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Some of the Modifications That I've Made to a Honda FL 250
1) AIR BOX MODIFICATION - The stock Honda FL250 airbox seemed to be a restriction with it's single, long snorkel. Some people just eliminate the stock airbox all together on their Honda FL250 and use a single open element filter (K&N or UNI). The problem with this idea (at least for me) is that the engine then breathes dirtier air closer to ground level and the air filter can be quickly plugged. In my area, there is a lot of dirt and flying dust when riding, so I knew that I wanted to keep the snorkel idea to help keep the air filter clean longer. In addition, no filter is 100% efficient. There is always a chance for some dirt to pass through or around the filter. This risk increases if you are dealing with a filter element is loaded up and the engine is pulling a higher vacuum behind the restricted filter. Eventually, it can draw dirt through into your expensive engine. In my mind, frequent air filter service is good, but even better is to also try to minimize the amount of dirt that the air filter ever sees in the first place.
So, I decided to keep the snorkel concept and just add one more. I actually had the flex snorkel tubing laying around anyway from past projects. I had originally purchased this flex tubing at McMaster Carr. Great company by the way. Huge industrial supply company that will also sell small quantities to the Average Joe. Great pricing and great service. Enough on that commercial... Having the snorkel tubing on hand, I then needed to come up with a way to attach to tubing to the stock Honda Odyssey airbox.
Additional 2" Pipe Brazed onto Airbox
I stopped by a local muffler shop and asked if they had a scrap of 2" exhaust pipe. No problem. They had lots of scraps and they gladly gave me a short piece of 2" pipe. I cut it to the right length and then I used a simple propane torch with MAPP gas to braze the tube in place. I decided to first braze the pipe in place and then to deal with cutting out the opening in the side of the airbox. In that way, I was able to use the pipe as a guide to drill a series of holes all around the inside of the pipe diameter. After removing the disc of metal from the inside of the pipe, I was able to take a die grinder and smooth everything out. Worked well. It was easier for me to do it this way than to try to make a perfect circular hole first, and then try to braze the tubing on later. I then used a hole saw to cut another opening into the plastic intake box up on the roll bar. So, now my Honda Odyssey has dual snorkels feeding into the stock airbox. No optimal for maximum performance, but overall best for my dusty riding conditions. I have a K&N and UNI stock replacement filter elements that fit in the stock Honda Odyssey airbox. I plan to run with the UNI since it's a foam element better at trapping dirt.
2) TUNED PIPE - The exhaust pipe on a stock Honda Odyssey FL250 was designed to help keep the Odyssey quiet (not an easy task) and to have a built in USFS approved spark arrestor. The stock exhaust is NOT optimal for performance. My stock exhaust was plugged up with so much oily carbon gunk, that I decided to cut it open and weld on a section of another stock exhaust that was not plugged so bad. When I cut it open, I got to see how bad the design of the stock pipe was in terms of performance. There was a gradual divergent area of pipe that went into a belly section and then an abrupt flat wall with a small hole leading into sound deadening baffles. There was no "convergent cone" designed into the stock pipe geometry. What this means is that the stock pipe was not a true tuned pipe design that is so critical to 2 stroke engine performance. I ran across someone nice enough to offer to give me an old Honda Odyssey tuned pipe. It was an old DG exhaust that had a true tuned pipe design. Since I was planning on riding in an area that requires a USFS approved spark arrestor, I began to look at options of how to adapt a spark arrestor onto the end of the DG pipe. It did not have a spark arrestor. I was looking at aftermarket spark arrestors like the Cobra Sparky that can be clamped on to the end of an existing tuned pipe silencer. While I was looking into this, I ran across another Honda Odyssey tuned pipe that already had a spark arrestor built in. It had the Krizman type spark arrestor that is less prone to clogging and also better for performance. I was told that it was probably also an old DG tuned pipe, but there was no DG markings on the pipe. I was able to buy it for a good price, so I decided to give it a try.
As it turns out, this Honda Odyssey tuned pipe is reasonably quiet - relatively speaking, because I don't think any Honda Odyssey would truly be called "quiet". In addition to being reasonably quiet and offering a spark arrestor, this particular Honda Odyssey tuned pipe also seems to help performance nicely. The only problem is the stock CVT belt drive system is calibrated to shift up way too soon and bogs the engine down. In other words, the engine can't rev up freely into the RPM powerband of the tuned pipe. This problem can be addressed by changing the CVT to allow the engine to rev up higher - which I am doing.
