Honda BF100 10HP overheats, but good waterflow

Hi- I recently got a B100S outboard 10hp 1983 model year. When I got it there was no water coming out the pee hole, so I replaced the impeller, which didn't work, so I replaced the T-stat and cleaned a bunch of gunk from the intake manifold. I blew air through the holes in the block. Put it back together and water came out nicely. Let it run for awhile in the tank and the water never got hot, but I figured it was fine. Compression tested both cylinders at 100psi (which seems a little low).

Took it out ran it for awhile, seemed fine. Then running under WOT it suddenly stops. I needed to avoid a barge so I got it started and motored off, it stopped again and I noticed some burning oil and the motor was really hot like it was overheating. At no time did the water stop coming out of the pee hole. Limped home and found the inside of the case covered in oil! Not sure where it was leaking from, maybe the cap or the plastic housing that holds the cap. Also noticed that the lower cylinder was leaking oil from the spark plug hole.

So I disassembled the engine thinking I had not cleaned the block out well enough. As I am taking it apart I find a wasp nest plugging the oil breather tube (unfortunately I found this out after I removed the block from the oil pan). Could this be the source of my overheating problem? Or is there gunk still in the block (I blasted hi pressure water through all cooling holes in the block and they seemed to pass water fine). Did I ruin the engine? Do i need to replace the head gasket or did the overpressure oil come through the rings? I don't have the tools to take it apart any further (can't get the flywheel off or the valves back in place I don't think). I have the shop manual and the Seloc manual but they don't mention this scenario!

Any advice would be much appreciated!

Thanks,

BigD14

Sounds pretty brutal. If I read your post correctly, you had low compression (100 psi IS low)before it even overheated. Was it a bit cranky to get started when cold? Anywho, it's hard to tell, from here, where the oil might have come from. But it sounds as if it got hot enough to start deforming things and...maybe more things.

With the crankcase probably building excessive pressure due to the breather clog, it's possible the oil got out past the valve cover gasket. With the oil drooling form the spark plug hole, it is possible there was some piston deformation on #2 but we knew, with the low compression, there was plenty of wear in both cylinders to begin with.

If you want to try to get it running again and go dodging barges then you may want to have it assessed by someone that has plenty of engine building experience. You can usually find the best machine shop in your town by going to the independent auto repair shops in the area and ask them where they send their heads out. The name that comes up the most is usually the best choice.

You can remove the power head without removing the flywheel and can unbolt the cylinder head from the block. Or, let them do it at the machine shop. At any rate, take them both to the machinist and just ask him to have a look and tell you what he thinks. I'm not talking about taking out the pistons and all. Well, at least not initially.

These don't have any provision for "oversize" cylinder bore machining and new piston matching. They allow about .006" wear limit and, if you can clean up the cylinder AND if the piston is still somewhat round AND the cylinder head and or block deck is not too badly warped, you might be able to put some new rings in, do a valve job, replace the bearings and seals and put the old girl back back in service.

They might charge .5 hrs labor or so for a "peek" but many will not. even if they do, a professional opinion is well worth it so that you don't throw money, time and parts at a project that doesn't leave you the winner.

That's what I would do if in your shoes. Whatever you decide, I wish you luck.

And, if you determine that she is now scrap, the parts that are still good should sell real well on Ebay OR, even right here. There's more than one guy that needs/wants a carb or ignition components right here on marineengine.com for an old B100.

Thanks JGMO! I removed the cylinder head and found this:




To my inexperienced eye it looked like there was some oil getting past the piston rings in the TOP cylinder, more than the bottom, and the head gasket seemed to have been slightly compromised, but there is nothing visibly deformed or warped. Obviously I need to have some real measurements made. There is a lot of carbon buildup everywhere. I will be taking it to a shop tomorrow to try and have things examined and measured by more experienced eyes. The camshaft seemed to turn roughly, not sure if this is normal or not. Maybe a sticky valve? The pistons move freely and the cylinder walls look pretty smooth. Since the cooling passages were not blocked, I think plenty of water was moving through the system, so maybe it was not really overheating too badly. Would the oil breather tube being blocked and not allowing proper flow of oil through the system cause a significant overheat condition?

Thanks again for the advice!

BigD14

Not sure about the answer to the oil breather tube clog causing overheating. I had never considered that before but if the crankcase was receiving a good deal of blow-by, which inherently contains heat from combustion, and it sort of "banked' up in there due to the wasp nest, then my opinion is a solid...maybe.

The good news is that you don't seem "afeared" to meet the problem head on and you are moving forward with gusto it would seem. And, other than obvious oil control issues in the cylinders, I agree that things don't appear to be all that bad internally. Be advised that if you are going to do a good deal of the clean up yourself, don't use metal scrapers or wire brushes to get the carbon broken up. I DO NOT recommend the use of abrasive type wheels or scrub pads for doing any of that either. Use plastic and wood for scrapers and clean motor oil and brake kleen for degreasing.

