Reading Spark Plugs

matthon said

I'm bringing the 600s, new plugs, and cam specs to a competent friend/mechanic on Wednesday. I'll share this thread with him.

What is the difference between the 390 and 600 carb?
Is it that on a 600 you can change air bleeds, idle feed restricters and emulsion, 390 you cannot, or is there more to it? Not that I've ever done any of that myself, but I can learn.

Click to expand...

The 390 doesn't have an accelerator pump (IIRC) and it's too small for a stock lawn mower.

Not a big fan of ANY vacuum secondary Holley carb. Also not a fan of a carb, especially in a 2x4 application that easily change the air bleeds, idle feed restriction, transfer slot and emulsion bleeds. You have doubled the needle and seat area, the jet area, the bleed area and the emulsion are. But you didn't double the air flow. You can't correct that with a jet. Also forgot, you need to be able to tune the power valve restriction.