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9899
Joined
8/17/2006
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CA
Edited Date/Time
3/7/2012 10:31pm
So....my buddy has a nice 85 CR500 and we are trying to make a fire breathing ice racer out of it. While I've successfully ported and other wise hopped up many two strokes with good results, I've never really touched any of the CR500s Ive owned. (never needed any more power) My friend wants to see just how much horsepower we can get from this old girl so who am I to argue? 
The problem is we have a very short season so there is no time to send the cylinder off for porting so what I'm looking for is porting specs. I know from experience that good tuners are notoriously tight lipped about port specs but maybe, just maybe this bike is old enough that someone might just share their specs. Any ideas?
We have already started ordering the various other parts we need but don't want to buy things like reeds, pipe, carb until we know what porting we are trying to match the rest to. I really just need the port specs so I can start grinding!
The problem is we have a very short season so there is no time to send the cylinder off for porting so what I'm looking for is porting specs. I know from experience that good tuners are notoriously tight lipped about port specs but maybe, just maybe this bike is old enough that someone might just share their specs. Any ideas?
We have already started ordering the various other parts we need but don't want to buy things like reeds, pipe, carb until we know what porting we are trying to match the rest to. I really just need the port specs so I can start grinding!
Action starts at 4.00.
https://youtu.be/nUSfYNFBx94
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The first one I did, I removed material on the exhaust port and also in the webbing. I matched the case webbing (I guess some people call this case ports) to the cylinder. I polished everything smooth on the exhaust port. The webbings were left with a slight "dimple" finish to promote fuel mixture swirl. I also shaved the head 0.010" and re-matched the squish band. This bike ran good. It lost a little bit of power down low and gained some on top. Mid was very strong.
The 2nd cylinder I did I didn't remove any material I just polished all of the cylinder ports. Again, I left a "dimple" finish on the crankcase webbing. I did the same mod with the shaved head. this bike seemed to run better than the first. I had no power loss down low. It was slightly stronger across all the powerband with a slight increase in over-rev.
Both bikes ran VP fuel and were jetted the same. They were both rebuilt from the carb to the sparkplug using the same parts and same pipe/silencer.
IMO, I think the 2nd configuration worked better. It was also less labor and easier than the 1st.
The second one the exhaust port has been raised 1.5 mm and widened 4mm each side this allows the gudgeon pin cut outs to open to the exhaust port by about 2 mm without performance loss, the hit of the power is intense and traction can be a problem. The 85 fueling is poor at best in mid range to cure this I switched to a1986 cr 250 airbox with a 1995 cr250 carb the carb has 2 top vent pipes like the 1996-2001 carb but without the electrical fuel valves .This cured the poor midrange fueling. The wide exhaust port does reduce piston life but I get 2 seasons out of a piston before the piston wears at the intake side to the wear limit.There is a lot of dead air space inside the reed block to adress this use a 85-88 reed block with a 89 onwards carb boot and integral reed stuffer .Use medical grade dental impression putty to smooth the mismatch beween the boot and reed cage and open reed petal windows in the cage towards the flange to match the boyessen dual stage reeds. this sorts the fueling supply. I have tried several pipes and found the FMF gnarly works well with this port arrangement.Arrow pipes reduce the hit and has good over run the dep boosts the bottom end but kills the peak power.Currntly working on a barrel and have matched te boost ports to 2006 2006 style still to be tested. apparrently the 1984 250 cdi improves the hit and rev ability but may make it a hand full on the ice. I will try this after the barrel just ported has been prooved then it is a long rod conversion to boost bottom end.
Regards
Griffel
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