3) RECALIBRATE CVT - The Honda Odyssey FL250 has a Constant Velocity Transmission (CVT) automatic belt drive similar to what is commonly found on snowmobiles. From the factory, the Honda Odyssey CVT system is calibrated very mildly. In fact, it felt like driving a stick shift car and starting off in 2nd gear and then shifting up to 3rd, 4th, 5th gear way too soon. The engine felt like it was constantly bogging and not allowed to rev up freely before the CVT system was shifting up to a higher range. In addition, when slowing down and then accelerating again, the CVT system would not "downshift" properly and it felt like it was really lugging. Since there is not much that can be done with the stock Salsbury front drive clutch, I decided to try to find a Powerbloc clutch for my Honda Odyssey...
The Powerbloc clutch is fully adjustable with changes in springs and weights in the pucks. It's a great upgrade for the Honda Odyssey, but the only problem is that they are now discontinued. So, the search began on eBay for a used or NOS Power Bloc for my Honda Odyssey. Fortunately, I was able to find one from an Odyssey racer that was slightly used and almost brand new. Finding a Powerbloc clutch for a Honda Odyssey FL250 can be a real challenge since they are no longer being made, but even more challenging can be the process to try to remove the old Salsbury drive clutch!
"Redneck" Spanner Wrench to Help Remove Old Salsbury Clutch
At first, I tried to use a powerful impact wrench to remove the center bolt on the old Salsbury clutch. Good luck! I tried and tried. I attempted to hold the clutch with a rubber strap wrench while I hammered away with the impact wrench. It would not come loose. So, then I decided to make a "redneck" spanner wrench to help hold the clutch more securely while I tried to loosen the bolt. I made this crude tool with a long piece of steel bars stock that I had. I drilled with 2 holes through which I mounted two Grade 8 bolts. At first, I tried to use it with the impact wrench, but it was no use. The impact wrench could not loosen the bolt. Then I grabbed a LONG breaker bar and began to try to use some serious leverage to loosen the bolt. My redneck spanner wrench began to bend. That could have been eliminated with some bigger/stronger steel, but I decided to try to add a few Grade 8 washers to support the steel bar around the areas where I drilled the holes. The steel was bending near the area where I drilled the bolt holes since those areas were weakened. The large washers strengthened my redneck "spanner wrench" enough, so that I was able finally apply enough force to loosen the center bolt. Once I got the center bolt off, that was just the beginning. Next, I used a 7/8" - 14 tap to clean up the internal threads in the hub of the Salsbury clutch. Then, I used a big 7/8" Grade 8 bolt as a "puller" to try to pull the clutch off. Only problem was that my bolt was not quite long enough to reach the end of the crankshaft, so instead of looking for a longer bolt, I ended up using a short socket adapter (most any short piece of steel that would fit through the 7/8" hole would work). I reinstalled the bolt and I was able to use the impact wrench to tighten the 7/8 bolt and hammer off the Salsbury clutch. FINALLY, the old Salsbury clutch was off, and now I could think about installing the Powerbloc clutch on my Honda Odyssey. At the time of this writing, I'm still waiting for some parts before I can put everything back together.
UPDATE: As is turns out, the guy who sold me Powerbloc clutch made a mistake and sent me one for a FL350. Unfortunately, he did not have the proper FL250 Powerbloc clutch. As a result, I needed to install the stock Salsbury clutch, but fortunately I found a set of stiffer springs and some new rollers to rebuild my old Salsbury.
4) REAR CVT DRIVEN CLUTCH MOD - As mentioned in the page about the Honda Odyssey FL250 front CVT clutch, the stock tuning on the Odyssey transmission is not great for optimal performance. In stock form, the Constant Velocity Transmission (CVT) shifts up too soon and bogs the engine too much. To help the engine rev up more freely before the CVT begins to shift up, the rear driven clutch can be recalibrated. A stiffer torque spring and a new cam (32 degree ramps vs. stock 41 degree ramps), helps to delay the upshift of the transmission and results in higher engine RPM's. This can help keep the engine in it's RPM powerband - especially when using an aftermarket tuned pipe.
As usual, the high performance parts do not install themselves (unfortunately), and when you are working on an old machine that has sat outside for decades, rust and corrosion are a big challenge to deal with. Just being able to take things apart so the new parts can be installed can be most of the work. In my case, the old cam was STUCK on the rear driven clutch real good. I soaked it well with PB Blaster penetrating lube, but it really did not want to come off. I used a 3 jaw puller and tried to crank it by hand to pull the old cam off. That didn't do it. I then used my impact wrench to hammer on the 3 jaw puller, and that finally broke it loose.
Removing Driven Cam from Honda Odyssey FL250
After removing the cam, the old buttons on the fixed face of the driven clutch need to be pulled out. Be prepared to have them break off. That can be remedied rather easily by carefully drilling out the remaining plastic out of the holes. New buttons are then pushed in and everything goes back together in reverse of disassembly.
