NonPower over 3800rpm

[Ship-Faced]

Newby here, need some advice.
I have a 2000 Johnson 70, J70PLSSD.
Just purchased the pontoon with the motor. Starts right up and runs great.
Runs VERY well, smooth, until 3800 rpm.
Than, it bogs down, looses rpm and will almost die.
I can pull back on the throttle and it will come back alive, runs good again.
I can pit it in neutral and run the rpm up to 5500 and 6000. I do see the rpm with a slight
miss at high rpm, over 5000. BUT, in gear, same issue. Loses all power and RPM after 3800
The carbs have been rebuilt, all fuel lines replaced. Local repair shop worked on the motor before I bought it. They replaced a cable by the carbs, didn't help.
Any Ideas ???

 

[Scott Danforth]

first, welcome aboard

just an FYI - your engine will run on 2 cylinders in neutral to 5500/6000 that means absolutely nothing from a diagnostic point of view. your engine will also run to 5500/6000 RPM on 2 drops of fuel in neutral where it will take cup loads of fuel while under load. again, reving in neutral means nothing.

start with a compression reading
then make sure you have spark
then go thru the carbs

however it sounds like a fuel issue (or lack of fuel issue) - also look at your fuel pump and the tank/anti-siphon/fuel/etc.

 

[Ship-Faced]

first, welcome aboard

just an FYI - your engine will run on 2 cylinders in neutral to 5500/6000 that means absolutely nothing from a diagnostic point of view. your engine will also run to 5500/6000 RPM on 2 drops of fuel in neutral where it will take cup loads of fuel while under load. again, reving in neutral means nothing.

start with a compression reading
then make sure you have spark
then go thru the carbs

however it sounds like a fuel issue (or lack of fuel issue) - also look at your fuel pump and the tank/anti-siphon/fuel/etc.

I checked with the shop that worked on the motor before I got it. The compression is good. Shop said they have gone through everything over and over again. They did rebuild the carbs, but the motor sat for 8 months before I bought it. The shop is telling me the carbs are the problem. They think they are just worn out. Do you thing that could be true ?

 

[racerone]

How big is the boat ?----What is the pitch of the prop.-----Some will argue, but " good compression " means nothing.----Post the actual values.----Sorry mate , but the carburetors on these motors do not WEAR OUT.-----I would take " big steps in rapid succession " and never set foot again in a shop that tells me that story.