V6 150 performs differently on muffs than in the lake

[sly_karma]

Late 80s V6 150 ready to go in the water. Adjusted idle speed and throttle cable barrel adjusters at the shop using muffs. Idling nicely at around 850 rpm, shifting in and out of gear cleanly. Off to the lake we go... different result. Won't idle, engine dies below 1400 rpm - can't run without the warmup lever still in use so it won't even go into gear. I get that idle speed will be different with the motor in gear and under load, but I can't even idle down enough to put it in gear.

The only difference I can think of between the two situations is water temp. Even though it's warm here in southern BC now, our water lines and mains are buried at least 5 ft deep to prevent freezing in winter. That means water comes out of the tap all year at around 8-9 C (46-48 F). I know engine temp gauges aren't wonderfully calibrated, but mine shows operating temp on the muffs to be around 125 F.

Lake temp is warmer now as the snow melt is well over and air temp is warming the water surface. Yesterday it was 16 C (61 F), and the engine temp gauge seems to agree, operating temp in the lake was more like 140 F. This is confusing, surely the thermostats should open at the same operating temp regardless of incoming water temp?

So, what to do? I've been through the process for a new-to-me motor: compression test, ignition tests (replaced switch boxes and spark plugs), new water pump kit. I'm frustrated with this rig at the moment.

 

[racerone]

Water temp likely has nothing to do with it.----What are the actual compression numbers ?---On a hose they all sound good because even in gear it is still in neutral.---The prop has no work to do.----There is / may be a poppit valve that opens with water pressure and that may be the reason for running cooler on a hose

 

[sly_karma]

I took the compression readings before I bought the engine last December. From memory they were 105-108 in all holes. Engine hasn't seen any service since, just shop testing. I guess I should borrow the compression tester again and check.

It occurs to me that there is a minor additional load when the boat goes in the water, as the water pump now has to actually work rather than just allowing mains pressure to supply water to the powerhead.

 

[dingbat]

Water creates a lot of back pressure in the exhaust system. Idle should be adjusted in the water

How does the timing look?

 

[Faztbullet]

Motor should idle around 1150rpm on muffs.

 

[sly_karma]

Thanks guys. I had timing set at the shop after we installed new switch boxes. Sounds like I set the idle too low and the additional load and back pressure were enough to stall it out. I went to the ag supply place today and got an empty plastic 50 gallon drum, I'll cut the top off that and do some tank tuning.