I wrote this section as an article that originally appeared in a late Spring 1996 edition of "Sport Twin News", published by John Sweeney.
Every big magazine seems to have a project bike or two under construction, and there is no reason why the biggest little magazine on John Sweeney's block should be any different. I offered to do a series of articles on a bike I planned to build, and John, being the perceptive person he is, jumped on the opportunity to not be forced to write the entire magazine himself.
I enjoy riding small bikes, and have several Honda 90s, 160s, and a Vintage 250GP Honda in my small accumulation of bikes. Over the past few years I've been mildly intrigued with the thought of doing an EX250 Kawasaki project, but not to the point of doing more than idle thinking. I was busy racing my 650 Ducati in 650 Twins/750 Superbike, and finishing my custom framed Vintage F750 Laverda twin.
Several weeks ago, I saw an EX250F2 offered for sale on the Internet. Since the owner wasn't getting any action and was starting to sound desperate, I dropped him a line telling him I was interested but his price was too high. Several days later I owned a $200 15,000 mile EX250. The tires were shot, the fairing came off in several more pieces than Kawasaki intended, and the forks and both handlebars were a bit tweaked.
There are a number of AFMers running EX250s in Modified Production, but when I saw that the stock bike was nearly the size of my 650 Ducati (and at an estimated 340 pounds, based on a curb weight reported by Cycle magazine, only 50 pounds lighter), I wasn't too wild about this class. Plus the stock carbs/exhaust/bodywork didn't appeal to me. Next class up is Superbike, where I could change the carbs and pipe, change the 16 inch wheels to 17s, and ditch the stock tank, seat etc to save weight. This sounded more like it. Then it hit me, "how much does a Suzuki RGV250 or Yamaha TZR250 weigh, and how much horsepower do they make"? Uh oh, doesn't sound too promising. I'm all for being the underdog, but let's not get carried away with it. I picked up my AFM rule book and began looking at the different class requirements. "Eureka" I cried, "I can build an EX250 for Formula 3 (a.k.a. 125GP)". I figured that if I used 125 specification running gear, the only difference between the EX and a 125 would be in the motor weights. Time to start weighing the motor (did I mention I had already picked up a spare motor?).
On my digital shipping scale, which reads to the nearest .5 pound, the weight of the complete EX motor less water pump is 89 pounds. Add in a pound for the water pump and call it an even 90 pounds. From this can be subtracted:
The motor is now down to 73 pounds, with potentially another pound or two to come off by milling away the decorative fins on the cylinder and some selective lightening of other components, such as trimming back the end of the crankshaft where the flywheel was located. The important item in the weights is the reduction of roughly 11.5 pounds of ROTATING weight. When you consider the flywheel has a diameter slightly over 5", its effective rotating weight at 14,000+ rpm is much greater than its actual weight. I don't have an RS125 engine less carb to put on the scale, but would be very interested if someone who does would let me know how much it weighs. I'll hazard a guess that it is about 40 to 45 pounds (a report from an RS125 rider in August 1996 indicates around 35#) which puts the EX250 about 30 pounds heavier. Still, a bike weighing about 185 pounds should be plenty entertaining to ride. I'll be building a lightweight frame for the bike that will be stretched out a bit to let a 6' tall rider get on with the minimum number of hip and leg cramps.
Next, let's look at the horsepower numbers. RS125s appear to have about 38-43 bhp, depending on state of tune. A B-kitted RS125 has been advertised recently as having 43.5 bhp. Lets figure 40 bhp for a well tuned mildly modified bike.
A stock EX250F2 made 28 bhp at 12,500 rpm when tested on the Kerker dyno by Cycle magazine. I've been told that the best of the Modified Production EXs are doing about 34 bhp. Remember, that is with stock carbs and exhaust. Current F1 car engines are making about 220 bhp/liter and World Superbikes about 200 bhp/liter, so if I shoot for 180 bhp/liter I'll have a 45 bhp engine. My engine tuner, Craig Hanson of Hanson Racing Technology, and I figured that with big carbs and a free flowing exhaust 40 bhp should be achievable with a reasonable amount of work. Maybe we can edge up towards the 45 bhp number with more time/effort/money.
