Sophomore Rider
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Joined: May 6, 2013 8:28:08 GMT -5
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Post by keltex78 on May 2, 2014 8:57:28 GMT -5
All,
As posted elsewhere here, the motor on my 244cc Jonway YY250T just decided to start shredding itself internally. What started out as metal flakes and dust in the oil has turned into metal shards.
I've started the tear-down process to remove the motor and repair if possible. While I have the motor out, would I benefit by the installation of a BBK? I've not researched those closely, but I've seen a couple that claim to upgrade from 244cc>260cc (not much difference). I've read on other forums (mostly buggy related) and the general consensus seems to be that the CN250 motor is best kept stock.
What does everyone else here think?
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Post by flyangler on May 2, 2014 9:09:36 GMT -5
I believe with the 244cc motor , you can get very close to 300cc and that would be a considerable upgrade . If you have the time go to the old Scoot Dawg forum and you will find info on this.
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Post by alleyoop on May 2, 2014 13:19:03 GMT -5
Here is what a guy by the name of JACINE put up on the old SD on some mods that can be done to the 244cc Helix motor.
Alleyoop
1: Malossi aftermarket pulley and weights
2: hi-performance reground cam and performance valve springs
3: shaved head to give 10.75 compression
4: 34 mm carb Mikuni/ the 30cc Keihein will do well or a larger Keihin
unit.
5: Ported and polished head.
6: +4 degrees over stock ignition advance
the following parts are common to the Helix (CN250) and the Elite CH250:
Malossi part # Description
2911326 SPRINGS SET x ORIGINAL CLUTCH
5111262 VARIATOR MULTIVAR 2000
61 9584 TOOTHED BELT KEVLAR
66 9915.D0 6 HTROLL D.23,8x18 gr23
66 9917.C0 6 HTROLL D.23 x18 gr14
The stock cam was reground from "webcam" in California 909-369-5144
approx $130.00 + price of a new cam from Honda (approx $45.00 cheap).
Lift on the stock cam is .310, on the performance cam .365.
Stock Cam (lift .310) Reground cam (lift .365) intake duration 220
degrees 250 degrees exhaust duration 220 degrees 250 degrees
intake open 0- TDC 15 deg BTDC intake closed 40 deg ABDC 55 deg
ABDC exhaust open 35 deg BBDC 50 deg BBDC exhaust closed 5 deg ATDC 20 deg ATDC
The springs can either be purchased or custom made. Inner and outer springs, shortened valve guides and titanium collars for $130.00 (Regrind was $110.00)
3: shaved head 2 mm to give 10.75:1 compression
4: 30-34 mm carb You can even use an older 34mm Mikuni from a junk yard. Stay with the CV type for smoothness .
5: Port and polished head. Ported and polished considerably. The
exhaust seems to be very restrictive.
6: Advanced ignition +4 degrees Weld a tab on the flywheel for the
electronic Pickup to "hit it sooner"( 3/16" long)
Results: bottom end makes wheelies and burn outs and top speed went from 70 or so to 87+.
NEXT:
Next add the Malossi popup piston that adds compression to 11:1
Expected mph increase should be 8-12 or add 295cc overbore and 11.5 :1 piston for 10-15 mph more.
Italian scooter mag (issue 68) had the following modifications to the
Honda helix engine.
" ...the 2 Roman technicians have worked on obtaining a compromise between performance and reliability, as befits a road going vehicle, and it's based on the standard Honda. The stroke is the same, but the cylinder has been lowered to obtain a C.R. of 11.5 to 1 (geometric value). The piston crown has also been turned down to make sure the squish band is right for the lowered cylinder, and two pockets have been milled into the crown to avoid
valve interference at high revs due to the new camshaft. The piston - cylinder clearance is in the order of 0.02 mm...
The original cam has been reprofiled. Lift is up by 1.2 mm for both
valves, and the phase diagram for this asymmetric cam is 55 - 20, 28 - 53 (degrees), with a value for the 'lobe centre' of 113 deg. obtained with a valve play of 0.2 mm for both inlet and exhaust, as opposed to the 0.08 and 0.13 of the original.
The head, which is supplied already assembled, is the part that has been most heavily modified in order to optimise gas flow... The valve seats have been enlarged by 1.5 mm, the tracts enlarged and smoothed (with fine sanding). The exhaust collector has been enlarged to aid evacuation of spend gas with the larger tracts. The valve guides have also been modified/replaced in line with the other modifications, and the gaskets are also new, including the head gasket, made especially for this kit by Spesso.
The chain tensioner support has also been modified to ensure more
accurate tensioning with the lowered cylinder. 2 main jets (for the standard carb) are also supplied with the kit, for summer/winter use, to optimise carburation..."
Max power is said to be 28 bhp and max revs 1500 above the
standard motor ( they claim reliability is unaffected). The tester rode a bike with the 11" Hexagon rear wheel and a Bettella Varimax variator, which comes with a 'revolutionary' spring slider to 'aid spring operation' (that's a thrust bearing to the rest of us). The bike is also fitted with Hagon shocks <end>
And finally some figures for those trying to figure what engine
displacement can be gotten from what bore size. The stock CH/CN250 is 244 cc with a 72 mm bore and 60 mm stroke.
The piston sizes listed are availiable from wiseco <http://www.pro-flo.com/> (piston manufacturers)
and are designed for other Honda (XR250/TRX300/) 2 valve SOHC motors with a 17 mm wristpin, the same as the Elite 250. Wiseco offers "high compression" pistons for those who have increased duration cams that want to keep the stock compression ratio. Increasing the bore will automatically increase cylinder pressure if the stock cam is used or restore cylinder pressure if a modified duration camshaft is used. As mentioned Malossi offers a 77 mm big bore "Foresight" piston.
Cylinder Bore Displacement
(with stock 60 mm stroke) 73mm 251cc 75mm 265cc 77mm 279cc
78mm 286cc 80mm 302cc
According to my friend, it can be done.
I don't have the expertise in the boring and cylinder head adjustments and advancing the spark. But apparently it;s not magic. So it can be done. Even if the 28 bhp is off, and increase of 8 to 10 hp can be expected. Interesting>>>>
gokartsusa.com/300pistonkitcn250.aspx
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Post by flyangler on May 2, 2014 13:54:06 GMT -5
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Post by rockynv on May 6, 2014 4:12:53 GMT -5
It depends on how the main bearings look when you tear it down. You may have had an oil pump failure and even if you didn't you probably should replace the oil pump anyway since it has most likely injested some of the metal. If the engine has not sustained too much main bearing damage you might get away with standard however if the mains are too far gone and the main bearings are cast in place then the engine is most likely history unless you can find a shop to bore out them out and press in sleeves. If the main bearings sustained zero damage then you might get away with a BBK but would still need to confirm the crankpin did not get scored by the metal shards then requiring a new crankshaft and connecting rod.
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..BAD TO THE BONE..
