Long time lurker but here is my first post...I have a 21' Laveycraft with a very newly rebuilt Berkeley jet drive and powered by a 2 season old completely rebuilt GEN V BBC TBI motor controlled by a MEFI 1 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 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 been working very closely with Offshoreonly forum member Scott "TRASH" on this issue who I would describe as nothing less than a MEFI GENIUS and have actually gone out on the lake twice and logged data & thrown about 10 tunes at it with him using the MEFIBURN software but to no avail. We are both stumped on this issue and Scott 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 Scott and I to be working as they should. The TBI is a 750cfm custom setup with adjustable FPR and 85# 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 Bob at OBD Diagnostics and had him look at it and he didn't notice anything out of the ordinary as well. He then referred me to this forum and Scott 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. 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 ( if I remember correctly - Scott?) 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. 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.

Pic of motor.

Check to see if your intake vacuum shoots ski high at that throttle position, or a c hair (that's a measurement that stands for an almost in - meausarable small amt...lol) before that spot.

Reason for asking, I've run into this with a bunch of 383 small blocks using stock size (read: restrictive) small block tbi's.

The computer see's the intake vacuum rise, and pulls fuel and adds timing. Rising vacuum means (to the computer) less of a load. When in fact, in this case, it's because of a restrive air inlet (not enough tbi air flow).

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What distributor ? GM/Merc/etc EST ?

BTW: With a 'custom' EFI motor, you are pissing in the wind without a compression check, fuel psi check, tuning (programming) the motor 'live' and a Wideband.

You've done all but one.

Explain this further. Do the 2 rpms always vary greatly at different actual rpm spots or is it only right here at this exact rpm spot.

Which one is correct - tach or diag software ?

SB, I'll add a little detail. It does not pull more vacuum as the throttle opens. TPS sweeps key on/engine off and key on/engine on all appear to be normal. MAP sensor appears to run the full range too. IAC operating along with knock sensor. The problem point lies in the 80 kPa region and about 4000 rpm. Stumble begins but eventually motor will pull through and take off. MAP will continue to rise until motor 'break out' (my term). Without wide band I can't tell what's going on (as expected). BPW is increasing however. He intends to get wide band in the future so we can either rule out or fix any lean/rich condition (not that that is necessarily the issue here).

He is running a larger bore TBI, bigger injectors and fuel pressure is stable from the looks I've seen.

I believe the computer scan shows the correct rpm. Idles around 750 ish on the scan, which is what the load calls for. Tach says either 500 or 1400 rpm depending on the detent setting on the back of the tach. With the tach set to the lower value (500 rpm idle indication) we did a small sampling of computer to tach readings. Best case at idle is about a 200 rpm difference with the two readings diverging as rpm increases. WOT computer shows 4800 ish while the tach is about 3400 or so. My ear says the motor is doing 4800 rpm and not 3k. The computer rpm would also more closely correlate to the GPS speeds we've seen.

It is a curious situation but I'm sure we'll eventually tackle this. I noticed no holes, dips, or other crazy things in the fuel or spark tables that might lead to this and I'm not the only guy to have said this in regard to the tune.

I would only add that Bonzen must have been drunk posting as he referred to me as a 'genius'. I, along with friends and cohorts, can assure you I am at the opposite end of that intelligence bell curve.

SB, I'll add a little detail. It does not pull more vacuum as the throttle opens. TPS sweeps key on/engine off and key on/engine on all appear to be normal. MAP sensor appears to run the full range too. IAC operating along with knock sensor. The problem point lies in the 80 kPa region and about 4000 rpm. Stumble begins but eventually motor will pull through and take off. MAP will continue to rise until motor 'break out' (my term). Without wide band I can't tell what's going on (as expected). BPW is increasing however. He intends to get wide band in the future so we can either rule out or fix any lean/rich condition (not that that is necessarily the issue here).

He is running a larger bore TBI, bigger injectors and fuel pressure is stable from the looks I've seen.

I believe the computer scan shows the correct rpm. Idles around 750 ish on the scan, which is what the load calls for. Tach says either 500 or 1400 rpm depending on the detent setting on the back of the tach. With the tach set to the lower value (500 rpm idle indication) we did a small sampling of computer to tach readings. Best case at idle is about a 200 rpm difference with the two readings diverging as rpm increases. WOT computer shows 4800 ish while the tach is about 3400 or so. My ear says the motor is doing 4800 rpm and not 3k. The computer rpm would also more closely correlate to the GPS speeds we've seen.

It is a curious situation but I'm sure we'll eventually tackle this. I noticed no holes, dips, or other crazy things in the fuel or spark tables that might lead to this and I'm not the only guy to have said this in regard to the tune.

I would only add that Bonzen must have been drunk posting as he referred to me as a 'genius'. I, along with friends and cohorts, can assure you I am at the opposite end of that intelligence bell curve.

What is the kPa with KOEO (key on, engine off) ?

What is the kPa at WOT and top rpm ?

80kPA is approx 23.6"inHg

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And why am I thinking about things without knowing what the A/F ratio throught the rpm range and especially this troubl spot.

Why Oh Why am I getting sucked in early. LOL.

Im sure Trash will be able to answer what those kPa values are when he gets back on here being that he's a GENIUS with MEFI stuff haha

No I was not drunk when I posted this as I did it on my lunch break at work and there is no way I could get hammered in that short amount of time!

Wideband will be coming in the next few weeks...but until then I'm looking for some "out of the box" ideas that could be just a simple setup thing or replacing some part that I may have overlooked?

the Kpa at wot better be close to 100 or there engine needs a bigger throttle body.100Kpa = 0" of vacuum give or take the absolute Atmosphereic pressure.

I'm also leaning to a wideband to see if theres a hole in the fuel curve,rich or lean.