Merc 350 mpi not producing full power

[jdubslopro]

You guys have been very instrumental in my efforts to get this thing going. It's now 95% complete and running with no issues. The other 5% comes from a lack of power.
Compression is good, plugs are new, half the injectors are new. Cap rotor and wires look good, although I ordered new cap/rotor just to eliminate that as a possible issue.
Serial number 0m0112557
The issue is this:
Engine has a hard time over 4,000 rpms. It gets up on plane pretty good, but once it's there it takes a while to accelerate further. I know there is a rev limiter around 5,000 rpms and it will reach 5,000 rpms. Top speed is 50mph at 5k with a 19p 4 blade prop. My v6 had the exact same performance as this, and the new motor is over 100hp increase. I put a 25p prop on it and it struggled at 4,000 and 46mph. I know neither of these are the correct prop, but the 23p 4-blade I bought is actually a LH rotation so I can't put that on.
I don't think my issue is completely prop related as it takes a LOT for the motor to go from 4,000 to 5,000 rpm with the 19p.
There is no backfiring, the timing is set perfectly to merc spec (8• btdc@1,000rpm with diagnostic terminals a&b Jumpered) the plugs are perfect upon inspection.

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

Also note the outdrive is original v6 gear ratio 1.62:1, fuel pressure is constant 40psi, and the reason for injector replacement was the plastic clips broke on 4 of them

 

[alldodge]

Re: Merc 350 mpi not producing full power

Sounds to me this is a prop issue since it will reach 5K. Are you saying it gets to 4K with ease and takes quite a bit more to get to 5K or something else? If your turning 5K then the prop is about right, but the more blades on a prop increase the resistance and drag. If you reach 5K with a 4 blade, you may overtach with a 3 blade. The 4 and 5 blade props give higher lift but loose top end with heaver boats.

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

This is the exact same prop and the same "performance" as my 4.3 v6 last year. It should be much more powerful with the extra 2 cylinders and 105hp. I understand that with the same prop at the same rpm it's going to go the same speed. But it should reach that speed much more easily. And with that gear ratio it should be able to turn the 25p with a little more ease, it seems like its struggling on the top end.

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

It should want to rev to 6,000 with what's on there now...but it's struggling at 5,000. It doesn't bounce off the rev limiter like a car that's in too low of a gear. It just "tops out"

 

[500dollar744ti]

Re: Merc 350 mpi not producing full power

You need to try a 21p prop, though you're over-estimating the effect of 100 more horsepower, propped correctly you should see a little better hole shot and some more top speed. What type of boat is it and what is the empty weight?

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

It's a 98 20' Mariah Shabah open bow. A 260hp 4bbl 350 pushes 62mph in the exact same boat with a 21p (1.5:1 gear ratio or thereabouts whatever they used in that vintage with alpha one). Now the prop will inherently turn slower due to gear ratio, which should give a quicker jump in rpms, and reach top rpms quicker. Not sure of the weight, but I know this same motor in a 21' wellcraft Excalibur pushes 65mph. I don't feel like I'm overestimating the expected power increase. I expect at least 60mph...
Do I need to go this fast? No. But if I wanted v6 performance I would've rebuilt another 4.3 and put that in...

 

[QC]

Re: Merc 350 mpi not producing full power

Did you change your tach from the 6 cyl to 8 cyl setting? How are you getting your speed number?

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

Yes tach switch changed and speed is GPS and speedo gauge they're not far off

 

[Tail_Gunner]

Re: Merc 350 mpi not producing full power

http://forums.iboats.com/mercruiser...mpi-wont-stay-running-605089.html#post4245442

Again i will ask did you reflash the ecu. I will tell you a 4.3 use's differnt timing curve's and fuel pulse duration's. If you are using the 4.3 program it is amazing that you havnt leaned out that 5.7 and popped something. But then again the curves are very conseravtive..Actually i cant see how it could run if you have not reflashed it.

And this post seem's to point at the problem

Tech said he set the timing according to merc spec. When I got it running it would not get on plane (even with a 4blade 19p on a v6 drive-1.62:1- that would shoot right out with my 4.3). At that point I advanced the timing and got instant throttle response. But once the new TPS is in they will reset the timing and go from there.

