Increase the performance of your stock engine

Discussion in 'Modifications' started by MonkeyBikerCanada, Apr 8, 2018.

  1. MonkeyBikerCanada

    MonkeyBikerCanada New Member

    Joined:
    Apr 4, 2018
    Messages:
    22
    Likes Received:
    0
    Hi guys, I'd like to know what would be the simple ways to increase the performance of my 1970 honda ct70 with a stock engine. I do not want to increase the size of the cylinder and the piston.I would also like to know if anyone knows the maximum speed that a honda ct70 1970 should reach and I would like to know which gear I should buy and how many teeth it should have to slightly increase my speed.
     
  2.  
  3. lukelaw1

    lukelaw1 Active Member

    Joined:
    Jan 28, 2013
    Messages:
    525
    Likes Received:
    47
    Why are you against changing the cylinder and piston??

    Changing the sprockets would probably hurt you in the end, motor will run out of usable power.

    Your only option would be a "hotter" cam profile, and at just that you wouldn't gain much.

    A healthy stock ct70 should run about 47mph depending on how flat the rode is, how long the rode is, head or tail wind, and rider weight.

    You may have to face the facts on this one more power equals more displacement (CCs)

    Racerx I hope will be chiming in to give you the complete run down. I think he has these ct70s calculated out to 4.7 mph X 1,000 rpm, so 47mph is 10,000 rpm.
     
  4. mikejana

    mikejana Active Member

    Joined:
    Apr 16, 2015
    Messages:
    420
    Likes Received:
    64
    These little Hondas last so long and work so well because they are mildly tuned.
    You can wring a lot more power out of the same CCs, but at a cost of reliability and longevity. The best way to add power without losing reliability is by adding some size.
    Others will be chiming in soon-
     
  5. MonkeyBikerCanada

    MonkeyBikerCanada New Member

    Joined:
    Apr 4, 2018
    Messages:
    22
    Likes Received:
    0
    Thanks for the reply guys.I would like to know what would be the benefit if I would install a larger carburettor on my stock engine
     
  6. Gary

    Gary Active Member

    Joined:
    Jan 17, 2006
    Messages:
    920
    Likes Received:
    95
    My 50 was alot faster when I got it than it is now. I was 100+ pounds lighter though. ;)
     
  7. racerx

    racerx Administrator
    Staff Member Administrator

    Joined:
    Jul 10, 2005
    Messages:
    12,639
    Likes Received:
    1,076
    Lukelaw is correct. These engines are geared to deliver 4.77mphy/1000rpm, in top gear. Unless you're an ultra-lightweight rider or/and, every road in your locale has a descending grade + strong tailwind, your engine will run out of torque before gearing can reach 5.0mph/1000rpm. And, a stock K0 will run out of breath around 9000rpm...as evidenced by typical top speeds in the low 40s. The 4-speed, H-models, tend to do a bit better; advancing the ignition lead seems to extend the power curve by ~1000rpm.

    HP = MPH

    There's no way around this, it's basic physics...and reliable enough to be used as "the poor man's dyno". And, airflow = HP potential. So, you have to increase airflow and your choices are: increased displacement (more air volume per cycle), increased rpm (more cycles per unit time), or a combination of both. Leaving everything beneath the head alone, that leaves: hotter cam, portwork, larger carb, ignition. The simplest mods: SL70 (a.k.a. "fast road) cam, set the point gap at the practical limit (~0.018-0.020") to advance the timing. If you go any wider, the points won't close. Remove the cone restrictor from the inlet side airbox boot. Beyond this, life gets more complicated...port work, hotter cam with matched valve springs, CDI conversion to get spark advance and an aftermarket 18mm carb. Gearing will remain constant, for any of these tunes. The only parameter that will change is the power peak...you're looking for another 1000-1500rpm.
     
  8. MonkeyBikerCanada

    MonkeyBikerCanada New Member

    Joined:
    Apr 4, 2018
    Messages:
    22
    Likes Received:
    0
    Thanks a lot for these big amount of information racerx.
     
  9. MonkeyBikerCanada

    MonkeyBikerCanada New Member

    Joined:
    Apr 4, 2018
    Messages:
    22
    Likes Received:
    0
    RacerX,
    Could you explain to me what is the cone restrictor from the inlet side airbox boot on my Ct70 1970 ans do you have a pics of this part?
     
