1977 xs650 - rev's hanging up when i get off thottle

ljevans

ljevans

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I've recently purchased a 1977 xs650 from a friend with 35,000km's on it. So far I've pulled the carbs apart and cleaned them as well as I can (torch tip cleaners throgh the jets, checked the diaphrams, cleaned out float bowls etc), sorted out the charging system (new battery, new plugs, new regulator, new rectifier, new alternator rotor) as well as some issues with lights and i've gotten to the point where i was ready to take her for a run around the block. It starts fine and idles pretty well with the choke off (a little black smoke puffs when cold) and it seems to run decent through the gears (no stumbling etc) but when i get off the throttle i can feel the bike still pulling. I pull the clutch in and the rev's stay around 4000rpm. I can snap the throttle several times but nothing seems to bring the revs back down. eventually (maybe 15 seconds or more) it'll eventually settle back into a decent idle. They are (from what I can tell) the stock BS38 carbs. My friend owned it for about 8 years before I bought it and he rode it seasonally until about the last year that he had it when he bought another bike so i don't think that there's necessairly an issue with the jets as he rode in the same climate as I am all those years with no issues. The throttle linkage and cable are definately not hanging up as i can see the linkage sitting against the idle stop screw while all of this high revving is going on. I've tried spraying ether around the intake and that didn't cause the rev's to climb from idle so i suspect there's no intake leak? I also drained all of the fuel out of the fuel tank and the carbs at the same time that I went through the carbs (within the last 2 weeks) It also has the stock airboxes on it with decently clean air filters in them. I have yet to go through the cam chain tension, valve lash, timing proceedure so that's probably going to be my next stop unless anyone can shed some light on this for me?

thanks :D

edit - the exhaust has been chopped off at the muffler area. that's the only mod that I can think of but my friend did that right when he bought it 8 years ago and it always ran good for him so I don't think that's necessairly part of my issue.
 
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Have you serviced the advance unit and advance rod? They need to be cleaned and lubed or the timing can stick in the advanced position which can cause the revs to hang.
 
There are quite a few things that can cause high hovering idle:

1) The carb butterflys must both be able to freely close all the way. Synchronizing the carbs is part of doing this.................use a manometer (make your own).

2)The ATU fly-weights and advancer shaft must be working properly, clean and lubed.

3)The air/fuel mixture needs to be correct (pilot screws)

4)Points gap and timing must be correct

5)Air leakage around the carb inlet area.

6)Last but not least, the 70 to 79 year carbs have an O-ring on the carb needle jet. If that O-ring is worn out, it lets unmetered gas bypass the needle jet, and cause a high idle.
 
Great thanks for the info guys. I'll check into these items and report back. Doe anyone have some good info on how to check the timing? I've gone through the forum and check youtube etc but I haven't found much info on how to check the timing with a timing light. I've read some info on how to get it close with a multimeter or a light bulb but i haven't seen a good breakdown on doing it with a timing light. can you adjust the timing while it's running like with the cars or do you have to stop the engine each time and make an adjustment the fire it up again to verify? thanks.
 
Start by adjusting as outlined in the repair manual. Then with the bike idling hook the timing light to the right cylinder. The rotor mark should be between the two marks by the F. Now rev the engine slowly to about 3000 rpms. The rotor mark should move smoothly left to the full advance mark. To but never past the full mark. Now repeat for the left side.
You can adjust the timing with the engine idling but it will be easier to do it with the engine shut off.
Leo
 
great, thanks. I'm going to go through the proceedure that's been outlined on here before. Cam chain tension, valve lash, timing then carbs. Hopfully somewhere along the way it gets sorted out. It's been improving greatly so far with the recent improvements so hopefully I can get it dialed in.
 
If your advance unit isn't functioning freely and properly then you won't be able to do the timing. This is one of the most over looked parts on this bike. I find it in need of servicing on pretty much every 650 I look at. Two I helped guys with this past summer both needed it. One was all rusty and missing a spring. The other had the unit itself lubed but not the advance rod running through the head. That's what gets over looked most often. If the rod is dry and binding, the unit can't do its thing (advance and retard as engine RPMs change).
 
