79cc Predator 4-stroke with Q-maitc on 29" Macargi Fatal Love

Discussion in 'Photos & Bicycle Builds' started by KCvale, Oct 24, 2013.

  1. KCvale

    KCvale Motorized Bicycle Vendor

    This build was originally going to be a 212cc Predator with GTC TC1002P torque converter but when all the parts were here it looked to be way to big so I exchanged the 212 for 79cc.

    I went with Quentons special Q-matic for the 79cc Pred from E-Z Motoribikes.
    The engine shaft is a bit too long for the thin profile cover (which is good) so I just cut a hole for it but you could cut 1/2" off the end of the shaft.



    The bike is a 29" Macargi Fatal Love I put a pair of caliper brakes on to help the coaster brake and of course HD tubes and liners in each wheel.

    The engine mount base and wide cranks were left over from a 49cc HS jackshaft shift kit build. By leaving the engines mounting plate off the engine it mounted to the plates front slots but we had to drill rear mount holes.
    Note we tack welded the plate joints on the underside of the plate as we didn't any adjustment after it was fit.

    The exhaust was removed and we fabed a new pipe using the flange on the stock exhaust welded to a spare section of SBP expansion chamber pipe and then a 2-stroke Cat muffler with spiffy heat shield.

    If that pipe proves to be too restrictive I'll pop the end cap, pull out the Cat Plenum and make the end covers hole a little bigger.


    We redrilled a 56T Maniac Maniac #41 chain sprocket for a 9 hole rag joint fit.

    The gas tank was removed and the linkage from the manual throttle/governor, then we fashioned an adjustable throttle cable end to the engine.
    The normal pull for a throttle is ~ 1.5" and 1/3 throttle turn, the throw of the carbs throttle tab would have made the throttle about 1/8 turn from idle to wide open so we extended the throw by adding an 1.5" tab to the carb top.
    This worked out great! It gives the full 1/3 throttle twist response and lined it up perfectly with the cable stay.

    Then we just used an old NT carb spring for a very nice 'snap back' throttle action.


    The gas tank is 1.2 gallon Whizzer tank that was mounted with a combination of 1.5" muffler clamp stays, frame tube brackets, long bolts and spacers.

    It has a drive side chain tensioner for future adjustment but with use of a #41 chain 1/2 link it is darn near direct drive.

    Last but not least the hard rubber grip was removed from the throttle barrel and BMX foam grips with spiffy end caps installed as well as nice new brake handles from a bike that got a dual pull for the front and rear calipers.

    We still have some odds and ends to do today like put the pedal chain guard on then get some test riding to decide about the exhaust but that's easy stuff.

    If you are an experienced builder with welding skills and lots of spare parts around this project isn't too bad (~60 man hours).

    BUT, if you are an MB rookie or just don't have the tools, skills, and misc parts I highly recommend you just start with a 49cc HS with 4G tranny complete MB kit for ~$360 as just the 79cc Pred and Q-matic were $400 and you still need a mount plate, rear sprocket, wide pedal crank set, gas tank, custom exhaust and throttle to make it go.

  2. zippinaround

    zippinaround Active Member

    looks sweet! nice work!
  3. KCvale

    KCvale Motorized Bicycle Vendor

    Thanks. A lot of work but it turned out nice.
  4. rasmith3530

    rasmith3530 New Member

    Wow KC, that is one sweet looking build. A tip of the hat to you!
  5. KCvale

    KCvale Motorized Bicycle Vendor

    Thanks rasmith3530, that was first 79cc 4-stroke but I have built quite a few 49cc 4-strokers.
  6. chainmaker

    chainmaker Member

    Thats a nice looking bike, that was my first 4 stroke set up and it is pretty solid.
  7. KCvale

    KCvale Motorized Bicycle Vendor

    Since this engine is meant for something like a generator it has a manual throttle and a governor that limits max RPM to about what? 5400 RPM?

    We don't need no stinking governors, you just have to be smart and not try to run it at 9800 RPM's for too long and burn it up ;-}

    Nothing I found on the net was a simple solution to bypass all the stock throttle stuff and hook up a 1 1/2" pull 1/3 turn throttle cable so I came up with my own which is my favorite part of builds like this.

