help with engine bog/misfire

[Blackbird66]

I have fallen into a black hole from hell..............
Problem: Engine bogs/misfires between 1/4 and 3/4 throttle. Idle is good with around 18 vacuum. Engine runs great pass 3/4 throttle as carb transitions to secondary. Spark plugs are white, and problem is worse as air temps drop. ( This means lean)
I am running a 350ho crate motor with holey 600 carb jetted to gm recommendations. 10 degrees advance on unilite with Malory coil gaped .040 My spark seems weak, but that should effect full throttle/ high rpm and not low throttle low rpm right?
Anyway, I can find no vacuum leaks, nor any problem with holey. I tried a new edelbrock 600 last night, and it ran the same. I don't see how I could be so extremely lean, my tune should be very close.
Ideas?
Thanks, James

[68gmsee]

You say the enigine misfires at 1/4 to 3/4 throttle... You've covered just about all the common areas that should be addressed so now you're at a point where you may need more advanced equipment to check your engine. Maybe take it to a speed shop or even local dealer and see if they have any way of analying your engine further.

Here's what I would do first, though.

First, I would check all ground connections and clean and retighten.
Made absolutely sure that the gas was good, clean with no moisture. (lots of crap fuel these days)
Regap the plugs to the recommended gap or get a different brand.
Make sure I had good, high quality sparkplug wires.
Checked compression.

I may replace the spark plugs (all) with a different brand and regap.
Play with the air/idle settings to see if it made any difference.
Move the timing to see if it affected the missing.

After that, I'd take it in to get another opinion.

[Ole Blue 68]

Since you covered carb problems and vacuum leaks, I'd see if adjusting the timing makes any difference. Or it could be bad gas like gmsee said...

When did you start having problems?

[Blackbird66]

I've had these problems since install. New wires, plugs, cap rotor, good compression(new motor) ect. I just tested the ignition with my new spark gauge, and have very week spark. The bare minimum for electronic ignition that shows on the gauge. By comparison my Focus shoots spark over twice the distance on the gauge. This is an old coil so I'm try that next. I still think that week spark should effect low rpm and load less than high rpm and load.....but what do I know......
Bad gas..... The truck is plenty drive able, and I've run a few tanks of high test. No ping under load or any other signs of bad gas. I do need a posi rear end. It seems that full throttle test in first or second only cause lots of right rear spin
If I could just drive everywhere at WOT I'd be in great shape!
Timing makes no diffrence on the bog, but richning the idle up an extra turn helped a lot.

[cdowns]

if you are using the original resistor wiring in the truck it will cause a weak spark problem

[Blackbird66]

A mallory unilite uses a ballast resistor as original. I'm using the mallory pro-master coil as recommended for this distributor. This coil requires around 7 volt ( I forget the exact numbers). I've used this set up in this truck for years with the previous engine. I think the coil is just showing age. It seems that aftermarket hi po items simply don't last or have the reliability of OEM.