5) BETTER ENGINE COOLING - If there is one area where the Honda Odyssey FL250 needs some help, then it's in the area of engine cooling. Of course, the lack of a rear suspension is a big issue too, but I'd say that the engine overheating issue is a bigger concern. With the engine tucked in behind the seat, it's no wonder that it tends to get hot after extended runs. I mounted a Digatron Tach and Cylinder Head Temp (CHT) gauge to my Honda Odyssey, and I was seeing over 400F head temps after extended runs with a lot of wide open throttle. At the same time, I could hear detonation (NOT good!) at the higher head temps, even though I was running with a 96 octane mix of higher octane fuel. Carb jetting was also set rich just to try to reduce temps, but the engine still would run hot after an extended time. To make matters worse, the stock Honda Odyssey cylinder head comes from an older Honda dirt bike which had some of the cooling fins shortened to clear the tuned pipe as it came up and over the head. These shorter cooling fins do not help matters when it comes to cooling. There are other options.
73-74 Honda CR250 Cylinder Head & 76 CR250M Copper Head Gasket
Fortunately, a 73 - 74 Honda CR250 head will bolt on to a Honda Odyssey FL250 engine. The 73-74 Honda CR250 cylinder head has taller cooling fins and they are taller across the entire head. No shortened fins to clear a tuned pipe since the earlier CR250's had the tuned pipe go down and along the bottom of the bike. What this means is that 73-74 CR250 head can help provide a little better cooling capacity than the stock head. They can be picked up on eBay for a reasonable price if you keep your eye out for them. In addition to the 73 - 74 CR250 head, it's also possible to run with a 76 Honda CR250M copper head gasket. The advantages of this copper head gasket are that it helps conduct heat better between the cylinder and head. In addition, this copper head gasket is around .020" thick (around half the thickness of stock FL250 head gasket). This thinner head gasket boosts compression and also tightens up the squish band a little. On the downside, the copper head gasket is less forgiving to imperfections on the sealing surfaces and is more prone to leaking combustion gases. The stock Honda Odyssey FL250 gasket is more forgiving and will seal better in many cases. You'll know the copper gasket is leaking because you'll see oil wetting around the gasket area between the cylinder and head. That's what happened in my situation when I first tried the copper head gasket. So, I bought a new copper gasket and carefully cleaned the top of the cylinder deck. Then I tried to clean up the cylinder head and noticed some nicks and dings on the gasket sealing surface that were definitely not helping matters at all. Since I don't have convenient access to a machine shop, I just decided to try a "redneck" method of cleaning up the head sealing surface. I took a scrap sheet of flat stainless steel sheet metal that I had laying around. I then taped a sheet of fine grit sandpaper to the sheet metal and placed it on a known flat section of concrete floor in my shop. I used a 1/2" drill with some adapters and a spark plug socket to slowly spin the cylinder head on top of the sandpaper. I went slowly because it's hard to control to spinning of the head if the drill is running too fast. I used a soapy dish soap solution to lubricate the sandpaper (it was wet/dry sandpaper), and I frequently stopped to check the head surface to keep an eye on progress. I just ran it long enough to clean up the entire gasket sealing surface of the head.
In the end, things turned out pretty well consider the redneck method I used. Obviously, if you are not careful, you could screw things up worse if you don't have a flat surface to do this lapping of the cylinder head. Doing all this definitely helped with sealing. I still need to run it some more to make sure that I have no more leaks long term. If I do, then I will just switch back to a stock Honda Odyssey FL250 head gasket and call it good enough. In my opinion, more important than the advantages of the copper head gasket is to ensure you get a tight seal on the combustion gases. And, better than using this redneck method would be to have the head professionally resurfaced by someone that has access to a machine shop and knows what they are doing. For most people, it'd probably be easier to just run with the stock head gasket and not worry about it since it is much more forgiving of sealing surface imperfections. The CR250 head on a Honda Odyssey helps a little with cooling, but the engine will still run hot because of how it is positioned behind the seat and shrouded from a direct blast of cooling air. Probably the best way to knock down engine temps is to get one of the water cooled conversion heads available for the Honda Odyssey. These water cooled heads are not cheap though, so the 73 - 74 CR250 head is still an option for those with a Honda Odyssey FL250. If you are interested in vintage Honda dirt bikes, then here is a video showing a nice 1973 Honda CR250. It is the cylinder head from this bike that will fit on a FL250 engine.
6) MORE ATTEMPTS AT BETTER ENGINE COOLING - With the FL250 engine tucked right behind the seat, Honda Odyssey overheating is not an uncommon problem. Liquid cooled heads are available for around $300+ and then you mount a small radiator up above where it can get some air flow. A small electric water pump can be used, but many people just route the hoses in such a way as to take advantage of the thermosiphon effect. The temperature differences in the system will cause the water to flow naturally - without the need for a pump. Regardless of whether you add an electric water pump or not, adding a liquid cooled head adds cost. For my old Odyssey that I bought for $500, I didn't want to spend another $300+ to go the liquid cooling route. Out of the desire to save money comes the $20 forced fan redneck idea shown below.
