ct70 gear upgrade?

[jsroark]

Is there any way to upgrade my 3 speed ct70 to a 4 speed? If I could get 5 mph more out of this engine without revving it up so much it would sure make my commute a little better. I can get it up to 37 or 38 mph but it seems to be rapped up quite a bit. Thank you.

[racerx]

It is possible to install a 4-speed tranny in place of your 3-speed and make it a 4-speed/semi-auto. The question is does your engine make enough power to pull a taller top gear? Your engine, unlike a 4-speed H-model, has no spark advance and that restricts performance somewhat, as compared with an H-motor. Sourcing a complete OEM 4-speed tranny isn't the easiest task anymore and new aftermarket trannies will set you back anywhere from $250-$900 all-in; it's a major job that can grow once you split the cases.

42/43mph is about the most you can expect from a healthy 3-speed motor in stock trim and the time it takes to get there can be measured with a calendar. If you're maxing-out 5mph below that, your motor may be down on power. I'd try replacing the 15t countershaft sprocket with a 16t and see what happens. If your speed picks up and the motor pulls along about the same (seat-of-the-pants judgement), you might try a 17t next. That, I expect, will be over-the-top. Who knows, a one-tooth larger c/s sprocket may even solve your problem for less than $15 and 10 minutes worth of work...it's worth a try. On the other hand, if the bike falls on its face it's probably time to freshen the upper end; then you may be better served by an 88cc kit, head rebuild and taller sprocket combo (which probably won't cost as much or require as much work as a tranny swap).

[fatcaaat]

My advice is this:

You don't need a 4 speed, you need more power and better gearing. So, on the cheap, you an do the following:

Buy
Mikuni VM20 Carb
Bolt on 88cc kit
Cam

All this is had on dratv.com and will run you about $200. Then put a 16T gear on the front and you'll be golden.

If you want more out of the top end, get a pipe too.

[jsroark]

Thank you both for your info. I appreciate it. I am interested in the 88cc kit upgrade. What all does it entail and how do I get it/ more info on it? Also, where is the best place to get the 16t sprocket?

[racerx]

May as well source the 16t sprocket from the same vendor supplying the 88cc kit. Installing a bore-up kit is pretty simple, it's not strictly necessary to split the cases.

However, you'll probably find some sludge inside the cases and that should be cleaned out. That is possible with the RH cover removed, using solvent such as brakleen. I'd plan on replacing the oil pump, cam chain & idlers, the rubber cushion in the cam chain tensioner piston, clutch friction discs and of course seals & gaskets. It might sound like a lot, but it really isn't; it's cheap peace of mind. That should be enough to return the lower end to like-new operating condition. Not a bad idea when building for added power.

The cylinder head should be disassembled, cleaned and inspected. The valves and guides should be in tiptop shape or you'll lose compression (and power). Its fairly common to find a worn intake valve; if the intake port is black, you probably have one. An OEM Honda intake valve is less than $20. Valve seat reconditioning & valve lapping adds another $45. Of course, you'll get new valve stem seals with the seal & gasket kit. Worn valve guides are relatively rare. If you're going with a hotter cam, new valve springs are in order, matched to the specific cam profile; it's easier than it sounds as there are only a few valve spring choices and the vendor supplying the cam should be able to tell you which springs to use.

Carb size should never exceed the diameter of the intake port. You should be able to make a VM20 carb work with this setup but it's a tad oversize unless you're running a race-type head with bigger ports & valves, a race cam, and have the ignition timing optimized for high-rpm. Advancing ignition and a new head would allow you to make enough power for ~55mph top speed, with a free flowing exhaust fitted. Unfortunately, those two items can easily more than double the cost of your project. An 18mm carb won't cost you more than 1-2mph, if that, will give you crisper throttle response (particularly at low-to-mid rpm) and be easier to tune. Although your top speed should be the same regardless of whether you have a 3 or 4 speed tranny, as long as the sprocket combo is dialed-in, having one less gear puts more emphasis on low & midrange torque.

With 88cc, hotter cam and 18mm carb + K&N air filter, I'd expect 45-50mph top speed with 40-ish cruising speed and a noticeable bump in torque. You can probably maximize your top speed with a hotter cam, but that may require the same gearing you have now to keep the engine buzzing along in its powerband. Thus, I'd select a cam with more lift and a bit more duration than stock, in other words, biased for midrange with a power peak around 9000rpm for a street-driven 3-speed bike with fixed ignition timing. That ought to get you a bit more than you originally asked and your engine ready for the next several thousand trouble-free miles for somewhere around $300 if you do most of the work.