High idle - no vacuum leak

[seestheday]

Hi,

So I've put my bike back together and taken it out on the road. Good thing is that she starts up strong, runs smooth even before I've sync'd the carbs, and pulls like a demon with that 900 camshaft.  The header wrap I put on my 4-1 exhaust also looks great.

Bad news is that I can't get the idle below 4500.  Idle screw is all the way down.

It was actually around 5500 at first, but I switched the main jets from 75 back to the stock 68's and it brought it down a bit.

I have a 105 secondary main, so I think my next step is to bring that back to stock to a 102.

I have confirmed that there is no vacuum leak with the spray test.

Before I start buying smaller jets to try - is there anything else that I'm missing?  I guess I'd just like a 2nd opinion.

[Lucky 1]

Which motorcycle? What year?

Main jets have no control over the idle, period.

Idle screw should be all the way UP.

Are you confusing the idle screw with the mixture screw?

[seestheday]

EDIT: Lucky is right - I'm not afraid to admit I'm wrong - the idle jet on these carbs is located beneath the rubber plug and is not removable without drilling.  I'm going to have to search a little to resolve this.  I hope I don't have to do anything major.

Original post below:
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You're right. Changing that 105 down to a 102 won't make a difference. In the dohc's the 68 is the idle jet. That is what I need to focus on if I'm changing jets. Anything else I should be looking at? I'm guessing the combination of the increased lift and duration from the cams and the increased pull on exhaust gasses from the wrap is causing the issue.

As always, my bike is in my sig.  It's a 1981 cb750K.

For reference, here are the carb specs for these DOHC carbs.  They only state "primary" and "secondary" main jets.  It is the "primary" in these carbs that is accessible at idle.  (THIS IS WRONG)

[seestheday]

(Note: I've cross posted this on cb750c.com: http://www.cb750c.com/modules.php?name=Forums&file=viewtopic&p=275437#275437)

Sigh, I figured it out.

Insulators mounted flush with the rail, verified that cables were operating smoothly and were not binding.

Pulled carbs, and verified that slides were operating smoothly, the choke plates were snapping shut as they should. Pulled the bowls off and verified that every jet was in the proper place (replaced the 68's with the 75's again when I did it). The slow jet in mine is non-removable so that isn't the issue.

I made videos and took pictures of everything so you guys could see everything that I saw, but I don't think it's worth posting now that I've figured it out.

I started to get worried that the ".10" guitar string I used to clear passages in the carbs when I cleaned it somehow bored out the slow jet somehow, but then I looked at the idle adjust screw and noticed that there wasn't air between the screw and throttle that would indicate it was all the way out.

I tried loosening it by hand and it would loosen a little, but when I released it to turn it again it would snap back tighter because of the spring. This was what was happening when I was trying to adjust it on the bike.

I grabbed a screwdriver and forced it loose. The spring un-binded and I was able to loosen it much more. There is now an air gap between the idle adjust screw and the throttle.

Put the carbs back on and it is idling at a nice 1,400 rpm. Not ideal, but much better than before.

I verified again that I don't have any vacuum leaks and will be syncing the carbs, dialing in the air/fuel mix & adjusting the timing now.