G1 not making horsepower after 4000 RPM

[Kevin trapane]

I’ve got a 3.5 L Oldsmobile in sandcar hooked to a G1 link plus motor pulls hard to 4000 and then stops making horsepower engine makes slight change in tone. 

[Kevin trapane]

And the AFR seems to be good

[David Ferguson]

Is the engine mechanically OK?   Have you done a compression, or leak down test?   Have you checked for broken valve springs (valve float can make an engine fall over and change tone)?

if this is the 3.5L V6 twin cam, I understand they can have some timing chain issues.