Elite with 85cc SK50 motor

Modifications to a 1996 SK50 Honda scooter motor. Parts from VT cycle in Hawaii are:

  • 85cc FM 51mm cylinder kit and KOSO head
  • CT Big bore "dual reed" manifold
  • 28mm OKO carb, throttle cable and POP airfilter
  • Hi-Speed pulley kit
  • 15/30 tooth gear kit (stock is 12/42 tooth). Increases top speed 
  • R-1 exhaust for large bore cylinders to 100cc
  • drive pulley 18 pack roller weight kit
  • driven pulley compression springs (1000 and 1079)

Some of the items were "on sale" at the time I purchased them and total cost was $525  (2003 prices)
SK50 motor with big bore kit, 28mm carb and dual reed manifold

    For the most part this was a "bolt -on" operation with substitution of  aftermarket parts for the stock ones. I did take the rear gear housing to a machine shop to have the old clutch shaft removed and the new 15 tooth shaft installed. I also increased the size of the rear boost port opening in the crankcase as the matching port in the cylinder was much larger.  The cylinders ports were "knife edged" and I bevelled the edges with fine emery paper so the rings had a better chance of passing the ports without inflicting damage. It may not be required but I do the same on all freshly bored cylinders. That huge carburetor and the offset of the manifold may cause some clearance issues with the body work.

    I'm going to use a 1991 SA50 "Elite" scooter as a testbed until I decide what scooter this motor will end up powering. Shown at right is the gigantic R-1 muffler which has quite a crackle to it. It doesn't look too different from the stock one but the inlet size is much larger.

    I'll use petroleum based two stroke oil to break the rings in and then switch to synthetic. I'll probably use a combination of the stock oil pump as well as oil in the fuel.  Stay tuned for more pics once the bodywork is mounted and the road test begins.  I want to thank Steve and Keoni at VT cycle for helping with the parts selection and recommendations.

1996 SK50 Honda motor in 1991 Elite frame

Some notes on fitting the 1996 SK50 motor to the older 1991 SA50

1.     The motor has a 10mm sized bushing in the engine mount whereas the SA50 motor mounting bolt was 8mm. After rummaging around the shop I found a Helix cylinder alignment dowel is 10mm outside diameter, 8mm inside diameter and is the right length for the motor bushings. Slip the alignment dowels in the engine bushings and the mounting bolt fits right inside the dowel.

2.     The cylinder has a recess for a round exhaust gasket. The stock SK 50 gasket is too small but a 1983/84 Aero NH80 gasket was just about right after filing the inside to make it larger.

3.    The spark plug on the SK50 is at an angle towards the rear of the scooter whereas the SA50 cylinder places the spark plug straight up. Two modifications were necessary to accomodate the different spark plug position. First was to trim away some of the tray that sits below the gas and oil tank and acts as a mudguard. A section was taken  out of the tray to allow the spark plug and cap to pivot without coming into contact with the tray. The rubber mudguard strip was also disconnected from the tray. The spark plug lead should also be checked to ensure its long enough. The coil may have to be re-mounted to ensure the lead is long enough taking into account the amount of engine travel..

4.     I'd originally intended to use the stock throttle cable but it was permanently attached to the carburetor. The 28mm OKO carb came with an adjustable throttle cable so I used it instead. I'll pre-mix the oil and gas but still hook up the oil pump so it feeds oil at its lowest setting. It usually increases oil flow as the throttle is increased. I could have had another cable

Elite LX with 85cc SK50 motor

~May, 2003~

    I've put about 80 km (50 miles) on the motor so far and it drives effortlessly around town. I replaced the stock SK50 spark plug with an NGK BPR6HS and am running a 138 mainjet. I'm using the stock idle jet but may go smaller (I've ordered a #35 keihin slowjet). The temperature is running around 15 deg C and I'm running at sea level. The plug looks light brown with a blackness around the edges. I doubt whether I've cracked it past 1/3 throttle and I'm thinking for the break-in I should have probably not installed the gear kit as I usually like to vary the engine speed on a scooter more on break-in but this scooter would shoot well past double the speed limit without hesitation. I've probably run it up to 70 km/h at most without any strain whatsoever on the motor.

    For those of you who have to maintain 70 km/h in normal city driving I suppose this sounds foolish but I live in a city with many winding roads and there are only a few main roads where theres four or more lanes that run straight. I haven't taken it out on the highway as yet.  The hills are effortless and I'm thinking this would be the ultimate wheelie machine and stoplight warrior with the stock gearing. The exhaust has a bit of a crackle to it when accelerating but is not as noisy as an expansion chamber. The carb emits a WAAAHH sound when the throttle is opened but is fairly quiet when just cruising at a constant speed.

