Motorcycle test tach?

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Motorcycle test tach?

Postby Crazy5 » Sun May 22, 2011 6:20 pm

Anybody have one I can borrow? I have an automotive one, but it only has settings for 3, 4, 5, 6, 8 cylinders. I'm working on a single cylinder engine with a wasted spark ignition system (so it sparks once every revolution instead of every other time like a car).

I tried to figure out the math to use my automotive tach on the four cylinder setting, but I just gave myself a headache.
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Motorcycle test tach?

Postby joshp xj » Sun May 22, 2011 9:25 pm

2 stroke?


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Re: Motorcycle test tach?

Postby Crazy5 » Sun May 22, 2011 9:27 pm

It's a four stroke, but with the wasted spark ignition system, you would use the 2 stroke setting on a tach that had one.
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Motorcycle test tach?

Postby YeeP » Mon May 23, 2011 7:36 am

So the plug fires on the intake stroke? Weird.
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Re: Motorcycle test tach?

Postby Crazy5 » Mon May 23, 2011 12:01 pm

No, it fires on the exhaust stroke. This is on amy's scooter, but a lot of motorcycles do this too.
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Motorcycle test tach?

Postby joshp xj » Mon May 23, 2011 12:49 pm

Is it a cdi ignition? I used to work on little go carts all the time with cdi and those little black cdi boxes went bad left and right. What kind of scooter is it?


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Re: Motorcycle test tach?

Postby Crazy5 » Mon May 23, 2011 3:52 pm

It is a cdi ignition. The scooter is a Chinese import using a 139qmb engine. It's branded as a Baja Sun City SC50, but it's the same mechanically as all the other 139qmb scoots.

Symptoms are a high rpm, full throttle misfire when cruising at top speed. Spark plug looks good. Full throttle power seems good until I get up around the current top speed.

I installed a big bore kit (and adjusted the valves while I was at it), which also included high-flow air filter (uni filter), high flow fuel filter (visu filter), new main jet, and new roller weights for the transmission.

I've been playing around with the roller weights to help with balancing acceleration and top speed, and like how it feels except for the misfire. I also have a main jet kit and experimented with different jet sizes to see if it was starving for fuel, but it ran slower when I put a larger jet in it, so I went back to the jet that was included in the kit.

One of the reasons that I wanted to check it with a tach was to see if I might be hitting the rev-limiter (I think factory limit is 9k?). The miss doesn't sound like that tell-tale rhythmic rev-limiter sound that I'm used to hearing on cars, but this engine doesn't rev that fast either...

I know that I can get an aftermarket cdi that doesn't have a rev-limiter, but I don't want to push this made-in-china engine too hard. If my rpm's are too high, I'd rather change the rollers a bit to bring them back down instead of blowing the engine.

What kind of symptoms would a bad cdi give?
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Motorcycle test tach?

Postby joshp xj » Tue May 24, 2011 9:09 am

From my experience I either had a no start or a bad miss


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Re: Motorcycle test tach?

Postby jollyroger » Wed Jun 01, 2011 11:58 pm

Like JoshP said it's usually a no start or occasional start or a bad miss when the Capasitive Discharge Ignition box shit's the bed.

If it's revving high at idle it might be because it's getting more air at idle due to the increased flow, less restiction, with the new air filter system. This would cause a lean condition that would account for higher idle speed. Sometimes the carbs are not able to compensate for this with adjustment alone. It likely needs an idle jet to be richened to get you back to throttle positions that allow you to set the idle to and at an acceptable speed.

When I'm tuning a carb i start with the idle circuit and work my way up the power spectrum from there......Carbs work in a cascading effect. Every power setting and adjustment works in concert with the lower setting.

Idle = closed throttle. Idle + tip in + accel pump. Idle + tip in + cruise + accel pump. Idle + tip in + cruise + mid power + accel pump. Idle + tip in + cruise + mid power + full power + accel pump = wide open throttle.

Your carb may not have as many circuits as that description but they all work in the same way.

So if you are lean at idle and have to richen the high jets to compensate for max power at WOT it will create drivability problems because you have an imballance in the mixture across the range.

So fix the idle problem and work up from there.
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Re: Motorcycle test tach?

Postby Crazy5 » Tue May 08, 2012 8:23 pm

Not sure how I missed your post last summer Johnny, but I haven't fixed this yet. It idles great (and at the rpm it should), mid-range is great, acceleration feels great. It's only misfiring when it's at WOT, and only when it's around it's top speed (so the engine rpms are the highest they ever get).
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