Yamahe XS400 carb. question

Hi all,

I'm returning to motorcycling after a 25 year gap. I picked up a 1980 Yamaha
XS400 as a play project and as a way to try out the waters before committing
big money on a "serious" bike. There's more detail, and pictures, on our web
site: http://perso.orange.fr/terryr999/Bikes.htm

I used to race sidecars and built my own racing engines. I'm not boasting
here, just explaining that, when I say a motor sounds "right", you can
assume it is. I've been working on the Yamaha and have it running 99% good.
There's just a couple of weird issues that remain and I'm pretty sure they
are carb. related.

When it's cold, it starts up (on choke) and runs well. It continues to run
well as it warms up, idles properly and revs up with the right "bark". It
sounds and acts the way an engine should when it is "right".

If I then turn it off, it won't start properly. Eventually it will run very
slowly (below idle speed) if I hold the throttle wide open. It sounds,
smells and acts like it is VERY rich. If I let it keep running like that, it
eventually soots up the plugs and stops. If I pinch off the fuel pipe it
will splutter on for about 45 seconds to a minute and then gradually rev up
and run clean. If I then release the fuel pipe it then continues to run
normally.

Float heights have been checked two ways - the line inscribed on the float
is parallel to the flange when the carbs. are upside down and, if I attach a
clear pipe to the drains and open the drain screw, the level is the same on
both carbs - just below the float bowl flange. If anything, the level is
slightly low. This also implies that the fuel valves are working OK and that
the floats aren't leaky.

The second issue may be related. Even when it is running right, it doesn't
respond to adjusting the pilot air screws - there is a wide band (about 4
full turns) that don't seem to affect the idle or throttle response at all.
I don't currently have access to vacuum gauges or an EGA but, as far as I
can tell, they are almost completely ineffective.

It also, intermittently, has a tendency to stick at 3000 rpm. If I flick the
kill switch so the revs drop below 3000, it then drops down to a normal
idle.

All air leaks have been fixed. Diaphragms are new and properly installed.
Slides are free and operate smoothly. Carbs are mechanically in balance
(butterflies open and close together). Ignition timing and tappets are
correct. Ignition advance seems to be working OK. Plugs are new and the
correct ones.

What am I missing here?

Thanks,

Terry.

 

Terry Richards says...

A different set of carbs.

 

The BS series mikuni CV carbs are a pain in the arse when it comes to pilot
air channels becoming easily blocked. *Completely* strip and soak the carbs
in clean brake fluid for about 48hrs, then flush out with carb cleaner and
blow through with an airline. When replacing the pilot air screws, make sure
that the small steel washer and (new) 'o' ring are in the correct order.
Adjust to ~1.5-2.0 turns out and start from there.
JB

 

Thanks JB,

That sounds well feasible. The pilot air jets don't do that much at higher
revs but a blocked one would make it rich at very low revs. So rich it
doesn't have the oomph to get the slides up? Dunno, but it sounds possible.
It would also explain why the pilot air screws don't seem to do much.

I don't see how it could make it stick at 3000 rpm but maybe that's another
problem or maybe it will go away once I clear the pilot air channels.

I'll report back with the results.

You don't happen to know what colour it was when new do you? Also, what the
seat looked like? I've seen two on e-bay that both claim to be original '80
SE but they are different! One has wide "stripes" running across (L->R) the
seating area and the other is more of a block / quilted pattern.

Thanks again,

Terry.

 

One other thing to look out for on these is a perforated diaphragm on the
vacuum opeated fuel tap.

JB

 

The fuel tap is working and fuel is flowing when it should and not when it
shouldn't. There's no sign of petrol in the vacuum pipe (I have clear tubes
on both the fuel pipe and vacuum pipe). I stripped and cleaned the tap and
the diaphragm "looked" OK. I suppose I could have missed a tiny hole and a
small amount of petrol could be going down the vacuum pipe and dumping into
the right inlet stub but both plugs soot up equally.

Good thought (and one I hadn't considered) but I don't think this is it.

Thanks again,

Terry.

 

Another way of cleaning carbs:

http://www.randakks.com/TechTips.htm#52

--
platypus

"Merely corroborative detail, intended to
give artistic verisimilitude to an otherwise
bald and unconvincing narrative.”

 

In message <45e54f24$0$27395$>, Terry Richards

<Snip>

Nice, deep maroon with a 2-colour pinstripe round the tank. Bloody good
thick paint too; best I've ever had on a Japanese bike.

Show me the two seats on e-bay and I'll tell you which one.

 

....And you'll want a very heavyweight grab rail too, btw

 

That's yet another one.

Was this it:

http://cgi.ebay.co.uk/YAMAHA-XS400-...5QQihZ011QQcategoryZ10490QQrdZ1QQcmdZViewItem

because my tank is not that shape.

I've lost one but the other is:

http://cgi.ebay.com/ebaymotors/1980...sPageNameZWD1VQQcmdZViewItem#ebayphotohosting

Thanks,

Terry.

 

Terry Richards says...

That's an XS400SE seat

 

Because yours appears to be an XS400SE, not a straight XS400

That looks like a US spec (Heritage Special) rather than a Europe spec
(US Custom/Special) seat (I think)

Unless I'm going potty, what you should end up with should look like
this...

http://www2.yamaha-motor.fr/actu/IMG/jpg/ym50_doc-18-XS400se.jpg

I found a perfect picture of a maroon one via google last night and I'm
damned if I can find it now

Like

http://www.jpwalkenhorst.de/images/cruiser_moppeds/xs400se_custom-m.jpg

but better.

If you're not in a hurry, I've got the original WhichBike? XS250SE vs
GSX250 Custom vs Cagiva SS250 roadtest somewhere; the XS250SE is
cosmetically identical to the 400, though you can tell the difference by
looking at the shape of the barrel/head finning.

 

Bingo!

I found two problems:-

1) There's two air jets in the bellmouth. The Haynes manual calls them
"starter jets". Both were blocked - one completely, the other partly. I'm
pretty sure this was the main problem.

2) The spring is missing on one of the pilot air screws. This is probably
causing a small air leak on that side and, indeed, that side is now showing
symptoms of being lean (spits a bit when cold).

Anyway, it now starts and runs fine when hot and it doesn't seem to stick at
3,000 any more. It still hesitates there on the way down but I seem to
remember my other vacuum carb. bikes did that too.

I think it needs a good run now to clear out some of the cobwebs and then
I'll go over everything again.

Thanks again,

Terry.

 

Yes, it is. It's a 4G5 which seems to be a March 1980 onwards SE. It was
first registered in March 1980 and has a low engine number (994) so that
ties in.

Hmmm, you may be right. It's priced in Canadian dollars.

That's it! Everything I have matches that picture perfectly.

Interesting, the bike is maroon now but has obviously been repainted. I
can't find any evidence of another colour so maybe the painter stuck close
to the original colour.

Or, did you find *my* web site? http://perso.orange.fr/terryr999/Bikes.htm

That one looks black to me. He says he bought it in '83 but, judging by the
price, it wasn't new. I'll drop him a note and see if he knows what year it
was.

I'm not in any hurry at all and would appreciate a look at that whenever you
have time.

Thanks again, you've been a great help.

Terry.

 

That's not a great surprise...

Only when I followed your link, not by googling for it

I think it probably is. The colour of yours is too light, with the
original being at the bordeaux end of maroon. I'll try and (find and)
scan a few pics of mine if I can

I'll let you know when I find it