Low vacuum readings in carb sync

Ok, working out the bugs.. I bought a 4 cylinder vacuum gauge for syncing carbs.. On #1 I got 5 inches, #2 12 inches, #3 10 inches and #4 12 inches at 950 rmps.. The gauges say's too much valve advance.. Did my chain jump? The bike seem to be running ok but maybe a bit slow from what I remember..
I have not done a compression check, I need a gauge..

Post edited by: vach, at: 2007/06/10 22:36

vach wrote:

Ok, working out the bugs.. I bought a 4 cylinder vacuum gauge for syncing carbs.. On #1 I got 5 inches, #2 12 inches, #3 10 inches and #4 12 inches at 950 rmps.. The gauges say's too much valve advance.. Did my chain jump? The bike seem to be running ok but maybe a bit slow from what I remember..
I have not done a compression check, I need a gauge..<br><br>Post edited by: vach, at: 2007/06/10 22:36

its been a while since ive synced a set of carbs but from my personal experiance in refrigiration work 28" is a perfect vacuam and your numbers are much lower. would check your carb boots and clamps for vacuam leaks,then i would run a compression test and check your valve clearances to make sure something isnt hanging open.if its not smoking/burning oil id lean towards valve adjustment before screwing with carbs too much;)

Ya Kevin...make SURE Your valves are set properly,and then try synching again...very important.

The carb boots are in excellent shape..They were brand new 10 years ago..:blink: I did do a starter fluid vacuum leak check and it seems to be in good shape. I need to take the tops off the carbs and manually sync em, I did do a bench sync but I need to fine tune it. I do think though that the cams are out of time.. I had em off about 9 years ago and I think I might have installed them incorrectly.. Off a tooth.. The bike has been sitting in my garage for 8 years. I'm bidding on a compression tester right now as i speak.. (Type). The bike is not smoking a bit though. When I goose it it takes a bit to come down to idle.. I know this could be vacuum, but all 4 are plugged with new caps except the #2 which has new line going to my new petcock. Could advaned cams cause the slow return to idle?

if the cams are out of wack it could hang valves open at wrong time,worse case being bent valves :S which would cause low compression and low vacaum.pop your valve cover off and bring #1 up to tdc and see where your cams are at,if they are right check your valve clearances.

Right on.. Thanks my fiend.. :blink: I mean friend.. Why are those words spelled so closely? Time for this stickleheadedwhiphenpoof to hit the Hznay!! Night my peeps..: Alrighty then.. :side:

that should be a quick look at the lines on the gears to see if they are in time, have you checke the cables to see if they are in a bind and not letting come back to the stop

vach wrote:

Ok, working out the bugs.. I bought a 4 cylinder vacuum gauge for syncing carbs.. On #1 I got 5 inches, #2 12 inches, #3 10 inches and #4 12 inches at 950 rmps.. The gauges say's too much valve advance.. Did my chain jump? The bike seem to be running ok but maybe a bit slow from what I remember..
I have not done a compression check, I need a gauge..<br><br>Post edited by: vach, at: 2007/06/10 22:36

Lower vacume in the one can be the the throttle blade is open more in that cylinder causing that one cylinder to force the engine to run at higher rpm till the other cylinders pull it down to slower speed.
If your cam time was off or bent valves I dought it would run without missing or even idle at that.
Im not sure what the vacume should be on a bike, but on cars its 28" vacume. It might be lower on a bike.
I have round slides on my bike, the way I synced my carb is with a drill bit. It runs and idles perfect now.
I have synced carb on blade type carbs with no problems by looking through the carb with a light on the other side a see how much light shines through at closed throttle.

If the valve timing is ok and compression is about equal.
The readings you have show that #2 and #4 are open the same, #3 it open just a tad further and #1 is open even more. Adjust #3 down a little so the reading matches 2 and 4, readjust the idle, the readings will all change some. Then adjust #1 down until it matches the other 3.
Ride safe.

Post edited by: The Milkman, at: 2007/06/11 08:33

Thanks guy's, I can't get to the bike for a few days. It's in Indiana and I'm back in Chicago.. I'll pull the head and see what's up. Get my compression gauge and then resync the carbs and hopefully that will get it right.. I have a 77 motor in the back of my truck I'm gathering parts for the 810 kit.. Already have the 80 Kz750 head and am working on cams. By next year I hope to have this thing sreamin..

I set all my carbs at 10 on vacuum gauges
even if I have to idle it up some.Some after
market cams will lower your vacuum because of
the overlap.If your compression is good and
the exhaust note sounds normal ( a cam off a
whole tooth usually makes the bike sound totally
different),I would just set all the carbs the same
and see how it runs.

tjetim.. The manual say's 19-24 cm Hg. Have you done a compression check? I'll do the compression check.. Then check my cam timing and valve clearance, I did have the cams out a while back. Then I will do the carb sync and get em as high as I can.. Anyone else have vacuum this low.. Any other Kz650's have 19-24 cm Hg vacuum?

Post edited by: vach, at: 2007/06/11 12:28