Mixture Screw and ratio issues

Again with the 1982 Maxim XJ650 frame/Maxim XJ750J engine.

It runs, it turns over, it's got amazing power once on the road, but it has a mind of it's own when it comes to idling.

Symptoms:
1) When first started, with the choke closed, it idles very high for a few seconds and then drops to an even idle. Then a few minutes later, will die.

2) On the road it hits a high RPM and will stick with it even after the throttle is closed. I can coast at 35 in 2nd gear with my hand off the throttle.

3) Sometimes it will idle fine, sometimes it won't idle at all and will just die.

4) It does all the same stuff when first started or after a 30 minute ride.

5) Adjusting the idle mixture screw doesn't seem to do much


The carbs have been cleaned out and I can see light through all the jets. Shot carb cleaner and Brakecleen through all the ports. The foam air filter has been pulled and re-oiled.

The engine is a 1982 Maxim XJ750J. I do not know if the carbs and exhaust are from the 650 or the 750.

Any ideas?

 

Did you rebuilt the carbs or just cleaned them?
My XJ700 was doing the same after a carb clean marathon so I decided to replaced al the o'rings and throttle seals. When I pulled them apart the throttle gaskets were pretty bad. I assume that was sucking air at some point, it was very inconsistent.
I didn't put them back yet but I think that was one of the problems. Also one mixture screw was 4 turns open, another one 1 turn open an the rest was decently 2.5 turns opened.
Just a though but I think your mixture ratio is changing for some reason and one could be "extra" air in the mix.

 

DO THE CARB SLIDES GO "CLUNK?" as in CLUNK? Have you checked for vacuum leaks using an unlit propane torch or carb cleaner?

 

Item #2 sounds indicative of an air leak through your carb-to-head rubber boots. Confirm using an unlit propane torch on these boots and see if your idle speed or character changes.

 

I've always been confused by this statement. What are the carb slides? And I suppose once I know what they are the "clunk" statement will make sense, too.

 

Rubi check in the FAQ section, there are some post that will explain to you about the importance of the CLUNK TEST and how to do it.
The carb slide are located inside the carbs and to get access to it you need to remove the carb hat (top). Once you removed the hat you will see the rubber diaphragm which is attached to the slide cylinder. This cylinder moved up and down due the vacuum created by the corresponding cylinder in the engine. When I get home I can send you a picture of one of this slides.

 

You don't need to remove the hats to do the "clunk" test just a carb to airbox boot. Stick your finger in and lift the carb slide up and then let it go. It should go back down with a soft CLUNK. If not, carbs (specifically the slide bores) need cleaning. Once you take them apart to clean that you'll discover ALL sorts of additional things that need to be cleaned as well.

 

You are right Fitz, you can do the test "in-place" but they will not slide (free fall) as Rick describe in his post. I was trying to do not confuse Rubik more.

 

True. In place=soft clunk, hats off=CLUNK. The point is that the slides have to immediately fall once released.

 

Oh, okay. I got it now. I actually already knew about the sliders and the clunk test, just didn't know it by that name. When I took the carbs apart to clean them I cleaned the sliders off with Brakeleen and a tach cloth as well as the cylinders. They rise and fall like a hot knife through butter.

 

OK, if the "clunk" test is good, then you are probably looking at a vacuum leak somewhere, which could also unfortunately include the THROTTLE SHAFT SEALS. Do the unlit propane torch test to check for vacuum leaks and see what it tells you. You also might want to VERY carefully inspect the carb diaphragms for pinholes.