XJ900 throttle shaft housing oval - bike won't idle

Hi All,

I've bought an old XJ900 1985 model to turn into a café racer/scrambler as a long term project. Having a nice perimeter frame, carb and aircooled engine and nice wheels, I thought it would be a good starting point.

After sourcing a tank from a local mechanic who deals with old bikes, I've dropped the bike into him to get it working to register the bike before I start. After a short time, I've being advised that although the bike runs, it won't idle due to the throttle shaft housings becoming oval overtime. The mechanic has told me there is no fix for such an issue. Has anyone had experience with this?

I've got to go down and have a look. Otherwise the bike is yet to reveal other dimensions of its 30 yr history...

I'm not concerned as this is not going to be the bike I ride until done. I would be happy to source another set of carbs if needs be. I've got time on my hands with this project.

I wanted to learn a few things and do a bit of the work myself, but as I'm reading, it seems that the mechanics don't bother too much with cheap bikes.

If you guys can you point me in the direction of where I could get help or advise on this issue.

Regards
Adam

 

I had the same problem on my XS650 and it caused a hanging idle, not NO idle. I don't understand the relation between an oval throttle shaft housing and no idle, honestly.

 

Hi Quebecois59

I've yet to go down and have a look for myself. Terms are a bit different out here in Aussie land and I'm not a mechanic. I have seen the hanging idle issue on youtube. The mechanic claims air is bypassing through the ovality when the bike warms up.

I'll go down to have a look tomorrow.

How did you resolve your issue?

Regards
Adam

 

Question here, could it just need new throttle shaft seals , or am I misunderstanding the problem?
Alan

 

I bought another set of carbs, it was cheaper and less work than rebuildong the other ones. XS650s have only two carbs, and carbs were relatively cheaper and easy to source.

 

Well, the xs650 carbs are different, too, so you're gonna be in a world of hurt.

Get the right carbs, or rebuild yours. I would suggest that you probably just need new throttle shaft seals.

Also, it's more like many mechanics now don't want to work on OLD bikes.... They can't hook a computer up to them and therefore actually have to either KNOW what to do or be able to actually THINK and figure out the problem. There are many legit mechanics out there though.....

Dave

 

Did he take the carbs apart to discover this issue? If not, he is blowing smoke, or sucking air, or whatever analogy you wish to use. I've never seen carbs with oval-ed shaft holes, and although anything is possible, his description is not very probable.....unless the bike has, say, 500,000 Km on it.....

 

I've seen it a grand total of ONCE..... Ever.

 

i had that happen to a lawn-boy mower from the 50's

 

Ok, got a bit more of an explanation.

The carbs are actually leaking fuel out the sides of the slide rails for the throttle shaft. The carbs are Hitachi ones.

Any recommendations? What other carburetors work for this bike?

 

Rebuilt ones work nicely, and will cost you less than picking up other used carbs and rebuilding them ( in my opinion all used carbs should be rebuilt, unless you're the sort of person who enjoys a hangover), and a lot less than buying new carbs and working out the proper jetting.

From the sound of it you have sticking floats or bad float needles. You might also have dried out fuel rail o-rings, and maybe (just maybe) worn out throttle shaft seals.

Even if the throttle shaft bores were worn (and there would be no way to tell wighout disassembling every carb) the worts case would be to buy a replacement rack of carbs, or have a machine shop plug and re-work the bores (yes, it can be done, but it will cost a bit).

 

Thanks so not impossible, just takes time. I note all the really great info here on undertaking the task - I'm up for it. Might just need to post a picture or two etc.

 

An XJ900 should have Mikuni (BS36 series) carbs.

Easy way to tell the difference between Hitachi and Mikuni carbs is to look at the chrome covers, if the raised dome is perfectly round, it's a Mikuni; if the raised dome has a 45* down-angle cut off on one side, they are Hitachi carbs.

 

Possibly the same one that Dave saw............

 

That explains a lot--- you have the wrong carbs, too--- you should have Mikuni carbs. Ask Len at xj4ever ( resident expert and parts supplier ) which size.

