ZRX1200 Carb Conversion

[IanDMacDonald]

I'll try to make tire tomorrow. I'll need to pull the carbs and increase the size of the pilots. The Keihin 138's had me at 3.5 turns out, with more needed. I'll bump them up to 140's, and sync the carbs.

[Bonz]

I didn't see the alert, bummed I didn't see this post quicker...

Good work. Float height for the ZRX carbs is 18.5mm +/- 2mm. Curious what you went with.

Sync the carbs as soon as you get the airbox on and idling. Don't expect a steady idle at first, but the sync will help dial that in. As you sync, may need to turn the idle down or up depending on how it responds.

Goes without saying, use the first set of plugs for the sync. Go for a ride, see how it feels if it is rideable at all or if it's decent. Make the first round of changes based on how it feels/seat of the pants. I would go for another ride with the first set of plugs, once the second round of change(s) are made. At that point I would put the new plugs in if I was doing it.

Last week I took a big swing at the CVK40's on the ZRX 1200. For those that might not have read the whole thread, I adapted those on instead of the stock ZRX carburetors. Did to the ZRX with ZZR 1200 carburetors as this thread is doing with the ZRX stock Carburetors in place of stock XS 1100 carbs.

The cruise air fuel ratio would not lean out much even when I leaned the needle clip out. Lean the needle clip enough to help light throttle cruise and then it's lean above that. It's certainly a balancing act.

Pilots control fuel flow more than I realized at light throttle cruise. Swapped down to 38 pilots from 40s. Man, what a difference in air fuel ratio. It's in the low 13s to low 14s now, instead of low 12's to high 12's. 12s are where power is made, mid-13s into low 14s are the balance for economy.

Hopefully you can flesh those things out by the seat of the pants and the plug reads. I share what I did above as information that might help in your continued work.

Heres hoping you've had some time over the weekend to make progress. Look forward to hearing!

[IanDMacDonald]

okay, got it started. I'll have to pull the carbs back off, and put the airbox on to start getting an idle, then swap the plugs to see what's going on with the tune.

[IanDMacDonald]

Okay, a couple things to confirm:

1. The carbs WILL mount to the OE airbox
2. The carss will mount to the OE intake manifolds without issue
3. The carbs take a decent amount of gas to keep the float bowls full

[Bonz]

That's awesome. This is going to be a great revisit of that original conversion.

[IanDMacDonald]

Did you drill the vacuum port? I always understood the vacuum advance on the XS1100 helped with fuel mileage when it pulled in the advance at high vacuum cruise.

Thanks for the walk-thru! And, yes, sir. I drilled per Tad's write-up.

[Bonz]

You got it on the pilot, if you go past 3.5 or 4 turns for sure, it needs to be larger. 1.5 or less and go smaller.

Even if the main jet isn't perfect, the needle clip is what you're tuning for mid-range response, roll on, fuel mileage at cruise.

It's definitely subjective based on the seat of the pants. If it feels rich, raise the clip to put the needle further into the main jet to lean it out in the mid-range roll on, and cruise conditions. If it feels lean lower the clip on the needle to get more of the needle out of the hole.

With the AFR gauge on my ZRX and the ZZR 1200 carburetors, a half clip position was worth .5 air fuel ratio either way all else equal, making one change at a time and comparing.

Fuel mileage can be a great help as well with respect to needle position. If gas mileage is in the high 30s or low 40s, you're probably pretty close. I would expect the more modern ZRX carburetors would get you into the lower 40's on the whole.

From what I've read in the old thread when this was first done, the performance increase should be noticeable regardless.

Did you drill the vacuum port? I always understood the vacuum advance on the XS1100 helped with fuel mileage when it pulled in the advance at high vacuum cruise.

[IanDMacDonald]

Once the bike will start and idle, sync the carbs. Mid-range and top end don't matter at that point, gives a consistent baseline to start.

Then find the main that pulls best at high rpm, once the needle has cleared the main jet.

Then work with the needle position clip, then back down to the pilot and adjust anything with that.

How do I tune the needle? How will I know to drop the clip or raise it?

Then, the pilot? I presume too many turns out on the A/F screw equals pilot too small, and vice versa?

[Bonz]

Once the bike will start and idle, sync the carbs. Mid-range and top end don't matter at that point, gives a consistent baseline to start.

Then find the main that pulls best at high rpm, once the needle has cleared the main jet.

Then work with the needle position clip, then back down to the pilot and adjust anything with that.

[Bonz]

Modified airbox that breathes much more with an automotive frame filter. I also have the Chinese 1179 kit.

The carbs had the original Keihin jets in it. Just the needles were the dynojets.

Gotcha. Factory pro has a kit for the ZRX that uses the air box with a big ring K&N Filter inside. It jets the same more or less as pods. 130-132 mains, 35 pilots 3-ish turns out as baseline fwiw. ZRX 1200 big bores are 1224 (1164 stock), 1100's are 1109 (1052 stock). Jetting doesn't change for either of the big bores. At least with the ZRX.

My thought is 140s and 38's are going to be on the large side. But that's the half the fun of being a guinea pig in the newer era of working on older bikes.

Will be interested to see how she runs.

[IanDMacDonald]

Can't recall, pods or airbox? You're starting jetting fit decently close with pods on ZRX. Typically, ZRX with pods is 130 Mains and 35 pilots as a general rule.

What size mains and pilots were in the carbs when you got them seeing as it already had a Dynojet kit based on the needles?

Modified airbox that breathes much more with an automotive frame filter. I also have the Chinese 1179 kit.

The carbs had the original Keihin jets in it. Just the needles were the dynojets.

[Bonz]

Can't recall, pods or airbox? You're starting jetting fit decently close with pods on ZRX. Typically, ZRX with pods is 130 Mains and 35 pilots as a general rule.

What size mains and pilots were in the carbs when you got them seeing as it already had a Dynojet kit based on the needles?

[IanDMacDonald]

Looks great!

I have them ready to mount, but won't get to this till next week.

I'll test with a remote fuel supply, as I need to have the tank off to fab the throttle cable.

I don't expect the tuning to be an easy process.

[Bonz]

Looks great!

[IanDMacDonald]

Making progress. I'll pull the choke bracket back off and paint it black. My days of access to a mill are over, so it's good enough from a workbench.

I have 140 mains, with 38 pilots in, to start. I have the A/F screws out 1-1/2 turns.

Three sets of new BPR6ES plugs ready to tune with.

Next up is mounting and throttle cable fitting.