setting idle w/vacuum gauge
 

Did the following after close to 100 miles break in on BB 351:

After doing my BB 351 installation I wasn't sure where to set the carb idle mixture screw so I kept it at the my old setting of 2-2.5 turns out. With a new #38 pilot jet that setting turned out to be way too rich.

On a warm engine, to find the "sweet spot" where the idle is best, I placed a vacuum gauge at the front carb fitting where the smog stuff on my bike used to hook up (removed all that a while back and put a blocking plate on the cylinder). Along with the gauge, I also put a canister in line with it to help reduce the needle fluctuation at idle (without the canister the needle is all over the place, this is a single cylinder bike, after all). Even with the canister in place there was still about a 3 psi swing in the needle but I could live with that.

Ran the bike at idle, and adjusted my Kouba screw in (lean) until the top reading of the gauge needle began to drop. Then I went the opposite direction (rich) until the needle reading peaked and began to drop. After that I set the Kouba screw back to the place where the gauge showed highest reading, about 1.5 turns out for my situation. Then I adjusted the idle screw on the right side of the carb for desired rpm as best as I could (damn those wide bars on the tach).

This all can be done by hearing without the vacuum guage but my ears are not calibrated that well anymore and there was some local noise at the time, too.

Hope this may be helpful to folks in KLX land...

Great idea, i've always tuned by ear with the mixture screw(s) but this would be a great way to get a visual representation of what is going on while adjusting the mix.

Thanks for sharing.

good stuff. I like it. My vapor computer gives a very good digital readout of rpm. Could I use the computer to find the best rpm, or would we expect this to be different from the highest vacuum point?
thanks

Rpm on the tach would get the job done too, as long as it has narrow enough increments. My tach is the stock one where the bars are 250 rpm wide. Useless for any precision in setting idle. That's why I dug out the vacuum gauge.

Is this something that you can show us in the diy section some time? If so, that would be a great addition. Thanks

I'll see if I can't work up some photos and a few details over the weekend.

In the meantime, here are a couple of links for great info on CV carbs in general:

Biker eNews: How Carburetors Work

Care & Feeding Of The Keihin Carb

Neither one is for the CVK34 carbs on KLX bikes, but give great general info, especially the one for the CVK40.

Interesting, to say the least. Do we find such vacuum gage at auto part stores?

Yes. Even WalMart has them.

My vacuum gauge is part of a pump kit I got at O'reillys a while back.

You can also just use one of your carb synch gauges for this and the throttling control on the hose makes the fluctuation issues almost non existent. Walmart carries single vacuum gauges as well. I have used equus gauges with good results with automotive and with rides. But as mentioned, you need to throttle the pressures to make it easier to read on the gauge. The liquid filled synch gauges are supposed to not have this issue, but I still prefer my analog dial gauges. Here is a set that I use now for all of my rides.

http://i41.photobucket.com/albums/e2...1243542_tp.jpg