XS400RK

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Ron Gibson - 01 Sep 2007 02:42 GMT

About a year ago I bought this 1983 400CC Seca (XS400RK). I got it pretty
cheap, $800. I just wanted to start riding again and save some gas. last
bike was a 1981 Seca 750 (XJ750 Seca).

I was really surprised at how peppy this bike is for a 400 twin...

http://bikez.com/motorcycles/yamaha_xs_400_dohc_1983.php

Anyway, the project has gone well, giving it new brakes, replaced fork
seal, new tires, etc.

I got a carburetion problem. The manual says it has twin Mikuni BS34.
Basically it was running too rich and the carbs would load up badly
while idling especially on a hot Florida day. In cool weather the problem
is greatly reduced.

I don't like to do carb work if I have to get inside but it looks like
that might be necessary. In fact I had the local Yamaha dealer balance
and adjust them and it actually got worse but it needed an air filter and
they did not have one in stock. Ordered one from Bike Bandit and what a
difference that made !

Considering the problem is almost eliminated (I'm left with ever so
slight loading and a few flat spots at high RPM in 4-6 gears with full
throttle - Have to back off for a sec))

I have three questions, guys...

1) In looking at the manual I see no idle mixture adjusting screw or
mechanism. Am I missing something there?

2) Any guesses as to why it's still running too rich?

3) Do you think giving the dealer another $150-200 to disassemble and
clean them is a good idea?

Right now it's dependable and I'm considering buying something newer in
three months so I don't want to put too much money in it unless it really
pays off. I'm looking at getting a 1999 FZS600 Fazer...

http://bikez.com/motorcycles/yamaha_fzs_600_fazer_1999.php

BTW, anyone had this model Fazer and if so do/did you like the bike?
Strong points? Weak points? Easy of care and maintenance?

TIA.

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Albrecht - 01 Sep 2007 03:12 GMT

>I got a carburetion problem. The manual says it has twin Mikuni BS34.

These carbs look like Hitachis to me because of the shape of the diaphragm
caps,

http://houseofmotorcycles.bikebandit.com/partsbandit/ShowSchematic.aspx?deptId=6
33246&machineId=8258

>1) In looking at the manual I see no idle mixture adjusting screw or
>mechanism. Am I missing something there?

There is a cylindrical sprue forward of the diaphragm caps. There may be a
brass plug concealing
the screw which needs to be piloted drilled and pulled out by screwing a
sheetmetal screw into the pilot
hole. Then the screw and plug are pulled out with a pair of pliers.

Alternatively, the screws are sealed by a red goop.

>2) Any guesses as to why it's still running too rich?

High float level, sticky float valve, or somebody may have already tweaked
with the idle mixture screws.

Or the idle ports and passages are so plugged up the last mechanic who tried
to synch the carbs opened the throttle butterflies so far the transition
ports are uncovered and the engine is getting
excess gas that way.

>3) Do you think giving the dealer another $150-200 to disassemble and
>clean them is a good idea?

Absolutely not. Most $tealer$hip "mechanics" don't have the experience to
adjust a carburetor correctly, and you've already had problems with those
guys.

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Steve Burgess - 01 Sep 2007 16:43 GMT

> About a year ago I bought this 1983 400CC Seca (XS400RK). I got it pretty
> cheap, $800. I just wanted to start riding again and save some gas. last
> bike was a 1981 Seca 750 (XJ750 Seca).

Hi,
I'm trying to finish up work on it's sister bike, a Maxim 400 (XS400K)
Overall very close some cosmetic differences and you have a 6 speed vs my 5.

> I was really surprised at how peppy this bike is for a 400 twin...
>
[quoted text clipped - 22 lines]
> 1) In looking at the manual I see no idle mixture adjusting screw or
> mechanism. Am I missing something there?

fuel mixture is on top of the carbs, towards the air box ,may be covered
with a plug (from factory, EPA crap) :) )you need to pull the tank to
see it (one of my few complaints about the design, that and the location
of the ignition coils)

> 2) Any guesses as to why it's still running too rich?