    An airbox is on my list of things to do when I get around to accumulating a bit more aluminum sheet metal. While I'm primarily wanting to reduce noise, I also want to make the scooter useable in the rain. Others who've mounted aftermarket airfilters have said they don't work well in the rain due to all the spray created by the back tire. I still ride my scooters when it rains as they all get used for transportation ( although I tend to use one of my "utility scooters when it rains hard). 

    Anyways thought I'd put in a quick update and will put up more info as time goes on. One parting opinion is that I might go for a smaller carb if this was purely a commuter scooter. VT cycles had both 21mm and 24 mm carb kits for around the same price as the 28 mm I bought. The gear kit is certainly keeping the revs down and is sure to help the gas mileage in spite of the larger cylinder and more efficient intake and exhaust so I've come to think this may be quite a sensible conversion for those who have to do a cylinder rebuild in any case. It certainly outperforms my Aero 80 scooter in every respect.

June 2004

    Hard to believe I've been riding it over a year . I live on the west coast of Canada so am able to ride it year round except for the week that it snows.   I've fitted an aluminum airbox to keep out the rain and keep that airfilter as dry as possible. I've tried a few different exhausts (stock dio, LeoVinci SP3) but the R1 exhaust from VT is the only one that lets it rev up. I've switched over to running 3% Belray synthetic oil pre-mixed with the gas and have disabled the oil pump and removed the oil tank. 

    I'm considering switching the final drive gears back to stock for around town riding. The scooter just coughs and burps below about 40 km/h (25 mph?) and really doesn't begin to sound like its in the powerband until 50 or 60 km/h (30-35 mph?) so its not that much fun to ride in stop and go traffic . Put it on a road with no frequent stoplights and 60 km/h plus speedlimit and it just flies . I get lots of e-mail from people who say their local traffic is in the 40 mph+ range and they need something to keep up. This kit certainly fits the bill for situations like that.   Lotsa fun but  lower gearing  will allow me to ride in the local slow traffic and also make it accelerate like a rocket .  I've also ordered a smaller slow speed jet. Part of the reason for the poor slow speed running is because its running too rich an fuel/air mixture at idle. A smaller carb would provide better throttle response as well for those short bursts of speed necessary in city traffic.  I don't think I've ever had it past 1/4 throttle opening and that gives me about twice the posted speed limit in the city.  Anyways just thought I'd put up a quick post to say the cylinder is very reliable and has lasted quite a few thousand kliks so far.

I picked up a 1997 Honda Dio SK50 scooter with a blown motor so may use this motor as a transplant for that scooter. Just by way of comaprison, the Dio I bought had a 70 cc Eurocylindro aluminum cylinder with chrome bore and a LeoVinci SP3 exhaust. It had stock carb, malossi carbon reed valves  and the previous owner ran it frequently on the highway . He had originally just run it with the SP3 muffler without any problems. He subsequently fitted a number of 70cc cylinders starting with a Malossi but continued to have seized pistons throughout. This is a good example of why  fitting a "performance" part may do more harm than good unless you take into consideration the extra demands on the motor.

     There are quite a few 70cc kits out there and , if you want to keep the stock oil pump arrangement then ensure you take time to jet the carburetor. Also  remove, clean and flush the oil tank/filter/lines  to ensure they're in top operating condition and ensure you're using top quality two stroke "injection" oil.  

    While the cylinder is one of the least expensive "performance" parts, you can't fit it without also taking into account the effects  the increased engine displacement has on the rest of the motor. The oil pump is designed to pump out the correct amount of oil for a 50cc motor and a 12mm carburetor.  The stock carburetor isn't all that big to begin with and fitting a larger cylinder just makes it even worse. If some time had gone into doing plug chops and adjusting the jetting to suit the new cylinder, the engine on the sk50 may have lasted. With the 70cc cylinder it would have increased power considerably so highway speeds could have been attainable by changing the final drive gears to a higher ratio. 

    f you can't afford to adjust the jetting on your current carburetor or  larger carb and don't like the idea of pre-mixing your oil with your gas,  don't buy a larger displacement cylinder.  While the SP3 may have been great on the stock scooter and adequate on a 70cc cylinder, it definitely isn't suited to the larger 85 cc kit.  The aluminum Eurocylindro cylinder is a work of art but a cast iron cylinder like the FM is more "sensible" for day to day driving. 

    While it's easy to buy "bolt-on" performance improvements, the work doesn't stop after they've been "bolted on". Ensure they're broken in properly and the fuel/air mixture is correct for your geographical area and combination of parts.  If you're unfamilar with the process or have a short attention span, you may want to seek outside help.  A local scooter shop that sells performance parts may be of help.   If there's a shop in your area that tunes 2 stroke dirt bikes, ask if they can help out with the final set-up and tuning.   One advantage of the OKO carb is that it's a direct copy of the keihin PWK that's used widely in dirt bikes so jets, needles etc should be easily obtainable (if not in stock) at a dirt bike tuners shop.

Doug S


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