260 EFI Problem - Need to think out of the box

[PORT]

I have a problem on one of the 260s on my boat that I have been chasing for most of the summer. When I purchased the boat, both engines wouldn't run for more than a few seconds, but now I have one running mint and the other still giving me a problem that I have never saw. Motor starts and idles perfectly, but once under load I drop rpms. She doesn't stall or runs rough, but you notice the rpms drop once you give her gas. Typically she is 600-800 rpms lower than the good engine from 2500 up. She is holding the good engine back to only 7100 rpms.
You can hear that she isn't as crisp as the other engine

So far I have
1. New power packs
2. New stator
3. New trigger
4. Doesn't matter if it is Merc ECUs or Brucatos
5. New Air /temp sensor
6. Swapped fuel rail complete with injectors and 39lb regulator
7. New fuel pump, filters, hoses (41lbs after filters)
8. Removed throttle body (inspected reeds & changed hose barb to computer)
9. Pulled powerhead and changed seals in lower cap
10. Swapped all coils, plugs & wires
11. Swapped tachs, and also ran a wire from rectifier directly to tach
12. New rectifier
13. Removed regulators
14. Swapped flywheels
15. Tuner hasn't fallen in mid
16. Tried with fuel tank cap open
17. Removed Blue Optimum batteries and replaced with regular
18. Compression is 142-146 all cylinders, same as other engine.
19. Verified lower units are both same size gears
20. Timing 25 degrees, same as other.
21. Changed springs in timing arm
22. Props are merc and have the correct serial #s and stamped 14.5-32P
23. All plugs look the same (good)

Others have looked at her with no success

In the spring my next step is to swap power heads, and see if mid, harness or something else on that side is the culprit.

Any thoughts??

[Todd B.]

Hi Port, had a similar problem that was solved by replacing the motor side harness. The ground and a couple of injector wires were released from their pin in the bullet connector. Diagnosed while running on trailer at ramp by holding a long screwdriver against an injector and put your ear against handle and you should hear a steady clicking. Had two that were intermittent and stopped at higher rpm. Swap your harness on the motor or buy new. Switch boxes not properly grounded can cause weird problems too.
Do you run an exhaust gas temp gauge? It Helped diagnose a cracked exhaust divider spraying water on two cylinders. Hope this helps. Give me a call or text 989-289-9240 Todd Buchanan

[mach351]

Both powerheads have the same hours? Leakdown tests?

[BUZZIN' DOZEN]

Frank call me tomorrow

[laser_ED]

[QUOTE=mach351;3087752]Both powerheads have the same hours? Leakdown tests?[/QUO And check your gear cases ,make sure they are the
same 1.87,,,2.1,,1.78,,,, mismatched gear sets can also do what you have going on....

[Motv18]

Check the throttle body synch. A touch out of wack with the other can leave you short at wot. So can different tps volts. I assume you can say at 4500rpm balance by adding throttle to the slow one. That difference could be simply one turn of a barrel nut or a throttle stop adjustment.

[Mrzip]

Try a New 12v Battery.

[Markus]

You need to first try to figure out if it is spark of fuel related.

What do your EGTs say? Is there one cylinder bank that looks different from the other three?

If you put in some brand new spark plugs and make a plug chop at WOT, what will the plugs tell you?

[Jay Smith]

Hopefully no six pack style battery’s , my guess is key or EFI harness .

Jay

[Motv18]

One I forgot and fairly common a factory redo engine will do it. Basically it’s a fixed engine. Things like rebored from casting mistakes and 10-20 over pistons and other stuff.

More or less one sloppy and one tight in the original builds. As single engines no one notice and just pitch one down a inch and blame the boat or something or just call it week.

[PORT]

Pulled powerheads today and going to have a very knowledgeable builder give the pain in the ass motor a good looking over just to make sure the last person to rebuild her knew what they were doing.
I do appreciate all of the input, and I'll be going thru all of the ideas I received.

Thanks

[ChrisCarsonMarine]

Sorry for coming in late.The first thing to do in a case like yours is to determine if the power loss is confined to one cylinder or if all cylinders are contributing to the power loss.This is easily done by disabling one cylinder at a time,since you have no carbs to choke off one barrel at a time I suppose grounding one spark plug wire at a time is your best bet...not sure if disconnecting injectors could damage the ecu.Anyway,kill one cylinder at a time and record top rpm's...then at least you will know if the problem is confined to one cylinder...connection at injector,failing coil or plug wire ,or plug,water intrusion,...I would tell you to look at the top of your pistons but I've seen a few mis-marked...with the intake off you can see if the ports are where they should be.A piston on the wrong side or upside down usually will idle fine,and loose power as rpm's increase.
Since the powerheads are off,I'd be pulling the intake...I vote piston,Chris

[Mrzip]

Post the Out Come When You Solve It :D

[Cervelo777]

I am curious to the reason also, please let us know.

[eli]

Hopefully no six pack style battery’s , my guess is key or EFI harness .

Jay
what optima no good ok for carb motor

[PORT]

They were the blue ones, but were removed early on.

[Mrzip]

Any Success In Finding Your Problem ?

[PORT]

Nothing popped out, so I pulled powerheads and mids, will poke around over the winter seeing if somebody messed up. Probably will change both harnesses on the one problem engine. Also playing around with ways to sync the engines better, and will swap engines in the spring to different sides. Going to be a loooong winter.

[Mrzip]

I would try a New 12v Group 27 Battery :)