MPI timing is controlled by the ecu at predetermined rpm levels no twisting the dist to find the sweet spot.

Here both mefi and 555: http://www.mdmotors.ru/netcat_files...akteristiki_sistem__EFI__MEFI__MPI__alb_3.pdf

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

The v6 was a 2bbl carb...
Not even a possibility to use the ecu.

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

The post that you quoted was from another issue that I was having, this is the problem now, moving forward.

 

[500dollar744ti]

Re: Merc 350 mpi not producing full power

It's a 98 20' Mariah Shabah open bow. A 260hp 4bbl 350 pushes 62mph in the exact same boat with a 21p (1.5:1 gear ratio or thereabouts whatever they used in that vintage with alpha one). Now the prop will inherently turn slower due to gear ratio, which should give a quicker jump in rpms, and reach top rpms quicker. Not sure of the weight, but I know this same motor in a 21' wellcraft Excalibur pushes 65mph. I don't feel like I'm overestimating the expected power increase. I expect at least 60mph...
Do I need to go this fast? No. But if I wanted v6 performance I would've rebuilt another 4.3 and put that in...

You should get 60-65 mph, however you aren't going to get it from a 25p or 19p prop, which as far as I can tell are the only two you've tried, correct? Your key is going to be an aggressive 21p or 23p of your choice.

 

[jdubslopro]

Re: Merc 350 mpi not producing full power

I agree with that statement. But it just doesn't seem to want to make power over 4,000 rpms, it slowwwwly climbs to 5,000. I'm going to throw on a 21 and a 23 this Sunday and see how it goes but I think it's going to have trouble and fall flat around 45-4600 and 50-52 mph

 

[QC]

Re: Merc 350 mpi not producing full power

I agree with that statement. But it just doesn't seem to want to make power over 4,000 rpms, it slowwwwly climbs to 5,000

All engines take time to build RPM towards the top, so I am not sure how that alone can indicate anything. That is where they are reaching full load (prop turning faster) and they run out of steam (fuel and air). This is how this stuff works, when you increase load (prop speed) you slow acceleration.

I have to assume you know how to trim your boat. I assume the hull is clean. I assume the weight is the same as the "sister" you quote. I assume the engine is right. If all of those assumptions are true then the only difference is gear ratio and the prop itself. I don't believe the gear ratio would hamper you that much (10 MPH).

Where did the "260" Mariah combo's data come from? There was no 260 carbed Merc in '98.

 

[Tail_Gunner]

Re: Merc 350 mpi not producing full power

19p w1.62 @5000 nets 50 mph that 11%
25p @ 4000 nets 21%.. Now at 25 @ 5000 with 11% slip would net 65 mph...300 hp 5.7 20' boat not all all unreasonable.

 

[QC]

Re: Merc 350 mpi not producing full power

Sort of agree, but that does not explain why the engine won't spin past 5000 as is . . .

What RPM is rev limiter set to? Don't recall. My 6.2 will spin past 5200 easily.

 

[Fun Times]

Re: Merc 350 mpi not producing full power

Serial number 0m0112557

fuel pressure is constant 40psi, and the reason for injector replacement was the plastic clips broke on 4 of them

Looking back at some of you're previous post, you're engine serial number is actually 0M011557.

According to the service manual and going by the part number of the fuel pressure regulator of 807952A1 you have too much fuel pressure at 40 psi, you should be at 30 psi. The fuel pressure regulator should have a pink color mark on it somewhere indicating 30 psi.

Hopefully your new fuel injectors are the correct size/flow rate as well.

Maximum rpm (at WOT) should be 4400-4800 rpm.

