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FT500 Racing Motor Help

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I road race an '83 FT591 powered '89 TZ250, and I'm finding that I need to talk to all types of bike riders to keep my machine going. I hope you dirt diggers will help out a pavement pounder like me! This is a long post, and if you have something to say and don't want to type, please feel free to pm me with your phone number and I'll give you a call.

Some background on the motor: 1983 Honda FT500 w/ EDC0 591cc kit (piston, sport--not race--cam, titanium springs), balanced crank, and Carillo rod. There is no battery on the bike.

First, I need to limit the revs in some way. I told myself that I would be good and keep things around 6500 rpm, but I was consistently running near 8k toward the end of the straight (even after moderating the throttle before the kink), and my Tiny Tach indicates that I actually hit 9,200 rpm at one point. I'm glad I still have a motor! Am I being too conservative in my max revs?

I'm considering replacing the Tiny Tach (pretty useless because of its 1/2 second refresh rate) with a SPA shift light/rpm indicator At least I'll know when I "should" be shifting. I want to install a Dyna DRL-300CDI rev limiter, as well. Can these be powered by the bike's system, or would I be better off installing a separate lightweight 12-volt rechargeable battery?

That should take care of max rpm, but then I still have to figure out how to get speed with the rpm restriction. So, I need to look at gearing.

The countershaft sprocket can't be larger than 16 T due to space restriction. So, I have to work with the rear chainwheel. I ran a 38-tooth because my chain was too long for anything smaller. But that theoretically limits me to a max speed of 105 mph with the 6,500 rev limit (thus the 8k+ I was running to get near 125 mph). As I look at gearing charts, I'm seeing possibilities like 133 mph with 16/30 at 6,500 rpm max. If I rev out to 7,500, I would need a 16/36 for the same results.

So, am I crazy/ignorant in thinking of going small on the back chainwheel, or does this seem like a useful solution.

Thanks for any and all help. I need to order parts soon!

Kurt

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Wow that's a lot of speed out of that motor. I had an 81 XR500R and I never got the thing to break 100 no matter what I did with sprockets. No tach on the dirt bike, but I did calculate rpms off the speed. I had to aim for 7500 to 8000 rpm to get the max speed. I think that your 6500 is too low.

You are sure getting a lot more torque out of your engine than I did, but I don't think that you will have enough to pull that 16/30 gearing. It may be that your 38 is optimum. How was it pulling at 8000 rpm? The only thing to do is experiment. Get a shorter chain, a few rears, like 36 and 34, and try them out.

As far as a battery goes, the simplest thing to use is a small gel cell. These are lead acid batteries. A 12 volt 1.2 Amp hour version weigh a pound or so. Not the very lightest battery. If you still have the rectifier/regulator on the bike then just put it in place of the normal battery and the bike will keep it charged. If you don't make a charging wire that plugs into a cigarette lighter for your car/truck. You could also just use 8 AA batteries and replace them when they wear out. All of this depends on the current that the limiter needs. You need enough to power it during your race.

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You could just run a capasiter instead of the battery if you dont need a battery to run the bike as it sits. Then you'd have a 12V source with almost zero wieght.

I dont know why your so worried about the RPM on that motor. 6500 is about the bottom of the envelope Id think. I think the stock redline on that thing was about 10,500 and youve got better springs and a good crank. Am I missing something?

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I have been racing a Ascot since 1984. I pretty much have the same motor combo but I have a larger cam and oversize valves alog with a custom machined piston and head work. You need to have the overall diameter of the rear wheel. I ahve pulled as high as 136MPH on my bike. I run a 17inch rear wheel with a 140 70 tire. I have never had a problem over reving the engine. I run a battery powered tach from a racing gocart. It is kins of nice. It give RPM in a digital read out along with cyclinder head temp so it helps with jetting. The last dyno sheet I had I was at 64 hp and 55 ftlbs torque. I hope this helps. My CR is also 13.5 to 1

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