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Originally Posted by hgs_notes
Been trying to diagnose a problem with the new engine. It starts and idles fine and seems ok cruising but under acceleration it sounds aweful. Like it's misfiring. All of the EFI parameters seem to be normal. I had a bad plug wire, interference with the headers. I replaced all of the plugs with Accel shorty plugs and some of their best wire sets with porcelain boots. It didn't make much difference.
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I don't think it was your EFI, or fuel related at all. Do a compression check, or use an infrared thermometer if you have one and take temps of each header primary right at the cylinder head. I bet you'll find one or more cylinders are much colder than the others or have lower compression. This is the exact scenario I have on my '72 at the moment. My case is self inflicted, and I bet yours is due to trust in brand new parts.
Your valves need lapped. They aren't sealing properly and creating leak-by, losing compression, and ultimately causing the engine to run like poo. It will start and idle fine, but worthless under acceleration.
A lot of budget heads are budget for a reason. They are assembled and seem ready to run, but a test as simple as filling the ports with water and looking for seepage around the valves will often show that they are not properly seated because they need lapped. That test should be performed even with expensive heads from Dart, Brodix, AFR, etc. Look at the reviews for Summit Racing's as-cast performance heads, which are just re-boxed Dart heads, and most reviews say that they need lapped right out of the box.
I did not test mine before I bolted them on, and sure enough...exact symptoms you're having. I went through three carburetors and never solved anything. After talking to a buddy, it clicked that I never lapped the valves after grinding them before assembling. Like I said...mine was self-inflicted.