jaguar

~SPONSOR~
Jul 29, 2000
1,501
82
South America
I have a rebuilt '89 Honda CR250; new piston/rings, new bore job, HPP working good, stock pipe. It has plenty of low end power but is just plain linear up to its top rpm's. The port timing is 89 degrees exhaust (w/valve open) and 119 degrees transfer.
Any suggestions for more top end power w/o losing much trail riding low end power? I was thinking about raising the transfers a bit since I can't see changing the exhaust timing because that edge is where the cylinder meets the power valve.
 

Pete Payne

MX-Tech Suspension Agent
Nov 3, 2000
933
38
The original cylinder was a nicasil coated cylinder (non boreable--without a replate at least) So I bet if it has a resleeved cylinder that the port timing is not where it should be ?????
 

helio lucas

~SPONSOR~
Jun 20, 2007
1,020
0
jaguar said:
Any suggestions for more top end power w/o losing much trail riding low end power?

what are you willing to lose? cr500 engine? :nener:
more top for trail riding? i would not like it...
maybe an eye in compression and squish, maybe race gas will give you more performance... only my opinion of course... :blah:
maybe grinding the exhaust valve?
 
Last edited:

spark250

Member
Feb 7, 2008
128
0
Then just ride the thing and wait till you have some cash!

Unless you have a lot of seat time with a grinder you could really hurt your motor instead of help
 

Pete Payne

MX-Tech Suspension Agent
Nov 3, 2000
933
38
to change the transfer port timing you need a right angle grinder made buy Foredom. -Its the only way.

Just for a FYI exhaust port timing 90deg ATDC is bottom end oriented power and 80deg ATDC is top end oriented .

Then there is the blow down timing - that is the amount of time between the exhaust port opening and the transfer ports opening .

There is alot to the porting of as 2 stroke .

I have been doing it for 23 years -dang I am getting old !
 

whenfoxforks-ruled

Old MX Racer
~SPONSOR~
Oct 19, 2006
8,129
2
Merrillville,Indiana
What, like Eric does not have a phone, email? I do not think I have ever had a short call to him? Does it have a liner in it? Is Pete staying busy, call him!
 

Pete Payne

MX-Tech Suspension Agent
Nov 3, 2000
933
38
I normaly shoot for 29 to 31 degrees of blowdown timing -providing everything else is right. There are way too many variables to list . The blowdown timing is one small part of a big puzzle.
 

jaguar

~SPONSOR~
Jul 29, 2000
1,501
82
South America
OK. I went ahead and raised the transfers 1.4mm which gave the desired result of reducing the low end power and increasing mid range. The difference was slight but noticable and just what I wanted.
Too bad these bikes weren't made with valves on the transfers to also change their port timing in harmony with the change to the exhaust port from low to high rpm.
 

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