XS400 carb swap from later model

Thanks for your creative input. It is actually very fast for what it
is. Needs a rear disc though, the drum won't really stop it well.
I don't want the starter, that is my point. I have removed it. I just
want it to crank without a battery. But thanks anyway.

 

Oh and it was free. I have other bikes, this one is just for fun, and
helpde me learn some new painting, fabrication, and welding
techniques.

 

Nonsense. Torquey, but not fast. Definitely not fast. And *certainly*
not "very fast for what it is", which is a meaningless phrase anyway.

Fast for a 400 twin? No. Fast for a cruiser? No. Fast for a 1970s
design? No.

Not really. The drum is fine. You should be doing most of the braking on
the front brake anyway. If the drum isn't up to snuff, strip it, clean
it, fit new shoes.

Like I said, the damn things were hard to start when new. The problem is
the carbs. You can convert them to kick, sure - early XS400s had a
kickstarter. But they were always less-than-mediocre bikes.

As a free bike, it has its merits. And the engine was always tough and
reliable. But they were still pretty poor bikes. I've owned two. Oddly,
I have a soft spot for them, but then I like all types of crap bikes.

 

It IS an XS400. It IS much faster than it WAS. Hence fast for what it
IS. An XS400. I am not a huge Yamaha fan anyway, and I don't think it
is a great bike. I am trying to make it a better bike, for fun you
know? Thanks anyway for all your help dude, try to have some fun.

 

If you have a re-chargable 12 volt battery to power the transistor
ignition unit, it will probably
keep the engine running until voltage drops below ~11 volts.

The problem with a permanent magnet alternator with shunt type
rectifier regulator is that it still needs
a rechargable battery of some reasonable ampere hour capacity to
provide system stability.

The six diodes in the rectifier are getting up to 120 volts from the
generator, but the battery is pushing back the other way, and the
battery's substantial power keeps the alternator's output voltage down
to only about 20 volts maximum.

The voltage regulating circuit inside the rectifier regulator is a
small zener diode that triggers the gate of a silicon control
rectifier. When triggered, the SCR shorts one phase of the alternator
to ground, and that reduces the alternator power by half.

The SCR has enough internal resistance, so it's not a dead short to
ground and it survives shunting alternator power to ground many times
per minute at high RPM.

The earliest permanent magnet alternator designs were unregulated
beyond using the headlight, tail light, and instrument lights to burn
up excess "juice" from the alternator at higher RPM. You could run
those motorcycles with the battery removed from the system, but, if
the head light burned out, the tail light and all the other bulbs
would burn out too.

Suzuki added a tiny little diode to some models, it may be have been a
zener, and the British bikes had that great big zener mounted under
the headlight. It did what the zener and SCR in a modern rectifier
regulator did.

The old Brit bikes (and little Japanese singles) had what was called
an "energy transfer magneto". The alternator produced a rather low
voltage which was fed to the ignition coils and the ignition coils
tranformed the low voltage to high voltage.

But these motorcycles all had ignition points and a points type
ignition will start off of a rather low voltage, while the
transistorized ignition needs that steady ~11 volts or so.

It may be possible to convert your bike to ignition points in order to
get rid of the battery *and the transistorized ignition box*, but
there are still problems.

The state I live in used to require any motorcycle to at least have a
battery that was capable of lighting the tail light only for half an
hour, but some of my battery-hating friends bought Battery Eliminators
which were just a huge capacitor (or a bank of capacitors).

I don't know what the value of the capacitors was, but without a
battery to resist the excess voltage from the permanent magnet
alternator, your transistor ignition box is going to get hit with some
wildly-varying voltages.

 

Not what you said originally.

Not what you said originally.

Fair enough. OK: in practical terms, there's not much you can do with
the engine. Nobody made tune-up kits for them AFAIK. And they were not
the most powerful 400s anyway. They have one asset, which is the nice
wide torque spread: far better than the other 400 twins of the era, with
the possible exception of the Kawasaki 400/440cc twins.

So leave the engine alone, and sort out the carbs as best you can. These
are the notorious weak spot of the bike. Absolutely everything needs to
be spot on.

