I've been thinking of a way to make a cheap rev limiter for my dirt bike. I think it would be cool to make one, and then watch other people do the same after posting the design. The problem is, I can't test different setups, and I don't know if it would work.

The design I came up with involves grounding the power to the coil with a conditional switch. This presents a problem though, because the bike's wiring is so simple. It's actualy its simplisity that causes the problem.

The problem is, theres only two wires on the whole bike. One goes from the AC generator to the coil, and has another wire branch off of it to the kill switch. This means that if I draw any power from the wire it will shut down the bike because it will go to ground.

I came up with a design for a switch though, that might alow the system to work. The generator's output power increases as engine RPM increases, So if I have a switch that activates at a set voltage, I can set it to ground when the motor hits redline. (10,500rpm)

The switch I came up with works on an electro magnet, as voltage increases, the electro magnet increases it's pull. The electro magnet is connected between the generator and coil and is next to a spring loaded metal pin that is grounded, when the voltage hits the level it would at redline, the pin is pulled so that it contacts the metal pad on the electro magnet, thus grounding the generator and cutting the ignition untill the rpm drops low enough to cause the pin to disconnect. The pin could be screwed in and out so it gets closer or farther away from the metal pad, this way you can adjust the limiter.

The only problems I can see with this is the electro magnet providing to much resistance between the generator and coil and causing the bike to just not run, or it heating up and burning out.

Tell me what you think, I don't have much experiance with this kind of thing so consider that I could just be completely off.

best thing to do is give it a go trial and error is the best things in this day and age when trying to do anything

I'm going to build a prototype, but I have to wait a while to test it because the bike is in storage for the winter. I've got my fingers crossed that it works, because if it does, I can make more of them and sell them.

Apologies for the noobishness, but could you explain to us non-technical minded why you need a rev limiter? Surely you'd want
all the revs you can get?

Something I've never been able to work out.

I don't think it would really matter on a offroad bike, as it's gonna get ragged with or without a limiter.

Theres not much more power past the peak power point so going so far towards a redline will make little difference in performance, just a few mph extra which you won't benefit from off road, all you need is torque really.

Someone with better knowledge can correct me if im wrong there.

There already is a manual rev limiter, it's called the throttle.

There isn't a rev limiter on it already, so I'm not trying to get it to rev higher, I'm trying to prevent it from reving higher than the motor was designed to.

The power band on this motor is relatively flat, so Its got almost as much low end as it does high end. It's also got a close ratio gear box, the bike's max speed is around 67km/h last time I did the top speed calculation. But it is capable of going faster if you put the right rear gearing in. with the shortest ratio at the rear, you can get up to about 84km/h. After that, you need to swap the gears in the transmission to go any faster.

However its a mini motocross, so going faster than 70 was never intended, its designed to accelerate like mad. Which it does a bit too well for what I use it for.

Basiclay, there are 2 reasons I want to put a rev limiter on it.

1. I'm not the only one who uses the bike, and I don't want someone comming back to me with a blown motor. I got the bike for free, and anything I spend on it is going to be for general repairs, not an engine rebuild. Which it just had a year ago.

2. It would just be plain cool to do it, its a project that would be interesting to work on.

On a final note, the bike is almost considered vintage, 1977 XRs arn't that common anymore. From what I read, about the only thing you can do to kill the motor on this thing is rev it like mad untill something breaks inside. Other than that, its practicaly bullet proof.

I'm imagining the bike going over some bumpy terrain, causing your pin to bounce against the contact. That is unless you immerse the pin in non-conductive fluid to damp its movement.

The pin can't be vertical, it would bounce up and down.
It has to move to the side to contact. It also has to be set so it doesn't deflect in any direction.

Also, the pin is spring loaded, so bouncing and g-forces woulden't have to much effect on it since it's so small. Even with a weak spring, it shoulden't contact when the bike is running under normal conditions.

I'm not sure what size engine you have but I assume its still a four, so I wouldn't really worry too much about it over revving, once you've hit peak they just run out of steam so theres very little you can do to damage the engine. Maintenance on fours are a bitch and you really need to keep on top of it, I used to change my oil and filter after every other ride (YZF426) even if I'd only been out for an hour.

Another important thing you need to know is not to idle with these monsters as they really get hot quick, if you're not moving then turn the engine off.

If you're still after more advice I suggest you head over to the Thumbertalk forum, they have a special XR sub forum and I'm sure they'd be able to help you further!

http://www.thumpertalk.com/forum/forumdisplay.php?f=253

Quote from Rooble :

I'm not sure what size engine you have but I assume its still a four, so I wouldn't really worry too much about it over revving, once you've hit peak they just run out of steam so theres very little you can do to damage the engine. Maintenance on fours are a bitch and you really need to keep on top of it, I used to change my oil and filter after every other ride (YZF426) even if I'd only been out for an hour.

Another important thing you need to know is not to idle with these monsters as they really get hot quick, if you're not moving then turn the engine off.

If you're still after more advice I suggest you head over to the Thumbertalk forum, they have a special XR sub forum and I'm sure they'd be able to help you further!

http://www.thumpertalk.com/forum/forumdisplay.php?f=253

thanks alot, I'll check it out.

It is a 4 stroke, but an older one.