Vacuum advance hole on ZRX Carbs

[motobrix]

I'm new to the site, but I've been following along and reading a lot to help with my own cafe racer/brat build.

I decided to attempt the ZRX1200 carb mod in order to run pod filters more easily, and in general just because I was enticed by the idea that I could run modern carbs on my bike

For the vacuum advance, I drilled the hole as described, using a long 1/16" drill bit, and it seems to be in the correct location, except I went too far and drilled all the way through. I had the throttle plates open, so I didn't damage anything, but I was wondering if the larger hole is going to affect anything. Basically it's still slightly uncovered when the throttle plates are closed on the engine side.



With the throttle plate open:


With it closed:


What do you guys think?

I have yet to try it out, but I'm still putting the bike together, so I thought I'd ask first before I start diagnosing a million problems at once. If anything I'll seal the hole and try again; I still have 3 good ones!

[Rasputin]

As I have very little to no experience with this I will let others answer you, however I can not see why a restrictor placed in the vac line close to that carb would not work.

[3Phase]

Motobrix, it'd be similar to putting the vacuum line on one of the manifold vacuum fittings so I'd fill it and do it over.

The vacuum advance takes very little vacuum, about 6in-Hg and it's all-in. Even with the small overlap on the hole you'd definitely have full vacuum advance on closed-throttle deceleration. You'd probably end up with full advance at idle too and that would drop back then advance again as you came off-idle.

Long-term, since the advance is only connected to one cylinder it'd pulse like crazy at idle and eventually beat the the advance assembly half to death.

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[motobrix]

Thanks 3Phase. What would you use to seal up the hole? RTV silicone?

[3Phase]

Silicone would work but the drill will want to go right back through the old hole and the silicone will just wriggle out of the way.

JB Weld or some of that AlumaWeld would be better and you'd be less likely to re-drill the hole in the same wrong place again, you could drill it wrong in a whole new place!

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[motobrix]

Silicone would work but the drill will want to go right back through the old hole and the silicone will just wriggle out of the way.

JB Weld or some of that AlumaWeld would be better and you'd be less likely to re-drill the hole in the same wrong place again, you could drill it wrong in a whole new place!

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Alright, makes sense to me. I may drill on one of the other carbs rather than trying to redrill the filled hole. We'll see how it goes. I would use a smaller drill bit, but it's hard to find a bit long enough to fit, and a way to hold a small bit.

[3Phase]

You could enlarge the hole you already drilled to move it a little more to the right side of the throttle plate.

Put a piece of tape over the hole in the carburetor throat, then grease the drill bit or a piece of wire and put it through the hole and the tape, then pour JB Weld or your favorite filler around it to block off the part of the hole you don't want.

When the goop starts to set enough to hold its shape, twist the drill or wire out of the hole and leave it alone overnight. Cut and smooth any blobs that dripped into the carburetor throat and it should be good to go.

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[IanDMacDonald]

Just curious about how these carbs would run without the vacuum advance mod, versus a set with them? I have my vacuum advance locked-out at full advance.

[TADracer]

Before you go plugging everything up I say install them and try it. Even with a very small hole you still get a bit of pulsing in the mechanism. If you use a plastic type fitting like I have done then an old pilot jet screws very nicely into that and acts as a perfect restrictor. If not then try to install it in the line.

Start her up and look at the advance mechanism. If it is at zero at idle and advances as you slowly open the throttle then you are pretty much OK.

Mine actually worked well without the advance but mileage dropped significantly cruising at highway speeds. That's why I did it....guess I am a bit anal that way.

[3Phase]

'Brix, remember that silicone and gasoline don't play well together.

You can use silicone to plug the hole so that you can drill another carburetor but the silicone you used as a plug will swell. You can trim the plug back out of the carburetor throat if it gets too bad but if you use it to fill around the vacuum advance hole it will close the hole and the vacuum advance won't work.


Ian, Mike,

The XS runs great with those carburetors. GarthXS used to live in San Diego and he had them on his XS before he sold it and moved to Seattle. I can't remember what he did but it was pretty close to plug-and-play and the power was good. The mileage was down without the vacuum advance.


