My new XS400 special II

So a few things I have noticed. Your miss the washer that goes between the spring and o-ring. Your carbs look to have the wrong covers on them. Those are found on the earlier type carbs that have the smaller diameter slides. They should have a tin top with a larger spring.
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Thanks again XSChris ... it's guys like you that make working on these bikes so easy :)

I would never have known about the caps/covers as I don't have any to compare ... I've definitely got the later springs (thicker). I can't imagine that's affecting the throttle response though ... still, I'll look for some later ones anyway.

As for the mixture screws, yep, it's missing the washer ... I will get one and sort that (hopefully this week), I've noticed it's not on all the parts diagrams
So ... I've been performing mental gymnastics to try to ascertain where my carb issue is, I think I'm on to something ... let me explain and please add any ideas or criticism:thumbsup:

My issue is runaway revs on opening the throttle past about 1/4 turn or 1800rpm. This is predictable, repeatable, always happens at the same place in the rev range and in the same way.

My thinking is that it is right at the point where the main circuit of the carb takes charge. I feel that the bike idles well, she needs minimal or no choke to start. So she idles rich and blackens plugs. I also know that she has series 6 needles ... significantly longer than the normal 5 series needles that she should have. I have a suspicion that these needles are running very lean as they are so long .... so when I open the throttle the main circuit engages and becomes very lean as soon as the idle circuit disengages ... this lean state drives the revs up until a significant fuel volume at higher revs brings the throttle to a controllable state at about 4000rpm ... in essence I'm losing my low end as shes so leaned out.

If I'm forgetting anything please let me know :)

My idea is to lathe down the existing needles to drop the e-clip point to that of a 5 series needle ... I'll then try her out.
Forget the above idea ... found some original needles in an old school motorcycle repair shop near me :) ... I lucked out on that one.

Fitted them today, along with new pilot mixture washers and O-rings. Before I fitted them I rechecked the carb for vacuum leaks around the boots, air filters and throttle control rod (none found). I started her up with the new needles and ... right cylinder refused to fire, turns out the plug had failed completely, replaced the plugs and still got the same problem :banghead:

The mixture screws were way way too rich at 3 turns out ... seemed better at about 2 turns. Re checked the carbs for sync and they're still spot on.

Here's how rich she was running, these are new plugs after less than 5 minutes ...

Seriously ... that's ridiculous.

Once I leaned out the mixture they're better but still dark. I have also dropped the float bowl level as I don't think that was helping ... this has also helped.

But, I'm kinda back to square one.

Starts easily, idles well, but throttle is running away from 1800 to 3500 revs :umm: ... really don't know what else to do
No time to work on the bike today but I made it to a local fastener supplier and indulged in a lot of stainless fasteners for my lovely XS. Bought a boatload of M6,M5 set screws and bolts along with hex nuts, acorns, and millions of washers up to M12 :cool:

It all came to about 40 bucks ... a bargain for quality stainless too (benefit of them being a wholesale/ retail operation). Also their customer service is awesome ... they'll spend 15 minutes measuring stuff and digging for what you need even though they specialise in big business orders. BRILLIANT

Not gonna get near the bike again til next week though ... I can't wait
Thanks buddy :bike:

I think it really pays off in the longrun. I was a metalworker/engineer by profession and now that I don't have to worry about build times and deadlines I get to indulge my bike builds and resto jobs. Lets be honest, other than tensile strength scenarios there is no reason other than cost that manufacturers don't use more stainless. If you factor in the hours I spend rebuilding the carbs alone you can justify spending extra on quality fasteners.

And in the long run it pays off from a maintenance perspective.

Here is my ongoing to do list:
- sort my rev/surging issue
- finish and remount the gauges
- fit stainless fasteners where possible/safe
- fit new footpeg rubbers
- polish/clean spokes (replace eventually)
- polish engine cases
- polish hubs
- consider changing the bars (I have some original flat bars in chrome from my 1980 GN400) ... the jury is still out on the buckhorns
Got a bit more done today ...

I've ordered some replacement needle jets/ emulsion tubes so while I'm waiting I've started doing cosmetic stuff. I've fitted the stainless stuff and some high tensile shock bolts and rear peg bolts. TBH it's unlikely that decent quality stainless would fail in these applications but why risk it...20220216_174020.jpg

I've also removed the kickstand, centre stand and exhaust for renovation ...
The crosstube is a bit ratty and the exhausts need a good clean and some preventative maintenance. I plan on wire brushing any rust, then polishing the chrome and VHT paint on the crosstube.

As usual, the cleaner the bike gets the more stuff stands out when it's not perfect :laugh:
Still waiting for my carb parts ... I've cleaned up a lot of parts but the one thing that was still standing out was the top yoke. It had a few scuffs and some wear through on the paint. So stripped it back, sanded it ...