A 32mm CV carb (early EX250 spec) probably flows about like a standard 28-30mm carb, as the throttle butterfly restricts flow. Hot ZX-6 Kawasakis are using 35mm Kei'hin flatslide smoothbore carbs on a 150cc cylinder. I decided to order a set of 33mm roundslide smoothbore Kei'hin CR Specials for my 125cc cylinders. The two types of carbs flow the same, and since the 250 intake ports are fairly horizontal there is no reason to order the noticeably more expensive flatslides. I called my Kei'hin distributor and placed my order. They called back in about 30 minutes and told me they had never had a set of carbs ordered for a 250 and needed the spigot and center to center dimensions from the stock carbs. After a light hearted 15 minute wrestling match I finally extracted the carbs from the stock bike and measured them. First time I've had to drop a rear fender to make enough room to move the airbox back to be able to pull the carbs out. Yet another reason for not running a class that requires stock parts.
Megacycle has only one cam listed for the EX, and R/D Spring has a set of valve springs with titanium retainers, so there are no big decisions needed on these parts. We'll have to wait until we get the head on the flowbench to see what porting/valve size changes will be needed.
With a bit of work, the EX should have equivalent horsepower to a pretty fair 125, maybe even a bit more. It may also have a broader powerband, making it easier for less than ace riders (that's me) to keep it in the powerband. Increased area under the torque curve compared to a two stroke might give the EX a bit of advantage in acceleration. Hopefully the EX250 will be less maintenance intensive than a two stroke.
I've noticed that much of the standard aerodynamic bodywork on the 125s doesn't seem to match up with what my research indicates is needed, so there are some potential gains to be made in this area. Less drag can often be a lot cheaper to find than more horsepower.
Now comes the part where the massive readership of Sport Twin Journal gets involved. I need you folks to keep your eyes peeled for killer deals on spare motors and 125 forks, brakes, etc. I'm not looking for the parts that everybody takes off because they didn't work, but then again I don't have to have parts just like the stuff on Waldeman or Aoki's bikes. If you find something let me know at 415-665-3363, or e-mail me at email@example.com. I'd also be interested in hearing about reliability problem areas in the EX250 motor.
An update note: Since the article was written I've purchased a set of Dymag wheels to RS125 spec - 2.5x17 front, and 3.5x17 rear. The front wheel bare is about 5.62 pounds and the rear a pound heavier. I haven't yet decided what to do for brakes, but for the chassis I'm tending toward a Hossack/Fior/Britten style front end with a tubular steel space frame and a fabricated steel sheet swing arm. Stay tuned!
01 January 1997
An Instamatic photo of Tom Laporta's chassis modifications
Today I picked up a 1989 EX250 project bike from AFMer Tom Laporta over in Berzerkley. He was suffering from a case of project avoidance depression which was assuaged by the application of a moderate sum of cash and the making of space in his lightly flooded garage (the rain has been bucketing down today, and his sump-pump went on strike).
Tom had sent a for-sale ad for posting to the Sport Twin News web site on Saturday night, and since John Sweeney knew that I'd be interested in adding to my supply of 250 stuff (much like some EX500 folks I know) he dropped me a quick email, resulting in an even quicker email to Tom doing the deal. Advertising with STN pays!
I had talked with Tom about his project earlier in the year, when he was having my friend Craig do a no-balance shaft lightening/rebalancing on the crank. I didn't realize until I got to his house the extent of his ambitions. He had removed the rear subframe, added in some short bracing tubes from the top of the steering head to the upper motormount area, and then some diagonal tubes from just behind the upper motormount to the swing arm pivot area. Tom and I talked about this for a while, and came to the consensus that what he had done was a fair amount of work, and Kawasaki hadn't gone out of their way to either make the work unnecessary or easy. I'm afraid the 250 chassis is going to be one of those sow's ears that no matter how much work you do, will still be a sow's ear (though a bit faster) when you are done. I'm going to avoid the whole problem by scrapping the stock chassis anyway.
