Tuning CT70s for street use

[Grant's trail]

Thats great you all ready have the carb and a performence cam. I have never ran the TB fast cam but It is the same one that Dratv sells, and the link you sent is the one I'm talking about. I think the Dr. sells a different flywheel that has a timing advancer that fits the automatic bikes. the H models have it so do the SL motors but the automatics have fixed timing. you can open the point gap .002 to .005 and that will advance the timing too, just watch out when you kick it so it doesn't kick back and hurt your leg. If you have a timing light you can check it when its runing and see how much advanced it is over stock with the points opened a little. you want another .125 to .200 more on the flywheel. have fun!

 

[kziolk]

... you want another .125 to .200 more on the flywheel. have fun!

Good info! Could you explain the last sentence about the .125 to .200? Thanks, Kevin.

 

[Grant's trail]

Sorry if it didn't make since, I meant set the timing about .125 to .200 further advanced then the stock firing mark on the flywheel and see how it runs. By opening the point gap a little the points break sooner and advance the timing. to the point that the coil doesn;t have enough time to build up for the next spark. but the small amount were talking about is if you have a larger cam and more compresion you might see a little more power from your combo. I use a 12v timing light hooked up to a seprate battery with the clip on the sparkplug wire, then with the engine running shine it on the timing marks. As the point block wears down your timing slows and you loose performance. With a little math if you measure the flywheel, that X Py or 3.1416 close enough take that number divide it by 360 that gives you how far the flywheel turns to travel 1 degree, now X that number by 40 for total degrees when runing and that number if you measure from top dead center mark forward
with a scale or tape wraped around the flywheel is were your total timing should be when the eng is rev-up. 40 degrees is a base number it might work a couple deg better forward or back but the automatic bike total timing is a lot less then this. you have to watch for it to kick back though when you kick start it.

 

[kziolk]

Latest modifications: Switched to a HK1 engine. 88cc TB cylinder and piston. Honda "hot cam". Changed the gearing back to 15/35 but it's way too high due to the taller 4th gear of the 4 speed. I still managed to run 47mph top speed. Next tests: 20mm carb and intake, 14t front sprocket, street tires.

 

[Grant's trail]

Thats good speed with a stock carb, Before I changed the gearing I'd put on a 20mm Mikuni and aftermarket air filter, I have almost the same setup with a 17 counter and 38 rear and its good for 52 to 53 mph.HTH

 

[kziolk]

Thats good speed with a stock carb...

I made some modifications to the air-box to increase flow thru the stock carb. On the 90 degree rubber elbow that attaches to the rear of the air-box, I took out the cone shaped restrictor and shortened the elbow (cut it in half). This increased the airflow enough to go from a 58 main jet to a 70 and I think I'm still a tad lean.

 

[YellowBeetleSB]

Okay, so even with the 16tooth switch my top speed is 43mph.
When I shift into 2nd it boggs down and after a few seconds of puttering and lull picks up. What do you think is causing the bogging? I'm not sure why my top speed isn't hitting the 50mph mark.

Remember my set-up:
1970 CT70 KO, 3spd auto
TB 88cc big bore & race head/ race cam
20mm carb & airfilter
Thumper Exhaust
16tooth front/ 35 rear (or whatever stock is)
Knobbies, but soon to be Pirelli SL26
Me; 5'4", 115lbs (I'm a female, not a tiny male)

What should the valve clearance be? And the Point & spark plug gaps?
Perhaps this is causing the bogging & lack of top speed?

Do you think a stroker crank would improve engine power?
Which Stroker will fit TB88cc kit without modification? (Just pop right in..)
I looked on TB site and I don't see any stroker cranks, but DrATV has several
COMPONENT ENGINE PARTS_ONLY

 

[P.C.]

when was the last time you replaced the points? when the rubbing block wears it slowly detunes your bike closing your points.especially a 3 spd that has no timing advance to begin with.change em dont just gap em.i would gap em at .16 then tell me if its still a dog.and we can go from there...put the flywheel at tdc stone cold and set the valves @ .3

 

[Grant's trail]

Is the Carb that came with the trail bikes kit a shingway carb by chance? It sounds like the main jet might be to small. But P.C. has a good point if you havent replaced the points ever it might be the problem and sure woldn't hurt. If it's a shingway carb your better then me if you can get it to work.

 

[YellowBeetleSB]

The points were replaced exactly one year ago.

As soon as we come upon a day without precipitation here in Western WA, I'm going to adjust the valves and check the point clearance & timing by following AlexManVideos on youtube.

As for the carb. On TB's page it says it's an AFT 20mm carb. I've never heard of that brand.
AFT 20mm carb

 

[fatcaaat]

A properly tuned 88cc kit, and properly geared, should take your bike out to about 55-60mph. The last 88 kit I did was in a Sachs Madass, which weighed in at 190lbs and I also weigh 190lbs and the 88kit took that to 53mph on the flat. I also realize that you purchased a complete kit...I'm curious to know if you got the right piston. The pistons for the modern motors have different deck heights than the pistons for the older motors. If you assembled the motor and the piston was for the modern bike, you are leaving the piston about 3mm in the hole at TDC, thus significantly reducing your compression ratio, and thus, power. When you assembled it, did the piston come all the way to TDC level with the top of the cylinder deck?