    I simply removed the linkage and spring from the stock system so all I had was the throttle actuation tab on it.

    That left finding a way to attach the throttle cable and there is a tab right there on the stock engine.


    With the pull of the throttle cable and the range of motion for the carb arm hooked up direct would mean about a 1/10th of a turn of throttle from idle to wide open and that's no good.

    The solution for full 1/3 throttle action was leverage on the carb arm so we used the 2 holes on it attach our own longer plate and our own spring as well as drilling out the engine mounted tab and installing an adjustable cable stay for the throttle cable.


    This the system at idle with the stock air cleaner on.


    You don't even see a spring and the engines idle screw sets idle.

    This is the throttle full pull 1.5" 1/3 turn which is full throttle on the carb so perfect.


    It took some trial error but should be cheap if not free to duplicate for any of you future builders.
    BigBlue likes this.
  8. keatonx

    keatonx Member

    I read somewhere that if you ungovern the engine and leave the governor system loose to flop around, the governor mechanism in your engine can fly apart. Which makes sense because on my 140cc 2 stroke lawnmower bike there were little balls that flung out against a washer for the governor, and if the carb linkage was unhooked the balls were barely held from rolling out of their holes! When I removed the governor on that motor I put the governor arm in what would have been the throttle-closed position, and tied it to the frame in that position with some metal wire.

    I know this is an old thread but I figured you might want to know this.
  9. keatonx

    keatonx Member

    Personally I wouldn't ungovern a single speed or CVT, because If
    (I mean when) I get the "need for speed" the only way to go faster is to overrev the engine. And I would probably end up revving all up and down the rev range trying to find a "safe rpm" and never make up my mind!

    My lawnmower bike had 5 gears. I only removed the governor so I could run out the gears more without hitting a "ceiling", and because acceleration with a governor seems awkward to me. The bike basically has 2 powerbands. It starts making full torque right at the speed where it's about to stall (around 500rpm, no joke), and the power's pretty linear up to maybe 8000rpm where it maxxes out.

    Did I mention that's a really clean looking build! Something my bikes lack:jester:
  10. birdmannn101

    birdmannn101 Member

    Nice build KCvale ... Any problems running the stock carb? Mine took 3-4 pulls with the choke on to start initially. Now that it's cold it doesn't start but that could probably be a fouled plug since the fuel flow looks good through the filter. Too cold right now. Just heard others asking about the HF 79cc carb.
  11. professor

    professor Active Member

    I opened up the main jet on my 79 a couple of thousandths and it runs better.
    All these motors are too lean - govt interference again.

    By the way, I do not know the limits of the stock flywheel.
    With the governor intact, it will never explode. On mine, I loosened the stock lever pivot nut (that attaches a throttle cable to) tweaked the gov a little higher and went with it.

    One of the nice benefits of a gov is under increased load- it automatically compensates and gives it more umph.
  12. KCvale

    KCvale Motorized Bicycle Vendor

    That was a customer build I did in summer so no problems starting it, here it is the end of January with mid 60 lows and mid 70's for a high ;-}

    I don't recall what I did with the governor but it it wasn't just 'flopping around'.

    Thanks for the Kudos guys.
  13. birdmannn101

    birdmannn101 Member

    Professor, for us older folks "couple of thousandths" means how many portions of a turn are we talking about?
  14. keatonx

    keatonx Member

    Ya you should be fine, and a flywheel explosion is pretty rare. It's just if it does happen it could easily kill you

    The rod's more of a problem, I'll take a wild guess and say that anything under 4000rpm is totally safe for continuous use
  15. Robie Osborne

    Robie Osborne New Member

    Looks nice! How fast does it go? And what's the MPG like?
  16. KCvale

    KCvale Motorized Bicycle Vendor

    This bike was like 30 or 40 builds ago and it didn't have a speedometer but well over 30 MPH with those big 29" wheels.
    The MPG wouldn't be as good as a 49cc shifter but over 60 MPG I would think.