Results 1 to 4 of 4
  1. #1
    Sly Fox DKFox's Avatar
    Join Date
    Feb 2009
    Huntsville, Alabama

    Ever heard of making mods to the rotary valve?

    The previous owner of this 800 SPX was into racing. I’ve now got the engine out on a table and doing all the regular maintenance just to make sure I know what all mods he did. Have any of you ever hear of grinding the angle of the rotary valve to something other than stock? He did that to the rotary valve that is currently installed and also to a spare one that I found in a box of misc parts!

    This is a 1998 model with the 787 engine. Specs call for Opening BTDC at 147 +/- 5 degrees. Closing at TDC is 65 +/- 5 degrees. I can get the Closing to fall within spec… but due to the wider angle he placed on the rotary valve the Opening BTCD falls at 155 degrees which is 3 degrees out of spec. (Flipping the rotary valve over puts both marks way out of spec so this is the only possible way it can go.)

    This configuration allows the port openings to stay open a little longer whenever a piston is on the upstroke. Guess that allows a tad bit more fuel into the chamber and thus a competitive edge when racing??? I don’t know and I can only guess at the logic. My local dealer wanted $85 for a new rotary valve… ouch…. so I’m putting this one back in. I only gave $900 for the two jet skis and a double trailer!

  2. #2
    96XPSS's Avatar
    Join Date
    Feb 2006
    Warner Robins, GA Home of the 2007 All-Star Little League World Champions!
    If he only removed 3 degrees I would slap it back in there and forget about it. On piped setups above 7600 rpm it may make a difference. The price quoted for a new one is for sure pricey.

  3. #3


    A good new aftermarket rotary valve can be had for 50.00 or less.A few degrees one way or the other wont be noticeable ,unless you have a ported engine, pipe,larger carbs,etc.Then it becomes very noticeable ,by moving power up or down.JMHO>Marvin

  4. #4
    tweeker's Avatar
    Join Date
    Jul 2007
    Denver NC
    Dont use those junk carbonfiber rotary valves either

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Deep grooves under the rotary valve - 787
    By wes35 in forum 2-Stroke Performance
    Replies: 12
    Last Post: 11-08-2010, 07:10 PM
  2. Would the 163 degree rotary valve help my 96xp mods??
    By 1fastxpp in forum 2-Stroke Performance
    Replies: 3
    Last Post: 08-17-2010, 09:53 AM
  3. New mods for the winter
    By buddydoo in forum 4-Tec Performance
    Replies: 5
    Last Post: 11-12-2005, 10:41 AM
  4. Finally bit the bullet..bought first mod,and "the list&
    By Shibby1485 in forum 4-Tec Performance
    Replies: 10
    Last Post: 10-12-2005, 11:33 AM
  5. Got my first mods in the mail today!
    By 05RXPnoob in forum 4-Tec Performance
    Replies: 1
    Last Post: 07-30-2005, 06:54 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts