97 spx 787 ..throttle problem top end only

[mike23]

i have a 97 spx with the 787 carburated. when cold it runs fine but once it warms up when i hold full throttle it goes into the power band then drops just out of it then back in to the power band and drops back out over and over. its quick out of the hole it just dont have top end ..when looking things over i found 180 compression in front cyl and 160 in rear.also i found that the back cyl was missing the spring for the rav valve and the rav on my water box looks to be in bad shape corroded. (its a TDR water box)..i installed new plugs and i did get some sea foam to try......please help...
mike23

 

[Nowak]

Not sure Why your compression is 180 but when that happend to my dad's 97 the rear cylinder was shot the rear seal went out on his allowing water to spray in cylinder. He had it happen twice the first was improperly winterized from dealer happened the first year he had it and the second was years later I guess just age.. hope that helps