Where should I start

[saxart]

I have a question about tuning the crate engine in my '70 C-10 and could use your help.

Engine is a relatively new GM (SBC) crate engine. It has a "new" quadra-jet that was reconditioned by National Carburetor on top and a Skip White HEI distributor. (Trans is a SM465 manual) . Timing is set at 11deg BTDC.

The engine starts and runs fine, but it has a miss that will show itself near the top of the RPM range. (just before shifting out of one gear and into the next)

I'll admit, that I don't know much about tuning these quadrajet carbs. (or even if this issue is carb related?) It seems the only adjustment on the QJ's is the idle air mixture screws and the internal fuel metering rods. Correct?

When driving the truck, I'll put it in gear and accelerate, and just before shifting from one gear to the next, it will miss (chug) once or twice in the upper RPM band. Once you get into the next gear (lower RPM) the engine pulls strong until it gets into higher RPMs again. This condition can sometimes be made worse by applying full throttle. (It will chug more than once or will be more pronounced)

Any thoughts on where I can start looking/adjusting?

[geezer#99]

Pop the valve covers off and check all the rockers for proper movement.
Those are symptoms of a cam lobe going flat.

[saxart]

Originally Posted by geezer#99

Pop the valve covers off and check all the rockers for proper movement.
Those are symptoms of a cam lobe going flat.

Oh gosh, I hope not. This engine literally has about 30 miles on it.

I did think of one thing that may be the issue. The truck got new Accel plug wires with the new engine and dizzy, but we had an issue where two of them were resting pretty tight against the exhaust manifold during the 20-min/2,000rpm cam break in. (Boneheaded move on my part, I cut them too short). They didn’t melt, but you can see a small spot on both of them where the insulation is now tan instead of dark blue. I wonder if spark is breaking up as it’s being driven because of those hot spots?

[cadillac_al]

It sounds like it is running out of fuel on the top end like a weak fuel pump. If it was a six it may have smaller fuel lines.

[Dead Parrot]

Your symptoms also sound like weak spark. High throttle and/or rpms stress the ignition.

Find a really dark spot at night, start it and look for any blue sparks where there shouldn't be. Best if your let your eyes adjust to the dark first.

Did that truck have a points set up originally? If so, was the original resistor wire/ballast replaced with a full 12V wire? HEI reacts poorly to being run off a resistor wire.

Plugs gapped at .045?

BTW - there is a lot more to tuning a Q-jet then idle mix and metering rods. Books worth in fact.

And if the motor only has 30 miles on it, follow the proper break in sequence before worrying too much. Rings may not be fully seated in yet.

[Wrenchbender Ret]

Sounds like spark plugs or wires to me. You may have cracked a plug putting them in or handleing the engine.
George