johnson 40hp won't rev past 2500 rpms

[brisboats]

I have an intermittent problem with a 2003 johnson 40 VRO. Motor will run fine and then drop to a max of 2500rpms and stay there. I am convinced this is a fuel delivery problem but I am missing something. The motor has systems check and does not seem to be running hot. I have cleaned the carbs, replaced the fuel filter, new spark plugs, checked advance, throttle plates are opening fully. VRO pump check valve is clean, checked and blown out all fuel lines. Using clean fresh gas and have tried a separate fuel tank. Yesterday I replaced the fuel filter, disassembled the carbs (they were spotless) and took the boat for a 1 -2 hour test run without a hiccup. This morning coming out of the creek engine fell off plane and back to same running condition. Will a faulty VRO pump run like this? If I use the enrichener the motor will briefly pickup rpms. Have tried pumping the primer bulb while running the engine with no positive result. The engine starts easy and idles very good. Compression is 135psi on each cylinder. Took the carbs apart again today and went through each furl line looking for anything suspect, nothing.

brian

 

[karlow1]

Re: johnson 40hp won't rev past 2500 rpms

I would try adding a fuel pressure gauge between the pump and the carbs. If you have a fuel issue it will show up on the gauge.

KP

 

[tashasdaddy]

Re: johnson 40hp won't rev past 2500 rpms

i would check the over heat sensor. it is throwing your motor in to slow mode.

 

[brisboats]

Re: johnson 40hp won't rev past 2500 rpms

Thanks for the replies, I had ruled out SLOW mode because I have no alarms and the engine seems to be running cool. But it sure does seem to be limited to 2500 rpm. Will check the overheat sensor and vro pump. Looks like I can disconnect the tan/blue lead to the temp sensor and as long as it doesn't touch ground the switch is out of the equation and therefore will not trip the SLOW mode, please correct me if I am wrong.

brian

 

[brisboats]

Re: johnson 40hp won't rev past 2500 rpms

Update, VRO pump and temp switch tested OK. but yet same running condition. Went ahead and replaced thermostat and inspected water pump impeller was still like new so left it. Then went ahead and pulled VRO check valve and replaced it too. Ran boat for over 10 hours now and problem still hasn't returned so maybe just maybe it is fixed. I may have a power-pack going bad but for now the engine has run great over a two day period.

I will admit that in the in the moments of anger and frustration I was ready to switch over to a mechanical fuel pump. Probably would have been a mistake as the oil injection is working fine. It would have made things a bit easier, simpler and cleaner on the engines port side and saved me a lot time as I have lost count on how many times I have pulled the fuel system off. But the mixer is working just fine. Thanks for the help and replies on this one just thought I would report back on what I hope was actually the culprit.

brian