Very high idle after bike gets hot

Maintenance, repair, troubleshooting, etc.
User avatar
Crimson
One of the Regulars
One of the Regulars
Posts: 127
Joined: Mon May 01, 2017 9:58 am
Region or State: London
Motorcycles Owned Currently: XJ600S
Contact:

Mon May 08, 2017 10:49 am

Anyone have any ideas what could cause this? When the bike is cold, it needs choke pulled out to start. This is normal, I know. The choke makes the bike rev up to 3k rpm. Also normal. Then, when the choke is pushed in, the bike idles at just under 1k rpm. It's possible to adjust the idle speed at this point using the big idle adjustment knob behind the carbs. All well and good!

The funny business starts when the bike has been fully warmed up. It won't exhibit any strange behaviour if you leave it idling for 5 to 10 minutes. The idle will go up to about 1.3k rpm. But if you then take it for a ride, in stop and start traffic, interspersed by fast, short countryside blasts, the engine gets hot and the idle sits at 2.5k rpm, threatening to creep up even higher. At this point, you could fully unscrew the idle adjustment knob. It won't do anything at all. Oh, and I've checked the throttle cable. It isn't catching or sticking. I've even removed the 'push' side throttle cable, to reduce the load on the spring, so it is working and the throttle does snap back properly.

Really strange. All I've done with it is flush the carbs out with carb cleaner (rough and ready chemical method) and crudely balanced 1 and 2 vs 3 and 4 using a pair of vacuum gauges. I've squirted carb cleaner all around the boots of the carbs, and around where they connect to the air filter. No changes in revs, which means no air leaks. What could cause this?

Thanks
A record of maintenance items carried out on my bike:
https://crimsonxj.wordpress.com/

User avatar
radare
Site Founder
Site Founder
Posts: 8476
Joined: Mon Nov 07, 2011 12:42 pm
Region or State: Rocky Mountains
Motorcycles Owned Currently: '92 XJR600, '92 Seca II
Location: Denver, CO
Contact:
United States of America

Mon May 08, 2017 12:20 pm

When the bike is warm, it needs less fuel. A high idle when hot indicates a rich condition. Start by checking all the usual culprits: Enrichment plunger leaks (choke), pilot jet settings, etc. Give the bike a good carb sync too.

User avatar
Crimson
One of the Regulars
One of the Regulars
Posts: 127
Joined: Mon May 01, 2017 9:58 am
Region or State: London
Motorcycles Owned Currently: XJ600S
Contact:

Mon May 08, 2017 12:52 pm

OK, thanks man. I'll try setting the pilot mixture screws back to Haynes spec, and will do a proper carb sync.
A record of maintenance items carried out on my bike:
https://crimsonxj.wordpress.com/

User avatar
Crimson
One of the Regulars
One of the Regulars
Posts: 127
Joined: Mon May 01, 2017 9:58 am
Region or State: London
Motorcycles Owned Currently: XJ600S
Contact:

Tue May 09, 2017 9:57 am

Unfortunately, it didn't work. I turned in all the pilot mixture screws, and then turned them out one and a half turns. I then properly checked the sync of the carbs. The carbs are actually perfectly synced (according to my meters, although to the naked eye 3 and 4 look much more chattery when the bike's idling). Running for a long time on 1.5k rpm, dead steady, and I thought the problem had gone away, and to check I went for a ride. The revs crept up quite quickly during my test ride, and it was idling at 3k rpm by the time I got it back home (30 miles, fairly hard riding). Tried out the choke, and the choke will cut out the bike if pulled out when idling, which presumably means the choke is working OK, if it's flooding the engine enough to cut it out when hot. Left it with the engine switched off for 5 to 10 minutes, then turned it on again. It idled lower.

I wonder whether this could be an electrical fault to do with induction in the circuit as the engine gets hotter. I would be grateful for yourr opinion on this. I have new spark plugs on their way to me anyway.
A record of maintenance items carried out on my bike:
https://crimsonxj.wordpress.com/

User avatar
Crimson
One of the Regulars
One of the Regulars
Posts: 127
Joined: Mon May 01, 2017 9:58 am
Region or State: London
Motorcycles Owned Currently: XJ600S
Contact:

Wed May 10, 2017 6:13 am

As Inspector Clouseau would say, the problem is SOL-VED. :D

Here's what was on my to-do list after reading Haynes and searching the internet:
Check plugs' colour first.
Spray lots of WD40 around to look for vacuum leaks.
Clean pilot jets.
Flush fuel filter.
Pilot mixture screws reset (again).
Check diaphragm in petcock.
Check float bowls aren't sticking.
Bench sync carbs.

Guess which one did the trick? Yep, the last one. LOL
For simple yet complex reasons... OK, here's what happened. When I set up my apparatus (which consists of one pair of vacuum gauges) and did my dynamic sync operation, I did it according to Haynes (1 vs 2, 3 vs 4, etc.). But I made a noob error. I hadn't set the idle screw in properly. It wasn't even seated all the way. So, because I was using just two gauges and not 4, I got near-perfect readings every time, but didn't observe the changes in vacuum in opposite pairs. So, according to the laws of fluid mechanics, here's what I think most likely happened:

Idled normally on 2 cylinders, and idle was adjusted on the basis of 2 cylinders (3 and 4) working. However, the butterfly opened right up when the throttle was opened. So after actually riding, and it didn't need to get very hot at all, the vacuum in cylinders 1and 2 sucked in hot combustible mixture even while the throttle was closed. This meant that these 'lazy' cylinders started idling, while the other two were on a fake idle, i.e. the main fuelling circuit.

