Maybe someone HELP ME!!! I have 1971 Honda CT70 that I can't get to go pass 29mph. I have put on 88cc top end geared 15/35 have tried everything to get it to go faster stiil no luck. Dont know if maybe something wrong with gear shift drum or transmission or shift forks if anybody can help me let me know it would be greatly appreciated
No power after 2nd gear go's a little faster in 3rd but not much
- Thread starter Slowgo
- Start date
Old Guy Too Many Bikes
Well-Known Member
Not a lot of info here. Why do you think it's a transmission issue, rather than a engine power issue? Have you checked the compression and ignition timing and advance, Have you double checked the cam chain to make sure the mark lines up. How did it run before the top end rebuild? A stock CT70 should do 40+, with an 88 you should get 45 out of it.
Wasn't going over 25mph when i got it. compression is good tried adjusting timing and advance runs fine in 1st and 2nd, 3rd just really doesn'tgo any faster act's like it's in a bind. I get real good torque in 1st and 2nd 3rd just can't get any extra speed out of. highest is 30mph.
Old Guy Too Many Bikes
Well-Known Member
Not sure what you mean when you say it's in a bind. You mean there's something wrong with 3rd gear. Have you tried a push start in 2nd, and then a push start in 3rd, and see if there's a different kind of drag before the engine engages. When my CT70 was stock, it could lift the front wheel off the ground with a power shift to 2nd. Obviously 3rd gear won't pull as hard. When riding in 3rd, does it feel funny when you let off the gas? Are you sure a carb adjustment isn't needed with the increase in displacement.
dirtbkr188
Active Member
These little engines usually have a compression range of 160-180 PSI when new or after a complete top end rebuild.
OLD CT
Well-Known Member
90 psi even before you fire a fresh rebuilt engine is unacceptable. I see at least 150 before the break in, fire up. If your mechanic installed a new head that can be another red flag as they may have installed the new dome version that will give you around 90 psi because of the incorrect combustion chamber. Time to remove the head...
Did we find a solution to this? I've recently been doing a resto on a 74 XL70 and couldn't get it to run correctly to save my life. After checking everything I did a compression test (thought I did it last year but I'm guessing not) and found it had the wrong piston in. Someone had rebuild it with the newer style piston. (Pictured below)
my CT with 88cc kit, 20mm carb and performance exhaust gives me an honest 50mph (I'm 215lbs). That's with absolutely everything to spec and a 16/35 sprocket setup.
Things I would look at.
- compression test. When I do my test I use a short tube attachment not the plug hose because i find the volume of the hose eats up some of the psi from the small volume cylinder. That 72cc rebuild I did was with rings at max spec and a used cylinder and I still got 110psi before break in run. I have never seen higher than 125psi
- what color is your spark plug (main jet size?) (Correct spark plug heat range?)
- are you running 91 octane?
- tappet adjustments to spec
- ignition timing checked with a timing light
- confirm you haven't got absurd gearing on the bike (aftermarket sprockets)
Please post results as I love to hear about how people solved these issues and in 10 years someone might come across this with the same issue.
my CT with 88cc kit, 20mm carb and performance exhaust gives me an honest 50mph (I'm 215lbs). That's with absolutely everything to spec and a 16/35 sprocket setup.
Things I would look at.
- compression test. When I do my test I use a short tube attachment not the plug hose because i find the volume of the hose eats up some of the psi from the small volume cylinder. That 72cc rebuild I did was with rings at max spec and a used cylinder and I still got 110psi before break in run. I have never seen higher than 125psi
- what color is your spark plug (main jet size?) (Correct spark plug heat range?)
- are you running 91 octane?
- tappet adjustments to spec
- ignition timing checked with a timing light
- confirm you haven't got absurd gearing on the bike (aftermarket sprockets)
Please post results as I love to hear about how people solved these issues and in 10 years someone might come across this with the same issue.
Attachments
airblazer
Active Member
I grew up in Boulder, CO and live in UT now. Never had a problem hitting +40mph on stock 72cc motors. You haven’t answered @JR86 questions in post #13 above. We can help but we need more info. Pictures would help. At 90psi your bike is barely running, maybe we need to verify how you did that test? You should have nearly 2x that compression.
did you replace the clutch pack? Ive had a bike kick start fine without "slipping" but when driving bike wouldn't "go" very well.
Does the bike rev out at high rpms when you hit 30mph?
I know this was asked before but what brand/who's big bore kit?
remove the head and confirm the piston is at the top of the cylinder at top dead center
Does the bike rev out at high rpms when you hit 30mph?
I know this was asked before but what brand/who's big bore kit?
remove the head and confirm the piston is at the top of the cylinder at top dead center