Pulsing on the vacuum advance is actually a great way to tell that the carburetors are not set correctly. At idle, the vacuum advance assembly should be at rest with 0 degrees of advance and no pulsing.

The exact amount of mercury that must be moved depends on the year/model but on a stock XS1100 the throttle plate on the #2 carb is closed far enough at idle to move completely away from the vacuum advance port and drop the vacuum signal to the advance diaphragm below the 2 or 3in-Hg it needs to start moving the ignition timing plate.

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[IanDMacDonald]

'Brix, remember that silicone and gasoline don't play well together.

You can use silicone to plug the hole so that you can drill another carburetor but the silicone you used as a plug will swell. You can trim the plug back out of the carburetor throat if it gets too bad but if you use it to fill around the vacuum advance hole it will close the hole and the vacuum advance won't work.


Ian, Mike,

The XS runs great with those carburetors. GarthXS used to live in San Diego and he had them on his XS before he sold it and moved to Seattle. I can't remember what he did but it was pretty close to plug-and-play and the power was good. The mileage was down without the vacuum advance.


Pulsing on the vacuum advance is actually a great way to tell that the carburetors are not set correctly. At idle, the vacuum advance assembly should be at rest with 0 degrees of advance and no pulsing.

The exact amount of mercury that must be moved depends on the year/model but on a stock XS1100 the throttle plate on the #2 carb is closed far enough at idle to move completely away from the vacuum advance port and drop the vacuum signal to the advance diaphragm below the 2 or 3in-Hg it needs to start moving the ignition timing plate.

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I'd be intetested in eventually trying this mod. My fuel mileage sucks due to keeping my advance locked-out, but I did go back to stock at one point, and saw quite a difference in throttle response.

[Diskman01]

Sorta off topic..

Since the ZRX carbs have a throttle position sensor.. All I need to find is a cheap electric valve to modulate the vacuum and some simple code on an Arduino microcontroller.

Instant adjustable vacuum advance curve.... Maybe use the controller to monitor other items on the XS also. Wouldnt need to drill that pesky port out then. Simply pull straight vacuum to the valve and let the controller adjust accordingly.

I smell a weekend project.. Controller is about 10 bux.. Small valve for gasoline environments..?

[Bonz]

I have a ZRX and am very familiar with the K-TRIC TPS that Kawasaki uses. Using throttle position to regulate a vacuum advance device won't work to my mind. You can have high load/high throttle, low load/high throttle, low load/light throttle, all of those combos at different rpm, etc and any combo in between.

Vacuum advance has to be done by vacuum source straight from the carbs. If you want electronic advance, then you need the ignitor box to control that based on throttle position and RPM, which is what the KTRIC works in conjunction with. By disconnecting it, it defaults to wide-open timing advance on the ZRX based on whatever rpm the engine is running.

I disconnected the KTRIC on my stock carbs and with the aftermarket jet kit, pods and full exhaust system it made a nice bump in response by not retarding the timing in the lower mid range, which is because the KTRIC was designed to work with stock carbs and jetting, to get best lean emissions in that range.

I have since put ZZR 1200 CV carbs on the ZRX, and it is a transformation in low end, mid range, top end and throttle response that cannot be described. Guys that have done the ZZR carbs and also flat slides say they are almost identical, but the CV carbs kill the flatslides for drive ability and mpg.

[Rasputin]

So can someone explain to me why a restrictor of the same size and length added in, immediately off of the carbs would not suffice in this instance?

[3Phase]

So can someone explain to me why a restrictor of the same size and length added in, immediately off of the carbs would not suffice in this instance?

You're suggesting an inline restrictor added immediately where after the carburetors?

Anywhere after the throttle plate is manifold vacuum and it's inverted 180 out phase at smaller throttle openings from the ported vacuum signal used for the XS1100 and the XJ1100.

Some of the stock #2 BS34 carburetors already have a restrictor in the ported vacuum port. I don't know if that's because they're a different design than other XS110 carburetors, if the restrictor fell out or a PO took it out for, "More power! r r R!"

The restrictor damps the vacuum signal so the advance diaphragm on the XS1100 and the vaccum boost sensor on the XJ1100 don't thrash at low RPM but can still react quickly to changes in load and throttle position.

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