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pw50 weak spark

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Hi, I'd like to ask a question about what normal spark for the pw50 is like. I could not find in either the service manual nor Clymer manual, if the switch position makes any difference. They say the spark must jump a 6 mm gap.

When testing with the switch in the "start" position, I get a very strong and fat spark between the spark plug wire and the engine fins for example. When the switch is in the "run" position the spark is a lot weaker. I could not make the spark jump between the wire and the engine fins. Using a spark plug, I can see the spark is blue in both cases, maybe a little bit stronger in the "start" position but it does not seem "fat" at all. This is with a new spark plug.

I'll get a spark tester and see how far the spark jumps, but I doubt it will jump 6 mm with the switch in the "run" position. Is this normal?

The engine starts usually at the second or third kick in the "start" position and keeps running ok in "run", meaning the bike is ridable. I want to do a top end rebuild, as the compression is below 90 psi, it smokes, rattles the piston, and it does not make enough power. I thought I would do a spark test first, and then I saw the spark was weak, so maybe it does not burn the fuel very well either.

What do you think? Any help is greatly appreciated.

Thanks much!

Sorin

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Spark is really tough to test and get real numbers while doing so. CDI parts usually work, or they dont. Very little in between. I would proceed with the top end rebuild.

Its never a bad idea to open up the connections and clean them. Clean the coil mount as well. Thsi may avoid problems later.

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The reason your spark decreases when the switch is changed from "start" to "run" is that it is a totally different ignition curve. Start is only giving a lot of spark very low in the RPM range, where as the run setting is providing enough spark to run up until the rev limiter.

doing the spark check without the plug cap on is only showing you that you have spark, the actual cap will decrease spark significantly. Using an adjustable gap spark tester like this is the only real way to test for the http://www.thetoolwarehouse.net/shop/THE-404.html

This is the best way to test spark and a whole lot more to do with the ignition, but it's pricey at $4-500.

http://www.klsupply.com/ProductCatalog/ProductDetails/tabid/57/Default.aspx?pid=62

You mentioned that your compression is down to 90psi. I don't recall the specs, but even with it only having a 6:1 compression ratio it should still be higher than 90psi.

hope this info helps a bit.

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you dont have weak spark you need a new top end compression on the pw50 should be over 100 at all times if its a new top end you will read about 115,

Thanks much. I've found the following formula to derive the compression from the compression rate: http://www.type2.com/library/engineg/comrat.htm

So at 6:1 compression rate, the formula yields a compression anywhere between 86 and 101 PSI. I'll go ahead with a top end rebuild and see what it reads after the rebuild.

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Thanks much. I've found the following formula to derive the compression from the compression rate: http://www.type2.com/library/engineg/comrat.htm

So at 6:1 compression rate, the formula yields a compression anywhere between 86 and 101 PSI. I'll go ahead with a top end rebuild and see what it reads after the rebuild.

you can go by that a little but that does not relly apply to a 2 stroke motor or single cylinder but to air cooled 4 strokes with 4 cylinders,

I bet money you will see a 115 range on compression after rebuild, the pw50 should never drop below 100, it will still run below 100 but it wont have the power it should.

good luck on your rebuild 👍

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