Detmar 4" (P/N: 751RC) 240CFM Marine Blower
I bent some aluminum flat stock to make a simple mounting bracket that attaches with the seat belt bolts. I bent the bracket so that the Detmar fan blows on top of the cylinder head. I already had the CR250 cylinder head with taller fins, but those taller fins don't do much good when they are not getting a good supply of cooling air. The Detmar blower is rated at a decent 240CFM of air flow at a modest current draw of 4.5 amps. The part number on the Detmar blower that I bought was 751RC. The "RC" denotes a water resistant blower - which I would recommend for this application. My 83 Honda Odyssey FL250 came with a slightly larger alternator output than the earlier FL250's, but even so it's output is very little at best. Even so, on a ride at the dunes that last around 4 - 5 hours, the fan continued to blow and never drained the battery during the whole time. The blower ran almost continuously while the Odyssey was driven. I was always able to use the Goki aftermarket electric starter to re-start the Odyssey. In other words, even with such a weak alternator output, the Detmar blower did not consume too much electricity. Of course, this idea would not work on a stock FL250 because the alternator output is AC. In addition, the earliest FL250's put out 6V, so that would not work for running a 12V blower. In my case, I wired my blower directly to the battery with a switch mounted near the Goki electric starter button. The aftermarket Goki electric start system has a simple rectifier circuit (diode) that converts the FL250's alternator ouput to DC current to charge the battery. If your FL250 has the Goki electric starter and has an alternator that puts out 12V, then you should be able to run a similar setup.
Now onto the results... prior to adding the blower fan, my Digatron Cylinder Head Temp (CHT) sensor would show over 400F after running for a while. When temperatures would reach around 350F, I could hear engine detonation which can quickly destroy an engine. If I continued to drive, the temps would continue to climb until they would exceed 400F and then I would shut down to let the engine cool. I don't know how high the temperatures would have climbed if I didn't stop. I didn't want to find out (engine seizure or piston meltdown due to detonation). After I added the Detmar blower, the CHT would read right around 300F +/- 10F. Even on a hot day at the dunes with outside temperatures in the 90's, my Odyssey would run right around 300F even after hours of driving. I was very pleased with the results given the simplicity and low cost. I would call the $20 redneck solution to Honda Odyssey overheating a success.
7) RAND WATER COOLED CYLINDER HEAD - Well, I finally decided that an air cooled Honda FL250 engine sitting right behind the seat is not a great combination. For a while, I used a bilge blower fan blowing on top of a CR250 head rated at 240CFM, and that did really help. However, even with the blower fan, the engine would get too hot after extended run times in warmer weather. I had a cylinder head temp sensor which showed me that it still wanted to get hot at the dunes while running hard for extended times in hot weather.
Honda FL250 Rand Water Cooled Head and Yamaha Banshee Radiator
Thankfully, I picked up a Rand water cooled head on eBay for a good price and a used Yamaha Banshee radiator cheap. I used some galvanized steel flat stock and bent some simple mounting brackets. These brackets were attached with some pipe clamp hardware purchased from McMaster Carr industrial supply. The radiator mounting setup turned out to be very solid. Another note related to the hoses. The Yamaha Banshee radiator has 3/4" hose fittings. The Rand head has 5/8" hose fittings. Initially, I found some thin rubber sleeves to slip over the 5/8" fittings to make them a tight fit with the 3/4" hose. I didn't like this arrangement, so I ordered some 5/8" to 3/4" brass hose barb adapter fittings to make sure the hose connections are all more secure.
Yamaha Banshee Radiator Mounted on 1983 Honda FL250
I also have a Delta computer cooling fan rated at over 200CFM with a modest current draw of around 1.5 amps. I was going to mount this to help pull more air through the Yamaha Banshee radiator. I will see if this is really even necessary. I have also considered adding a small water pump (auxiliary heater pump from certain vehicles) to circulate the coolant. For now, I am considering just running with a thermosiphon arrangement. The routing of the hoses and the vertical rows of the Yamaha Banshee radiator is supposed to allow the coolant to circulate naturally. The hot coolant rises and then as it cools through the radiator, it is supposed to drop back down into the head. The simplicity of this arrangement (no pump to fail) is appealing. Time will tell if it's really sufficient. There you have it, some of the modifications that can be made to a Honda FL250. These machines are old and outdated in many ways, but there is one thing that a Honda FL250 offers that never goes out of style. FUN!