Mechanical Advance

[vanimal5k]

I have a 79 Special with an 80 3J6 engine. 4 into 1 with a Jardine crotch rocket style can. 2000 ZRX1200 carbs jetted 137.5/42.5 with oval pods, vacuum advance thru #2 carb sync port using a jet drilled to 1/2mm. My question is should I or can I run either a 2h7-10/11 mechanical advance? What should I expect? Thanks!!

[skids]

I have a 79 Special with an 80 3J6 engine. 4 into 1 with a Jardine crotch rocket style can. 2000 ZRX1200 carbs jetted 137.5/42.5 with oval pods, vacuum advance thru #2 carb sync port using a jet drilled to 1/2mm. My question is should I or can I run either a 2h7-10/11 mechanical advance? What should I expect? Thanks!!

Everything you did could change the jetting requirements - air and fuel supply. The mechanical advance goes with the vacuum advance timing plate with the timing coils, coupled with the black box. I am not sure about what you are saying "vacuum advance thru #2 carb sync port using a jet drilled to 1/2mm". I suppose you did this because of the ZRX carbs. I would think that the timing would depend on all of the original timing parts and the location of the drilled vacuum advance nipple. Yes, you would need the mechanical advance... ALTHOUGH I have no experience with what you did.

[BA80]

I have a 79 Special with an 80 3J6 engine. 4 into 1 with a Jardine crotch rocket style can. 2000 ZRX1200 carbs jetted 137.5/42.5 with oval pods, vacuum advance thru #2 carb sync port using a jet drilled to 1/2mm. My question is should I or can I run either a 2h7-10/11 mechanical advance? What should I expect? Thanks!!

You can expect that if you modify everything you can expect to be on your own. Good luck.

[Bonz]

Since you have done all that work, what does the bike do at this point? I assume there is an advance unit on the bike and it can be ridden.

Follow the recipe other guys have used and you should be pretty darn close. I have not run the ZRX carbs on an XS but it should be a great mod.

I put ZZR 1200 carbs on my ZRX 1200 and on the dyno was worth 12 hp and 9 ft/lbs. That is flatslide-like power gains with 50+ mpg.

[vanimal5k]

Yes, it has an '80 3H5-10 on it. Yes, it can be ridden.

[Bonz]

Well, how does it run?

Do tell... Very interested to offer thoughts and suggestions based on what you're finding.

[vanimal5k]

A lot better with the ZRX carbs. With the old carbs, it seemed sluggish until around 5500 rpms. Now it's more steady thru the entire power band. However if I'm not "hot rodding around" it did seem to load up slightly. I was going to try repositioning the needle in the slide. I'm looking to run more street light to street light, that's why I was wondering if anyone had played with the different mechanical advances.

[Bonz]

If loading up under light Cruise, turn in your pilot screws 1/4 turn at a time.

If it's loading up under normal acceleration I would raise the clip on the needle if it's adjustable, to lean out the needle circuit. From Kawasaki the ZRX carbs did not have adjustable needles. You could put a washer under them to shim them up but you couldn't lower them. If the needles are adjustable, take a picture of them and post it so I can tell you what jet kit is installed in there.

[TopCatGr58]

Hey Vanimal,

I know I've read about the ZRX carbs mod, but I don't necessarily agree with the use of the metered pilot jet insert in the vacuum line on the synch port.

The vac. advance does provide a little advance during low rpm running, but it's mostly to provide more advance during cruising rpm/speeds to provide a bit more fuel economy. During hard throttle input/strong acceleration, especially changing from cruising rpm/speed, the vac. adv. actually retards a bit to allow the timing to return to the max CENT adv. level of about 35 degree BTDC which is the point of strongest affect.

With the vac. adv. connected to the synch port, even with the metered pilot, it's still providing vac. signal and could actually be TOO MUCH, and may NOT release quickly enough under strong throttle input which could also be contributing to the bog response.

A while back, a fellow with an XJ had a broken TCI, and with the XJ both the vac. and cent. adv. curves are programmed into the TCI. He couldn't find an XJ TCI but was able to find an 81 4RO Tci which still had the CENT adv. programmed, but normally used the mech. vac. adv. for that part. He put it on his XJ without any Vac. Adv input, and it performed just fine with plenty of power through to redline. I think he did report that it may not have had as much MPG, but not a great deal difference.

Moral of the story, You've got a great set of MODERN carbs that provide much better atomization and mixing of the fuel than the OEM XS11's, and so I would suggest plugging the VAC Adv. deactivating it, and just rely on the CENT adv. and see how it behaves and what your mileage is. You might need to make a small adjustment in the static timing setting it slightly more advanced than stock to get the best overall affect without the vac. adv.
You may find that it may respond better to throttle input, and may not loose much mileage wise. YMMV, JAT.

T.C.