So I got a motor with a fresh valve job and fresh lightened/rebalanced crank, an adjustable ride-height Works Perf damper assy with only 2 hours of use (it doesn't appear to have any external damping adjustments), Lindeman-modified forks with clipons and Telefix fork brace, factory manuals, a used 1000T Grimeca caliper (the small one) for the rear wheel, an RS125 seat, and a box of misc stuff that I haven't managed to go through because I wasn't going to lug it into the garage through the rain. I did spot a set of stock headpipes, which will go immediately on my street 250 once I see what I need to do to adapt the mufflers from my 500 Guzzi to them.
All in all, everyone seemed well pleased by the deal, and it was all made possible thanks to John Sweeney, from whom all Sport-Twin blessings flow.
I'll take some pictures of Tom's modifications to the chassis when I can (did I mention that it was pouring down rain?) and post them to the web site (see the link at the beginning of this section).
I can hardly wait until next week when another 250 motor I'm buying from Andre at Out & Out arrives. Just can't get enough stuff....
Some comments on Tom's chassis modifications:
I would guess that some of Tom's angst over the project was because the engine doesn't appear to fit back into the modified frame (the rocker cover hits one of the diagonal tubes). This is probably due to some distortion in the chassis during welding, which may have been done with the engine removed. Tom brought the top of the diagonal frame members into the existing frame right behind the front motor mount brackets. He also put a (very hard to see in the picture) short bracing tube from the top of the steering head to the front motor mount bracket area. I think he was hoping to avoid having to make new front motor mounts. He would have been better off removing the mounts and running the diagonal tube a couple of inches farther forward, reducing the offsets in the tubing junctions, and making the frame stiffer. This would have given a straighter line to the diagonal tube as well (and might have given a bit more room to fit the motor back in the frame). A triangulating tube from the back of the original tube extending from the top of the steering head to the diagonal would also help.
Examination of the photo shows some of the drawbacks to the stock EX frame (as far as building an all-out racer goes). Notice the huge gap between the front tire and the engine? There is room to move the engine a good 5 inches or more closer to the front wheel, increasing the loading of the front end. The whole bike is also too high - even when the forks are collapsed the steering head is quite a ways from the top of the tire, and the engine can be lowered quite a bit as well, which will increase the massive amount of clearance already existing between the top of the engine and the frame. Moving the center of gravity down and forwards will help retain stability when the steering angle is steepened and the trail reduced. The stock upper rear damper mount is cantilevered from the unsupported center of a cross tube - pretty much as bad as it can be built.
I won't even start on the 16" wheels that GREATLY limit the choice of tires (none of which will let the bikes go as fast as the fast riders are able to ride them).
25 March 1997
Several weeks ago I went up to Craig's shop and took these pictures of the cylinder head. Note that Craig has removed the non-functional fins from the cylinder for a modest saving of weight. The new intake stubs aren't yet welded in - Craig is porting from the middle towards both ends to avoid using an extremely long shank cutter in his grinder. After welding he'll blend the two sections together. At this time Craig doesn't plan to increase valve sizes. The valves already overhang the cylinder block, and going bigger would increase the masking. The exhaust is flowing pretty well, so it is now necessary to get the intake flow up to match.
The downdraft intake ports on the head - back view
The downdraft intake ports on the head - side view
08 August 1997
There hasn't been much happening on this project, as I've been selling books and working on vintage road racers instead (and even getting a little racing in too).
I just had Craig send me some flow bench test information. He supplied me with flow numbers for both the EX250 head as received (stock) and a Honda XL125 two-valve single (stock). The XL is the later two-piece head which has bigger valves and, I think, a different valve angle from the earlier one-piece heads. The flow bench tests were run at 15" of water.
