Thread: Sprocket Advise Please

I fired up a rebuilt Z engine today. Took long enough, it's been a busy summer. Put a bigger cam than stock in it. Seems to have more power than the others. Sputters a little through the mid range. Any ideas? The real reason for this post is; what is a good combo for sprockets w/ only a bigger cam? Right now it does seem like it has power to spare.

Sputtering in the mid range is likely a jetting issue. You might be able to raise the needle and make most of it go away, though bumping the main a size or two is probably worth trying as well. Just don't lose the little clip on the needle. It's easy to do.

Gearing is the kind of thing you kind of have to play by feel to some degree. My old '71 Z50 had 12/35 gearing when I got it. I swapped a 72cc top end onto it with a cam from a Honda S65 (a little hotter than stock CT70s) and a bigger, free flowing exhaust. Once I got it jetted I went up two teeth on the counter shaft to 14/35 gearing and it was still strong at slow speeds but it had more top end as well. I could have geared it even taller, but it was getting a lot of woods riding and still needed the "grunt". Doc ATV has all kinds of sprockets for Z50s so the possible combinations are pretty extensive. Mostly, the final gearing depends on what you intend to do with it. (Drag racing? Motocross? Supermoto? Maybe a record run at Bonneville? ) It's part of the fun. If you want to experiment, swapping countershaft sprockets is usually the cheapest place to start. Order your chain 4 links longer than stock and get an extra master link or two so you can handle the increased length for the larger CS sprockets.

-kevin

Once I got it jetted I went up two teeth on the counter shaft to 14/35 gearing

You began with a 12T c/s sprocket?! That bike must have eaten through chains like Al Gore at an all-you-can eat lobster buffet

Midrange bogging is usually the result of a lean spot. If it's high enough in the rev range, a needle height adjustment will cure it, especially if we're talking about the stock carb (assuming it was functioning correctly before the bore-up kit). Usually, finding the rich limits for idle and main jets (at WOT) will get you close enough that needle height tweaking will get the job done, even with a larger carb. Of course, that's about the limit of carb tuning circa 2008 Back in the good old days, tuning slides, air jets, needle jets with different internal profiles and jet needles with varying tapers were available for small carbs.

Originally Posted by racerx

You began with a 12T c/s sprocket?! That bike must have eaten through chains like Al Gore at an all-you-can eat lobster buffet . . . .

Yeah, I'm afraid that's what it came with. It only did a few weeks of kid hauling around the yard before I tore into it. And it always went through chains. Thank goodness they've improved considerably since then. Even after the 70 top end it still spent more time going slow than going fast. All of my kids rode the tires off of it, and then it went to a good home with a whole bunch more kids. I guess that's what they were made for.

-kevin

Thanks guys,
The spark plug showed a touch lean. Bumped the needle up one notch last night & seemed to do better. I think it has a 13/35 on it now. I may just get a 15 and call it good.