Tigerx7

Hey all, as the title says, I'm having a severe sputter/loss of power below about 3K rpm (the actual rpm that it stops varies). During the sputter it is running extremely lean 18:1+ AFR. As it nears 3K rpm the AFR starts to drop and by 3.5K its completely normal. It idles a bit rough and is a tad lean at idle (about 14:1 AFR). in neutral it revs normally with very slight throttle, but if I get into it at all it sputters. when driving it sputters and runs lean no matter what unless I can keep it above 3K at which point it runs great whether cruising or full throttle. Makes full boost of 10 psi, and there's no vacuum leaks. It does have an exhaust leak from a crack in the manifold, and the radiator fan switch stopped working a few days after the car started acting up. It used to run at about 1/4 on the temp. meter but now its running at about 1/2 way up. other than that everything is normal.

It started acting up while on the way to a meet, cruising at about 80 on the highway and it was fine, then I got off of the highway and as I was accelerating away from the first light is when it started acting up. at first it was just below about 2k rpm and I kept driving it because I was only a few miles away from the meet. By the time I got there it was acting like it is now. The only thing I changed was that as I left for the meet I filled up on gas and I switched to Benol oil for premix. I've switched back to quicksilver now and it had no effect.

Its an 88 TII with an RB turboback, Rtek 1.7 and 720cc secondaries, OMP deleted and premixing at 1 oz per gallon with quicksilver, all emissions removed.

I did search and didn't find anything like this. Anyone have any ideas? I'm stumped.
thanks in advance

Tigerx7

I forgot to mention that I've done the throttle body mods, gotten rid of the e-shaft thermowax, and deleted the twin scroll system. All of this was done long before it started acting up.