Carb sync Negative Reading?

[system9]

Some info first. Saturday I received a set of VM34 carbs. After some basic tweaking bike was idling ok. It seemed like it idled best with the mix screw all the way in, so I'm guessing I need a little bigger pilot jet. But, it still started, and idled pretty well.
I took it out for a test drive. I was worried as to what may happen when I gave it some gas. I was thrilled when it took off like a bat out of hell. It ran fast, really fast. I was hitting 80 mph fairly quickly. So , happily I parked it, went online and ordered a few jets.
Yesterday I went to start it, it started up, but I realized the right cylinder was not firing. Turns out one of the cable ends that goes into the 2 way splitter was hung up a bit. Its hard to explain if you don't use one of these. Anyway, I got it started again, the right cylinder coughed and sputtered back to life. So now I hook up my vacuum gauges to sync the carbs more precisely,
and something weird happens.
When I give the bike throttle, the left side makes the needle go up, but the right side makes it go down! I have never seen this before. But, at idle the needles are in the same spot, which is good. So I take it for a ride today, and it is behaving completely differently than the maiden voyage. It is pretty smooth all around, but not nearly as aggressive. It doesn't scream at all now, but still cruises ok.
What could make a vacuum reading go the opposite direction? And is that reading related to why it no longer blasts off like a rocket.
Will keep this post updated as I get the time to mess around. I will have the carbs off when I get the new pilots, and I will also borrow my friends compression tester on thursday. Any ideas ahead of time would be great. Thanks.

 

[TwoManyXS1Bs]

Sounds like the throttle cable for the right side is pulling before the left side moves.
Double check your cable end fitments into the 1-2 splitter, and the slides...

 

[system9]

I have balanced the cables. Both slides are lifting at the same time. The needles are moving at the same time, just in opposite directions. If I swap the vacuum sync hoses, the problem moves, so I know its not the gauges. Right side will make the needle go backwards even when the left side is not running (spark plug out)

 

[TwoManyXS1Bs]

The gauges are reading manifold vacuum. As a slide is raised, it lets in more air, and the manifold vacuum drops. If the engine rpms go up, and a slide doesn't raise, manifold vacuum for that side will go up. Keep playin' with it, it'll eventually make sense...

 

[retiredgentleman]

Why not just use a simple manometer. Very inexpensive and very accurate. Vacuum gauges are, not surprisingly, meant to measure vacuum; not to balance carbs.

 

[5twins]

With individual throttle cables, one running to each carb, you need to do two syncs, one at idle and one at speed. Do the idle sync first with the idle speed adjustment screws. These work against a ramp cut in the side of the slide. They raise or lower the slide a little and that varies the idle speed. That also changes the amount of freeplay in the cable. Syncing at speed is achieved by matching this freeplay between the cables. That's why you must sync the idle first.

After idle sync, set cable freeplay on each carb about where you want it (usually about 1/8" to 3/16"). Then move on to the "sync at speed". To sync at speed, crack the throttle open a little and run the motor at a steady moderate RPM, say something in the 2500 to 3500 range. Pick one throttle cable and fool with it's freeplay adjustment until the gauge readings match. No need to mess with both cables.

I wouldn't be real concerned with that dip you're seeing just off idle. It is probably just an out of sync indicator. The readings should be more stable up around 3K.

 

[system9]

I found out that the right cylinder rings became washed out after the 1into 2 cable splitter malfunctioned. I added some oil into the spark plug hole, burned it off, and seems to be ok now. But the info regarding carb syncing is super helpful.
I have used the vacuum gauges to great effect on CV style carbs over the years, but this is my first use of them on the VM34 style carbs. I have worked on VM style carbs in the past, but only single carb applications for 2 stroke bikes. So i'm learning something new again.

retiredgentleman- I will definitely try a manometer in the future.

5twins- Thanks for the super detailed sync process. I still feel like the 2 way splitter is a little clumsy, and it seems to be a little different with each visit to the bike. I will look into ways I can keep the cable settings from mysteriously changing. The problem started because the cable splitter failed, and I flooded the right cylinder badly.

 

[5twins]

Dealing with dual cables can be a P.I.T.A. but it's not too bad once you understand how things operate. An important thing to remember is that if you ever need to adjust your idle speed, adjust each carb equally, don't just do one. Remember, adjusting the idle speed changes the cable freeplay. Doing just one will throw the sync off, both at idle and at speed. Adjust each one equally and hopefully the cables will stay matched.