SYNC PROBLEM

[chiz]

Howdy folks
  I have 3 Mikunis in a row that fit a Yamaha of mine. I understand a carb sync on these old carbs will only last a couple of hours before they go out of tune. Pretty much proved this by doing a sync  and having the engine start to pop back the following day. 
   Just wondering what is the cause I understand even Locktite on the adjusters is not a solution so I wonder if the carbs are just worn out?

  Thanks Chiz

[Pat Conlon]

Chiz, I have also heard of this, I wonder if the threads on the adjuster screws gets so sloppy (along with the female threads on the brackets) so with vibration, they rotate out.
So can you run a tap thru the buggered threads and go up one size on the threads on the adjuster screw?

The springs holding tension on the adjuster screws are very important. Several years ago we had a forum member report all kinds of problems when he reassembled his carbs without those springs.
We know springs lose their compressive tension over time....maybe a fresh set of springs?

Those Yamaha 850 triples are a very smooth engine. Much less vibes than our FJ's.

I would love to hear what others think.

[RPM - Robert]

A problem that we have encountered here, is the throttle seals being shot. They get to where they seal until you move the throttle plates and then when you put them back they aren't sealing. Virtually impossible to sync them when you encounter this.

However popping back through the carbs usually does not indicate a synchronization problem so you may have two different issues here.

[chiz]

Thanks for the help good suggestions... I try to avoid replacing seals simply because I don't know how to remove those bleeping butterfly screws. Does each carb need 2 seals or is it just one?

  Chiz

[RPM - Robert]

The FJ carbs have 2 per body. So 8 total on the bike. The real problem is getting the plates re-aligned after you take them out.

[Pat Conlon]

Robert, are not the back side of those throttle plate screws peened over the shaft?

[RPM - Robert]

Robert, are not the back side of those throttle plate screws peened over the shaft?

Staked, not peened over. They thread out hard and the new screws thread either easy or hard. That is why loc-tite is required.

[Pat Conlon]

Gotcha, thanks.

[JMR]

 Just an FYI.....an engine with out of spec valve lash will not stay synched as well.