I am working on a 50cc Cafe, power is from a MB5, 6 speed. Stock bare wheels weigh 13lb rear and 10lb front. I have built new wheels with weigts of rear 9lb and front 3.4lb. Any ideas as to what kind of useable power improvements I can expect. A Stock MB5 weighs right at 175lbs dry, I have my tatol weight down to 136lb. Thanks for any input
Power gain ?
- Thread starter stanyon
- Start date
Swapmeet Louie
Put some metal in yer eye....
You'll find the most power for the buck in a after market chamber/pipe. And ya, you should feel an increase in acceleration with lower rotating mass. Also, if that motor is a reed valve unit. After market reeds would make a difference too.
Those bikes have quite a cult status. A common upgrade seems to be a 100cc motor, but IIRC the exhaust has a restrictor that's a PIA to remove. M/C Mechanics did a project MB5 many years ago. I may have a copy lying around in my "filing system".
Google MB5 and you'll find a couple of useful sites out there.
Google MB5 and you'll find a couple of useful sites out there.
Swapmeet Louie said:
+1 to this. 2 strokes love a good expansion chamber. Any changes you make to the engine will require a rework to the exhaust to get the most out of it.
Aside from the usual stoke and bore changes, look at getting the transfer ports worked on. Also, not sure if that bike already has one or if there is an aftermarket solution, but look into electronic ignitions as well. At a certain point in the RPM range you actually want timing to start being retarded again and that's best accomplished with some well-tuned electronics.
And some recommended reading material:
http://www.amazon.com/Two-Stroke-Performance-Tuning-Bell/dp/1859606199/ref=sr_1_1?ie=UTF8&qid=1296754272&sr=8-1
http://www.amazon.com/Two-Stroke-Performance-Tuning-Bell/dp/1859606199/ref=sr_1_1?ie=UTF8&qid=1296754272&sr=8-1
boingk
Standard gearhead.
I know a bit about these. Do you know if it is a restricted model or not? A restricted model will have very mild porting and the expansion chamber will usually be bypassed with an internal solid header pipe. You can easily remedy these, and the expension chamber is worth doing because it is actually quite a good unit.
The carburettor is (if I recall correctly) a 14mm unit - you could upsize this to around an 18mm unit for a bit more topend gain, or even just bore it out. To do this you will need to see what the diameter of the slide is, and bore the carburettor to about 1~1.5mm less than that... or the slide will not function correctly. I believe 17mm is possible. You will most likely also need to upgrade the mainjet to one or two sizes over as soon as you do this - plug chops will tell you if you're close. After mainjet is correct, see if raising the needle height one notch helps.
You can advance the timing (if not ECU controlled) by opening the points gap. I imagine it would be running around a 1.8~2mm gap at TDC, and you could open this out to maybe 2.25~2.5mm at the maximum. Try in 0.1mm stages once past 2mm. This will give you more power up top but at the expense of lower rpm running.... which, lets face it, there is bugger all of to start with.
The compression ratio can be raised by machining the head. I would recommend 0.5~1mm off the head to start with, and check that you will not run into problems with the piston contacting either the head itself or the sparkplug. A more finite way of tuning is to get a squish band of around 0.8mm~1.0mm - this will provide optimum characteristics for most small stroker engines. The squish band is the shallow part around the outside of the head, and the clearance to that when the piston is at TDC is what you measure. Stock it will most likely be something around 1.2~1.8mm - possibly more.
With porting, try taking a half millimeter off the top of the exhaust port. This will allow a greater pressurised charge to enter via the exhaust port, giving you a boost in topend power when the engine is 'on the pipe'. I would not go further with this or you may risk holing a piston... especially if you don't check the jetting afterwards.
If you get all that reaosnably simple stuff done you might be surprised at the results. The character of the engine will change as well, becoming a fair bit more topend oriented than stock. Any more questions feel free to ask.
Cheers - boingk
EDIT: Here you go! Because these are restricted by law in the UK for 16 year old's (or used to be) there is some tuning info floating about if you know where to look. A few choice keywords into google picked up this little gem:
http://www.mb5.co.uk/14.html
Enjoy!
The carburettor is (if I recall correctly) a 14mm unit - you could upsize this to around an 18mm unit for a bit more topend gain, or even just bore it out. To do this you will need to see what the diameter of the slide is, and bore the carburettor to about 1~1.5mm less than that... or the slide will not function correctly. I believe 17mm is possible. You will most likely also need to upgrade the mainjet to one or two sizes over as soon as you do this - plug chops will tell you if you're close. After mainjet is correct, see if raising the needle height one notch helps.
You can advance the timing (if not ECU controlled) by opening the points gap. I imagine it would be running around a 1.8~2mm gap at TDC, and you could open this out to maybe 2.25~2.5mm at the maximum. Try in 0.1mm stages once past 2mm. This will give you more power up top but at the expense of lower rpm running.... which, lets face it, there is bugger all of to start with.
The compression ratio can be raised by machining the head. I would recommend 0.5~1mm off the head to start with, and check that you will not run into problems with the piston contacting either the head itself or the sparkplug. A more finite way of tuning is to get a squish band of around 0.8mm~1.0mm - this will provide optimum characteristics for most small stroker engines. The squish band is the shallow part around the outside of the head, and the clearance to that when the piston is at TDC is what you measure. Stock it will most likely be something around 1.2~1.8mm - possibly more.
With porting, try taking a half millimeter off the top of the exhaust port. This will allow a greater pressurised charge to enter via the exhaust port, giving you a boost in topend power when the engine is 'on the pipe'. I would not go further with this or you may risk holing a piston... especially if you don't check the jetting afterwards.
If you get all that reaosnably simple stuff done you might be surprised at the results. The character of the engine will change as well, becoming a fair bit more topend oriented than stock. Any more questions feel free to ask.
Cheers - boingk
EDIT: Here you go! Because these are restricted by law in the UK for 16 year old's (or used to be) there is some tuning info floating about if you know where to look. A few choice keywords into google picked up this little gem:
http://www.mb5.co.uk/14.html
Enjoy!
crazypj
Split personality, I fake being smart
boingk said:
I had quite a lot of practice modifying restricted bikes in late 70's, early/mid 80's Britain
The UK model MB5 only makes around 2.1bhp stock
They are restricted really bad, from the airbox all the way through to end of exhaust.
Porting cylinder doesn't really do much good until you cut up airbox and fit decent carb
It's a pretty good series of articles, would have got a lot more power by doing some serious work on airbox.
If you have unrestricted version, it makes around 6.5bhp
The Suzuki RG50 and Yamaha RD50 were a lot easier to 'fix' but the Fizzer was always the best (I clocked a slightly modified Yamaha FS1E at 62mph)
Kawasaki 50 was 'fast' when modified but everything on it wore out real quick, from wheel bearings to shock seals, they were junk in a year or less.
Disc valve motors were always faster than reed valve motors but anything could be (and was) modified, (I once built a 120mph Suzuki RG125 Gamma, admittedly, rider was only 5ft 4 and weighed around 90lbs ;D )
FS1E for the win. 
Also, stanyon, if you're serious about power and are looking to put some money into your ride, contact www.rexcauntracing.com in the UK. They're mainly a BSA specialist, but they deal with two-strokes of all types and have been in the racing business for decades. Tell them Matt sent you.
Also, stanyon, if you're serious about power and are looking to put some money into your ride, contact www.rexcauntracing.com in the UK. They're mainly a BSA specialist, but they deal with two-strokes of all types and have been in the racing business for decades. Tell them Matt sent you.