1981 XJ550 Maxim dies when warm

Hey all, I'm having a bit of an unusual problem.

My bike will start up just fine (and was running just fine until this morning), then as it warms up the revs start to drop until it stalls. While it's running, it hesitates on throttle, and pulling the choke once it's warm kills it instantly. I pulled the plugs and it's definitely running rich, but I'm not sure how to fix it. My first step was cleaning the carbs, and my next will be rebuilding the petcock to make sure it's not leaking (I've gone out to my garage to the smell of gas many times these past few weeks). Any suggestions?

 

Open your oil cap and take a whiff. If it smells like gas then STOP running it. it sounds like a stuck float that filling your crankcase with gas. Not good. And you gonna probably have to check your valves if they haven't been checked already. How many miles? Any other history will help the others people trying to help you. Pods/stock ...

 

David,

Open the oil filler cap and sniff for gas. If your oil smells like gas your bike has a valve(s) issue. Put rags under the fuel line and dis-connect the line from the carbs. Turn the petcock to prime. Does fuel flow? Turn the petcock to on. Does it stop completely? You shouldn't have any fuel flowing when the petcock is turned to the "on" position unless your bike is running. Install a in-line fuel filter if you do not have one. Hope this helps.

Roc

 

It has 13k miles on it, and I doubt the valves have been adjusted. It's all stock, and the oil DOES smell like gas. Given I just cleaned the carbs and it's still having this issue, would that confirm a valve adjustment issue? I just got the bike in the fall and haven't had the chance to do any maintenance items yet, and given the attitude of the PO, I doubt they did either. Edit: Some gas drips when the petcock is ON, but it's a VERY slow drip. Would a valve adjustment issue really present so suddenly?

 

So to my knowledge valves out of spec doesn't fill your crankcase with gas your fuel levels do maybe a petcock problem also. So here you go...

http://xjbikes.com/Forums/viewtopic/t=2 ... build.html
And
http://www.xj4ever.com/setting%20fuel%20levels.pdf
And your gonna need to do this at some point soon
http://xjbikes.com/Forums/viewtopic/t=1 ... xj550.html
And for good measures
http://www.xjbikes.com/Forums/viewtopic/t=14692.html

Did you replace any seals in your carbs when you cleaned them?

This is some info from some of the great people here. Your in the right place...

But it time for you to start reading!

 

David,

Yes, it sounds like you have a valve(s) issue. Do not try to start your bike. You need to change the oil along with several other scheduled maintenance items including getting the valves in spec. Do you have a service manual yet? Have you done a compression test on the cylinders yet?

Roc

 

I only have a Clymer manual. I did rebuild the carbs when I got the bike, but not the petcock (for reasons unbeknownst to me). I figured as soon as there was gas in the oil I'd have to change it, but I wanted to solve the source of the problem first. I have not done a compression test, but I'll do that tomorrow to confirm the valve adjustment issue. I also ordered a petcock rebuild kit, so that'll be fixed soon. The floats are not stuck, I checked them. Thanks for the advice!

 

Did you bench sync the carbs? Wet set the floats? these need to be done when you have the carbs off the bike or you'll start way behind the curve.

You have a sticking/maladjusted float and a leaking petcock, this is the only way to get fuel into the oil unless you left the tank on prime.

I'm not convinced it's a valve issue, I'd more lay money on not having a proper sync and proper float height/float operation.

If you don't do the things in order, then you'll just be doing them over and over.

And definitely DO NOT START THE BIKE with fuel in the sump. What level did you fill the oil to? Is it still at that level or does it have a gallon of fuel in there as well now?

Oil is lubrication, fuel thins out oil making it not so much a lubricant, which means quickly seized engine parts.

The correct order is
compression test
valves in spec
carbs clean
carbs bench synced
float heights wet set
carbs on bike
running vac sync
colortune
running vac sync

That's just the very very short list to get it running, there will be more to come to make sure it's road worthy.

 

David,

Might want to add a in-line fuel filter, new fuel line and vac line. Also, open your gas cap and see if the seal is cracked or damaged. These will be future fuel issues if they haven't been tended to in 30yrs. Hope this helps.

Gary

 

Yup!!!

Who wants to talk brakes.

 

XJ,

If the valve that slips on the tang of the floats gets stuck in the open position and does not shut off the fuel supply it will cause a overflow of fuel.

Roc

 

Brakes were the first thing I did, since I knew it was a safety issue. New pads, checked rotors, bled.

I also wet set the floats when I did the rebuild, so unless one of them is intermittently sticking, that shouldn't be a problem, and as I said, when I cleaned them earlier today none of them were stuck.

As for syncing, I haven't done any of it yet, though once I have it running without fuel in the sump I'll head up to the lab at my school (I'm a motorcycle mechanic student) and get them fully synced. I highly doubt they'd fall out of sync in a day though, it seems more likely that the slow drip from the fuel valve has, over time, been pouring gas into the oil. I should have noticed the problem earlier, the oil level seemed to be rising, I was stupid for not paying attention to that. It's now above the sight window, so there's probably a decent amount in there. I'll definitely throw in fuel lines and a new fuel filter when my next paycheck comes in next week.

 

Rebuilding the brakes includes more than just pads and bleeding. There are seals in the calipers that only had a lifespan of 4 years, they need replaced. Same with the seals in your master cylinder.
http://xjbikes.com/Forums/viewtopic/t=41403.html
http://xjbikes.com/Forums/viewtopic/t=18377.html
Does it still have the original brake lines?
http://xjbikes.com/Forums/viewtopic/t=41400.html
What about the rear brake shoes, did you check/replace them?
http://xjbikes.com/Forums/viewtopic/t=15874.html
Once you tend to all this, THEN your brakes will be done.

As far as gas in your oil, you have a petcock issue AND a float issue. Even if the petcock is set on prime, the floats when working properly will stop the flow of gas once the bowls reach the proper level. Gonna have to pull those carbs off again.
Also the valves need to be in spec before attempting any carb syncing, so might as well tend to them awhile too.

 

Yes. "Starts fine cold, won't run once warm" is the classic symptom of too-tight valves. They should have been checked initially at 3K miles and every 5K since; at 13K you're into the "danger zone."

But now you already know you have carb float issues so you need to revisit carbs. And the petcock too; but it's only the enabler for float issues to fill the crankcase with gas.

In order to adjust the freshly-serviced carbs the valves will need to be in spec. You can't simply skip over this; it's the first step.

 

Oh man, I can't believe I forgot about rebuilding the brakes. Good thing they've been working so far, I guess. I found the issue with the floats, the pins on the "top" of three of the float needles were stuck, even though the floats and needles themselves weren't. I cleaned those out and got them freed up, changed the oil, and now it's running fine again. So now I'm down to:

-Valve adjustment
-Bench sync
-Vacuum sync
-Rebuild caliper and master cylinder
-Check for delamination of rear brake
-Rebuild petcock, new lines and filter
-Fork seals

Think I'm missing anything?

 

Way to go.