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The following is an edited record of an e-mail exchange between
David and myself about obtaining performance increases from his TGB scooter,
a 125 cc Honda clone . Regardless of the type of four stroke scooter, most
of the issues would remain the same. David's made quite a few changes to
his scooter that would be of interest to most (four stroke) scooterists
seeking increased performance. The most important issue is that tuning
of any scooter can be done whether or not performance parts for that model
are listed in a manufacturers catalogue.
The picture at left is Davids scooter.
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Doug,
I read your web site with interest, especially re tuning the Elite.
I run a Taiwanese 125 cc four stroke, a TGB. It has an horizontally oriented
single, air cooled. As far as I can ascertain, it is the engine from the
CHA125. I'm interested in upping the power a little, and am working on
a couple of things. The engine has a few nice features, like a camshaft
running on ball races a la C90E Cub. I wonder if you have any updates on
the info on the web site? Most of the info and specs I'm getting are from
Monkey bike tuners, I'd dearly love to get some accurate info for the Honda
application of this engine. Any thoughts or comments appreciated.
Regards,
David Miller
In my opinion, increasing the camshaft duration,
fitting a more responsive carb and welding up a proper exhaust header would
do a world of good without stressing the engine.
I especially hate automatic chokes........
....... You mentioned the Honda monkey tuners as having a large resource of information. The same sort of experience exists with the single cylinder Honda 125/185 cc SOHC dirt bikes and odds are good you have a dirt bike enthusiast close to you who has experience with tuning these bikes. They are direct relations to the Honda scooters and the odds are good you could apply their experience directly to your scooter. Bear in mind that they are dirt bikes tuning for gobs of low end torque and you may want more top end power if you're tuning for highway speeds. If on the other hand you like to do wheelies, this may just be the ticket. In any case you'll get an idea of local machine shops and parts suppliers who are familiar with the 125 cc Honda four stroke SOHC motor and its tuning peculiarities. Have you compared the camshaft from a Honda CH125 to yours? The one modification that could noticeably increase performance without turning your scooter into a "long term project" would be to get a camshaft with slightly greater duration.
Gawd what a small stock exhaust pipe. Check
out http://www.kamware.com/free.htm
and download the TopSpeed program
for an idea of what exhaust diameter and length
your scooter needs to breathe properly. I've sold both my CV carbs
and
mufflers; I'm going to replace with a VM26 mikuni
and a muffler welded up like a paper clip shape out of mandrel tubing..
An
equivalent size Dellorto or Amal might be easier
to find in your neck of the woods. Make sure its for a four stroke. With
the
exhaust and new pipe you'd create quite a different
scooter. The thing with the CVs is that they're too small and the venturi
size
is actually quite smaller than listed due to
the butterfly obstruction. They're made for leisurely acceleration but
traffic requires
quite the opposite sort of response.
Doug
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Left:
Sketch of a possible exhaust design for the Honda 125/150 cc four stroke vertical cylinder scooter motor keeping in mind the figures from Top Speed software at http://www.kamware.com/free.htm. The tubing in the sketch at left increases from 1 inch at the exhaust port to 1.5 inches at the final point where a muffler is attached. Mandrel bent tubing is used for the "U" bends and cones fabricated from sheet metal connect the different sized tubing. The software is quick to illustrate how the dimensions of an exhaust that works well at low RPM look quite different from a design that promotes top end power. It makes sense to think about what you want to fit your particular needs (acceleration vs top speed). The higher revving motors used larger diameter tubing and shorter lengths; lower revving motors used smaller diameter tubing at longer lengths. Even the high revving designs will demand a paperclip design in order to gain the necessary length before exiting the rear of the scooter. Adjustments to jetting and drive (weights, springs etc) components will more than likely be necessary and an increased duration camshaft would really benefit from a more eficient exhaust system. |
Hi Doug,
It's funny, we must think alike- I've explored the avenues you've
considered. Carb? 20 mm keihin CV. I've upped the main jet one size, and
lifted the needle about 1 mm. Exhaust? original 17 mm downpipe cut off,
replacement made up in 25 mm electrical conduit (22 mm O.D.) the downpipe
length seems about right for the 7500 rpm max power. The baffle has been
de-restricted also. Next on the agenda is to try a different CDI box to
confirm whether there's a restriction or limiter in it. Performance cams
seem to be harder to get for these roller bearing head machines. I've had
the head off and tidied the ports up a bit, but there's more to do. Don't
know about valve sizes, though. I've also ordered a tach for it- easier
to set up the transmission if you know what it's doing... It's 52.4x57.8,
horizontal, and air-cooled. Bore and stroke are the same as the Pantheon,
but it's water cooled..........
David
Fri, 10 Aug. 2001
Interesting news.
Head off, valves out, ports cleaned up. Intake, 25 mm, ex. 21 mm. Cam timing same as CH/ CG, assuming it's measured at a larger than stock clearance (20thou ish). Lift around 6.5mm. I've fitted a small rev counter, from www.monkey-bike.co.uk, and it shows that the bike runs at 8k. Max power is 7.5k. Added some additional weight to the rollers, ground a little off the spacer in the variator, and I've gained 3-4mph on my favorite test hill (43-46mph from standing start to a marker) . Also have found that there is a rev limiter in the CGI, 8.5k. This equates to around 70 mph, so we'll have to do something about that...
Later, David
regards
David
Doug,
The TGB (David's scooter) now has a reprofiled cam
from Phil Joy, and I'm in the process of dialing it in. I've also raised
the comp.ratio to about 10.5:1 by welding up the dish in the piston. A
great improvement!
Glad to hear you're still experimenting and
things are going well. I've received quite a few e-mails asking about Frank
but he sold his Honda some time ago and bought a Runner 180 cc two stroke.
