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Enduro Engine - 2 stroke Cylinder, Piston, Tranny, Bearings, Clutch, Pipes & Silencers, etc. |
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#1
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MC 250 port timing...
Ok, I'm in the process of total rebuild on my '05 MC 250. Right main bearing went...
So during disassembly I measured how my piston aligned with exhaust port. Top of piston .83mm above bottom of exhaust port. One .3mm base gasket. One might conclude they dropped the cyl. to increase compression on the MC. The parts fiche also shows a different head for the MC. So based on the conventional wisdom expounded on this site I mentioned to the engine guy pressing my crank that I was gonna raise my cyl. To align the exhaust port and would likely need squish band correction. Well... He said (emphatically) that aligning top of piston to bottom of exhaust port is WRONG... that piston should be aligned to bottom of transfer ports. Basically said bottom of exhaust port is irrelevant... He also mentioned that IF the piston is high on the transfer ports, correcting that will give me more top end... I got plenty of that now I just want a 'crisper' low to mid. My questions: - I didn't check piston related to transfer ports (and cyl. is at machinists presently) but why is the talk here about exhaust port and not transfers? Maybe bottom of trans and ex is equivalent on these motors? - If all I'm worried about is low-mid grunt is there anything to be gained by messing with altering port timing via base gaskets? The way I understand it one would actually want to lower the cyl. to move timing towards more favorable low end... - I will definitely get the squish corrected once this is back together so maybe that's sufficient? - Does this sound like I'd maybe like a 300?
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#2
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I just had a quick look at my 300 jug. The intakes and exhaust are both the same at BDC. Basically by changing the base gasket stacks you're changing the time and duration that the ports open/closed which will alter the engines performance and characteristics.
As it is running with the piston .8mm above the exhaust/intakes you have the engine set up to favour bottom end. Depending on the final compression ratio of the engine it may also be set to boost bottom or top. A higher CR will make better snap down low, where less CR will want to rev further. Basically your mechanic was right, that if you raise the cylinder more you will be taking from the bottom and adding to the top. Moving the cylinder lower making the overlap greater than 0.8mm and you will start to see significant losses to top end so I wouldn't recommend that. If you haven't had the squish corrected and the compression ratio set I would look at doing that. It will improve engine efficiency and you'll get a gain across the board. Other considerations. Does the MC model use a more aggressive CDI? 2k2 ignition? Have you tried altering the ignition timing? Flywheel weight to stop it racing off so fast? How many teeth on the rear sprocket? Are you using a povervalve cover to boost low end? Jetted on the richer side to improve torque? Last edited by Jakobi; 07-03-2012 at 08:38 PM. Reason: ..i cant speel |
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Quote:
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broken down over 60 slow rider.. |
#4
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Quote:
On the topic, when I changed from the 300 top end to the 250 I went from having the ports timed flush to a slight overlap. This was mostly because the 250 cylinder was an S3 Race ported one which had the exhaust raised a touch so I could do this without losing anything on the top. Like you I would choose a 250 over the 300 now. To the op, when you say you're looking for bottom end, where is the 250 letting you down? |
#5
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Quote:
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broken down over 60 slow rider.. |
#6
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Ha! Choice! I'm going to have two beers to celebrate!
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#7
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I install alwasy my cylinder with the thinnest gasket, 0,15mm.
And I modified once the cilinder head to get the right squish. The wrong thinking of everybody is, to align the piston with the bottom of the ports, but when the piston is in bottom dead center, there is now pressure any more and no gas will flow, means a adge between teh port and the piston makes no difference. After installing the cylinder the first time like this, I checked out that I reduced the loss of flushing.
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4 strokes are only good for power generators |
#8
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Changing the deck height certainly does have an effect on the power curve. Even if nothing is happening at BDC any changes effect the time in the stroke at which the ports are effective. IE having the piston sit higher in the cylinder means that the exhaust port will open later, and be open for a shorter duration. This will give more back pressure and low end power, but as such won't flow as well higher into the rpm (someone correct me if I'm wrong - just using logic and pretending that my beer is a piston and the desk is the exhaust port).
From my rebuild thread: Quote:
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#9
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And for the 300 its even more evident that dropping the cylinder too far will kill the top end..
Here is the last graph that Dave did up for Cols 2010 EC300 Euro Quote:
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#10
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Quote:
im not sure you should go with piston and ports flush on a 250, but for a 300 it seems to work... Really all that doesnt matter as much as making sure the squish is right at the end of it all...... |
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