Vacuum advance with ZRX Carbs.

[TADracer]

For those of us trying a carb upgrade to a rack of ZRX 36mm Keihins, the problem of correctly operating the vacuum advance rears its ugly head for the earlier machines. Here is what I did to solve the problem. It was fairly easy but it requires a steady hand and a bit of patience. Best part is if you screw one up, plug the hole and you have three more tries. I made it on the first try.

If you look at the bottom of the carb there is a cast-in port ready made for a vacuum tap. I used a digital caliper to take several measurements and determined that a small hole strategically drilled in the bottom of this well will exit right on the rear edge the butterfly when it is seated. The hole needs to be very small and drilled at an angle towards the rear of the well.



In this picture the hole is already drilled but the wire indicates the angle the hole miust be driled to exit in the right location. I wedged the butterflies open with a scrap of soft wood and placed a light in front of the carb spigot so I could see when the point of the drill started to break through. I used a 1/16" drill in my best drill motor that provided the slowest trigger speed. I would drill in a bit with very light pressure and then check for light in the bottom of the hole. As soon as I saw a glimpse of light I stopped drilling. Then I took a 1 mm airbrush cleaning wire and worked it through the hole to open it up a bit.



Here is a view of the hole location after drilling.



Here is another view



Then I took a common 1/4" to 3/16" hose barb fitting available at any auto store and the 1/4" side is a tight fit into the well. The original vacuum hose attaches easily to the 3/16" side. The barb is easily removed to replace if it should break. A 90 degree fitting would allow a bit more room under the carbs for the hose but this one is what I had at the moment. Also, it does not interfere with reaching in for an idle mixture screw adjustment.



Here is the finished product. I installed the carbs and the vacuum advance operates exactly like it is supposed to in that there is no advance at idle, a ramp up in advance as the butterflies are cracked, such as when cruising and then no advance when the butterflies are opened wider like under hard acceleration.



Have not had a chance to road test it much other than a trip or two around the block but it seems to be working as intended.

Thats a wrap.

[petejw]

your a good man mike,
ive got spare carbys that came with mine,
id like to see a vid if its possible with it connected to
the vac advance.
my mod didnt work so im glad someone figured something out.

[cywelchjr]

Hey Pete, from what he said you got three more carbs to try it on, so you should still be able to do it just like he did. You might wait till he's gotten it on the road first to see how it works under load first though.

[petejw]

It WORKS :-)

just took the carbys off
and attempted this mod,
this works beautifully,
theres no chatter at idle and works just as
well as the original mikuni carb, ive taken it for a ride
and the vac adv works exactly as it should.

Big thanx Mike for taking the effort for attempting
this mod and posting.

[petejw]

hey cy,
i had a spare set of incomplete keihin carbs
that came with the carbs i bought from a former
member on here,
so i replaced the one that i hacked into yesterday,
couldnt help myself so took em off again today
and did the mod as mike suggested,
and it works really well.

[yamahansolo]

This will be next on my todo list!!!!

[TADracer]

I am humbled

Thanks for the kind words. A deviant mind always seems to find a way.

We have been having a few days of rain down here in south Louisiana so yesterday was the first time I got a real road test. I like it. Pulls hard at WOT but has a nice crisp but steady feel while cruising at a part throttle.

The main advantage I find with these carbs is that they are so much more accurate. Adjustments seem to take affect easily and there is not any 'hunting and pecking' when the carbs are transitioning between fuel circuits.

I wholeheartedly recommend a 90 degree screwdriver (like Motion Pro sells) to adjust the idle mixture screws with the motor running.

[petejw]

i stuff a thick rag between the barrels and my hand
and use a small cut down screw driver to adj
the mixture screws.

been riding mine a bit and have noticed a difference
when cruising to cracking open the throttle.

mike,
what jets are u running?
im just rapped that the vac adv is working.

[TADracer]

Pete,

I have 38 pilots and 130 mains. All else is untouched. The pilots are good but the mains might need a jump up a notch. But I am sticking with the 130's until I get more time to diddle with jetting.

[Shuriken]

Nice work.

[petejw]

thanx mike,
im running 40 pilots with the mixture screws just over
1 and half turns out, ive also run 38 pilots with just
under 3 turns out, for mine the 40 pilots suited best.

135 main jets, ive run 130 and 140 the 135 suited mine
nicely.

ive had my slide springs cut and i think the slide
hole has been drilled as well which hasnt helped my cause.

if u remove yours again, could u measure the length of
the slide springs and if its possible the hole in the slide?

[TADracer]

Pete,

I am planning to take the rack off so I can remove that throttle position sensor on the right side and make a small aluminum cover plate for that. I know, I can do it on the bike but its just easier to take them out and do it on the workbench. Hell, only takes about five minutes to remove the whole setup, air cleaners, throttle cable and all. I will try and measure the spring and hole this weekend for you.

Funny how different these machines are from each other. According to the manufacturer, the correct setting for the idle mixture screw is around 3/4 of a turn open. With the 35 pilot it was around 3 turns out for a decent mixture but I could not get rich enough for a yellow with a color tune. I changed to the 38's and get a perfect mixture at 1 turn open and with another half turn can get a rich yellow flame with the color tune. So I will stay with the 38's but I know I need some tweeking on the mains at some point. Probably wait until spring (in the Northern Hemisphere) for that.

[petejw]

hey yamahansolo,
if i was to do it again id use either no 3 or 4
carb, this would make it a little easier to get to
the mixture screws for adjustment.

hey mike,
i was looking at the kawazaki forum, the majority there
are using 35 or 38 pilots, so i might go back
and try the 38's again and see how they go,
for me they just seemed a little flat at off idle.
i noticed u have the 79 model, mines the 81
so i wonder if the head/cams make any difference
to the tuning.


i also just shimmed the slide needles, that made a bit
of difference, could be something u could try before upping
the main jets jat.

[TADracer]

Did a bit of tweaking

It is getting cold here around New Orleans. I know, cold is relative and the 40's we have been having here might seem balmy to some of you guys up North. But I ride year round so I use a two spark plug system to help keep my motor temp where it likes it.

I decided to switch to my hotter plugs and while checking things out I noticed that when the throttle is cracked open the timing plate bounced back and forth a bit before it started a smooth advance. I put on my timing light and in that 'shake zone' the timing was all over the place. I took a ride with the cover off and it did the same thing under load. I did not notice this before so I figured that sone adjustment should be done here.

I took an old pilot jet (one of the funky ones from a K&L carb kit that dont work) and then put two layers of shrink tube around the body of it to make it want to fit tight inside the vacuum tube from the carbs and to close off the holes in the side of the jet leaving only the small hole in the end open. Then I shoved it about an inch into the tube at the dashpot end and gave it a try.

With the jet restricting the vacuum pulses in the line the advance now works smoothly throughout the range and elimated the 'shake zone' on the timing. I dont really notice any difference in rideability when just off idle when the bouncing was happening but I figure that concerning timing, smooth is better than rough.

The more I ride with these carbs the more love them and I would recommend them for someone looking for an easy and reliable upgrade from the 30 year old OEM carbs.

[Spootinay]

Anybody have any idea where i can drill a hole for a vacuum advance in these carbs?

http://s1188.photobucket.com/albums/...3DDSC03119.jpg