Centrifugal centrifugal vs manual clutch engine top speed

Discussion in 'Transmission / Drivetrain' started by Heathen, Jun 2, 2009.

  1. Heathen

    Heathen Member

    Hi guys!
    I have been using a manual clutch 70cc ht engine for a few years now and it just keeps on going, so just for fun i decided that it would be fun to buy a 70c centrifugal clutch engine with pull-start to use on days when i feel to lazy to use a clutch lever and pedel-start :whistling:.

    got it mounted on a new bike and it runs but i'm not happy with it's performance. could it be that these engines with centrifugal clutch have a much lower top speed/rpm then a manual clutch engine???

    the old engine on a much heavier bike does easy 50kph (31mph) but the new one struggles to get upp to 35kph (21mph). it also drinks alot more fuel, old one(manual) does 0.18-0.20 liters per 10km (0.047-0.055mpg) whilst the new one(centrifugal) does 0.25-0.30 liters per 10km(0.065-0.080mpg).

    I have the needle lowered as much as i can in the carb and it still gets abit to much fuel so i thought bad carb. switched it out for an other one with same result. ok checked electric if it gets bad spark, and yes a little weak.
    changed spark, same result, so switched cdi no better spark.
    now i have changed to whole electrical system with no improvement.

    I've checked for air-leaks in the intake ans sealed the carb/intake manifold good with flench sealant and o-rings so no air-leaks there.
    i have even tried an other exhaust system to see if it was a back pressure issue nothing seams to help.:confused:

    Anyone having any thoughts???
    Last edited: Jun 2, 2009

  2. BSA

    BSA Guest

    Well you need to gove your new kit a chance to break in. How many miles have you done?

  3. Heathen

    Heathen Member

    well not sure about milage but i'm on my 8 or 9 tank of fuel so i would guess 600+ km (370+ miles).

    i did not have a speedo for the first 3 tanks or so, and it felt like it was going faster then, but now with the speedo on i've only been able to push it up to 35 kph.

    never had this problem with any of the manual clutched engines i put into bikes, yes i have made a few for myself and friends over the last 3-4 years.
  4. solitus3989

    solitus3989 Member

    Did you gap the centrifugal engine's spark plug? Thats what i always forget to do after i replace the little chinese spark plug...
  5. i have a 48cc dualstart starfire gt-2 grubee 2-stroke in frame w 44t sprocket on 26" schwninn delmar cruiser. this engine has a manual and centrifugal clutch and can be peddal or pull started. i only need to pull clutch in to start it then the centrifugal clutch takes over,i can still pull in the man clutch if needed. i am getting 180-200mpg at 20-25mph cruising and topspeed 32-35mph.all stock except i have a ram air intake pipe installed.engine kit bought from gasbike.net for $186 and bike from walmart $99.
    and it goes up hills with no problem.
  6. Heathen

    Heathen Member

    ok so after much trouble shooting it is now running a little better. switched out the carb for a third one and that made it runs smoother and it is a lot stronger but i only manage to get it up to about 40 kph (25 mph) after 35 kph (20-21 mph) it is hesitant to rev any further.
    i have not yet checked gas millage so i dont know if it still is thirsty, but it almost feels like it is running a little lean this time.

    I'm gonna play with the carb and try an other type exhaust to see if that helps.

    @ cabinfever1977:
    Nice to hear that it can go a bit faster if properly tuned, i would like to be able to cruise at 20-25 mph too as all my moped friends seam to like that speed on their "weight compensated" mopeds. in Sweden old mopeds was only made to do 30 kph (18 mph) with 1 hp by law, but as we get older we gain a bit of weight so some tuning is necessary to give those old mopeds a chance to handle the extra load :)

    // Heathen
    Last edited: Jun 6, 2009
  7. Heathen

    Heathen Member

    resurrecting my thread as i still have problems.

    I have gotten it to run abit better then before by switching out the carb but it is still not good by any means. it drinks about 0.35-0.40L / 10km of fuel wich means i only can go about 50km before running out of fuel(with my old engine i can almost do twice that)

    it wont rev up properly it seams to bog down at 35-38km/h only downhill will it rev a little more not much though. and upphill it has no power what so ever have to help it along by peddling as soon as it sees an incline.

    is my only choice to rejet the carb or get a better replacement carb?
    is there any jets that fit these cheap carbs?
  8. KR4WM

    KR4WM New Member

    The carb that came with my bike allows me to move an "E" clip which changes the fuel/air mixture. There is no adjustment on the exterior of the carb. I have to unscrew and remove the carb slide, remove the "E" clip, and reset it to one of three positions, then reassemble and see how it runs.

    One way to see if you're too rich or lean is to do a nice, long, high-speed run (several blocks), and kill the ignition while the engine is still running at top RPM. Examine the spark plug. Deposits should be brownish/orange colored. If they're white you're too lean. If black or dark you're too rich.

    Read the docs on your particular carb for additional info about how to richen or lean out the mixture.