I have two whipplecharged 500EFI's in my 35 fountain. The starboard engine has suddenly started loosing power above 4000 rpm. The engine will rev up to 4500 - 4800 rpm's then suddenly fall back to 4000 rpm's. If i don't pull back on the throttle the rpms sometimes fall further and the engine backfires. I have changed the distributer, TPS sensor, MAP sensor and the plugs look fine. Fuel pressure at WOT is 55 - 60 pounds and steady. Any ideas would be appreciated.

Are you sure it's not detonating? Get a scan tool and see if you have any knock sensor faults.

Or be very careful - disconect the knock sensor go for a quick blast and see if it stops. If so, you found the problem, then you need to find the solution!!

check for codes. could be pickup coil in the distributor, or the module.

there are no codes in the computer and I replaced the entire distributor including the module. I disconnected the knock sensor and the engine just became more sluggish (didn't have much power above 3000 rpm). The plugs are on the black side and show no signs of detenation.

Are you too fat at high RPM and blowing/flooding out the plugs? Can you reduce fuel pressure by 5 psi on that engine and try it again?

I had this happen to me!!

Are you sure the blower belt isn't slipping and loosing boost then lugging the motor loading it up causing the misfiring,I'm sure you've probably watched boost gauge and checked but I thought I'd ask,Smitty

Is it the top or side mount?

the top mount is supose to have 52psi at wfo depends on the cal in the computer, mine is in the 40's but we dynoednd maped it at that presure ,Its doesent go above 50 even at 11.5 pds of boost.


I thinking valve spring or the computer retarding it ?retarding it far enough to back fire ,not sure ?Watch the timming at speed its will bounch some but ya might get an idear.

if plugs look good fuel should be ok

I am going to check the blower belt today as well as plug in the computer and see if the timing is being retarded by the computer. We will see...

Actuater valve?