Honda 7.5 (BF75) compression, rough idle, head gasket question

[wrwetzel]

Launched on Saturday, engine ran fine for 1/2 hour trip to mooring, a little above idle for the first half the trip, a little below wide open for the remainder. Started it on Sunday and it ran ok above idle but stalled at idle. Let it warm up above idle but no better at idle. The stream of cooling water coming out of the engine appeared cloudy, as if it has fine bubbles in it. I believe it was always clear in the past.

I had an overheating event at the beginning of last season from a clogged riser tube. I repaired the plastic dipstick plate (it was warped) on the side of the engine and it worked fine the rest of last season.

The cloudy cooling water suggests a blown head gasket and I believe the overheat condition could have caused it but I would think it would have shown up last year.

Measured compression today: top cylinder - 110 psi, bottom cylinder - 125 psi. Pulled on starter rope several times until pressure didn't change. Throttle position unknown.

Questions:

1) The shop manual says compression should be 142 psi at 600 rpm. Are my readings of 110 & 125 ok given I obtained them when turning the engine over with the rope starter?

2) Are they far enough apart to indicate a problem?

3) Given the compression difference and the cloudy cooling water stream is it most likely the head gasket?

4) Would a small leak in the head gasket explain the stalling at idle?

I obviously don't want to buy a new head gasket and pull the head off until I have confidence that this is the problem.

Engine serial: 1200493. I'm not sure of the age but it is likely at least 15-20 years old.

Thanks much,
Bill Wetzel

 

[wrwetzel]

Re: Honda 7.5 (BF75) compression, rough idle, head gasket question - follow up

Re: Honda 7.5 (BF75) compression, rough idle, head gasket question - follow up

I tested the engine further off the boat.

With the engine in a tank I applied compressed air to each cylinder at top dead center. I covered the above-water exhaust holes as well as the water drain tube that comes off the inlet manifold. I could see absolutely no air bubbles coming from the below-water exhaust or water inlet ports as I would expect if there was a head gasket leak into the cooling passages. I could hear air coming from the crankcase breather as expected from leakage past the piston rings.

I repeated the compression tests, this time paying attention to the throttle position as posts in other threads suggested. After squirting oil into the cylinders I measured 180 psi in both cylinders with the throttle wide open, 120 psi with the throttle closed, both by pulling on the starter several times until the pressure stopped increasing. The 180 psi readings are well above the number given in the shop manual and almost identical between cylinders.

I ran the engine in a tank. Ran fine at idle and the water stream was clear. Put the engine back on the boat - still fine at idle and the stream was still clear. Took the boat out for an hour at various engine operating points including quickly going to idle from power. Worked fine.

At this point I have no idea what caused my initial problem. Whatever it was, it cleared up by testing. This is not a satisfying resolution as I found nothing to explain the problem and hence have little confidence that it won't show up again. However, all tests are fine and it works fine so I can do little more now.