The advance rod rotates a little less than 1/4 turn between retard and full advance. With that cover removed and the motor off, grab the end of the advance rod, rotate it, and let go. It should snap back quickly to the retard position. If it doesn't, cleaning and lubing of the unit and rod are probably needed. The springs also sometimes get worn out and need replacing (or modding). Clean and lube things first. That's all it takes many times.
 
I just went out on my lunch break (i'm keeping the bike at my work while im working on it) and it seems that those timing advance arms are not returning properly. I rotate the shaft and they open up but when i let go they stay where they are instead of returning to the stops. It seems that the rod is binding as you suggested. Whats the best way to lubricate this assembly?

I've attached a photo of my advance system (with a cameo by my finger) I probably should have attached a photo of it stuck open but you get the idea :thumbsup:
 

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Yes, yours looks all dried out and I see some rust around the pivot posts and springs. Here's a thread that covers the servicing .....

http://www.xs650.com/forum/showthread.php?t=18187

Note that line scribed on the little disc above the arrow in your pic. That aligns with one on the ATU main plate, which is barely visible in your pic. You must re-assemble things like that or your timing will be thrown 180° off. There's a little locating pin in the side of the advance rod, you can see the slot in the little disc for it at about 3 o'clock in your pic. The hole in the advance rod for that locating pin goes all the way through. That means it's possible to have the pin in two positions. One is correct, the other will throw your timing off.
 
5twins said:
Yes, yours looks all dried out and I see some rust around the pivot posts and springs. Here's a thread that covers the servicing .....

http://www.xs650.com/forum/showthread.php?t=18187

Note that line scribed on the little disc above the arrow in your pic. That aligns with one on the ATU main plate, which is barely visible in your pic. You must re-assemble things like that or your timing will be thrown 180° off. There's a little locating pin in the side of the advance rod, you can see the slot in the little disc for it at about 3 o'clock in your pic. The hole in the advance rod for that locating pin goes all the way through. That means it's possible to have the pin in two positions. One is correct, the other will throw your timing off.
Click to expand...

that's great, thanks. I'll likely pull it apart after work today and see if I can free it up. Based on what I'm hearing it looks like this could be the culpret behind my RPM issue :banghead:
 
Indeed, very common problem with these bikes. It can't entirely be blamed on the owners, though. Yamaha never mentioned the need to service the rod or gave any procedures for doing so in any of the manuals. Guess they didn't figure these bikes would still be here 30+ years later, lol. Do a thorough servicing and greasing of the rod and you'll be good for 5 or 6 years, maybe longer. Then the rod will probably need to be re-greased, or more correctly, it's bushings.
 
:banghead:Well I removed the shaft, lubed everything up and put it all back together. It still seemed to hang up a bit so I modified one of the springs to increase the tension and that seemed to do it. I ran it and it seemed to cure the rev's hanging issue. I ran it for 5-10 min and just before I shut it off i blasted the throttle again and the rpm stuck at 5,000. I tried twisting the throttle a few more times, flipping the choke on/off and nothing seemed to help. after 5 seconds or so i just shut it off with the kill switch. I opened up the cover to the advance assembly again and the arms are still moving freely... back to the drawing board
 
Throttle shaft seals leaking? Intake manifolds rotton?
 
I tried pulling the throttle shaft side to side when this was happening and it didn't seem to affect it but that probably doesn't mean that they're necessarily ok. The intake manifolds seemed ok but I think I'll pull the carbs off and take a second look. I did spray ether on them yesterday when it was idling and the rpm didn't increase so I figured they were ok it's tough to say for sure
 
Now im far from a pro mechanic, but i would replace the springs. U said it ran right for a little while, maybe the tension of the spring is less than its supposed to be. I don't know tho lets see wat the guru's say....

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