HONDA XL125 EX250
LIFT INTAKE CFM EXHAUST CFM E/I% INTAKE CFM EXHAUST CFM E/I%
.050" 12.76 13.0 103 19.7 16.0 81
.100" 26.0 28.9 111 36.0 34.0 94
.150" 41.8 38.6 92 48.7 45.7 93
.200" 54.0 42.0 77 63.0 55.6 88
.250" 61.9 44.6 71 68.0 59.6 87
.300" 63.0 46.0 73 70.8 63.0 88
.350" 64.0 48.0 75 71.9 65.0 90
.400 66.0 47.0 71 72.4 66.0 91
Exhaust flow as a percentage of intake flow should usually be in the 80-90% range, and Craig says that on a modern 4 valve head they can usually run at the high end of that range.
Craig has done many of the two valve Honda heads and had them get up to an intake flow of around 80 cfm. He says that while the EX250 head is better than the XL125 head, it isn't as much better as it should be considering it is a much more modern four valve design. Actually, I think he said something about the EX250 head being pretty bad in stock form. He did a little smoothing of some of the glaringly obvious problems and picked up 3-4 cfm on the intake side.
Craig says that he thinks the EX250 head should probably be able to exceed 90 cfm on the intake side. At this time the intake is looking like much more of a hindrance to developing power than the exhaust side.
As mentioned above, the intake and exhaust valve sizes will stay stock or increase only slightly, as the valves are already right next to the cylinder wall and increasing their diameter will just make them more shrouded, requiring the cylinder wall to be cut away.
The hot Megacycle cam for the Honda single gives .370" of valve lift on both sides, with 274-275 degrees of duration and 106-106.5 degree lobe centers. The Megacycle cam for the EX250 gives .349" of valve lift on both sides, with 243 degrees of duration and 104.5 degree lobe centers. The four-valve head should be able to run a shorter timing cam than is needed on the two valve engine, and that is shown in the timings above.
When Craig was dynomometer developing the Honda singles for the F-150 class we tried to get started in the mid-1980s he was working with 150cc displacement and a rule-specified carb size of 26mm. This carb is noticeably restrictive, but even so the best engine developed about 14.5 bhp at the rear wheel on the dynomometer. A 30mm carb and the 125cc displacement would probably show around 17 bhp, and doubled up that would make a 34 bhp twin. Since my EX will hopefully have about 12-15% more flow than the best Honda head and a noticeably bigger and more efficient carburettor (33mm Kei'hin CR Specials) a 20% power increase over the doubled-up Honda would give 40-41 bhp. This is about where Craig expects to end up with maybe another 1 or 2 bhp to be found with continued development.
Once the AHRMA vintage National at Sears Point is over (22-24 August 1997) and the Laverda is running tolerably well the next project is to get my 560 ROTAX-engined Can Am Sonic dirt bike together for the winter and hopefully do the last little bit needed to get my street EX250 on the road. Maybe then I'll have some time to sit down and start figuring out the geometry for the Hossack/Fior front end for the EX racer, and when that is done the rest of the frame should be a relative doddle to draw.
In August 2000 I got my first American Federation of Motorcyclists (AFM) roadrace outing in a couple of years. The bike was my Kawasaki 1989 EX250 former streetbike. To prep it for the track I built an aluminum tank, dual megaphone exhausts with SuperTrapp discs for some silencing, a fiberglass H-D XR750 "Big Butt" seat, mounted Kei'hin 33mm CR smoothbore carbs, and built clip-ons and plates to relocate the footrests up and back. Oh yes, I also converted it to twin rear dampers from the stock Kawasaki mono-damper setup. I boxed in and gussetted the stock swing arm, added new damper mounts (and deleted the old ones) and also swapped over to bronze bearings from the stock needle bearings. I acquitted myself with honor if not distinction (beating about 25% of the grids in 250SB and F500Twins classes). The plates boxing in the swing arm were domed on my English wheel (pics of that elsewhere on the site). Here are some photos (click thumbnail for larger image):Under side of the modified swing arm
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