Then masked it up and sprayed it ...20220222_161301.jpg 20220222_162841.jpg

It looks a million times better ....


But then the risers looked dirty sooooo ....

Not sure what's next ...
The bike is looking really nice! Good job working through things and getting it ready for the season.

Regarding the surging issue - I don't have any direct experience to share, but just reading through your description leads me to think that it might be the vacuum slide. The only thing that moves by itself in the carb is the slide and the jet needle, so if there if there is still some very slight binding with the slide, a small leak in the diaphragm or incorrect profile of the jet needle, I could see these symptoms occuring. Just my two pence....
Thanks CaptChrome ... :)

I'm slowly ticking off the less interesting jobs... hoping to have it finished by next month. I still haven't decided on the bars either.

As for surging, I've got a set of needle jets and some other carb bits on the way and I'm hoping that sorts it. Testing the vacuum on the slides they seem ok but I'm probably going to replace them too if the rest of the stuff doesn't work (process of elimination) ...

Can't wait to get out and about :cool:
I'm finally back home from working abroad ... really looking forward to getting a further along on the project :)

Started the day by refitting the handlebars, gauge mount and gauge housings, refitted the lock and barrel, then onto the important stuff... rebuilding the carbs (again:eek:) ...

I have replaced the following:
- needles
- needle jets
- pilot jet
- mixture screws (+washers/O- rings)
- float valves
- float bowl taps

So this should mean that the only remaining original parts remaining to cause a problem are the CV boots and the throttle shaft seals.

I didnt have either of those bits to replace the originals but as it is now it will either work or if not it will indicate that one of those original bits are at fault (in theory)

We shall see :lmao: ...
I've decided that I'll probably refinish the engine (respray) as I'm doing so much other cosmetic stuff. So while the exhaust is off I've taken off the valve covers and stator cover plate for polishing... got some primary polishing done today:

They need buffing, but they're better than they were.

I've also refitted the gauges (at long last):

The suspense of trying the carbs out got too much for me ... I refitted the exhaust and carbs and the valve covers etc. and decided to have day messing about with tuning ...

On the plus side, she fired right up. However, I was getting a very lean run with 3 turns out and she was hanging and revving very high. I spent a couple of hours tweaking out the mixture then worked back in to lean it off again with moderate success.

It's interesting to note that when rich she runs exactly as she did with the old needle and needle valves (surging), however, I noted that leaning her off reduces the surge... after a lot of micro adjusting and balancing she's much better, still not quite there, but i can run up the revs without a strong surge ... however, she's very sensitive on the throttle.

Also found that she runs without surging and with much better throttle control when she's on either the left or right cylinder ????? ..... seriously, each cylinder running alone is perfect :lmao:

Bizarre ...

I got to a stage where I was starting to get tired of it and in my mind that is when it's time to stop or you make stupid mistakes, so I've left it until my next day off next week and I'll come back with fresh eyes. Unfortunately I still don't have a root cause for the surging, my guess is pilot adjustment :shrug:
Spent another few hours tuning today ... after a bit of work I've almost nailed it.

I lowered the needles down to the 2nd notch from top as she was a little too rich, this combined with dialling in the pilot mixture has got me to a nice steady idle, adjusted nicely to 1200rpm when hot. Balanced up nicely too and she seems almost there. There's still a small surge between 2000 and 3000 revs but it's much better than before and the throttle is less twitchy.

I'll be interested to see how she performs from cold tomorrow.

However, as per the 1 step forward 2 steps back rule I now have a duff rear turn signal (it worked yesterday) ... bulbs fine, so traced a worn out wire earthing on the frame ... rewired ... works off the bike, fitted it, but now nothing ... this was just before I was going to go for a ride, I'll have to sort it tomorrow.
Better luck today and a really really nice long ride in perfect weather too !!!

Sorted the rear turn signals (earthing problems), rewired them.

Then took the bike for a long overdue ride ... took her out for over an hour and loved it, I was grinnning like a fool the whole time. Much less choppy throttle, beautiful idling, great acceleration. I had a blast up to about 70 mph and she was smooth all the way through the revs... can even see in the rear view mirrors.

Checked the plugs again once back in the garage ... right cylinder is perfect, a lovely tan brown. Left cylinder is too lean, white and spotless. So I'm going to readjust the floats on the left cylinder when I get time and hopefully that'll sort it ... fingers crossed
Great job on sticking with getting the mixture right on the carbs. Tedious work, but will be worth it in the long run. The bike is looking really great with all of the cosmetic work you are doing. I can only hope to get my side covers that bright!
Thanks buddy, you're absolutely right. It's been really tedious but totally worth it ... she's a pocket rocket now.

She looks beautiful but there's still a bit more I'll do to finish her off. I'm gonna to use her to commute from next week (if she behaves herself)

I will add, that this site is a gold mine of information and it's wouldve been a whole lot more difficult without the input from you guys.