bike won't idle below 2,000 rpm

[Austin 4x4]

so here is the story. rebuilt carbs on a 1974 cb750.  i am running a 120 mainjet and a 40 slow jet, aswell as pod airfilters and a 4 in to 2 exaust.  and this is also at altitude here in denver.  other than high ideling the bike runs great.  if i try to set the idle speed screw any lower it just dies.  the bike was running ok before i changed the ait filters, exaust and rebuilt the carbs. so what ideas do you all have for me.

[TOOLS1]

Welcome to the forum. It sounds like the pilot jets are plugged. Does it run with the choke on?
TOOLS

[shinyribs]

I agree with Tools on the pilot jets. Another possibility is the idle mixture is set too lean.

[Austin 4x4]

the bike will run with or without the choke.

[seestheday]

If your pilot jet isn't clogged, you might want to try a larger slow jet?

[Austin 4x4]

what size slow jet should i try?

[seestheday]

I'm assuming that you have confirmed that your jets aren't clogged per tools/shiney.

I should also say that I know the dohc carbs reasonably well, but don't have experience with the sohc ones. If tools, re-run, lucky or shiney chime in you should listen to them over me.

You're probably going to have to start experimenting with jets if that is the problem. There is a thread where lucky recently mentioned that he went from 38 to 40 to 42 before he got them right, and that is with a stock airbox. Your elevation is different though.

Before you do that though, have you played with your air/fuel mix screws? What are they set at? Do they have an impact at all?

Can you pull your plugs and see if they're indicating a lean condition?

Sent from my BlackBerry device on the Rogers Wireless Network

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From: "Austin 4x4 [via Honda CB750'S]" <[hidden email]>

Date: Mon, 4 Jun 2012 19:37:29 -0700 (PDT)

To: seestheday<[hidden email]>

Subject: Re: bike won't idle below 2,000 rpm

what size slow jet should i try?

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[shinyribs]

Your jet sizes of 120/40 are a tad lean compared to my bike (125/44),but I live/ ride at 2-6000 feet above sea level. Going from down in the valleys to the tallest mountains we have the jetting seems to not be affected. Yours may be spot on with your elevation. Another possible cause of a high idle would be a vacuum leak. If you have a vacuum leak you will usually have a slow return to idle that goes with it. That would be easy to check,just spray carburetor cleaner,ether,etc... around your carb boots. If you see a rise in engine speed there is your leak.If not,I would really check out those idle jets? They are very small and can be easily plugged up.

[Jantz]

I’m having the same issue with my 83 cb750f I believe it’s the emissions but I might have too big of jets I bought a dynojet kit and it told me to use 130/74 ? Reading this I’m thinking that’s too much but I plugged my emissions temporarily and it idled fine but got hot even faster

[gareth06]

Maybe the slow circuit in the carbs went bad.