Carb Synk

Hey guys. Just cleaned the carbs from the church of clean, new pilots, new throttle shaft seals, plunger seals ect... The carbs are worth more than the whole bike now. Color tuned it and sank the carbs, only one hiccup. Three of the carbs behave properly. All synked perfect at idle but when I bring it up to 2500 rpm the far left cylinder lags about one water column inch or so behind the rest. And of course when all synked together at 2500 rpm the far left is not synked at idle with the other three. I'm not exactly sure what to make of it. Maybe the color tune is off? linkage issue? linkage spring maybe? Anyone have this issue at some point? Idles smooth either way, I checked the synk gauges (for accuracy/ all zeroed properly) and theyr spot on.

 

Did you check/set your valve clearances? That will affect your vacuum readings and won't be able to sync well until that is done. The sync should be done at idle since that is when it has the most effect on the engine running. Once you start to open the throttle, the effect is minimal. Out of sync bikes will usually run well at part to full throttle but idle and cruise poorly.

 

Thanks I did check the valves last year and didn't ride much, they were all on, but I will double check them again.

 

The more I thought about this, I've honestly never checked sync off idle nor am I sure it matters. There's a lot of factors in play that determine the amount of vacuum the cylinder will pull, once you're off idle the CV slide will take care of the mixture and it probably doesn't matter if there is a difference in the amount of vacuum.

I would say that if your valve clearances are good, you sync it at idle, compression numbers are within spec, colortune, and it runs well I wouldn't worry about it.

 

As mentioned, CV carbs don't need to be syched off idle. Once the throttle plates are set to match the cylinders, the vacuum slides take care of balancing things all the way to redline.

 

Great info, thank you gentlemen. I've never broke down on the road. Have ridden to Florida and back several times. (In my younger day) Also New Hampshire several times, Chillicothe, short trip and may many more. Always tune what ever I ride to its best ability. Ridin is fun, pushing, not so much. LOL

 

Double checked the valve clearance. All are within specs except for #1 exhaust is .002 too tight. Its .004 should be 006.-008. So its opening a fraction too quickly or before it should and I would imagine staying open just a fraction longer than it should. This may be enough to rob a little power and vacuum. 1-4 intake 004, 004,005,006 1-4 exhaust .004, 007, 008,007

 

All I need now is a 6th gear !!!!!!

 

The issue with a valve being too tight is not when it opens/closes, but as the motor heats up and expands, the clearance may close up completely and the valve will not close and can cause the valve to burn.

 

AHHH I see what you mean. So in that comparison all valves should be set to maximum tolerance rather than minimal if obtainable? Intakes can be .004-.006 Even though the exhaust you would imagine to be a tad hotter than the intake, the motor as a whole is a heat sink otherwise the inside cylinders would seize and valves would burn being hotter than the exteriors, or the tolerance for their valves and piston fitting would be greater upon the build, but this is not the case. But by tolerance settings on exhaust being .006-.008 there is only a .002 differential between those and the intakes being .004-.006. In theory then heated up on the exhaust valve set at .004 would still have a .002 clearance to the cam from the shim. One would think as the motor wears ie. cam and shim their tolerances will be greater the more mileage you put on the motor. Does this theory hold for the Maxims? Or do they tend to get tighter with age and wear?

 

All the info is much appreciated guys. Just trying to get a better grasp on where I want them to be before ordering new shims.

 

The tolerances close up as the valve "seats" into the head over time. Cam wear does not affect clearance.

Yes you want to try to shoot for the higher end of the range if possible.

I'm on a date with the wife and getting yelled at for posting, must sign off now

 

You don't need a 6th gear. 5,000 RPM is right in the middle of the rev range. I tour on my SECA and the engine runs comfortably at 75MPH all day long.

Use the Table to figure your shim needs. In spec is in spec, and out of spec is out. Shims come in 0.05mm increments. http://www.xjbikes.com/forums/threads/airhead-valve-adjustment-with-pics.14827/#post-122016

Cam lobes don't wear. Shims don't wear. They never come in contact with each other (unless something goes horribly wrong). They are always seperated by a film of oil. As pointed out, the clearance changes come from the valves slamming into the seats.

 

read this posting from xj4ever's Chacal to better understand what you are actualy syncing.
TALKIN' TECH: Various thoughts on various issues

 

5k is right in the middle of the rev range, BUT at 5k you're juuuust starting to get into the powerband