Below is an explanation of how to advance the ignition from a motocross
article on The Honda CR80 but I've also included a basic explanation of
Franks method. I'm sending you a photo of the flywheel and case from
a Honda CH150 but all modern scooters I've seen operate on the same principle
so if yours looks different its no matter as long as you can identify the
two key components.
The first is the flywheel and the small metal
square welded onto the outside/inside. This square acts as a "trigger"
so that when it passes next to the "pulse generator" , the pulse generator
sends the signal to the cdi unit. By advancing the point where the signal
is sent you are "advancing" the timing. Frank did this by welding a new
metal square on to the flywheel so that it activated the pulse generator
"x" degrees of flywheel rotation before the original square. In order to
use Franks method you have to know which way the flywheel turns (direction
of rotation) so that you know on which side of the existing metal square
you will be placing your new metal square. Next is to figure out how many
degrees of advance ( If you wanted to retard the ignition you'd want to
remove some of the old metal square) you will want. Lets say you want to
advance the ignition another 3 degrees. The diameter of the CH150
flywheel is 114 mm. The circumference of the flywheel is Pi x Diameter
or 3.14 x 114 or 357.96 mm. To find out how many mm you need to advance
the metal trigger you need to find out how many mm equals 3 degrees advance.
The circumference (357.96 mm) divided by the number of degrees in a circle
(360) is such that every 1 mm of trigger advancement equals about 1 degree
of advance. To advance 3 degrees I need to place the new trigger 3 mm in
front (taking into account the direction of rotation) of the current trigger.
The trigger plate needs to be the same thickness as the original to maintain
the clearance between the trigger and the pulse generator. Welding
is the means to attach the trigger but
remember that heat could affect the magnetic
qualities of the flywheel so don't use an acetylene torch~ use a tig /mig
etc.
The second key component is the pulse generator.
Its mounted by two bolts fitting through two mounting holes in the Pulse
generator and has a small metal rod on the side facing the flywheel. A
trick used by Honda motocrossers to advance the ignition could be applied
to the scooter motor. They elongate the two mounting holes. The nice part
about this is that the advance can be easily switched back to stock or
even retarded. Its much more easily adjustable. Its important to keep the
distance between the generator and trigger the same as stock so you can't
be too sloppy about enlarging the holes. I've done this with a two stroke
(Honda NH80 Lead) so I know it works.
After you've finished you're going to be the
world expert on Honda clone 125 cc tuning. If you ever get the time I'd
like to put up a web page with details of your mods. As far as the
cam, what lift timing and durations did you go with?
Doug
David
I spent the morning getting the 'scope out, and it does seem that
the CDI uses both ends of the trigger tag. I've attached a small graphic
that should be self-explanatory. The trigger causes a pulse at its start
and finish. At low revs, the box uses the trailing edge of the tag to time
the spark, and as revs increase, it pulls the spark to happen at the leading
edge. So for optimum advance at low revs, the tag might need to be moved
whereas for high revs, an extension will suffice. I've noticed that advancing
the timing a touch has increased the tickover speed. Looking for the sweetspot!!
An offset key might help also.
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.......If you measure the length of the tag and convert it
into degrees, it should match the amount of auto advance-
typ.10-17o. The fact that my idle speed raised after advancing the
pickup only indicated that my engine needs a bit more than factory advance
at low revs. Below is a copy of the email to various variator manufacturers,
c/w dimensions.
Crankshaft D15mm X 50mm long + splined part D14.6, 19 splines, 11mm
long + thread M12 16mm long.
Variator diameter 107mm, fixed half pulley with fan 112mm- Thickness
9.6mm.
Variator backing plate 2.8mm thickness
Central Bush 51.5mm long, 24mm OD.
Standard rollers 14x18x 11g
Belt VS846.20.30
Clutch Drum 125mm diameter
Rear pulley 137mm diameter.
the spline on the clutch and the crank are identical.....
........Latest mod is clutch springs from(of all things)
a Yamaha Aerox 50- takeoff at 5k rpm now!..........
........was bemused to find there was commonality between my parts and Yamaha, though. My clutch drum is bigger than yours even, yet the centre fitting( I'll be interested to see whay yours is...) is the same as the Yamaha Aerox 50 (minarelli engine) and the clutch springs were close enough to be interchangeable. Hmm. So I've ordered the variator kit for the Kymco Vivio/ Heroism, and a set of springs for the Aerox and the Kymco Dink. it seems that the attention to the cam and upping the CR have made quite a difference, although not moving the power up the rev range as far as I had first expected- I've been adding weight to the (drilled out) rollers again by welding, having gone from 12g stock down to 8g, i'm back up to 10g or so with good acceleration. The variator will improve things, though, as revs "on the rollers" varies from 7.5k up to 9k or so- I can either set the rollers for the start of the range or the end, not both. malossi seem to get things levelled out a good bit.......
March/02 Note: I had lost a lot of my e-mail when "upgrading" my computer. The following narrative was after David had fitted the Malossi variator to his scooter. After much correspondence David had obtained a variator designed for another model scooter that fit his.
I "adjusted" the rollers today- really critical adjustment!
Standard rollers supplied with the kit are 9g. Revs are 8.5-9k, the bike
feels a little breathless at top end. I epoxied some weight into each,
taking the set to 10.7g. That dropped the revs to 7-7.5k, feeling a little
sluggish. So I removed the weight from two opposing rollers, losing 3.4g
total. The result seems to be about the best compromise, with the revs
being between 7.5 and 8.5k.
I also went down a grade lighter in the clutch springs. It's interesting
to note the difference between the standard variator and the Multivar.
With the standard, I had to put really strong clutch springs in to stop
the revs from dropping as the clutch bites. But with the malossi, I notice
that the clutch slips more gently, smoothly transitioning into the variator.