OCHOHILL

I bought a 5.0 roller long block from a 91 GT. I installed and RPM Air Gap, Headman shorty headers, holley carb (4150?) and a mallory distributor.
I replaced the timing chain. The ports at that back of the head have been plugged. The spark plugs were checked out before installing in the 66. The PCV valve and hose are clamped. The timing is set at 15 degrees. The carb is about 1.25 turns out. The butterfly stop screw is all the way backed off. It idles at about 550 rpm and draws a little better than 15" of vacuum.

I replaced the spark plugs with new. #5 and 6 were fuel black. The rest looked good. Compression is between 170 and 180 on all cylinders.

The carb has been modified. The air horn was machined off. It is a mechanical secondary holley that is either a 600 or 650. I know that the primary jets are 64's. I haven't checked the rears. Both bowls are clean.

When I rev it without a load it doesn't miss. Under acceleration it does what is in the video. It sounds like there is an explosion taking place in the headers.
The conditions are the same with the timing at 8 degrees but it idles too low.
It also bucks if I am in too tall a gear.

Any advice?

Thanks,

Dave


http://s69.photobucket.com/albums/i5...nginemiss1.flv

hiboostwoody

I wonder if you have a broken valve spring. That sounds like valve float.

67t5ponycoupe

I would say valves too. Is it possible that the timing chain was put in one tooth off?

hiboostwoody

Or possibly jumped a tooth?

OCHOHILL

I installed a new timing chain. I don't believe I have it off a tooth. I made no attempt to degree and just lined up the dots. The chain was tight. I don't believe it would have jumped.

If the chain was off wouldn't the sound/miss be more consistent? I had not considered a broken valve spring. I am leaning at this as being more likely the cause. If you listen, It almost sounds like 1 cylinder is missing. I'll pull the valve covers and take a look.

I may have to install my holley heads and cam sooner than I thought.

OCHOHILL

I pulled the valve covers. I did not find any broken springs. I did find that the valves have one spring. I have new headers with about 2 hours on them. The tubes for 5 and 6 still have the paint on them while the others have burned off. Spark plugs 5 and 6 were fuel black. Is it possible that my 5 and 6 spark plug wires are bad?

hiboostwoody

Very possible. Also possible that your distributor cap is bad. I guess I just assumed you had checked those. I should have asked. My bad.

Heres one way to check the wires. Switch them with the wires from two other cylinders and see if the miss follows the wires. OR, grab ahold of them while you are grounded and see if you get bit. Good wires won't bite.

OCHOHILL

I have been shocked enough by distributors. I am not trying that. I will bring home a couple of new spark plug wires and try them. The cap is new. The wires are older MSD 8.5mm. A friend of mine also suggested I make sure that 5 and 6 are not crossed.

I just double checked the firing order. 5 and 6 are next to each other. I can't wait to get home and try it.

OCHOHILL

I crossed 5 and 6 several months ago when I installed the distributor. I haven't checked it since. The funny thing is the car idles absolutely the same with the exception of 2 more inches of vacuum.

Problem solved. Thanks for your help.

Dave

cprstreetmachines

I would say no to a broken valve spring, even an inner you may not be able to see ONLY because the rpm's are so low. Usually one breaks, and the other is enough until it gets up around 4-5k. But you may have already figured it out. It is a very consistent miss. Which does point to wires crossed.