CV carbs with plugs on vacuum ports?

hey all,

I haven't posted in a few years bc of work and all, so I'm not surprised if I'm practically a new face in here.

I have a question regarding my 1983 KZ750 L3. I'm pretty sure it has 750 mikuni bs 34, the stock carbs, which are constant velocity. Stock air filter, pretty much stock pipes, and I have taken apart the carbs and given them a cleaning. It wasn't very thorough, but that was because I thought they looked pretty clean.

I haven't tuned them at all, but this is where I stand now:

The bike will start very sluggishly (always have to use starter fluid unless bike has been running immediately before, then it starts right up). It will run quite well, maybe be a bit sluggish out of idle when I give it a bit of throttle (a second or two, but it does eventually respond). Sometimes, it will the rpms will run high, but when I ease the clutch out (essentially putting some load on the engine), the rpms drop down to normal idle levels (1k or so).

I was trying to find out the reason for this, and found that several of the vacuum ports on the carbs (I think two of the four carbs) were capped off. I'm not sure what these do, and was wondering if they could possibly give me some of the problems I'm describing.

Thanks for sticking with me this far! My bike is so close to being done but I'm so clueless on carbs and would love some help!!

Dave

On my 750 (it's in the garage and I'm at work) from what I remember there are ports on the engine side of each carb, and these should be capped off. (They are used to synch the carbs.)

Between carb 1-2 and 3-4 are T's that should have a hose attached for bowl venting; and should run to the rear of the motor or up by the airbox.

Hope that helps.

i can't be sure, because i've never worked on that specific carb, but I believe that there should be a crossover line (just a vacuum line from one to the other) on those two that are plugged off. Are there lines on the other two carbs? Where are they going? peek around and make sure there aren't any empty vacuum taps. Also a good way to go is to find a diagram of the complete carb assembly, it could give you some clues as to what goes where. try bikebandit.com or various other places for these.

This is how the vacuum lines on my carb are set The long center one goes to the vacuum side of the petcock..

Post edited by: CoreyClough, at: 2007/09/24 01:04

I just went out and looked at the L-3 I have, on #1 and #4 there are the hoses that operate the emission control system, #2 the hose runs to the petcock and #3 is capped off with a rubber plug. Hope this helps.

Gringo's explanation is correct. Please see the BS Carb Connections link on our website under wg's Tech Stuff Index for carburetor connections pictures. Not sure we are talking about the same thing when we talk about vacuum ports; these are located on the front of the carbs next to the mixture screws.

thanks for the help everybody.

I looked and the Gringo and WG are right - the three vacuum ports (on the engine side of the carbs) are connected to the petcock/emission controls and such, so that shouldn't be a problem.

I looked at the diaphragms last and they didn't seem to have any problems, slides were all clean. At this point, I'm wondering whether it would be worthwhile to rebuild the carbs with a rebuild kit from Jeff. Does my problem sound like it's the carbs (where it revs high but putting load on the engine brings the revs back down to normal)? Is it generally worthwhile to rebuild them or is that seldom necessary? I juts don't want to drop $80 needlessly. Thanks!

Dave