The kit is designed to be run with a fairly open exhaust. You should be able to mount the piston kit and head and carb and not need to rejet it. I've built a few of these in my time and never once had to rejet one. As a matter of fact, I have one in a box sitting here that's going to be installed this weekend on a bike.

If you purchased the correct piston and head combo, it should run well as jetted. If you need more power than that, its available in the form of a stroker crank. since you're running still the 6v stuff, you can go to dratv and buy one of the 52mm stroker cranks and make a 52 x 52 square bore 110 motor. That will get you into the mid-upper 60s. http://www.dratv.com/racr52stforp.html is the correct one for your application. And no matter how you slice it, you'll need to update your oil pump and shaft so that the crank ways will clear.

 

[kziolk]

The points were replaced exactly one year ago.

As soon as we come upon a day without precipitation here in Western WA, I'm going to adjust the valves and check the point clearance & timing by following AlexManVideos on youtube.

As for the carb. On TB's page it says it's an AFT 20mm carb. I've never heard of that brand.
AFT 20mm carb

Anytime you see AFT on TB's site, it simply means aftermarket.

 

[P.C.]

it really depends on mileage.if you did alot of miles i would change em.but going 2 years or more is really pushing it.if you do change em have air tools to put that nut on,i cant remember exactly what the torque setting is but its one of the highest on the whole bike.you do not want that baby to come loose!the carb you have is a sheng wey,not the best for your set up...they seem to perform better on chinese 110's then on 88cc custom built hondas.that would be next on the list to eliminate if the points dont do the trick.on ebay they have a REAL nice 19mm for 39 bucks delivered.i would buy one!it has a dual fuel inlet and choke on the left side.if you cant find it i will post the item# for you.

 

[Merz]

fatcaat, It says that a $140 flywheel is needed to use that stroker crank. Could someone get a used flywheel off of a 4spd motor instead of buying that one? $160 isnt bad for the crank, but having to spend an extra 140 doesnt seem worth it. thanks

 

[YellowBeetleSB]

fatcaat:
The kit I got was this one:
Honda Trail Bikes
And it says it is for the K0 so I figured it had the correct piston since my bike is a K0. I installed the kit 2 years ago (but replaced points last year) and I really don't remember about the piston at ADT. I feel like it did. I had everything lined up and have had the bike serviced at both a Honda shop and a local dirt & vintage bike shop several times since. I do recall one guy at a shop making a snide comment about me degrading the Honda engine with that aft Chinese crap....so I'm asuuming he looked inside. Maybe he would have noticed if the piston did fit?

 

[P.C.]

fatcaat, It says that a $140 flywheel is needed to use that stroker crank. Could someone get a used flywheel off of a 4spd motor instead of buying that one? $160 isnt bad for the crank, but having to spend an extra 140 doesnt seem worth it. thanks

you would need the matching stator also,the reason that the Dr recomends that you change to the 4 spd flywheel is cause it has a timing advancer.my thought is get the most advance you can right now through the point gap.16 maybe .17 the honda guy just saw a different carb and cylinder,he didnt look inside.he would of charged you for a head gasket and seals.

 

[Merz]

So if i got the flywheel and stator from a 6volt 4 spd sl/xl/ct70, I could use it with the dratv stroker crank on my 3sp auto?

 

[Grant's trail]

Yes, the 4 speed flywheel and stator works with the 52mm stroker crank.

 

[YellowBeetleSB]

on ebay they have a REAL nice 19mm for 39 bucks delivered.i would buy one!it has a dual fuel inlet and choke on the left side.if you cant find it i will post the item# for you.

I found the carb. 19mm Honda CT70 MINI TRAIL Carb CRF SL PASSPORT 70 CA21 : eBay Motors (item 170450228232 end time May-23-10 21:19:44 PDT)

But where do I get the gaskets, heatstop and mounting hardware? The air filters on TB's site are for 20-24mm, this is a 19mm.

Now... Back to the stroker & oil pump
The 52mm Stroker requires
1) Piston with a trimmed Skirt (Is the piston from TB 88cc kit trimmed?)
2) Trimmed Pump Shaft on the Oil pump (Is TBs trimmed or should I just get DRATV's HV oil pump that is trimmed? http://www.dratv.com/27hioutputoi.html)
3) And Dr. ATV flywheel stator combo (Which is out of stock- http://www.dratv.com/afstatwitfly.html)

 

[fatcaaat]

I think if you got a flywheel for a 4-speed, either sl70, cl70, xl70 or s65 you can get the rebuild kit from CHP for any backing plate.