Racing engine after warm up

I picked up an ’82 Yamaha xj550 for my son and we’ve been tinkering with it. It hadn’t run for a few months but was a daily driver before that, as the story goes. I was able to get it started right away with ether and after awhile it would run on its own, not quite right, but it ran. Dumped some Seafoam in the tank and have let it run a little each day for the last week. Today, the bike started racing after it warmed up, like about 5k rpm and wouldn’t idle down. It did not do that before.

We took the carbs off and the bowls were full of sediment. Jets were a little gunky, but not real bad. I polished the seats w/ brasso and we got everything cleaned out. Also we changed the intake boots. The originals were badly cracked on the outside, though we couldn’t see any cracks on the inside. I have a 550 parts bike that the boots looked like new, so we put those on.

Stuck the cleaned out carbs back on, and it fired nicely, ran well while warming up, but after a couple minutes it started racing again. We can push down on the carb throttle linkage and it nicely idles back down. Throttle cable seems to have adequate slack but it feels like the linkage is not returning all the way to the warm idle stop.

These are cv mikuni carbs. I don’t understand how the choke works as there are no choke butterflies in the intake side. But it acts like it isn’t coming off fast idle stop and after it warms up it just races. Does the choke mechanism employ a fast idle cam that possibly isn’t working?

Also, with the air filter out, but air box still in place, when I put my hand over the inlet opening with the engine racing, it seems like there should be significant suction, but there isn’t. Could the Mikuni’s be pulling air from somewhere besides the intake?

Any other thoughts? My previous motorcycle carb experience is with my Honda CB900 Kheins. I also have a fair amount of experience with auto carbs, though it was 30 years ago.

We plan on pulling the carb bank again tomorrow to check it out.

 

There aren't chokes, they have an enrichment system, with the motor off, when you twist the throttle grip & release it the slides should bottom with a 'clunk', lube the cable, check for kinks, sometimes the bolt on the boot clamp can foul the throttle quadrant, the butterflys could be catching on the body of the carb /s, or you're sucking air, boots, vac point caps, or throttle seals.
EDIT-BRAIN DEAD:
You lift the slides mechanically & they should drop with a 'clunk'

 

The Mikunis on the 550s are finicky at best. Everything is TINY.

Here's an exploded Mikuni of the same type: http://xjbikes.com/Forums/viewtopic/t=31061.html Clean isn't clean unless they've been completely apart.

You'll need to verify the float levels using fuel and clear tubing; the spec is only a millimeter or so each way;

and do a bench sync using thin strips of business card.

Once back on the bike, you need to do a running vacuum sync, with the YICS blocked. You'll need some sort of "comparative manometer" to do the vac sync, and either a special tool or other DIY remedy for blocking the YICS.

PRIOR to trying to vacuum sync, you need to check the valve clearances and be sure they're in spec. You won't get a good vacuum sync with a tight valve or two.

Welcome to the wonderful world of the XJ550. Sweet bikes but they do require some special attention if you want them to run right. It'll prove to be a tad more work than your Honda, I bet.

 

If you broke the rack apart you probly changed the idle setting....when it races back the idle adjustment off to bring it back down.

 

Problem fixed! Turned out to be multiple problem (ain't they always).

Took the intake boots back off, suspecting a leak at one of the gaskets as I had to reuse the old ones, and one of them tore. Sure enough, I installed the torn one upside down not realizing they were not symmetrical. I made a new one as the stealership is not open until tomorrow.

Put carbs back on and still high idle. This time it was obvious that the throttle plates were not fully returning. Took back off, carefully seated the butterfly plates, carefully bench synched, carefully loosened the rails and realigned the carbs on flat surface. Still things were catching. Discovered the springs were actually binding. Used some zip ties to pull them together, hard to describe exactly, but it worked, no more binding. Put carbs back in and viola, really smooth running.

1500 rpm seems to be about the lowest I can set it. But took it down the road and it flies.

Next issues are the chain is binding and fuse block is trash.

Thanks for your help.

 

Replace the chain with both sprockets, as a set.

Don't ignore the valve clearances; you'll just burn up the motor. The need won't go away, and damage will result.

Be sure to check the rear brake for delamination first off, before any riding: http://xjbikes.com/Forums/viewtopic/t=15874.html You can't go by feel or how it works, you have to LOOK.

It's gonna be "cold blooded" and you're going to get crappy fuel economy (should get 52~56mpg) until you go through the maintenance/tuning steps I outlined.

But now that it's running, get after those brakes. Original rubber lines had a four-year recommended lifespan; caliper and master cylinder seals, 2 years. Stuff's gotta be replaced, period; or the bike won't be SAFE.

How old are the (tubeless) tires?

 

I've got a parts bike that looks to have been better taken care of than the one we are restoring. Sprockets and chain are looking good on it so we'll use them. Will check rear brake when that is all disassembled.

Tires look pretty new, but haven't checked date codes. Have bled front brakes to push new fluid through. If people purged their fluid once a year, caliper problems would disappear.

Valve check is definitely on the agenda. I can't believe how much smoother it runs than my CB900 Honda. Would that be because of the YICS? My engineering mind is already designing a balancing system for my 900.

Thanks!

 

Linking the vac' ports isn't how the YICS works.

 

YICS:



Connects the intake tracts; the openings are just behind the heads of the intake valves.