two stroke ?'s

[Glarusguy]

My canam dirt bike says to check the timing at 5000 rpms. Two questions:
1. Why would it change? Which it does at different rpms. It is the simple magneto style.
2. What is a good source for an rpm guage that would work for this single cylinder?

Oh and a third question while I'm here. What is a ball park compression reading that I should expect on this two-stroke motor. It just doesn't run the same ever since I had the cylinder resized to my new piston.

Thanks all!

[MartyA]

2 stroke - half the power twice as often...

Must have a centrifugal advance mechanism. Like the XS, which in addition to the vacuum "advance" has a mechanical centrifugal advance which is at full advance at 5,500 RPM.

Most flukes or hand-held electrical testers have RPM setting. Not the first level $9 ones, the $19 dollar one I had had a tach.

If I recall, 6.5 to 1 up to 8.0 to 1 is fairly typical 2-stroke compression ratio. There is some weird method of calculating bottom dead center to top dead center volumes that makes the ratio calculation a little different from 4-stroke.

Not sure about the latest technology.

[Glarusguy]

Nope no vacuum or centrifugal advance. Just straight magnet and coils. This motor is even simpler than a lawn mower engine. I guess I didn't get far enough in electronic school to explain to me why the timing would change at a higher rpm. Unless there is a lag due to the magnetism.
I'll have to check the magazines for an rpm guage.


Just when you get a grip on one thing the other is needed.

[MartyA]

Well the timing does have to change or you wouldn't get too far too fast.

Might be in the electronics?

30 years ago I knew a lot about 2 strokes, adjusting the 5 exhaust sections for optimizing power in a certain band or RPM, timing, port timing, 'shaving' piston heads to increase volume and change the timing, etc. Frankly I don't remember most of it anymore. I do remember that the timing is not as adjustable as a typical 4 stroke, and there is usually a very narrow band of power and that band is adjustable up and down the RPM but mostly you cannot widen it much.

Some quick google search stuff:

http://www.bridgestonemotorcycle.com/documents/higher_compression6.pdf

http://www.eric-gorr.com/techarticles/basic_two_stroke.html

http://www.users.bigpond.com/jack_stands/motorcycle_advice/two_stroke/two_strokes_1.html

Have fun

[Glarusguy]

Excellent articles! It those kind of writings I spend way too much time searching. See...it does pay to be lazy sometimes. Thanks for including them.

Well. I guess I have some more reading to do.

Thanks Marty!

Sorry... but I have to say that 30 years ago I was just figureing out how to use the toilet.

[MartyA]

Well, me too. Sally says I haven't learned yet...

Actually it was more like 35 or 36 or 37 years ago. Yeah, I'm still learning math also.

Hope the articles help. I just glanced them quickly to see if they might help. There are probably more and better ones.

The one thing we did to jack up the power was redesign the exhaust. As mentioned there are 5 (I remember 5, anyway) sections, their sizes can be calculated to match where in the RPM band you want the most power. It always meant making the overall lenth longer, but the expansion chamber dimensions and all the other sections were fabricated based on either ratios or calculations. If you ever see land speed record attempts with 2 stroke engines you will see REALLY LONG - 10'-12' and longer - exhaust pipes. I think that was to get the power up in the very high RPM range.

We messed with pistons also, and reed valve timing.

[LoserShoes]

If the magneto says anything about CDI, it probably has an advance built into the pickup module. Most weedwackers and such that have a flywheel magneto nowadays have a built in advance.

Steve

[webbcraft2150]

Stihl used to offer a handheld tach for dialing in there two stroke chainsaws. Bout the size of a calculator, you held it next to the spark plug. Should be able to still find it from a Stihl dealer or check with the larger tool distributors. Seemed to work fine up to and past 8 grand, bout as fast a stihl will go!!

[GNEPIG]

.. hey Thomas, this will give you more power
the price seems a little high but these parts are not getting any easer to find and i remember parts for my can am being real expensive in 1988, $500 ! for a piston kit for an 84 500 MX

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1,1&item=4580425339&sspagename=STRK%3AMEWA%3AIT

[Glarusguy]

Funny how I just left Ft. Myers yesterday too. Could have saved shipping. I notice the compression too. I am not getting that and the tht is the enduro model. I still need to double check my squish area first, but I think my machinist screwed up. Thanks for the info guys. I know I can get a .030 and a .060 over for pretty cheap. Just need to find out whether I want to go to .060 or not. The mx cylindes are pretty thick,but,,,