zrx carb update 2.0 41mpg!

[petejw]

jat,

if your running pods,
these carbs with the vented top 't's do run better
with an airbox, former member pggg has the same carbs
with the air box and has his running very well.

i was running pods when i first started messing around with them
and found it hard to get it running well down low.

[yamahansolo]

Mine runs awesome through the whole band. The only problem I had was when I had the vac advance hooked up without a restrictor in there.
I have found that with the choke on and the petcocks set at prime it starts right up no problem.

I think pgg cut his springs and moved needles. Plus I don't know what pods he tried. I had the emgos on my xs 750 and they sucked.
I use the mikes xsones and they are great

[petejw]

that mod i did didnt work,
but im still messing around with it,

the bike runs much better with the vac adv disconnected,
with it connected to the carb nipple and restricted
im getting full adv at idle resulting in the engine
pinging under throttle and it runs a little sluggish.

with your setup is the vac adv fully adv at idle?

atm im running 130 mains
and 40 pilots,
ive just stuck a shim under the needles to see how it
runs, havent tried it yet, ill let u know how it goes.

atm it still runs better than the mikunis.

[petejw]

i changed the bolt to a inlet rubber barb
and pressed it in and then smothered with
silastic working much better now

[TADracer]

Pete,

Did you get the chance to put an unrestricted vacuum gauge on the nipple and see what the vacuum curve is as the throttle plate opens? I think at some point you should be getting a vacuum of around 6 inches in order to fully advance the mechanism.

I would be interested to see that as I have a set of ZRX carbs sitting on the bench since last Spring.

[petejw]

hi mike,

i did the first time when i put the bolt in
and didnt get a reading, when i put the barb in
i was getting a reading, but i hadnt sealed it properly,
it was getting dark so i put the silicon around it tonite and ill let it dry
and try and do that tomorow, i did use the vac guage on the mikuni carb
before i pulled them off tho i did it when the bike was stationary
and with no real load on it, even with the mikuni carb there wasnt
much vac without any load.

it was pretty tricky as there was no real room and the keihin butterflys
open the opposite way the mikuni butterflys do, hence the barb sitting at the bottom.

even without the vac adv fitted these carbys are much better than
the mikunis.
i ran out of daylight so ill be messing around with them again tomorow.

the bike runs better with the needle shimmed, i jumped the mains up2 140
but havent tried it yet to compare.

i think these carbs would work even better with the xj, as theres no vac
or mech adv to mess around with.

[cywelchjr]

IIRC the port/hole for the vacuum advance fitting should be BEHIND the butterfly on the non-vacuum side away from the engine, but should be positioned to transition into the vacuum zone shortly after the butterfly starts to move. This applies vacuum above idle, but provides none at idle if everything is adjusted correctly. I remember in the day having some cars that used ported vacuum for advance and some that used manifold vacuum, and I seem to recall that those that used ported tended to have a smoother idle. So, I'm thinking you have the hole too far forward, and I'm not sure what you can do to fix that at this point.

[petejw]

hi cy,
if you look at the pic,
youll see that i have the hole on/behind the butterfly.
on the mikuni carbs there is very slight vacuum but not
efficient enuff to actuate the adv. the hole in the mikuni carb
is very small and sits on the butterfly, but the butterfly is slightly
open at idle.



these butterflys open the opposite way than the mikuni carbs.

[TADracer]

About a year ago I disconnected the advance and put a vacuum gauge on the carb nipple and then took a short ride. It gives a vacuum curve that is zero at idle then ramps up to around 6-7 inches when the throttle is slightly opened and held there (like when cruising) and then back down to zero as you get on it.

Something is telling me that placing a port in the correct position on a carb that is not tapped already is going to be something that must be dead on accurate to work correctly. I am thinking that the tap hole must be very small, placed in an exact position in relation to the butterfly and then possibly tuned with the slot on the butterfly to give the correct vacuum curve.

There has got to be another way around this like perhaps dealing with spring rates on the centrifugal advance or something? After all, its just an economy and slight drivability issue when riding at part throttle. So, is the vacuum advance really needed?

[yamahansolo]

I didn't notice a huge difference with mine either way. Also I don't like the xj idea because they arnt as advanced as the e or f models. I have monkey spanked newer xs/xj's with ease before the car mod.
I have my vac advance hose on the number 1 with about a .5mm restriction. Also if you look at the way its drilled its angled on the inside. Maybe why it works ok on mine.
I'll have to pull the side cover again and check if the advancer is moving but I don't. Here it sitting next to it. Before it was slamming open and closed.

[petejw]

ive got the 81 model,
no mechanical adv on mine,
when i was chatting to pggg a while ago
he had the 78 model as well,
i think the centrifical adv has the advantage
of advancing under crank rpm.

tho the timing plate is fixed on mine, i might mess around
with adv the ignition a few degrees to see if that makes any difference.

your right mike with regards to drilling the hole,
u need to b spot on with it, no real big deal i just
block it off, for me it was worth a try.

the other thing i might suss out later on is changing the
ignition system.

yamahansolo, when u get a chance to remove the cover
can u check out if its fully adv at idle?

[yamahansolo]

No problem... everyone is sleeping right now... my bike is really loud at idle so I'll have to wait. I really wish someone with an 80 or 81 and a big bore kit would swap out the ignition for the e or f. I would be interested to see if with more advance they would be faster. According to tests done back along time ago the other ones were slower by about half a second. Or so.

[petejw]

ive elongated the holes in a timing plate
that i havent fitted as yet.
i was going to try and adv it around 5 degrees more at idle
to see how that would go,
i could do that with the vac adv and tie it off,
something else to try i guess.

i want to wait till i get the 135 mains before i mess
around with it 2 much more atm, and ill probably put the air
box back on as well.

i was also sussing out the xj module but after
chatting to todd (TRBig), it still uses the 2nd carbs nipple
so that idea is shot as well.

[yamahansolo]

Ok shot a qwik vid but 3g is slow as poop this morning... I'll have to wait till it get to pb before I can post it.

Now with the chocke fully on the vac advance pulses just a little not to full. Once the bike warmed up and the choke was off there is no movement at all till I whack the throttle. As you will see in the vid.
Another option you could try is to drill out the pot andplay with the screw inside. There is a post on the site somewhere showing you how. If you look at you vac advancer there is a spot that looks filled in. Carefully drill it and there is a screw inside.
Hope to have the video up soon.

[yamahansolo]