Rev-limiter on Chinese "Happy Time" 80cc?

Discussion in '2-Stroke Engines' started by MMedlen, Aug 6, 2008.

  1. MMedlen

    MMedlen New Member

    I installed the 80cc motor kit on a nearly new mountain bike. Everything is straight, well lubed and runs true. After about 100 miles I decided to test the top speed. Up to 25mph everythings great. Plenty of throttle left and when fully open engine sounds like it is going to gain speed rapidly. After about 27mph the engine acts like it is being controlled or held back by some kind of restrictive device. My son in law has the same engine kit and it responds the same way. I have installed a new carbon plug wire, plug and carb. o-ring. I modified the muffler by removing baffles and cutting the small exhaust pipe to about 3 inches. I would appreciate any ideas or suggestions to engine can continue reving. Thanks, Mike

  2. Simonator

    Simonator Guest

    I think alot of it has to do with the size of the intake manifold. You can buy larger intakes, but they are hard to find.
  3. stude13

    stude13 Active Member

    search, carb tuning or rpms
  4. Dude. Don't. 27 mph,44 tooth sprocket I would say that's redline,dawg.
    You don't want to go past that "rev point" or you'll start breaking parts.
  5. so what kind of engine is this guys? sounds like its not so good if you can over rev and break parts
  6. Simonator

    Simonator Guest

    True. after that speed the engine starts to vibrate like crazy. It can't be a good thing...
  7. you should not be able to over rev a two stroke and start breaking your engine cause it cant get over 30 sounds like its either not flowing enough fuel/air or is running rich
  8. graucho

    graucho Active Member

    Just an idea, If you need more speed, buy a kit from these guys. You can shift through your normal derailer gears ive heard.
    Otherwise you will blow it up. If you need proof, go out for a ride with it wide open for 2 miles and come back to the post with your results. LOL

    Or If your that set on the rrrr-ee-vvvvv, get one of these engines that can handle the RPM's :cool:
  9. Skyliner70cc

    Skyliner70cc Active Member

    The rev limiter is the carb's inability to provide enough fuel to the engine at those engine speeds. 27mph is not too fast on a 44 tooth, I've done that on a 48 tooth sprocket with no problems.

    Some adjust their float to allow more fuel, some adjust or bore out their main jet to a slightly larger size, and some do both.
  10. relaxxx

    relaxxx Member

    Like I've said before, MY HT is literally screaming at 26mph with 44 tooth. Most of that is the gearbox howling. I'd say 26-27 is definitely the red line with a 44 tooth and my particular type of engine.

    Now I've held it at 26mph for quite a long time (over 5 minutes). On a long stretch I usually get up to max speed, listen to it scream for a minute then release the clutch and throttle and coast down to 15mph and do it all over again. Over, and over for about 90 minutes strait was my longest continuous run so far.

    I don't think Max speed will kill the HT in a few minutes but It's definitely not good for the long term. While 26MPH may be my top seed, it feels like there is next to no load on this motor. I know this motor can handle way more torque as I've reached top speed even going up a slight incline. The motor even sounds better going up hill. That's why I'm installing a 36 tooth. I should be able to do 26 mph no problem without making the motor scream and increase my MPG. Any tweaks that just increase RPM will only shorten the lifespan of your motor. For speed, smaller sprocket is the only way to go.
    Last edited: Aug 7, 2008
  11. Skyliner70cc

    Skyliner70cc Active Member

    Max speed won't hurt a thing provided the bearings can take it. With a bushing engine, I never had a problem going full throttle down a steep hill and running full throttle ALL day long.

    You gotta give these HT engines a break, they aren't that bad. There is no reason why you couldn't run it at speeds as high as 8,000 rpm. The newer engines are much smoother and better balances than the older ones. I've got 2 stroke engines that are in the 25-50 cc range that I prop (they are on model airplanes) to spin 8500-9000 rpm at max throttle.
  12. HERPER

    HERPER Member

    my engine, an 80cc ht motor, when i go to my girlfriends house, its about a 10 mile ride and the whole time if not majority of the time is going about 20 or 22 mph. the max ive gotten is 30, and it didnt seem to be hurting too bad. maybe adjusting your throttle and ratio and mix could make your engine run better.

    and def check all your parts before and after
  13. Ozi

    Ozi Member

    If you have only done 100 miles

    I could be wrong but I think Its called 'not run in yet' mine did the same

    at about 150-180 mi that bogging down at wot dissapeared.

  14. MMedlen

    MMedlen New Member

    I just got a response from Powerking and they indicate that their 80cc kit actually measures 69.3cc. This may be of help to those of you needing the info to get a good fit on a boost bottle.
  15. Ghost0

    Ghost0 Guest

    Just a little background on these engines. The 50cc engines since they are smaller have been timed to produce good horse power over 6K rpm. The larger 70cc engines were designed to provide more torque so are timed to produce best power at a lower rpm. The "rev limiting" effect you are feeling is due to the port timing. The port timing on the 50cc is designed to Rev higher and the 70cc to rev lower. So your options are to change the port timing or to use the higher torque of the stock 70cc engine by gearing it taller.
  16. MMedlen

    MMedlen New Member

    Thanks for the info, I was not aware of the timing differences and that will be useful!
  17. fetor56

    fetor56 Guest

    100 miles is not enough as a run-in period to gain consistency in these engines.....see how u go after atleast 200 miles.
  18. try1897

    try1897 Guest

    Hi, I went from a kings 70 / 80 cc to a Dax 65 and installed a 36 tooth . the 65 has less hp but it hums right along at 25 and purs like a kitten. runs sweet. It will do 30 or better but I don't want to blow it up so I tend to keep it down to 25 and cruse. It's gone a long way .Many many many miles and still sounds good idels well runs smooth and has had no knock on wood problems so far. Its going on a year and loving life.. T
  19. wildemere

    wildemere Member

    A 26" wheel at 30mph needs 7000rpm when using a 44t sprocket

    6000rpm is 25.7 mph
  20. arceeguy

    arceeguy Active Member

    VTBikeman analyzed the CDI unit and found that it will produce spark up to about 30,000 RPM - so there is no rev limiter built into the CDI.

    Most of these engines now have roller bearings at both rod ends, and while this is mostly a good thing, it does make the engine less tolerant to over-revving. Once you hit the limit, the cage that holds the rollers breaks from the pounding and metal pieces start circulating through the crankcase. Bits of metal get caught in the transfer and exhaust ports and everything slows to a grinding halt. A bushing engine can be revved higher for longer without this type of failure, but the bushings are not particularly long lived in a two stroke because of the scant lubrication provided by the fuel/oil mix. The roller bearings need much less lubrication to survive.