4 BARREL HOLLEY CARB ADJUSTMENT

TRBenj wrote: Go gently to the stop (where they bottom out), then bring them 1.5 turns out... That's usually a pretty good starting point. Adjust idle level to ~700-750 and then adjust the 2 idle mixture screws for max vacuum. Then set idle level to 650 or so in gear.

I can follow this reply, with one exception...... how do I measure manifold vacuum? Best I can tell I don't see any ports on the engine manifold.

So I basically left the idle mixture screws on 1-1/2 turns out and set the RPM between 500 and 750 at idle in neutral.    I will go to the lake Saturday and tinker with the idle level at the throttle in gear. Where and how do folks measure manifold vacuum?

The dash RPM gauge limits the tuning. Is there a better way to measure RPM with no distributor?

Thanks,

Clark

malcolm2 wrote: TRBenj wrote: Go gently to the stop (where they bottom out), then bring them 1.5 turns out... That's usually a pretty good starting point. Adjust idle level to ~700-750 and then adjust the 2 idle mixture screws for max vacuum. Then set idle level to 650 or so in gear. I can follow this reply, with one exception...... how do I measure manifold vacuum? Best I can tell I don't see any ports on the engine manifold. So I basically left the idle mixture screws on 1-1/2 turns out and set the RPM between 500 and 750 at idle in neutral.    I will go to the lake Saturday and tinker with the idle level at the throttle in gear. Where and how do folks measure manifold vacuum? The dash RPM gauge limits the tuning. Is there a better way to measure RPM with no distributor? Thanks, Clark

Manifold vacuum: Find the barb / port at the BOTTOM of the carburetor or on the spacer between the carburetor and the intake manifold. Those ports will provide you with full manifold vacuum. Then hook up a vacuum gauge to the port. Make sure that you have a good seal between port and hose and then between hose and vacuum gauge. NO vacuum leaks.

Then you adjust your idle mixture screws in 1/4 turn increments and watch your vacuum gauge. You adjust both screws until you achieve the maximum vacuum reading on the gauge.

At this point, some will tell you to turn them back in (clockwise) 1/4 turn for your "final" adjustment.

I like to have the boat in gear and at idle. RPMs at 600 to 700 at this point for proper idle mixture screw adjustment using the vacuum gauge. You can do this tied at the dock or on the trailer.

I would start at 1 1/2 turns out (counter-clockwise) from LIGHTLY seated. Don't over tighten the screws and damage the seat. When I ran the Holley 4160, my best vacuum was at 1 1/2 turns out. Maybe that's why its a good starting point!

Vacuum gauge: I like the unit from Lysle:

Lysle Vacuum Gauge

JQ

In your photo, you are indicating the PCV hose. That port / barb on the carb spacer will work just fine. Be sure to use the proper size hose. NO vacuum leaks.

One of these will help when dealing with hoses of different sizes.

If your PCV hose is in good shape, try removing the PCV valve from the valve cover. Then remove the PCV valve from the hose. Your vacuum gauge can hook-up right up there at the business end of the PCV hose. I put my smaller vacuum gauge hose inside the larger PCV hose. I used a small hose clamp (lightly tightened) to ensure a good seal.

JQ