Hi there,
I believe I've diagnosed the following:
The diaphragm on the second carb has warped slightly and doesn't sit correct in the seating. This results in a poor seal, causing the bike to splutter and be unable to rev above 3000rpm.

I want to replace the part: 1JK-14940-01, but can only find the OEM for over £180
Anyone know of a alterative diaphragm which fits the 3aj YDIS carb?
Thanks,
Joe

Jsseppe wrote: Thu Jan 02, 2025 5:42 pm I want to replace the part: 1JK-14940-01, but can only find the OEM for over £180
Anyone know of a alterative diaphragm which fits the 3aj YDIS carb?
Thanks,
Joe

I found the part for sale in the US for half the price you quoted

https://www.partzilla.com/product/yamah ... 4940-01-00

Also Kedo sells an aftermarket version. As an aftermarket retailer Kedo is a good brand.

https://www.kedo.com/throttle-valve-wit ... 10141.html

But in my opinion OEM Yamaha carburetor parts are superior to aftermarket parts. I would grip my wallet and grimace and buy the Yamaha rubber diaphragm and be confident that it is the right fit

Do you know how the diaphragm became warped? Contact with adverse chemicals? Dropped or run over? Are you sure the carb body is straight and that the needle is running straight through the main jet? Done a carb sync yet? Tell us more!

Thanks for the part recommendations and advice!

Heres some more info!
The Problem: The bike is a bit tricky to start, but when running cant rev above 3000x rpm. Its a noisy affair, spluttering, bangs, choppy rev.
I suspect I've inadvertently caused this issue when I took the carb apart for cleaning...
The diaphragm is fairly old but I given its shrunk and warped in one specific place, it could have been caused by contact with carb cleaner.... Whoops!

However after cleaning out the main jet and resetting the diaphragm I managed to have it running above the 3000x but it reverted back to the 3000x limit as soon as i put the bike back together.....

Does the diagnosis of the diaphragm seem accurate to your expert eyes? Or am i missing something obvious?

Carb Syncing
I'm fairly amateur (Be gentle if I state something stupid) so I've never done a sync or have a syncing tool! Would this be necessary on a YDIS carb on an Xt600z? Would happily give it a go, but will do some reading first.
The YDIS carb seems to be a tricky system and whilst I have exploded diagrams and a service manual for the 3AJ, there little to be found about the YDIS carb...

Anyway thanks for your thoughts and fingers crosses replacing the Diaphragm is the key.

Hi there,
usually swelling of the rubber is caused by alcohol, and leaving it out in the (warm) fresh air reverts the process. If it shrunk, you can get it to original size again by dipping it into methylated spirit.

The carb is not to be synchronized, so there is no tool for that. Syncing carbs is done with on multicylinder engines with identical carbs supplying their individual cylinders.

The XT carb is a 2-stage- carburettor, where the primary carb (left) is actuated via throttle grip and slider, while the secondary (right) carb opens around 3500-4000 rpm (under load, so NOT when revving the enging) to assist the primary carb. The secondary carb is actuated via vacuum, which is initiated by opening a butterfly valve, which then causes the pressure difference to suck on the diaphragm and lift the slider. Also check the condition of the long vacuum line connecting both carbs. Slide should run smoothly inside its bore when pushed.

The starting point of that opening of the throttle (butterfly) valve can be modified with a set screw, and the default for that is when the primary carb slider opening approaches a set value (different per model). For 1VJ and 3AJ this is 7 mm, for 34L or 55W it is 5 mm. As with the idle mixture setting, this is a safe starting point, but the correct setting for the secific engine should be tuned to fit engine condition, altitude etc., so I would recommend to change the setting half-turn wise and do test rides with various settings.

The starting value is easiest set by inserting a e.g. 7 mm steel rod (aka spiral drill bit) under the slider, then turning the set screw so the butterfly juuust begins to move.

You can leave the carb in the bike when adjusting, only the air filter rubber boots have to come off.

Check the exploded view (open in new window): https://www.xt600.de/xt_werkstatt/_spec ... losion.htm

(34) vacuum hose
(45) set screw

Andy

Hi Andy,
Thanks so much this was an excellent guide to help!
I adjusted the carbs as you suggested to the 7mm measurement! The bikes response has improved significantly, acceleration feels more powerful and seems to run less hot! Really pleased with this so appreciate your guidance.

However the bike still stumbles, splutters at 3000 - 4000 rpm and cant seem to rev above this, on the road or in neutral........
I'm completely out of ideas for what this could be caused by..... If there's any ideas I'd welcome them wholeheartedly!!!!!!

Frustratingly it feels like I'm so close to having it in perfect running order
Thanks again for everyone's input and help so far!

As stated, 7 mm is a starting point, adjust and test drive from there. Also adjust and test drive the idle mixture setting.

Andy