Valve adjustment and carb sync together?

[mx916]

I recently posted about doing a valve aduustment and got good feedback and help as always. Question is; is it necessary to do the carb sync at the same time? I have yet to do either yet and dont want to open 2 cans of worms at the same time if I dont have to. Bike is a 2000 750 with 13,000 miles. I bought it with 3000 miles and know it has not had an adjustment since then. It runs great, and does'nt give me any trouble, but I want to do the valve adjustment to keep it that way. So if you have done valves without the carb sync and had no troubles or vice versa, please let me know. Thanks>

[UFO8MyCow]

Do them both the valves will deffinatly afect the carbs. and if yiou have acces to a sync tool why not do iot any way? it really is simple to do.

[Black_peter]

I only had to adjust my valves a little..
I only tweeked 3 tappets. So I can't say how far off it puts them. After the adjustment the carbs were pretty
darn close. However enveryone I trust says do both.

[tzortn]

The hard part is actually getting to the carbs and the valves. So while you are in there you may as well do both.

[The CyberPoet]

Do both, valve adjustment first. The carb sync only takes about 5 to 10 minutes once you already have the tank out of the way to work on the valves. If it's a money issue ($$ for the sync tool), then do the valve adjustment and come back and do the sync when you can -- OR borrow a sync tool from someone else.

Cheers,
=-= The CyberPoet

[tzortn]

...If it's a money issue ($$ for the sync tool), then do the valve adjustment and come back and do the sync when you can -- OR borrow a sync tool from someone else.

Or consider making your own tool. I have not done it myself, but a number of people on the board have. I am sure that you can get some direction on how to build your own sync tool.

[md86]

Yeah , but you check the valves cold , and the synch hot . So you gotta warm it up before synching it anyway .

[Special K]

Basic rule: If you adjust the valves, you synch the carbs. 8)

[KatanaMan]

on this subject where is a good place to look for the sync tool and valve stuff? i am going to have my roommate do the work for me when it starts getting cold out (almots there). How bad is that sync tool going to set me back.. (90 kat 600 if that helps)

[Black_peter]

Gee lookie here..
about $4...

http://www.airheads.org/content/view/183/49/

[DreKat]

do them both, and if you are taking the bike to a shop remove as many fairings as possible. you will save your self a lot on money in labor costs.

[harrye]

Do them both the valves will deffinatly afect the carbs. and if yiou have acces to a sync tool why not do iot any way? it really is simple to do.

Why does this happen? The carbs are controlled by the throttle cable. Why would the mere act of adjusting valve lash have an impact on the carburators?

I can see that since you have to remove the carbs to do the valve job, they might have to be synced to get the mechanical linkage identical for all 4 carbs. But I am getting the impression that this has nothing to do with the throttle cable. It has to do with vacuum or air flow, and maybe fuel flow too?

So on bikes where you can adjust valve lash without removing the carburators, you still have to sync the carbs after the valve adjustment?

[md86]

Well , when you adjust the valves , the vacumme that the enigine can create is changed , changing the amount of fuel/air that the cylinder can draw . So you synch the carbs to compensate for the vacumme change in the engine . The synch screws change the actual amount each butterfly is open , changing how much air/fuel the cylinder gets .

[The CyberPoet]

Well put, MD!

As the output from each cylinder changes with the vacuum draw for that cylinder changing, the engine output can now be rather uneven (let's say cylinder 1 was very loose before, and now it's nice and in-spec; as a result, it will produce more vacuum, but also higher combustion power).

Syncing the valves effectively balances the output from each of the cylinders by balancing the vacuum draw into them to compensate for whatever the current condition is (so they get even fuel-air mixtures). This virtually always makes the engine run smoother (in all cases where there isn't already significant internal damage), creates less wear on various bearings, and reduces engine vibration (possibly noticably, usually not).

Cheers,
=-= The CyberPoet

[harrye]

Thanks again MD and Cyber. You've cleared up a mystery for me. When I get the valves adjusted my 1981 Yamaha XS400, I'll have to specify a carb sync too. At lower speeds that bike is still a lot of fun for me. It's a twin with instant torque when you crack the throttle.