Long time lurker but here is my first post...I have a jet boat with a completely rebuilt GEN V BBC TBI motor controlled by a MEFI 1 ECM and it runs great except at 3/4 throttle when it seriously looses power at the same point every time like clockwork...engine RPM drops, sounds very rough, runs like A$$ and completely scares the hell out of you....but if I push the gas pedal "through" it to WOT it instantly picks up again and pulls hard all the way to a top speed of about 55-60 GPS. The dead spot exists only at around 3/4 throttle, or in the 80 kPa region and about 4000-ish rpm but then it's fine after that and builds quite a bit more power. Id like to figure this out so I can have a smooth powerband all the way. I have actually gone out on the lake twice and logged data & thrown about 10 tunes at it with my tuner who has a vast understanding of the MEFI software using the MEFIBURN software from www.http://mefiburn.com/ but to no avail. My tuner and I are both stumped on this issue and he cannot find anything out of place so far on all the program settings and ECM tables. It all "looks" correct or in other words nothing sticks out as a red flag to create the issue that the engine is experiencing. To an extent, we are somewhat tuning blind as I do not have a wideband 02 sensor installed so we have no idea of the A/F ratio however I do know hat the motor runs very rich and the spark plugs confirm that. Some motor information - 1994 454 bored .030 over using 10:1 compression SRP pistons. 91 octane used only with octane booster. Whole rotating assy was professionally balanced. Stock small port iron heads with all new valve train. 3/8" cromoly pushrods. Comp Cams 226/236 @ .050 112 LSA. Factory GM aluminum dual plane intake manifold. Engine shows 160 psi in all cylinders on compression test. The TBI is a brand new unit from SPR performance - www.tbiparts.com. No vacuum leaks. Brand new TPS and IAC were shipped with the TB and both have been calibrated correctly and verified by my tuner and I to be working as they should. The TBI is a 750cfm custom setup with adjustable FPR and 80# injectors. Fuel pressure is a steady 28psi, at idle and under load. Every engine sensor and the entire ignition system has been replaced at this point(Ignition coil, pickup coil, ignition control module, cap, rotor, wires, plugs) Spark plugs are not lean looking at all and gapped to .043. The timing has been set and checked and matches the timing value in the ECM (8 degrees BTDC locked out) Fuel pump is working strong, (high pressure pump) All fuel filters are new, water fuel separator is new. Gas tanks are venting properly. Basically the motor is brand new inside and out.

The ECM had been originally calibrated by Arizona Speed and Marine for the new motor/camshaft specs, after I verified that the newly rebuilt motor ran mechanically good with the "stock" tune(or whatever it was) last season. However, I had never set the timing correctly (locked out) while running with the "stock" tune so I always assumed that the ECM needed to be calibrated for the motor to run correctly. Once I learned about setting the ECM in service mode, I locked it out and timed it properly. The motor did gain some RPM and run faster but then almost immediately the "flat spot" started to appear at 3/4 throttle. So I do not know if this is an ECM issue that AZM screwed up or a hard part issue with my engine setup...or something ignition related maybe? A little over a month ago I sent the ECM into OBD Diagnostics and had the designer of the MEFIBURN software look at it and he didn't notice anything out of the ordinary as well. He then referred me to my current local tuner for further help. I am going to get a wideband setup soon but until then I am looking for an out of the box hypothesis as to what could be causing such a bad, but on the money every time dead zone in the powerband in the exact same spot. The one weird thing I will say, is that the tach on the dash reads 3,400 RPM while the tuning software is reading 4,200-ish on the laptop when the problem occurs. Every time at that RPM give or take 100. I will also note that this problem did not exist at all with the old motor which was your standard run of the mill marinepower 330hp 454 TBI. The issue also did not exist with the newly rebuilt motor until I had the ECM recalibrated by Arizona Speed and Marine and bolted on the new bigger bore TBI which were done at the same time. This issue has been haunting me for over a year, I am out of ideas and almost out of $$ haha. Thanks guys for any and all help, as to what could be causing this really weird issue. I've never heard of a motor running great to a certain point, then running like crap, and then running great again at WOT.