Usually they left the factory running lean, could have a clogged air
passage. They are not that bad to clean, I did mine, lots of carb
cleaning manuals online. Might want to find out when the valves were
last done as well. (gotta check mine too)

> 3) Do you think giving the dealer another $150-200 to disassemble and
> clean them is a good idea?

If you're not comfortable doing it maybe, but it's not that hard, if you
take your time disassemble carefully,get some carb cleaner and an clean
and blow compressed air through every passageway.
Check out xjbikes.com they have some good info in there for this type of
thing, just remember you got 2 carbs, they have 4 :)

> Right now it's dependable and I'm considering buying something newer in
> three months so I don't want to put too much money in it unless it really
[quoted text clipped - 6 lines]
>
> TIA.

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Ron Gibson - 01 Sep 2007 19:35 GMT

>> About a year ago I bought this 1983 400CC Seca (XS400RK). I got it
>> pretty cheap, $800. I just wanted to start riding again and save some
>> gas. last bike was a 1981 Seca 750 (XJ750 Seca).

> Hi,
> I'm trying to finish up work on it's sister bike, a Maxim 400 (XS400K)
> Overall very close some cosmetic differences and you have a 6 speed vs my
> 5.

You know I really prefer the styling of these 80's sport naked bikes.
Only new yammy I really like is the FZ6...

http://bikez.com/motorcycles/yamaha_fz6_2007.php

The seating position is great and doesn't make you hunch over in order
to ride it. Of course I love the performance of the R6's. That R1 is too
much bike for me. I'd kill myself on that puppy.

>> Considering the problem is almost eliminated (I'm left with ever so
>> slight loading and a few flat spots at high RPM in 4-6 gears with full
>> throttle - Have to back off for a sec))

>> 1) In looking at the manual I see no idle mixture adjusting screw or
>> mechanism. Am I missing something there?

> fuel mixture is on top of the carbs, towards the air box ,may be covered
> with a plug (from factory, EPA crap) :) )you need to pull the tank to see
> it (one of my few complaints about the design, that and the location of
> the ignition coils)

Thanx on that. I never would have found it. When I was younger I used to
tackle these types of problems but I'd rather let a pro do it now. I can
pull the tank and take a look see easy enough.

>> 2) Any guesses as to why it's still running too rich?

> Usually they left the factory running lean, could have a clogged air
> passage. They are not that bad to clean, I did mine, lots of carb cleaning
> manuals online. Might want to find out when the valves were last done as
> well. (gotta check mine too)

Yeah, valves of on the list, too but hopefully I can find a 99 Fazer in
a month or two.

>> 3) Do you think giving the dealer another $150-200 to disassemble and
>> clean them is a good idea?

> If you're not comfortable doing it maybe, but it's not that hard, if you
> take your time disassemble carefully,get some carb cleaner and an clean
> and blow compressed air through every passageway. Check out xjbikes.com
> they have some good info in there for this type of thing, just remember
> you got 2 carbs, they have 4 :)

This is one advantage about twins :-) I had an old 79 650 Special and I
tell ya, this little 400 would tear it up. But I did considerable
performance tuning on the 650 myself.

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Ron Gibson - 12 Sep 2007 01:55 GMT

>> About a year ago I bought this 1983 400CC Seca (XS400RK). I got it
>> pretty cheap, $800. I just wanted to start riding again and save some
>> gas. last bike was a 1981 Seca 750 (XJ750 Seca).

>> I got a carburetion problem. The manual says it has twin Mikuni BS34.
>> Basically it was running too rich and the carbs would load up badly
>> while idling

> If you're not comfortable doing it maybe, but it's not that hard, if
> you take your time disassemble carefully,get some carb cleaner and an
> clean and blow compressed air through every passageway. Check out

Well I'm sort of retired now and don't have my old workshop or space to
work in comfortably.

The good news is I let the dealer do it (after cussing and coaching them
a bit) and it runs damn near like it just rolled out of the showroom
now. I'm tickled pink. It's got new tires, brakes, fork seals,
recovered seat and running perfect all for about $1350.

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Motorcycle Forum / General / Yamaha / September 2007

XS400RK