Engine not running quite right after overhaul, weak spark maybe?
 

So I've rebuilt my car's engine to solve my terrible performance. I emulated Hot Rod magazine's budget 450hp engine with a better set of heads, but it's got nowhere near that much power yet. Can't even smoke the tires.

I've done the whole engine including a new Holley 670, Holley fuel pump, MSI HEI, replaced my sock filter in the gas tank, fuel pressure gauge, all that.

Except I still can't properly tune it because turning the idle mixture screws simply don't do anything to either my vacuum reading or my engine RPM unless I screw them all the way in.

I can not find any evidence of a vacuum leak using brake cleaner or starting fluid. I keep everything except my distributor, transmission (th400) and freshly installed brake booster disconnected from my vacuum.

If It's of any relevence. Here's a short clip I recorded of my vacuum gauge with the engine running. It's flicking around a lot between 2 and 5 inches
https://youtu.be/viDLjOvANhw

I did pull the plugs at the dizzy with the car running and noticed something odd. When I pulled #1 wire, the terminal was arcing steadily every time the cylinder was supposed to fire, but my #3 would only arc here and there roughly twice a second. The rest all went like that with different amounts of variation. Could this mean I have weak spark? As well, this is a brand new HEI and I have no tach filter so I'm not quite sure where else to look for the source of weak spark.

Any idea? I've been troubleshooting this engine for almost 2 years now :ack:

Car is a '75 TH400 Vette

When you installed the new heads, did you measure for proper pushrod length? 99% of the time, you can not use the stock length.

What is your Power Valve number and is it about 2" of vacuum below your cruising vacuum reading?

Tach Filter helps the tach take an accurate reading w/o spike signal and has nothing to do with the performance.

Disconnect the main vacuum line off the back of the carb and plug. Does it run better? (caution no PB)

Absolutely! I can't remember what size I ended up going with, but I measured it out with an adjustable pushrod tool.

Powervalve number, I can't remember. I believe it was 6.5, but I do know that I have the correct power valve for my cam.

Ah I see. I removed it to see if there was any change, but I'll put it back in place.

I'll try taking the main vacuum line off and see. While I'm at it, I'll do the same with the line to my transmission.

Just read your post again. Vacuum flickers at 2-5 inches or it fluctuates from 2-5"? Your Power Valve maybe open all the time. Sounds like late valve timing. Where was the cam timing mark lined up: Straight up, 4* advanced or 4* retarded?

I doubt its a IGN issue with a brand new HEI, but maybe.

Sure your valves are adjusted right? Hydraulic lifters or solids? They might be too tight. Maybe a bad HEI? Bad plug wires?
The list can get endless.
Hope you solve it!

I had a local classic car mechanic adjust my valves for me since I had messed up on that last time, so I'd have to imagine they're pretty spot on. But good news! Your advice led me to a vacuum leak. I hadn't checked the brake booster because I assumed it was fine, being brand new and all. Turns out the little valve connector on mine has a crack in it. Disconnected the brake booster and the needle vibrates, but the fluctuations are only around .5" which I assume is how it's supposed to look? The idle is also about 300RPM higher and sounds a lot stronger when the brake booster is disconnected.

Hydraulic lifters and the plug wires are nice new Accel wires.

Also, when I said that the gauge fluctuates I'm talking about a quick vibration type of movement, not a slow one.

Cam timing is straight up as recommended by Comp for my cam choice. Also, I have Comp 1.5 full roller rockers if that bit of info matters.

That vacuum gauge "needle bounce" is usually one valve that is not seating (closing) properly. Lord knows which one. So now you are back to the valvelash adjustment or maybe someone missed one valve. You won't get a clatter on a valve that's not closing, only on a valve that is too loose.
With the valvecovers off, engine off, watch for a rockerarm that appears over tight, while cranking or jogging the starter. If you find one, reset the lash using the method for HYD lifters.
Its very easy.

What should it look like if it appears overly tight? Pulled the valve covers, but I'm not quite sure what I'm looking for.

When a valve is fully closed, there should be a little movement in the rockerarm. Start inspection at one end of the head and work your way to the other end. If one rocker is super tight, that's the culprit. Back off the rocker nut a half turn.

But you need to learn to set valvelash using the firing order. With valve closed you basically spin the pushrod until you feel resistance, then tighten the rocker nut another half a turn with your socket / ratchet. Must be some YouTube videos to show you how.

I set my valvelash with engine running, using oil deflector clips. That way I don't have to mess with turning the crank several times.

As far as I can tell, my lash seems to be pretty good. Could the remaining jumpiness of my vac gauge needle be a result of the fair amount of overlap due to my cam choice? I have a XE274H w/ .490 lift rated for only 11" at 800rpm and audible overlap at idle

I vote no. Long duration cams give little vacuum, but it still should be fairly steady. Every time the gauge needle drops indicates a piston that is not sucking air (vacuum) for some reason.

Edit: Nevermind. Figured out my lash was alright. I think my issue was a combination of a crack in my PCV line and a bad vacuum fitting. I'll be wrapping stuff up and hopefully that means my vacuum is better!