Quick idle speed question - Weber Carb - Speed Screw 2.5 turns in, but 0 vacuum - JeepForum.com
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Unread 04-22-2011, 11:45 AM   #1
Registered User
1989 YJ Wrangler 
Join Date: Mar 2011
Posts: 103
Quick idle speed question - Weber Carb - Speed Screw 2.5 turns in, but 0 vacuum

I just tuned my Weber 32/36. The idle speed screw is at 2.5 turns to get it idling at ~650 RPMs. I checked the S port and I'm still getting 0 vacuum until I blip the throttle.

Does this STILL mean it's a lean condition? I'm under the assumption that a lean condition would require the throttle plate to be slightly open in order to achieve the correct idle speed. But since my S port shows zero vacuum, then my throttle plate is closed, right?

My mixture screw is at 1.75 turns.

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Unread 04-22-2011, 06:30 PM   #2
Web Wheeler
1983 CJ8 
Join Date: May 2008
Location: N.E. Alabama
Posts: 2,649
My suggestion is that you have another go at dialing it in. A reasonable assessment of the mixture strength can't be made until the amount of flow at the idle hole has been established. That can't be done until we are assured that there is zero flow from the enrichment hole... and that can't be done until the throttle plate is set as close to closed as is possible.

While using the Redline guideline don't regulate either of the screws by the curb idle speed. In other words what you get is what you should report. The most important finding/report is the resting position of the speed and mix screws.

A Lean Best setting is not the fastest idle speed, its the sweet spot that only an intermittent lean misfire is heard.

Once the engine is completely warmed so that the choke plate is fully opened set the speed screw to a point that is between 1-1/4 to a max of 1-1/2 turns in once the tip of that screw has made contact with the stop lever, so that the throttle lever is first caused to budge.

Then start with the mix screw at say 3 turns out and work it in at 1/4 turns allowing time between each adjustment so that the engine will have time to react to each adjustment. Once the screw reaches the point that a rather constant misfire is heard back it out to that point that the misfire is heard at say each 5-8 seconds.

The much published 1-1/2 maximum speed screw setting is in reference to that point that one should be assured to see vacuum at the 'S" port nipple... that one that is farthest away from the mix screw.

Here is a guide that will indicate the stop, painted red.
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