79 XS11 Idle Issues

[Schrock86]

I bought this bike last fall not running. I removed the carbs and gave them a very thorough cleaning. I changed the oil and threw in a set of new spark plugs and took it out for a ride. For some reason 15-20 seconds after I start it, The motor idles up to 2500 or so and doesn't come back down. I can turn down the Idle speed screw so that it idles around 1000 but then It hardly idles. If I turn it in until it runs well. It always speeds up after running a short time. Not sure what is causing this any suggestions. Other than that it runs smooth and accelerates nicely. Also I have run 3 tanks of fuel through it and i get about 90 miles before the fuel light turns on. Is this normal?

[natemoen]

Welcome!

For the special 90 miles till the fuels light is about right.

As to the idle, have you synced the carbs yet? When cleaning how did you do it?

[TADracer]

Yes, Welcome!!

Yes, Welcome! In case you have not noticed, you landed in the right place to learn virtually everything about these machines. This site is a wealth of information and knowledge. I am not sure what your experience is but here is my two cents.

Sounds like you need a synchronizing and general tune-up. But there is one thing in your post I see that makes me question a few things...."I bought this bike last fall not running". Raises several questions.

First and foremost...do you have a manual? If not, get one!
How long was the machine sitting?
How far down did you take the carbs to clean them (the pilot circuits in particular)? Did you soak them in a solvent or use spray cleaners?
What is your float setting and where did you measure from?
Did you get the chance to check timing, mechanical and vacuum advance mechanisms?
What kind of plugs (heat range) did you put in? If your bike is stock then stick with NGK BP6ES at first, which is the stock recommendation.
Have you checked valve clearances?

I guess what I am saying is that most everything else affects how the carbs operate. Normally I would say go for the carbs last but in this instance, you already paid some attention to them first. Tend to the other stuff and then tune the carbs last again. If you do not have a set of synchronizing gauges, try using the bread tie wire method for a good synchronizing starting point (I usually just prop open the slides and do it by eye but then, that's me). In any event you need to get everything set back to stock (provided your bike is still stock) to get a baseline tune-up you can rely on.

Again, welcome.

[jjz28]

my xs750 did this to me and it was the butterfly seals leaking air past them, or sucking air really. You may have a vacuum leak somewhere or maybe its something totally different. I am sure someone here will help you find whats going on.

[Schrock86]

I believe it was sitting for 15 years give or take. I stripped the carbs all the way down and "boiled" the bare carb bodies in a 50/50 mix of lemon juice and water. I cant remember where I heard about it but I have down this with quite a few sets of carbs and it is amazing how clean they come out. By boiled I really mean simmer for 15-20 minutes then thoroughly rise with just water then completely blow all moisture out with compressed air. I set everything inside just as the manual specifies and I am using the NGK BP6ES plugs. I have sync'd them and had not thought to check timing because the bike runs very well just idles up and wont return. I checked for air leaking around the carb boots, but haven't checked the butterfly seals. I have not checked valves yet.

[jjz28]

The way I checked my butterfly seals was spraying starting fluid or brake cleaner around them while running and see if it idles up.

[DGXSER]

Although the valves should not effect the idle as you describe, it is always a good idea to check them before you get to the "lets ride this beast" phase. As most people forget all the other stuff they needed to do once they get it on the road.

As to the idle issue, check for vacuum leaks, also check that the vac advance is plugged into the nipple on the no 2 carb body, not the one on the boot. Look pretty carefully at the throttle cable, it can get hung up on the clamps on the carb boots.

I would suggest using a bread tie for the synch, or eye balling it as to how much of one of the small holes gets covered for your bench synch. However, until they are synched while running, they are not truly synched and it will make a difference in how the bike runs.

Other things not mentioned is how far out did you start the mixture screws, and are they even across the board? What did the compression numbers look like across the board? After sitting that long, I would suspect some rings to be stuck and your compression not to be very good right now. But uneven compression can cause some pretty eratic running issues.

[Schrock86]

I have been lurking for sometime now, so much info available here. I finally had to post because this bike has me stumped. Its good to know that 90 miles before the fuel light is normal, I was worried about a possible over fueling issue. I checked compression last fall all cylinder were within 10% of each other and overall were right on spec. I liberally sprayed carb cleaner all over the carb boots, carb bodies and no change in the idle. So I doubt if there is a leak in that area. The vac advance is plugged into the number 2 carb. The mix screws are all at 1 1/4. I dont believe the throttle cable is getting hung up, because the idle speed will go from normal to higher without touching the throttle. I use one of these to sync, but I used ATF instead of water http://www.obairlann.net/reaper/moto...manometer.html

[natemoen]

Open those mix screws up to 2.5 turns out and then resync. See what you get. 1.25 is too lean for these now days.

[Schrock86]

Hopefully I can find some time this weekend to try that. What is the stock setting? I don't have my manual and now I'm curious.

[natemoen]

Hopefully I can find some time this weekend to try that. What is the stock setting? I don't have my manual and now I'm curious.

The manual says 1.25 but that was decades ago with very different gas. Either way what you set isnat is just a starting setting after you get it going you then need to set them further and more accurately and evertly cylinder with most likely be set a little different.

[oldnortonrider]

I bought this bike last fall not running. I removed the carbs and gave them a very thorough cleaning. I changed the oil and threw in a set of new spark plugs and took it out for a ride. For some reason 15-20 seconds after I start it, The motor idles up to 2500 or so and doesn't come back down. I can turn down the Idle speed screw so that it idles around 1000 but then It hardly idles. If I turn it in until it runs well. It always speeds up after running a short time. Not sure what is causing this any suggestions. Other than that it runs smooth and accelerates nicely. Also I have run 3 tanks of fuel through it and i get about 90 miles before the fuel light turns on. Is this normal?

Your carb sync "method" for "finallizing a sync" is your problem. I just got through rebuilding my carbs and did "your same type bench sync". After installing the carbs it started right up and ran well for about a minute, and then did the exact same as yours ~~ revved up and stayed there; when I would adjust to bring idle down ~~ the idle was really ragged ~~ no matter how much different adjustments I tinkered with, it stayed with same problem exactly as you describe. Finally before the engine got too hot, I set up some fans and got my old MotionPro carb stick synchronizer all set up. The mercury levels didn't really look that "far off" when I started it up~~ but when I got 'em all level, the problem, just like you're having, was gone. I did this today; running perfect now. Had been 10 years since I had last used my mercury synchronizer, and thought the hoses might be bad by now, and really had decided to get a CarbTune synchronizer. But last quick minute decision, I decided to try it ~~ still works like a charm. And did eyeball the hoses to make sure they didn't look cracked. What I'm sayin', is, you need to get one of the different type synchronizers hooked up to the intake boot nibs, to sort our your problem. The vacuum gauge synchronizer type ~~ I'm not all that keen on.

[Schrock86]

I turned the screws out and the idle issue seems to have cleared right up. I didn't have to much time to mess with it, but it seems better. Tomorrow I hopefully can devote more time to it.

[MarkD]

Hey Schrock-- maybe go to my recent thread--"unwanted revving issue". There might be some ideas you can use. These guys are very knowledgeable, and helped me get my '79 xs running sweetly. One suggestion ( from a lot of guys) was to replace K&L pilot jets with genuine Mikuni jets. That was the 'magic ' fix. My bike runs like a beast.

[jjz28]

I don't think if the bench synch is done right and the screws at 2.5 turns it would idle up unless it has a leak somewhere. I have never had a "real" synch, just a bench synch and am getting 33 mpg with pretty aggressive riding. I cant wait to see how much better a real synch will make it. I couldn't imagine it running any better than it does.