Carburetor advice sought please and am I the only Brit?

SillyBrit

SillyBrit

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Hello everyone!
I am new to the forum and own a 1981 XS400 SH imported into the UK from the USA. I have only a very elementary level of mechanical knowledge so am very pleased this forum exists.
My bike was bogging above 7,000 revs so I took it to my local motorcycle garage. After cleaning, rebuilding and balancing the carbs, the problem persisted. After a lot of investigations, the mechanic has diagnosed what he believes the fault to be, which is air being drawn into the carbs through the butterfly bushes housed within the carb body. Please see photographs.
Has anyone encountered the problem and/or can anyone suggest a fix please? I could buy a set of second-hand carbs but may end up with the same or other problems, so thought I'd give the forum a go. All advice welcomed.
Thank you
 

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Shaft seals. I just replaced mine from an ebay kit. They are a little fussy to do and I had one of the butterfly screw shear off. Use a Dremel to grind off the knerled threads first before removing the screws.
 
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Fantastic - good solution and thank you for the advice. Could I possibly trouble you for the link to the item you purchased or the ebay item number so I know I am buying a seal that works? Thanks again!
 
 
Greetings, silly English kanigit.

:D


You can still buy the seals new from Yamaha. I always get OEM ones. Part number 256-14997-00-00.

Yamaha does not list them for this bike, but this it the correct number.

Two things you need to know:

The screws that hold the butterflies are peened, and may not come out without filing the tips.

The screws are not Philips, even though they look like it. It's JIS, a Japanese driver. Buy a JIS screwdriver, a Philips can strip them out in a second, and you will regret it. The metal is the consistency of warm butter. You can get JIS as screwdrivers, or as bits that fit in regular bit holders.

https://chapmanmfg.com/blogs/news/phillips-bits-vs-jis-bits-whats-the-difference
 
Did you check this with break cleaner ?
From my experiance bogging over 7000 RPM means not enouph fuel, you can see that also that right spark looks much more in the directoin to white
check the hole fuel line beginning on the tank where the fuel has been taken in (air must replace the fuel which should be burned)
 
Did you check this with break cleaner ?
From my experiance bogging over 7000 RPM means not enouph fuel, you can see that also that right spark looks much more in the directoin to white
check the hole fuel line beginning on the tank where the fuel has been taken in (air must replace the fuel which should be burned)
 
possibly a larger main jet.kill the engine at 7000rpm .check plug for colour. white too lean ,black too rich.go up or down on the main jet .
 
JPaganel said:
Greetings, silly English kanigit.

:D


You can still buy the seals new from Yamaha. I always get OEM ones. Part number 256-14997-00-00.

Yamaha does not list them for this bike, but this it the correct number.

Two things you need to know:

The screws that hold the butterflies are peened, and may not come out without filing the tips.

The screws are not Philips, even though they look like it. It's JIS, a Japanese driver. Buy a JIS screwdriver, a Philips can strip them out in a second, and you will regret it. The metal is the consistency of warm butter. You can get JIS as screwdrivers, or as bits that fit in regular bit holders.

https://chapmanmfg.com/blogs/news/phillips-bits-vs-jis-bits-whats-the-difference
Click to expand...
Belatedly, many thanks for your advice. Unfortunately I was unable to obtain the kit suggested by tstidham as the supplier does not ship to the UK from the US. I have now found a supplier within Europe that should be able to ship to the UK - fingers crossed. Top tip re. the JIS screws - thanks.
 
RadlagerMike said:
Did you check this with break cleaner ?
From my experiance bogging over 7000 RPM means not enouph fuel, you can see that also that right spark looks much more in the directoin to white
check the hole fuel line beginning on the tank where the fuel has been taken in (air must replace the fuel which should be burned)
Click to expand...
Belatedly thanks for the advice. The mechanic doing the work tells he he has tried everything re. fuelling and is absolutely convinced its air getting past the seals. He did demonstrate this to me using fluid. I will still show him your post though to be sure. Thanks.
 
SillyBrit said:
Belatedly thanks for the advice. The mechanic doing the work tells he he has tried everything re. fuelling and is absolutely convinced its air getting past the seals. He did demonstrate this to me using fluid. I will still show him your post though to be sure. Thanks.
Click to expand...
did you get it fixed 🤔
 
SillyBrit said:
Top tip re. the JIS screws - thanks.
Click to expand...
In general, if you own an old Japanese bike, JIS screwdrivers are a godsend. You probably already have some buggered screws from someone who didn't know about it.

SillyBrit said:
Belatedly thanks for the advice. The mechanic doing the work tells he he has tried everything re. fuelling and is absolutely convinced its air getting past the seals. He did demonstrate this to me using fluid. I will still show him your post though to be sure. Thanks.
Click to expand...
The seals quite likely have never been replaced, making them a 40 year old piece of rubber. These bikes are getting to an age where I would be surprised if they weren't bad.
 

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