Carb Synchronization problem

JKMike · Aug 22, 2020

hey,

After making a jetting change to my carbs this morning and double checking the synchronization, I noticed something really odd. While the carbs are perfectly synced with the butterfly valves closed, once I start to open up the throttle they are clearly out of synch. Here is a shot of what they look like at WOT. Anyone ever seen this before or have any recommendations? I would imagine this has to be causing some performance issues.

Jcary85 · Aug 22, 2020

That pic is terrible lol. It almost looks like the butterflies are mounted on opposite sides of the throttle shafts though? Maybe just a bad pic?

JKMike · Aug 22, 2020

Jcary85 said:
That pic is terrible lol. It almost looks like the butterflies are mounted on opposite sides of the throttle shafts though? Maybe just a bad pic?

Something deffinatelt happened to the quality of the pic when I uploaded it. Maybe because it rotated it. Idk. Try this one.

JKMike · Aug 22, 2020

I think that one came out better.

I thought the same thing about the throttle shaft at first but, they go through the middle and can only go one way. It looks like the issue was that the shafts of the two carbs weren't aligning when everything was bolted down.

JKMike · Aug 22, 2020

You can kind of see it here.

JKMike · Aug 22, 2020

It was the mounting plates that bolt to the bottom of the carbs that was causing the issue. There is enough slop in the bolt holes shown below that when all bolted up it was causing the butterfly shafts not to align perfectly between the two carbs. Some minor adjustments to the position of those plates and its way better.

I'm really curious to see how this affects performance. But none the less this is something that could easily be missed and might be worth checking for others.

wmazz · Aug 22, 2020

It is more difficult synchronizing a set of carbs that isn't held together with a rack.

If your superjet runs like other Yamaha's, the slave carb that doesn't open all the way
will make a little more power than the master that is wide open.

Did you reuse any gaskets?

Bill M.

JKMike · Aug 22, 2020

wmazz said:
It is more difficult synchronizing a set of carbs that isn't held together with a rack.

If your superjet runs like other Yamaha's, the slave carb that doesn't open all the way
will make a little more power than the master that is wide open.

Did you reuse any gaskets?

Bill M.

If I had a rack I would have definitely caught the misalignment sooner as I don't think I would have been able to get the carbs on the studs, because the adapter plates were too far off. I think if I had one of the aftermarket carb couplers that might have also helped. None the less, I was able to get them pretty well synchronized and it definitely made an improvement when I took the ski out today.

As for the gaskets, I did reuse the intake manifold to carb gasket, but its fairly new. All the other gaskets are new.