holds revs when hot

I bought an 82 xj550 maxim for my girlfriend to learn to ride on. Figured something cheap and old that if it gets dropped or something no big deal.

Well, I replaced the coils and my father has done the valves and carbs for me now. It runs great at idle, but once it gets hot if it is revved it tends to want to stay at 3000rpm's. He said when he got it all balanced and set his test ride was a little scary.

He said something about a vacuum leak.

Is that right? What could keep the revs up at no throttle input?

 

Could be a vacuum leak. Get a propane torch, and turn it on, but don't light it.'test the area around the carbs with the gas. If the revs go up, you have a leak.

 

the most likley place would be the rubber boots inbetween the carbs and the head. they can go bad and leak

 

Looks like this guy had the same issue, might be worth a read. Title says he solved it http://xjbikes.com/Forums/viewtopic/t=16551.html

 

The intake boots were replaced. I am going to get new vacuum line for the petcock, some gasket sealer for the engine side of the boots, and new nipple caps today at lunch.

 

You can also reach in and turn the idle adjustment screw out while it's hanging. That may bring it down. I had that happen to me when I first got the bike. It wasn't hot enough when I did my tuning but after a 30 mile interstate ride would hang a bit high.

 

Ok. I replaced the vacuum nipple caps and got a new vacuum line for the petcock.

Here is what has been done:
carbs removed, disassembled and cleaned. Dad soaked them in carb cleaner and blew them all out good. diaphragms soaked in silicone or armor all, not sure which.
carb kits installed
new engine side intake boots
valves set
mixture screws set to 2.5 turns (what they were when disassembled)
as above, new vacuum caps and line to petcock.
.5-.75 full tank of fuel.
throttle shaft seals not replaced (propane test passes)
carbs synced


What it does:
Once warmed up, idles wonderfully. Small blips on the throttle are good. Anything over 2500-3000 and it will speed up to 4000-5000 and stay there. If you hit the kill and turn it back on before the engine completely stops it idles good again.

We tried the propane test at idle and found that the idle speed did not increase with that.

The clunk test - I assume this to mean I lift a carb slide and drop it. All but number two passed. I pulled number two out, examined it, replaced it and now it passes. What can I polish these with??

The slides did seem awfully wet with fuel. Should they be? I realize that fuel is running under them and all, but the slides shouldn't be wet, should they?

We didn't set the floats as there wasn't a spec to be found. I have found a thread on why to use the clear tube test, but need further directions on that. Dad was looking for a float spec as that is how things are done. This has apparantly changed, so we would like directions and advice on that for a 550 engine.

If anyone has any idea as to what we should check further, please let us know. My girlfriend (who the bike is for) is quite pissed with this all as she wants to ride and can't.

Thank you much,

Matt N

 

You're halfway there.

Float levels ARE critical; how-to and specs are here: http://www.xj4ever.com/setting%20fuel%20levels.pdf

Once the floats are set, and you're sure you got a nice accurate bench sync, THEN they go back on the bike, for:

The running vacuum sync, with YICS blocked.

Oh, and don't polish the slides with anything. Be sure the BORES are shiny clean, as well as the emulsion tubes, etc.

Mikuni exploded: http://xjbikes.com/Forums/viewtopic/t=31061.html if you didn't get the carbs truly clean, you're going to have issues.

 

So, can I run something like 1000 grit wet sand paper through the bores to touch them up? The number two slide has a little knick in the very bottom that I think I should buff or polish. There doesn't feel to be an edge to it, but its there and would affect nearly the whole range.

Thanks again,

Matt

 

yep, you're getting close. The bores should SHINE like a mirror. I use 1000, 1200, followed by Mothers Aluminum polisher. With the caps off, the clunk should drop without any hesitation. With the caps on, the clunk should drop slowly & smoothly and still make a slight noise once it hits bottom.

Can't get anywhere after this, keep asking. We're all here to help one another, and you describe your problem in great detail for us to understand.

 

"The clunk test - I assume this to mean I lift a carb slide and drop it. All but number two passed. I pulled number two out, examined it, replaced it and now it passes. What can I polish these with??"

This leads me to believe you haven't been here: ?

http://www.xjbikes.com/Forums/viewtopic ... art=0.html

 

Yay! I figured it out. I need to wet set the floats. I came to this conclusion by capping the vacuum port for the petcock and removing the fuel tank. I fired it up and as it ran the bowls down it quit holding the revs high.

So, is there an easier way to remove the carbs than to take half the bike a part and shoving the air box side boots half into the air box?