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How often does spark fires on WR250F?

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Hello, still trying to trouble shoot my 05 WR250F. I think I may actually be nuts and that I may not have an RPM rev limiter problem. I think my Tiny Tach could be a 2 stroke model, and secondly the grey wire mod gave me a bit more mid range that gets me to rev limiter quicker with out as much of a "hit" at the top.

Basically the Tiny Tach rev limits at 6000+ which if it was 6500 then 6500*2 puts it pritty close to 1300rpm.

Some one mentioned that a spark occures on every motor revolution, though I just talked to Yamaha dealer's parts guy and he said its every 2 revolutions.

Again I think between the false info from RPM gauge and different power band after snorkel out and grey wire removed I might just think its not reving as high.

Unfortunately with work I have not had but one ride with bike before mods were done.

Anyone know for sure if we get a spark every 2 or 1 revolutions?

:)

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That's what you get for getting your tech info from parts guys. It has a Power Stroke every other time around, true, but it makes a spark EVERY revolution. The spark is triggered by a signal point on the flywheel. If it were triggered by a signal generator on the cam, it would be every other stroke, but it isn't.

If the tach is set up for a two stroke single, it should read right.

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Oka then there me be a problem after all. :)

I just did some speed test a found that I hit rev limiter at:

27 mph in 1st

40 mph in 2nd

52 mph in 3rd

This data is generally averages and obtained via a good hand held GPS.

Would this indicate a early hit of the rev limiter?

Blue wire disconnect DID require me to use clutch to start. Re engaged blue wire and now I can start in neutral again. So gear switch seems not to be the problem....again if there is one......

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This is basic engine theory.

2 stroke = once per 360*

4 stroke = once per 720*

and now for basic magneto theory and operation: the CDI system typically used on small engines like your 250F cares not whether the engine is at TDC at the end of the compression stroke or at TDC at the end of the exhaust stroke. it fires either way, ignorant of the fact that it's firing into exhaust gases in the second case.

ergo, on a 250F you get one spark per revolution.

or in your terminology, one spark per 360*.

jim aka the wrooster

'01 wr250f

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and now for basic magneto theory and operation: the CDI system typically used on small engines like your 250F cares not whether the engine is at TDC at the end of the compression stroke or at TDC at the end of the exhaust stroke. it fires either way, ignorant of the fact that it's firing into exhaust gases in the second case.

ergo, on a 250F you get one spark per revolution.

or in your terminology, one spark per 360*.

jim aka the wrooster

'01 wr250f

Exactly. The signal trigger is crank mounted (in the flywheel), and it passes the pickup coil once each trip around. There's nothing on either camshaft to tell the ignition how to tell the difference between TDC compression and TDC exhaust, so it is, as Jim says, ignorant of it. One spark per revolution, same as a two stroke.

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Makes sense to me. I never thought about it firing at BDC. You learn something new every day here on TT.

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Hmmm so the ignition system is not as hi tech as that in a modern car. Well then if I got a Tiny Tach variant that was made for a regular 4 stroke.....then maybe it is dividing the sparks by 2 when it should not be. Hence an abnoraml RPM indication!

:)

Do those speeds I mention seem right for the gears?

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Makes sense to me. I never thought about it firing at BDC. You learn something new every day here on TT.

it's not firing at BDC.

jim aka the wrooster

'01 wr250f

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Makes sense to me. I never thought about it firing at BDC. You learn something new every day here on TT.

It's still TDC (or the same advance relative to)... it's just TDC on the end of the exhaust stroke.

I suspect this may be the reason thumpers backfire through the carb.

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27 mph in 1st

40 mph in 2nd

52 mph in 3rd

Given that the bike peaks at about 85 in 5th, I think you're right in there hitting 52 in 3rd.

When I'm cruising a fire road, I'm comfortably in 4th at 40-45.

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It's still TDC (or the same advance relative to)... it's just TDC on the end of the exhaust stroke.

I suspect this may be the reason thumpers backfire through the carb.

DOH! :)

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