Pre 1997 Honda BF 10 Overheated? No compression

[w2much]

I am trying to decide whether or not to rebuild this motor or switch the powerhead. I have not rebuilt powerheads before but seems like a winter project worth while. Looking at the parts diagrams for the piston and crank shaft I see no bearings where to connecting rod joins the crankshaft. What is up with that?

 

[Sea Rider]

Can you disassemble the lower rod portion where connects to crank shaft, are there 2 lower bolts to take rod out or it's a one piece connecting rod ?

Is that a new to you, second, third hand outboard ? As to deduct if previous owner had an overheat issue and sold the outboard as it is. If so, could have a severe warped cylinder head along blown head gasket that gives low or inexistant right cylinder compression.

Happy Boating

 

[w2much]

Thank you for your reply
The motor was sold to me as a parts motor. Actually two were purchased from the same seller at the same time. One had no compression one had a cracked lower unit but runs fine. My first thought was to just switch the power heads. The motor with the bad compression is just so clean I think I should give the thing a chance and rebuild. For now the 7.5 with the cracked lower unit is put aside. I will just deal with the 10 HP and the compression issue.
I have not removed the power head yet but I have taken off the head. I filled both cylinders with thin oil and left it for overnight. The bottom cylinder held the liquid the top just leaked past the rings in a few hours. Neither cylinder seemed scoured or otherwise damaged. Both had just a bit of rust which may be from sitting or possibly a bad head gasket. The head gasket was not obviously blown although it may have been ,if it was it is not plainly obvious to me. I will check the head today for warpage. I plan to remove the power head and possibly rebuild it. If not I will go back to plan A which was to just switch power heads with the 7.5.
I am not even certain that overheating caused the low compression in the first place,. As you mentioned it may have been a warped head or a blown head gasket but something took out the rings on the top cylinder. If only 1 cylinder /piston had bad rings the other one should have had some compression unless there was also a bad head gasket or other issue. The valves appear to be well seated and in good condition. The motor looks to have very low hours.
Looking at the parts diagram I was surprised to see no bearings between the connecting rod and the crankshaft (I think its called the journal) .I thought all motors had bearings there.
By the way Sea Rider in your reply you mention the right cylinder compression. This is a ten HP 2 cylinder which I would describe as top and bottom cylinder.
The rebuild seems like a good way to go as perhaps I may get two engines running should I find a lower unit for the other motor and rebuild this one.

 

[Sea Rider]

We're talking about 2 or 4 strokes OB's as Honda is a brand that don't work with. Can you sit a 7.5 and a 10 HP powerhead on any of both middle leg units ? With which one should stay ? depends on how much work you need to do along costs. As I like doing partial and full restaurations would go for the larger 10 HP, if cylinder walls are impeccable perfect round and within factory specs will only need to change piston rings, head gasket, base gasket. Worst scenario, if not within cylinder specs, rebore cylinder to next super size.

The only way to warp badly a cylinder head along blowing a head gsket is by means of a severe overheat due to lower leg picked debris, algae, plastic bag, even a banana peel while OB was revving high and you weren't aware of this condition, usually will find a melted impeller inside liner due to running dry for extended time.

If head gasket has not been re torqued after break in period has ended expect some water intrussion on cylinders which produces corrotion, oxidation if OB was left siting for extreme long time periods. Worst if OB was used on salty waters.

Is it possible to weld cracked 7.5 to original condition and use while the 10 undergoes restoration ? At least will not be left stranded at home while other boaters are having great boting time. LOL!!

Happy Boating

 

[w2much]

I tore down the rest of the engine. I find that there are actually no bearings on the connecting rods at the crank shaft. OK so I am past that.
What I did find was that the rings on the top cylinder were stuck in place thus the compression issue on top. I am pretty certain that the head gasket allowed water into the cylinder thus making the rings have enough rust on them to stick. The bottom piston was a lot cleaner than the top one. I am guessing that water leaked between cylinders starting on the bottom then the top. Motor was then probably stored and rust settled in.
The crankshaft bearings and journals look really good, no scouring ,pitting or rust.. I plan to just put new rings in and call it a motor..
I think you hit it on the button SeaRider .Thank you
Will post other questions as they arise.
Still do not know the year nor why 2 different head gaskets are listed on this motor

 

[raczekp1]

iworked on few this ype powerheads. if anything just shoot