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Old 08-18-2016, 11:21 AM   #1
motoxvetII
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Default Anyone try raising the compression on a DT-250?

I was thinking of trying to raise the compression on a '77 DT-250 to try to improve the squish area and the compression will probably go up a little. Am I looking at a jetting nightmare if I do this? If I have to completely re-jet the carb, I think I'll probably skip it. Has anyone had any experience doing this?
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Old 08-18-2016, 01:58 PM   #2
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I haven't done it but found this interesting article which is somewhat related. How many pounds compression are you shooting for and squish thickness?

Taken from KIWI BIKER:

Simple, the changes you have made will have contributed to making 1 or more of the 3 main variables, rise in value.
That is Trapping Efficiency,Scavenging Efficiency and or Delivery Ratio.
What accompanies an increase in 1 or more of these, is usually that you change the Brake Specific Fuel Consumption number ie Lbs/Hp/Hr - Kg/Kw/Sec
and thus make the engine more efficient at using the injested fuels energy to make power.
The bottom line, where say making the stinger bigger is a perfect example, is you have reduced the fueling needed to keep the engine alive ie using fuel to cool
the engine and prevent deto or seizure, and using it instead to make Hp.
Thus dropping the BSFC, means a leaner jet will make more power , safely - its a myth that more power automatically needs more fuel.
In fact depending upon the mods done, the carb velocity may well have DROPPED, but the air ingested is being used more efficiently to produce Hp.
And finally all of the above effects are easily seen and graphed in EngMod - not " just " the final effect of more Hp.
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Old 08-18-2016, 06:36 PM   #3
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This is just my theory. Increasing the compression works differently on 4-strokes and 2-strokes. Cylinder filling on 4-strokes is dictated by and large by what the piston draws in. 2-strokes have positive pressures by and large from the crankcase. The 4 draws in what it needs, the 2 still fills the cylinder the same as before, so the excess fuel/air causes the need to lean the incoming fuel mixture. What you need of course is someone who bumped their compression and either had to re jet or didn't. Even more concern might be the issues associated by higher compression and squish velocity.
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Old 08-18-2016, 06:48 PM   #4
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No edit- anyhow I believe I should have said the swept volume of the 4 and 2 have changed with higher compression. Since less fuel/air in the 4 the fuel/air mixture needs to be increased while the lesser swept volume in the 2 requires a leaner mixture because the same amount of fuel or delivery ratio from the crank case remains the same.
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Old 08-19-2016, 01:02 PM   #5
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I was going to be quite conservative of the squish dimensions. I was thinking of checking it with a piece of solder and then taking it down to maybe .055 to .060, maybe depending on what it first measures. I thought all of the DT-250's I've dealt with, they didn't feel like they had much of a compression ratio. Since the non-eth I always use is 91 octane, a small jump in CR I don't think would be a problem.
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Old 08-22-2016, 10:34 PM   #6
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Default RT-1/RT-1MX compression

I know the '70/'71 Enduro and MX models had different heads (smaller combustion on the MX) and different carbs. In '72 I bolted an MX head on my 360 RT-1B enduro, no other changes at all and it ran great. A little hard to start but it did improve performance as I could climb hills that I couldn't prior to the swap. I would think you could do the same with a 250 based upon a percent calculation? Get you in the ball part anyway.
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Old 08-24-2016, 09:00 AM   #7
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Hmmm. Maybe, I'll just try to find an MX or YZ head.
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Old 08-24-2016, 11:01 AM   #8
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I should have asked this before. Does anyone know the interchange of 70's cylinder heads? I'm putting it on a '77 DT250.
Thanks.
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Old 08-24-2016, 01:41 PM   #9
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Here is a list of model code breakdown-

http://www.cmsnl.com/help/faq/95_yamaha/

Looks like the head gasket for your 77 was first used on the 74 TY250. The parts list below of compatible head gaskets was sort of unclear. Looks like a 434 gasket. These gaskets should net you the cylinder heads that will work on your cylinder in theory. Still don't know why a few 400's were included.

http://www.cheapcycleparts.com/oempa...ylinder-head-1
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Old 08-24-2016, 01:44 PM   #10
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Don't know if some of the heads will have pipe clearance problems or angled spark plugs or head mounted compression releases, etc.
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