs not manufactured properly with the main well plugged by air bleed orifice, but you said you changed carb.

 

It does have reasonable vacuum, considering it will hardly run. Vacuum falls off as it stalls. I'm going to try a known good dizzy and coil and have another go. I use the steel bathtub intake gasket so I don't think it could fail that badly.

It's been some time since it was rebuilt, 20,000miles. flushed out the fuel system. I run a Snow Performance water/methenol system to compensate for my compression. I've kept it turned off since this started.

Thanks for all the suggestions. I think I need a priest.

 

Well there is a small idle feed restriction brass jet in the primary carb metering body. The driver side one had backed out into the gasket which closed it off. Half of the cylinders were not getting any idle/transfer slot fuel. Screws everything up up to about 2000rpm plus. It was moving around giving various problems.

Also somewhere along the line the ignition module failed in the MSD street billet dizzy. Hard to diagnose two things at once.

Lessons learned:

Inspect the carb parts "very" carefully when rebuilding. I've never seen a brass bleed jet come loose before. It wouldn't come all the way out of the metering body, just sliding in and out.

Disconnect the ignition when running compression test!

I guess I'll go back to draining the swamp now.

Otto Mechanic

 

Hi beartracks,

Well done for finding the problem, and many thanks for posting the solution.

I would never have suggested the jet as being the issue (Or maybe a million miles down the line).

Google Mechanic