Engine riddle

[chef]

So here is the story, I have been restoring a 88 grand waggy and I am having trouble with the engine. Specs are stock bore, heads ported and polished three angle valve job, 27 thou off heads (est 9-1 comp ratio). Performer Edelbrock intake, 0-5500 Edelbrock cam, shorty headers, single 3 inch exhaust. DUI ignition, live wires, champion plugs, Howell EFI. Problem is it runs like crap, backfiring through intake, stumble poor idle, dies idling. I have checked timing, there does not appear to be any vacuum leaks. I do however have what I think is low vacuum at idle 8-10HG and the needle is bouncy, bring it up to 1800-2500 its's about 15-19HG. When I ease off it goes up slightly and then drops back to 8-10. If it's idling and i punch it I get the backfire or if I load the engine up, again backfire. Any ideas?
Thanks

[tufcj]

Welcome to Bulltear!

.027" is a lot to machine off the heads. Did you machine anything off the intake manifold? If you didn't, chances are you have a vacuum leak on the underside (inside the engine). With the Howell system, you might be able to verify that with an OBD I scanner. You can read the O2 sensor values and tell if it's lean. Pop back thru the carb (or throttle body) is usually a lean condition.

A general rule of thumb is that if you machine the heads or block the deck, the same amount should be taken off the sides of the intake, and 1.4 times the amount taken off the ends. So you should mill .027" from each side of the manifold and about .038" from the ends.

Bob
tufcj

[Old Rugged Crosser]

Have you verified the firing order of your spark plug wires? Also it sounds like your ign. timing is off.

[MC]

Could also be that our timing set is off as well

[juggernaught]

You only need to be off one tooth on the dizzy to cause havic similar to your symptoms. Vacuum should be at least 14Hg at 800 rpm min.

Look up links on "how to read a vacuum gauge".

[chef]

Welcome to Bulltear!

.027" is a lot to machine off the heads. Did you machine anything off the intake manifold? If you didn't, chances are you have a vacuum leak on the underside (inside the engine). With the Howell system, you might be able to verify that with an OBD I scanner. You can read the O2 sensor values and tell if it's lean. Pop back thru the carb (or throttle body) is usually a lean condition.

A general rule of thumb is that if you machine the heads or block the deck, the same amount should be taken off the sides of the intake, and 1.4 times the amount taken off the ends. So you should mill .027" from each side of the manifold and about .038" from the ends.

Bob
tufcj

If I do not use the end gaskets, do the ends have to be machined? Just trying to save a buck.

[tufcj]

I really don't know. I've always used the end gaskets, although I know some people don't. Measure an end gasket, if it's more than .038" thick, you can probably get away with it. If it's thinner, there may be metal/metal interference on the ends when you tighten the intake.

Bob
tufcj

[chef]

Also someone had mentioned my push-rods maybe too long now with the .027 off the heads? Is there an acceptable tolerance/ speck for this. When the valve is in the down position, I cannot rotate them by hand?

[msalaba]

I had my heads shaved 0.045"

I had a machine shop mill 0.045" from each side (the intake to head ports) of the intake manifold.

I did not have the ends machined. I am a firm believer of "take those rubber %$^@#*$ and throw them away" Buy not machining, I use less "The Right Stuff" for sealing.

I can't remember what length rods I used. There is a measurement of lifter pre-load (so many thousanths) when the lifter is on the cam base circle. I used an adjustable push-rod and lengthened it to put the lifter pre-load on the light side. I checked every lifter and used the length of the longest measuement for all push rods. IIRC ~0.040" less than stock.


I wish I could remember what the tolerance for the lifter/push rod was. @ 0.027" you should be close. I would look for the port match from the intake to the heads. At 0.045" my ports (or bolt holes) didn't line up at all.

[juggernaught]

My preload tolerance was .020 to .040 for my 360 / 9.0:1