Midnight Maxim 750 runaway idle

Hello all, I recently purchased a 1983 midnight maxim 750. When I purchased it, it had no airbox. So I sourced one and installed it. She fires right up but the issue is she immediately revs to the moon! Extremely high idle. I had to put her in 1st against the wall of my garage at partial clutch just to troubleshoot.

Looking for suggestions of what to check here. Idle screw seemed to have no effect. Throttle and choke cable are not stuck open. Boots seem nice and tight. Could it be something in the air box maybe? carb issue?

any help would be appreciated.

 

oh forgot to mention bike has no mufflers, just headers, could that be a factor?

 

Carbs might need a good cleaning, jets and tubes in carbs could be plugged, throttle shaft seals probably need replacing causing an air leak.

 

None of the above are likely to be causing your high idle. For high idle you need air and fuel in the right ratio, so blocked jets? No.
You need to remove the accellerator cable, adjust the idle screw to close the carbs butterflies. The trouble is you can’t see them, or tell if they’re all aligned. So it’s carbs off to do a bench synch. Do this using the edge of the first hole to line them up to. Whilst out set the idle mixture screws 3 turns out from bottom.
Refit, warm up, adjust idle speed, then mixtures - do you know how to do this? Then adjust idle speed again, then running synch.

 

One more thing - the intake boots each have a vacuum port on them. Three of them should have a plug cap, and one should have a hose to the fuel tank petcock. If any of those plugs or hose are missing or leaking you're going to have a bad time. Not sure if it's enough to cause the runaway idle that you are describing, but it's not going to help.

I tend to agree with @Minimutly - the proper course of action is probably to pull the carbs to do a full inspection of the carbs and intakes. If you don't do a full clean and rebuild, then probably at least wet set the float height, bench synch, and then install and running synch.

In any event, you are probably at the start of the process where you learn to get good at removing and installing the carb rack!

Other than that, how are things in Michiana?

 

Well I think I clogged up the carbs a second time, did not use an inline filter and basically clogged back up in 5 minutes or so it seems so need to tear into them again.

Is the bench synch where you take guitar string in the butterfly valve and turn until it slips out?

While I have you guys here, Are these two screws identical? Like is there a way to put them in backwards?

 

Yes, I believe the threads are the same but the jets are different sizes. Not sure what carbs are on that bike but someone will be along to straighten out which goes where.

 

Brian check this how to bench sync your carburettors.

https://www.xjbikes.com/forums/threads/how-to-bench-synch-your-carbs.6366/

 

that pic if

thank you that helps

 

The pic looks like Hitachi but I was too lazy to take the time to look up what exact carbs are on that bike.

 

There is a ton of information on the xj4ever site regarding exactly which models came on which bikes, jet sizes, etc: http://xj4ever.com/catalog/c.html