Dave F

 

Ok went down to the mechanics on the weekend and got my bike back. The carbs are Mikuni, they are leaking from the throttle shafts. I determined this by spraying a small amount of starter fluid at that point. The fuel leak was from a leaking old line from the petcock, now replaced. A plug cap was broken and only working intermittently now replaced.

The bike runs once warmed up at idle. When I open the choke and the bike warms up the revs come up. The choke housing is broken so I can see how it may not have turned completely off. Take off the choke when warm the bike sat at about an 1100-1200 rpm idle. The guys pulled off my pods and put the airbox back in so I now need an airbox door... Anyway a work in progress, I reckon those mechanics had there eye on the bike as telling me it was a lemon so they could use it to fix there XJ or use for parts... Ok now I'm convinced to do things myself.

I have a full set of mikuni carbs spare, that are completely stripped of jets but still in the rack.

 

Okay, sounds like you are heading in the right direction.

Just be aware that 1983-84 XJ900 models used the BS35-series Mikuni carbs, and the 1985-on used the slightly larger BS36-series carbs. Although many of the internal pieces will interchange between the two series of carbs, the carb BODIES are different and you do NOT want to mix-and-match 35-series and 36-series carbs in the same system!

 

Thanks Charcal I'm thinking to take off the ones that work and rebuild them. At least the ones on the bike worked.

How do you check the sizing? Are they marked?

What is your recommended plan of attack?

 

The later ones were slighter larger, and the actual cyl displacement was a little larger too.

 

The carb throat ID and vac piston OD are the "smoking gun" measurements to distinguish the 35's from the 36's.

Also, on the 35's, the fuel inlet T-fitting is between carbs #1 and #2, while on the 36's the fuel fitting Is between carbs #2 and #3.


Mikuni BS35 (900RK/RL) intake throat ID: 34.95mm
Mikuni BS35 (900RK/RL) intake manifold flange OD: 42.00mm
Mikuni BS35 (900RK/RL) airbox boot flange OD: 54.90mm

Mikuni BS36 (900N, F, FN) intake throat ID: 35.97mm ID
Mikuni BS36 (900N, F, FN) intake manifold flange OD: 42.00mm
Mikuni BS36 (900N, F, FN) airbox boot flange OD: 54.90mm
Mikuni BS36 (900N, F, FN) vacuum piston OD: 30.90mm

 

Ok got the carbs off. Thank god they are 36mm Mikuni's - phew... as my spare is a 35mm. Thanks for the dimensions Chacal.

Now for the fun part...

Removing the float bowls I was surprised to discover no gasket on float boat 4... now that would cause leaking fuel to spill everywhere and suck air.

Within another 5 minutes I notices the butterfly 4 open when 3 was shut... 1+2 slightly open, so not set properly = more and more air and running on.

In an effort to stop the bike running on someone had wound down the throttle screw on 4 all the way in. Which is the possible cause of no. 4 not firing at idle (when it starts and idles on the choke).

Wow what you can see in spending 10 mins with your carb... off the bike

Otherwise the internals of the carb are clean as whistle, shinny and appear well cared for. Looks as though someone had attempted the rebuild and failed at the last post.

Time for a carb kit, throttle seals, valve adjustment, vacuum test, colour tune etc etc. The information here is just great.

In terms of the diaphragms for the sliders - they look ok, no holes and reasonably flexible. What are the ways of determining that they are worn or too tired?

 

Just because the butterflies were not all the same when you took it off is not a problem in and of itself.

They are set the same for initial bench syncing, but when the screws are adjusted during a running sync, those butterflies ARE changing.

 

Thanks Hogfiddles,

I am about to enter the world of tuning...

I understand that they would need to be sync'd to equivalent vacuum pressures. Is this done whilst at idle and then through the rev range? Should there be no situation where the butterflys are fully closed ie no air?

I'm a bit of a way off for tuning, but I am increasing my understanding.

 

It's done at idle, adjusted in small increments with the throttle blipped and rpms allowed to settle down before next adjustment...... Adjust blip watch adjust blip watch adjust blip.... Etc...

Usually done in well under 5 min. You'll get the hang of it quick---

You don't "adjust anything through the rpm range".