Handling: you can improve the front end feel by jettisoning the rubber
mounts on the handlebars (if that hasn't been done already). You'll get
more vibes through ther bars, but I'd experiment with bar end weights if
it irritates you.

However, ideally you want to throw away the OE cowhorns and fit much
flatter lower bars. The OE bars make the front end too light.

Back end: decent shocks will help, and also decent rubber. There's not
much you can do about the huge heavy 16" rear wheel.

 

Thank you. I think that is pretty much what I am doing. Doesn't seem
to be a lot you can do with the engine, and that isn't a big deal to
me. It is fast enough for what I want it for. I think the stock motor
claimed 14 seconds in 1/4 mile, that is OK. I tossed the crazy bars,
and have some clubman bars on it (ebay junk). I wanted clip ons but
man they are higher than a cat's back. I got a nice pair of lowered
shocks from a guy on ebay, really stiff. I have some XS360 fork
cushions, about 1.5" lower than the XS400 (although I can't seem to
find out how much oil they take, anyone?) And they are very stiff. The
lowered center of gravity make the bike feel better and more stable,
al least to me. I will mill some aluminum bushings to replace the
rubber ones when I can get back to the machine shop, see how that
does. I saw an XS400 rear wheel on ebay with a disc, it "said" it was
18". I don't think that is correct, anyone verify that? Looked like
the same wheel as mine, but with disc. I thought they were all 16" on
the rear? I think I have it running pretty well, once it warms up the
popping stops and the plugs have a nice tan appeal when they come out.
Only problem now is I can't seem to get any damned voltage to the
headlight, even with the bike running. Going to test the relay and see
if it is bad.
I appreciate all of the help, have a nice one. Beautiful riding
weather where I am right now. Hope the same is true for everyone else!

 

That's about right.

<snip>

I saw an XS400 rear wheel on ebay with a disc, it "said" it was

No, the roadster (ie: not cruiser) version had an 18" rear wheel and a
single disc brake at each end.

 

I also see there is a 16" disc version, I might try and swap it over.
I have the MC and caliper already for the rear...
I am still having the "runs but won't crank back up after it is hot"
problem. I have decided it must be a constant open in a circuit
somewhere that can't make once it heats up. It can bridge the air gap
while running, but not after it is shut off. Possible the plugs or
proximity sensor or coils? Anyone know which is more likely? I am
goign to try new plugs today, and the coils maybe next. I can't seem
to figure out a way to get to the sensor, the cover doesn't wat to
come off of the locator pin. The screw on the left side comes out, but
how do you get the right side loose? 1980 XS400.

 

Ah, we never got that oen in the UK.

IIRC you want to pull it off the bottom locator, and then lift it
slightly. I think there are L-shaped lugs that hold the top edge in
place, but ICBW.

 

Thanks for the info. Tried swapping the plugs, that seemed to helped
it start and idle a bit better, but it didn't fix it. Now it acts like
it is running out of gas, and eventually dies and will not crank up.
There is gas, and gas in the bowls. No vapor lock in the tank. Next
time I will pull off the tank and check the coils for voltage while I
kick it over, if they aren't getting anything from the sensor I guess
I know that is it, if they are then I guess that means the coils.
Might be overlooking something. It is fun to ride while it lasts, as
you say very torquey. (sp?) Sounds good too, almost like a real
bike ;-)
Have a good one, and thank you.

 

I think I have isolated the problem to defective plug wires. I tested
the resistance in both coils, wires, and connectors, and found one
wire 3 times the resistance of the other, and the other wire's
connector appear to open intermitantly. After replacing the wires with
some auto ignition wires i had lying around, the bike started right
up.
I never suspected the end would be bad, but now that I think about it
the bike was dropped on the right side, and that is the side with the
bad plug end (duh!). Thanks everyone.

 

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That reminds me of a quote from Dirtcrasher:

"You can't polish a turd, but if you drink a quart of olive oil, it
will come out nice and shiney."

Sorry for the interruption.


Wudsracer/Jim Cook
Smackover Racing
'06 Gas Gas DE300
'82 Husqvarna XC250
Team LAGNAF

 

No apologies needed. I rather like that.