Took it out for a 'spirited' ride ;)
Doesn't idle high any more, no matter what. It always comes right down...
WHEW
A record of maintenance items carried out on my bike:
https://crimsonxj.wordpress.com/

User avatar
TonyKZ1
XJ Enthusiast
XJ Enthusiast
Posts: 859
Joined: Tue Nov 12, 2013 12:15 pm
Region or State: Missouri, U.S.A.
Motorcycles Owned Currently: 1997 Yamaha XJ600s Seca II
Location: Marble Hill, MO. U.S.A.
United States of America

Wed May 10, 2017 7:43 am

Glad to hear you got it sorted out. This might a good reason to get a 4 cylinder vacuum gauge set vs the homemade one for 2 cylinders that I've got.
1997 Yamaha Seca II - mostly stock, Racetech upgraded forks, FZ6R rear shock, Oxford Adventure Style Heated Grips, Barkbusters Blizzard Cold Weather Handguards, a Scottoiler vSystem chain oiler. My Mileage Tracker Page.

User avatar
XJLiverpool
Lookin' Around
Lookin' Around
Posts: 70
Joined: Fri Jul 28, 2017 6:51 am
Region or State: Northwest, Liverpool, UK
Motorcycles Owned Currently: 1992 Yamaha XJ600S (seca II)

Fri Jul 28, 2017 1:38 pm

My XJ does the exact same thing but doesn't Idle so high as 2.5K more like 1.9k-2k So I was thinking the bike has been stood for a very long time from the guy I bought it off, So a card Sync might do the trick..... Plus gives me an excuse to buy one :D Shhh don't tell the missus hahah

User avatar
Crimson
One of the Regulars
One of the Regulars
Posts: 127
Joined: Mon May 01, 2017 9:58 am
Region or State: London
Motorcycles Owned Currently: XJ600S
Contact:

Fri Jul 28, 2017 2:58 pm

XJLiverpool wrote:
Fri Jul 28, 2017 1:38 pm
My XJ does the exact same thing but doesn't Idle so high as 2.5K more like 1.9k-2k So I was thinking the bike has been stood for a very long time from the guy I bought it off, So a card Sync might do the trick..... Plus gives me an excuse to buy one :D Shhh don't tell the missus hahah
Carb syncing definitely fixed it for me. The high idle when hot isn't normal behaviour for the bike. There has been no recurrence of the problem since. However, I also did a lot of other stuff to the bike. Replaced the air filter, adjusted valve clearances, etc. IIRC, the carb syncing fixed the high idle when the bike got hot, and, later, valve clearance shimming made the engine go more quickly to the normal idle speed when the bike was started (i.e. before valve clearances were set, it used to take more than a few seconds for the bike to idle properly when started from cold). So I would do the valve clearances first, then carb syncing.
A record of maintenance items carried out on my bike:
https://crimsonxj.wordpress.com/

User avatar
XJLiverpool
Lookin' Around
Lookin' Around
Posts: 70
Joined: Fri Jul 28, 2017 6:51 am
Region or State: Northwest, Liverpool, UK
Motorcycles Owned Currently: 1992 Yamaha XJ600S (seca II)

Fri Jul 28, 2017 3:07 pm

Crimson wrote:
Fri Jul 28, 2017 2:58 pm
XJLiverpool wrote:
Fri Jul 28, 2017 1:38 pm
My XJ does the exact same thing but doesn't Idle so high as 2.5K more like 1.9k-2k So I was thinking the bike has been stood for a very long time from the guy I bought it off, So a card Sync might do the trick..... Plus gives me an excuse to buy one :D Shhh don't tell the missus hahah
Carb syncing definitely fixed it for me. The high idle when hot isn't normal behaviour for the bike. There has been no recurrence of the problem since. However, I also did a lot of other stuff to the bike. Replaced the air filter, adjusted valve clearances, etc. IIRC, the carb syncing fixed the high idle when the bike got hot, and, later, valve clearance shimming made the engine go more quickly to the normal idle speed when the bike was started (i.e. before valve clearances were set, it used to take more than a few seconds for the bike to idle properly when started from cold). So I would do the valve clearances first, then carb syncing.
Thanks for the heads up, I will look into doing the Valve Clearances first, quick question.
How hard is the valve clearances to do and would I need any specific tools?

User avatar
Crimson
One of the Regulars
One of the Regulars
Posts: 127
Joined: Mon May 01, 2017 9:58 am
Region or State: London
Motorcycles Owned Currently: XJ600S
Contact:

Fri Jul 28, 2017 4:28 pm

The valve clearances are easy to do - you just need to be careful with the whole process and be prepared for the bike to be out of action while you wait for new shims. The order in which each cylinder gets to TDC is 1, 2, 4 then 3. It's good practice to drop a bit of oil over the cam lobes as you turn the engine over. You just open it up, measure the clearances, pop out the old shims and read their values, decide the ones you need, then order or go out and get them. The bike will be out of action for as long as that takes. To take out the shims I used a horrible tool called a Laser 6519, and a small hand-held spike and a retractable magnet. Don't bother with the 6519 - it's ridiculously tricky. Use something else, maybe like this:
https://www.youtube.com/watch?v=wI4LznrDzcg
Don't forget to have a set of feeler gauges ready.
A record of maintenance items carried out on my bike:
https://crimsonxj.wordpress.com/

Post Reply