Check your spark plugs to see if they are in a fouled condition.
Spark Plug Type
AC - MR43LTS
Champion - RS12YC
NGK - BPR6EFS
Spark Plug Gap .045 in (1.1 mm)

DESCRIPTION: NEW MerCruiser OEM Fuel Pressure Regulator 807952A 1.
Regulator mounted on fuel cooler. Pink color mark. Fits MCM/MIE Gm V-6 & V-8 (#05, 350, 377) EFI engines & MCM 575
New Mercruiser Fuel Pressure Regulator 807952A 1 | eBay

V8 5.7L, 6.2L, MEFI-2, -3, (MPI, Cool Fuel on Port side, Gen+ engines, Magneti-Marelli injectors).
350 Mag MPI Pink 30 psi (207) 807952A 1 Regulator on fuel cooler. No ring on hose
Alpha, Bravo, fitting.
Inbrd, Ski, and
MX 6.2 MPI
Bravo, Inbrd

Also this engine would typically come with a gear ratio of either 1.50:1 or 2.00:1 for example. FYI, This engine did come with a 2.00:1 Bravo 3 when new. So ultimately you are fighting prop size/pitch, diameter, design, etc. etc. to get it to work correctly along with an unusual gear ratio that is typically found on bigger, heavier cruiser type boats with V8 power.

It has been my experience for best all around/overall performance that using an OEM Mercruiser propeller will usually best fit your needs without the need of having the propeller re-worked at a propeller shop for fine tuning purposes. In my opinion, Some (not all) aftermarket props have been known to work well in some areas and work slow in other areas like you're experiencing mainly due to design.

Hope you get it figured out, Good luck.

 

[Tail_Gunner]

Re: Merc 350 mpi not producing full power

Well a engine that will spin to 5000 has to be running well....If it cannot and should spin a 25 to 4800 i would suspect timing first. A timing tape would tell one quickly if the timing is advanceing correctly as she spins up. second would be fuel pressure at high rpm..then it could be a dirty hull.

When i first picked up my current tub it had a 19p alum and only occasionally would it ping the rpm limiter....I later added a 20p agresively pitched 4 blade ss and it pulled to 50 to 55 and still held to 4900. There is little cam in a stock marine motor he may just have to keep adding pitch until it pull's it down to 4900. I would check the timing close with the tape however.

Really dont know enough about the yr and makeup to this engine is there a link.

 

[Tail_Gunner]

Re: Merc 350 mpi not producing full power

Looking back at some of you're previous post, you're engine serial number is actually 0M011557.

According to the service manual and going by the part number of the fuel pressure regulator of 807952A1 you have too much fuel pressure at 40 psi, you should be at 30 psi. The fuel pressure regulator should have a pink color mark on it somewhere indicating 30 psi.

Hopefully your new fuel injectors are the correct size/flow rate as well.

Maximum rpm (at WOT) should be 4400-4800 rpm.

Check your spark plugs to see if they are in a fouled condition.
Spark Plug Type
AC - MR43LTS
Champion - RS12YC
NGK - BPR6EFS
Spark Plug Gap .045 in (1.1 mm)

DESCRIPTION: NEW MerCruiser OEM Fuel Pressure Regulator 807952A 1.
Regulator mounted on fuel cooler. Pink color mark. Fits MCM/MIE Gm V-6 & V-8 (#05, 350, 377) EFI engines & MCM 575
New Mercruiser Fuel Pressure Regulator 807952A 1 | eBay

V8 5.7L, 6.2L, MEFI-2, -3, (MPI, Cool Fuel on Port side, Gen+ engines, Magneti-Marelli injectors).
350 Mag MPI Pink 30 psi (207) 807952A 1 Regulator on fuel cooler. No ring on hose
Alpha, Bravo, fitting.
Inbrd, Ski, and
MX 6.2 MPI
Bravo, Inbrd

Also this engine would typically come with a gear ratio of either 1.50:1 or 2.00:1 for example. FYI, This engine did come with a 2.00:1 Bravo 3 when new. So ultimately you are fighting prop size/pitch, diameter, design, etc. etc. to get it to work correctly along with an unusual gear ratio that is typically found on bigger, heavier cruiser type boats with V8 power.

It has been my experience for best all around/overall performance that using an OEM Mercruiser propeller will usually best fit your needs without the need of having the propeller re-worked at a propeller shop for fine tuning purposes. In my opinion, Some (not all) aftermarket props have been known to work well in some areas and work slow in other areas like you're experiencing mainly due to design.

Hope you get it figured out, Good luck.

Is this a TBI motor...i think its mpi and a merc 555 40psi should be right. You can refereice the link i posted it cover's it all.