My new XS400 special II

We’ve been working on ours for a looooong time. Tinker, find a problem, order a part, wait, install part… rinse and repeat hahaha.

We’re also going for (mostly) stock. Definitely stock engine/exhaust.

Those dampers in the gauges don’t come cheap; we may install an Acewell and ditch the (41 year old) gauges.
"We’ve been working on ours for a looooong time. Tinker, find a problem, order a part, wait, install part… rinse and repeat hahaha"
... ain't that the truth :)

Those dampers were a bit rich for my blood ... I'll buy them if I have to but I've got an idea to use large O-rings and foam tape but I'll have to see how that pans out ... I'll keep you posted.

I do think that I'd prefer either smaller gauges or none at all to improve the look of the bike ... but I'm going to go stock for now and see how she is in regular use... I've already got plans to maybe get another XS400 to mod.
Maybe ditch all the lights. Lol. None of these ideas are good ones. I would rather be seen as much as possible.
Agreed XChris ;)... On a more serious note, I've got some 90mm I.D. O-Rings that are 5mm thick to use around the gauges (they are 88mm at the "body" and 100mm around the bezel) ... I'll be interested to see if these work as they cost me 3 bucks for 5 :cool:

Then I'll use foam tape around the gauge body as a damper.

Fingers crossed...
Tried out the O-rings to replace the gauge dampers ... success :D

I used a 90mm I.D x 5mm width rubber O-ring and it's perfect. They cost under 5 bucks for 5 and that suits me :cheers: ... If you do use these you will have to use something the dampen the internal wall of the gauge housing ... I am using foam tape. I haven't finished the gauges yet but here's the trial fitting ...
I also fitted my new main jets today (135) and synched the carbs ... the carbs syncing went well and after a bit of messing around I got them totally even without any slow changing in level :)

However my problem with my runaway throttle remains the same :banghead: ... she idles well at about 1200rpm when warm... then, on increasing the throttle slowly once she hits 1800rpm she revs up all the way to about 4000rpm :shrug:

I spent the rest of the time trying out things that might eliminate a cause or diagnose an issue, here's what I found:
- I believe she's running rich overall
- Plugs blacken fairly quickly (I'm mostly idling so I'm not surprised)
- The carb. Jet needles are wrong (I think) they are 6zg5 (very hard to read) and should be 5z1-4 (haynes)
- I ran each cylinder independently and found that the left idles at about 1000rpm and the right idles at about 1700rpm ...:umm:
- carb "boots" are good and have no air leaks
- my throttle shaft is perfect with no play or wear
- the throttle cable is new and unimpeded with no excess bends or kinking
- the twist/spring action of the hand throttle is snappy and smooth

I'm really unsure as to what's causing this ...

The jet needles are single position, so I shimmed them up about 1mm (the only washer I had to hand) ... this didn't change the runaway happening, instead it moved up the rev range to begin at about 2000rpm

I'm not suspecting a control issue, I don't feel there's an air/vacuum leak as she idles smoothly, my suspicion lies with the fuelling.

I'd really appreciate any ideas/thoughts ... it's really bugging me:shootme:


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5gz9 is what they should have in it. The 5z1 is for the earlier type carbs. No shim needed. What are your pilot mix screws set at? New shaft seals, mix screw O-rings, and pilot jet plugs? Are your diaphragms in good shape without any leaks? Make sure all your tests are done with the stock air boxes on the bike.
No problem AmbientMoto.

Thanks XSChris, you're a legend :cool:, my Pilot mix screws are still covered as stock ... I'm going to drill out the plug (as seen in many other threads elsewhere), so that may be an issue as I had to clean around them before and it's possible there is some blockage/dirt or more likely the O rings are shot. The throttle shaft seals all looked good (they look like they've been replaced) the pilot jet plugs look worn and aren't a tight fit ... so that's a possible issue too. The diaphagms both seem good, I've inspected them carefully and they have no visual damage, if I push up the piston and cover the large intake aperture on the top of the carb mouth they come down slowly ... so possibly a leak (on previous CV carbs I've had they stay up if you block the intake).

Looks like I've got more :twocents: to spend and work to do ...
Set the mix screws to 3 turns out as a starting point. And replace the jet plugs they need to fit tight.
Did a couple more hours trial and error on the carbs today ...
I drilled out the pilot mixture blanking plugs and removed the pilot mixture screws ... they were a little dirty but not filthy ... they have no O-rings fitted (? Stock has no O-rings) ... put them back in after cleaning the passages and added O-rings
I also used a manometer to set the float heights ... turns out they needed to be about 22mm rather than the 26mm to get to it right.

I might fine tune them a bit further but they're good ... the 26mm was making the level very low in the bowl.

I checked the pilot jet plugs too, they're actually tighter than I remembered, I put them in with a bit more force and a twist and achieved a tight fit.

Once I'd done the floats and set the pilot mixture screws to 3 turns out I started her up... she starts easily, still has the problem with the runaway revs in the same rev range as before though. I then decided to check the individual cylinder performance. I removed the right then the left plug and on running them they both now idle at about 1000rpm and both runaway from about 1600rpm as before (a little slower, but that's from a single cylinder) ... the plot thickens !!!

I'm stumped ...

Anyone got any other ideas?
Those o rings get stuck on the bottom of the hole. They need to be removed manually.
Thanks XSChris ... I checked the hole and there didn't appear to be any at all, I shone a light into the hole and used a fine tool to check it out ... :umm:

I'm going to tear down the carb completely again this week, starting from the very beginning (again) ... so I'll hopefully have better results then.