  10. fatcaaat

    fatcaaat Well-Known Member

    Joined:
    Aug 1, 2007
    Messages:
    1,671
    Likes Received:
    154
    I think i wrote about what I've done in the past on this topic. It definitely allowed me to add a tooth to the front sprocket and pull it fine. I ported and cammed the head, swapped the intake manifold and port matched it, added a vm20 carb, swapped the exhaust, and messed with the points. I believe I got that motor to 53mph like that, but it definitely got there a lot faster than getting to mid 40's with the stock setup.

    Truthfully, if you want to do something to your bike on the cheap that will make a big difference, do the following:
    this cam: http://dratv.com/bez5atcttrxl.html
    this piston/cylinder: http://dratv.com/88ctalalc7.html
    This carb: http://dratv.com/mivmcateouto.html
    This sprocket: http://dratv.com/spfr70.html in 17T
    You will also want to change your intake manifold and swap the airbox out.

    And then make sure your clutch is adjusted properly or you'll have some slippage and kick-through on starting.

    You can retain your stock exhaust. This change will be night/day over what you have now.

    I would suggest not porting it unless you know what you are doing. There is definitely some extra power in these heads with the right port job, but it's also easy to mess up
     
  11. fatcaaat

    fatcaaat Well-Known Member

    Joined:
    Aug 1, 2007
    Messages:
    1,671
    Likes Received:
    154
    On Friday, I took my stock 1975 out for a lunch. On the way back, I took the 4 lane road with the 50mph speed limit to see what the little sucker could do. This bike is pretty much stock all around. 670 original miles. Everything works including the speedo. If we are to believe the speedo, I was able to get 47mph on the flat. When we were going down a small incline, it registered 55mph. Again, if you can believe the speedo.

    The only thing that is different on this bike than a stocker is that the points were set the fatcaaat way. I never have ever set points using feelers...100% by this approach.

    Prep...get out a little flex ruler and line it up on the F timing mark and scribe a small pencil mark every 1 mm for 8 mm counter clockwise. This is your advancing indicator.

    Now, turn the flywheel around for 1 turn until it lands about an inch before your first scribed mark.
    Loosen up the points
    Rotate the engine a full revolution
    Now, rotate the engine until it is at the 5mm mark.
    At this point, use one screwdriver to hold the points until the cam thing is resting on the flywheel cam and tighten the screw.

    Points set! The other mm's you can experiment with but I have usually wound up on 5mm.
     
    stangconv21, farm dog and cjpayne like this.
  12. lukelaw1

    lukelaw1 Active Member

    Joined:
    Jan 28, 2013
    Messages:
    525
    Likes Received:
    47

    Have you ever checked the timing with an automotive light once set with this procedure? I'd just like to know once set with your procedure is "F" is right on case mark retort or advanced.
     
  13. cjpayne

    cjpayne Well-Known Member

    Joined:
    Jul 26, 2012
    Messages:
    5,422
    Likes Received:
    449
    I was kinda wondering what the measurement is when it's put on the "T".
     
  14. racerx

    racerx Administrator
    Staff Member Administrator

    Joined:
    Jul 10, 2005
    Messages:
    12,639
    Likes Received:
    1,076
    That's really healthy, imho. Honda claimed 47mph top speed for these bikes, that's rarely been seen.

    FYI, with stock gearing & OEM Trailwings, 55mph requires the motor to spin 11,500rpm(!). That's right at the ragged edge of clutch basket failure. Your speedo might possibly be a little generous at the top but, in my experience, the earlier round speedos (K1/HK1) tend to get stingy above ~45mph.

    You do realize that this calls into question the need for a 20mm carb, for anything below ~90cc.;)
     
  15. fatcaaat

    fatcaaat Well-Known Member

    Joined:
    Aug 1, 2007
    Messages:
    1,671
    Likes Received:
    154
    I haven't felt the need to confirm the speed with GPS, but it might be a good exercise to do so. Racerx, to your point, calls into the question the 20mm carb...there is a difference still. The top speed is the top speed...that's power and gear restricted. And a carb change will have little influence on that. However, my experience does tell me that a carb does have impact on how fast you get to that speed. I have a lot of examples that dicate this...from stock engine/carb to VM20, VM26 vs MJN22, 17mm trx90 carb to VM20, and most recently a VM26 vs a chinese pumper 30mm. I have even experienced this on my Jeep Scrambler. I had the original carb on it and it ran fine, but when I added a new 2bbl webber as a direct replacement, the response, power, and acceleration were night and day and didn't consume much more fuel.
     
    kirrbby likes this.
  16. stangconv21

    stangconv21 New Member

    Joined:
    Aug 1, 2018
    Messages:
    8
    Likes Received:
    0
    fatcaaat any chance you can do a video on how you set the points like that? I have to do mine here pretty soon.
     

Share This Page