When I slowly, steadily bring the RPM up on my Elky's 350, under no load (in Park), there's a slight but consistent stumble at ~ 3200 RPM. The carb is a 600 Holley model 1850, modified with secondary metering block and center inlet fuel bowls. Could anyone advise re the cause of this stumble.

Thanks,

JW

That particular Holley is calibrated a little lean for what you are doing with it. It's not the main jets you are having trouble with, it's the idle channel restrictions. Once you get a book that shows you where they are, drill yours out to .034" and the stumble should go away. Opening the throttle slowly in park with no load runs the engine with just the fuel supplied by the idle/transfer circuit. Yours is a little lean.

Louie

Makes sense. I have info on the Holley. Thanks Louie

Wow, .034" seems little big Louie, what is the motor this is on? Isn't the stock like .028 or so? I know last year when you helped me with mine, you said not to go over .031" or you start to lose control of the idle?

I have mine at about .031" now and it helped, but still feels like it could use a tad more!

Thanks,
Bill C.

Bill, you're right. The circuits in a Holley were never designed for this situation. Some recent tests have proven to me that as long as the camshaft isn't so long in duration that it causes an idle stability problem, most of the common Holleys will somewhat tolerate up to .036" to .038" diameter idle channel restrictions. The biggest problem created tends to be the transfer fuel ratio. You know what it takes to change those little restrictions. It's not a lot of fun. I usually do hesitate to drill out anything except as a last resort and also, you haven't expressed a necessity for running your engine at 3000+ rpm in neutral. The starting line of a dragstrip is the only place I can think of that might require this but, heck, maybe JW has a reason.
I should have said drill them out to .033" because that is a #66 drill, the next drill size up is #65 and that's .035". To get exactly .034" you have find a metric circuit board drill.

Louie

Hi Louie;

Thought I'd reply again to elaborate. My purpose re the Holley is to optimize for street driving. The cam in my 350 is a single pattern ISKY 270 (221 at .050 lift), .465 lift. Lobe separation is a tight 108. The 600 is small for this but if it can be dialed in I'd expect better throttle response, torque and mileage for typical driving. I've got a 750 that I could use as well.

Thanks again,

Jeff