1983 XJ 750 Midnight Maxim Idles 4000 rpm when it warms up.

Hi I am new to this site . I have a 1983 Midnight Maxim 750 , my problem is it will idle at 4000 rpm when it warms up . I have replaced the intake boots from the carbs to the motor, and the vac. line , and the rubber nipple caps on the intake boots . I cleaned the carbs really good even though they looked very clean before i started cleaning them. I cleaned the jets ,emulsion tubes ,everything. The diaphrams are not torn , and don't have any holes in them. The slides drop fast when the carbs are disassembled but are just a little bit slower dropping when back together . I don't have cable problems ,they are fine . Would there be anything else that could cause this 4000 rpm idle problem . The bike runs smooth ,no bogging ,sputtering . I'am the original owner , it has always ran great until the one season that that I didn't ride it - so I was sure it was carb problems . :?:

 

Check the obvious things first:

- make sure throttle cable isn't binding, or that the throttle linkage hookup wasn't "trapped" open when you re-installed the carbs.

- make sure the choke cable isn't binding, and that the choke/enrichment levers are fully closed.

- make sure that the idle adjustment screw---the large one with the big knurled finger grip---isn't adjusted all the way open.


Assuming none of the above works, you're going to have synch the carbs and make sure the synch screws aren't opened up too far. When you cleaned the carbs, did you take the carbs apart from each other?

ANother question: did you remove the pilot and main AIR jets when cleaning the carbs (up top, under the slide diaphram) and if so, are you SURE you returned them to their proper positions?....smaller one (lower number) is the main air jet and it goes in the center hole....the forward hole (forward meaning closest to the cylinder head) has the larger, higher-numbered pilot air jet.

You can always try to use a small propane torch and see if there is a vacuum leak somewhere....the boots, the intake-to-head gaskets, the throttle shaft seals, etc. Running propane over those spots will result in a bump in rpms if and when the propane gets sucked in.

 

Re: 1983 XJ 750 Midnight Maxim Idles 4000 rpm when it warms

No the throttle is not trapped open , cables are not stuck. enrichening valves are not sticking and the enrichening valve levers are all the way down . The jets are not mixed up as I removed them and cleaned them one jet at a time and one carb at a time and also did not seperate any of the carbs apart from any other ones . Thanks , anybody else have any other idea's ? O , I did use propane to check for leaks and it did not change rpm .

 

Well, either your butterflys are open far too much (idle adjustment screw turned in too far) or the synch screws are opened too much.....both of these screws will open the butterflys.

You say the carb diaphrams are sliding closed fully, how did you verify that?

That or you have a massive air leak somewhere!

 

Re: 1983 XJ 750 Midnight Maxim Idles 4000 rpm when it warms

The sync screws and the idle screw have not been turned , all I did was clean the carbs . The slides I determined were closing by the clunk sound they make if I put my finger in and lift them and let the slides drop . As far as a massive air leak I cannot find one using a propane torch with the propane turned on , no leaks by throttle shafts or carb intake boots, or the covers on top of the carbs . When this first started idling high I replaced all 4 carb intakes as I thought it was leaking air past them . It did not help .

 

First thing I would do is to try turning the idle screw down.

You should always adjust the idle when the bike is at normal operating temprature.

If you want, take the carbs back off and clean and free up the sync screws then bench sync the butterflys.

Build/borrow a YICS tool, build a manometer (tube and 2/s oil) and sync the carbs (look in the suggested FAQ section).

 

I have the same problem, and found no leaks. I will try and turn the idle screw down. Mine are 3 turns out. Carbs are clean...

 

If I turn the idle screw in too far, bike will die. Once I get it right and it warms up the idle starts to rise by itself.