Yeah, you could have a sticky valve or you could have a scored cam bearing. The big questions will be answered with a proper straightedge and a set of feeler gauges. If the block is flat and the head isn't warped, then it will be time to get out the snap gauges and micrometer to see if the cylinders are not too worn, egg shaped or unduly tapered. You just don't have much "meat" to play with on one of these. And, if all else is good, you could still just throw a new set of rings at it and see how long she'll last.

Good luck with the exam and let us know how you make out.
regards,
jimmyd

Thanks again jimmyd- So I took it to a local head shop and they didn't want to touch it, then took it to a place specializing in outboards. They told my they could sell me a new powerhead, or another running motor... They did tell me that they saw wasp nest problems all the time, and that the extreme oil pressure building up in the crankcase could cause the motor to spontaneously stop. Still not entirely sure about why it was overheating since all the water passages were open.

Anyway, I decided to strip the entire motor down. Took it all apart and cleaned it. It cleaned up well, and all the measurements I was able to make were pretty much in spec, but I wasn't able to measure everything. So I ground the valves and have ordered new rings and gaskets. I am just going to put it together and see how she runs. At the very least it has been a great learning experience. I'll report back in a few weeks when I can get it all back together.

Well, I guess I was right about you not being a "frady cat" but I didn't count on the machine shop not having the moxy to even give you an opinion. Hard to get anybody to help these days. Too many lawyers and not enough fishermen.

I do hope for your sake though that you are working with a block and a head that aren't too warped. I always get the "yips" when one has overheated like that and stopped on it's own.

Anyway, it sounds doable and here's wishing you success with the project. I'll be looking forward to your next post saying how great it runs!

Finally got the motor all put back together. Seems to run ok, I ran it in the tank for half an hour. With new rings, valve job and all new gaskets I am still only getting around 90psi. Guess it needs time to bed in the new rings. Or maybe the cylinders are egged out too much. Anyway, I will try to test it this weekend at full power and see how it goes. At any rate it should run reasonably well even with the low compression since everything has been gone through and replaced or cleaned up. I'll post an update after the test run!

Now, after re reading your post, I wish we had spent more time discussing how you were going to go about the work. I suppose when you said you had ground the valves I just assumed you knew all the procedural things to do. You obviously had the pistons out for ring replacement but did you knock the glaze off of the cylinder walls with a hone? I can see a good deal of glazing in the photos above and if you skipped that step, then that would account for the initially low compression readings. Other than that, then yes, out of roundness and possibly excessive taper would allow leakage.
Does she smoke a bit? I'll be interested to hear how the test run goes and I hope she "comes around" for you as you get a few hours on her.

Forgot to mention that I took it to a motorcycle shop and they guy honed the cylinders and got the pistons properly back in. So that is all good. Let you know more after I have tested it!

Update on the motor. I took it out and ran it for over an hour and a half at a variety of speeds. Everything worked perfectly no overheating, no smoke, no hesitation, no drama. Pulled the plugs afterwards and they looked good and dry. Compression still tested at about 90 (warm). That still seems low, but everything else works great so I am not going to worry about it. Its just for a dinghy anyways. Thanks for your advice jgmo. I learned a lot tearing it down and putting it back together.

Doug

Hi Doug,
Thanks for the pat on the back buddy but you did all the work. I have to say though that 90 psi compression on an engine that runs as good as you say just doesn't sound real. I have not gone back and read all of this thread so I don't remember if we discussed how you are performing the test or if you have ever checked the calibration of your gauge but I just have to believe that you have better compression than 90.

I guess though that the old saw "the proofs in the puddin" will apply here. If it works, IT WORKS! So I guess instead of fretting about a few pounds per inches cylinder pressure we should just say HOORAY!

Cheers! And happy boating my friend.

Thanks JGMO! Well, I just bought the compression tester a month ago and my test procedure is thus (with engine warmed to operating temperature):

1. remove spark plug, install tester, tightening lightly with a wrench to compress gasket
2. while holding kill switch down, pull hard 6 pulls.
3. read gauge.
4. replace spark plug, repeat procedure on other cylinder.

Is that more or less the correct procedure? Maybe the tester is bad?

Aha! I knew something wasn't "kosher"! Here's the procedure:

Start and run the engine out of fuel. This would be with the fuel tank disconnected, obviously.

Pull BOTH spark plugs. This is important in that it allows you spin the engine over at the "test" rpms of over 750.

Install the gauge in one cylinder.

Lock the throttle in the WIDE OPEN position. Do not hold it open with your "free hand". I will explain why below. This allows for the maximum amount of air to enter the cylinder. You can't compress what you ain't got. This is the one step that most people omit, giving them low compression readings

I advise you to use a jumper wire to ground out the ignition if you want to do that. There really is no need though since the engine cannot start with both plugs removed. I don't want you to be holding the kill switch with your "free hand and this is why;

Give that sucker as good a three or four pulls on the start rope as you have in you. Don't hurt yourself but give it as good a go as you safely can to achieve the elevated rpm that an ACCURATE compression test requires.

It is always a good idea to have a fit helper do the pulling while you look at the gauge. This is because you are not only interested in the ultimate pressure that the cylinder can attain. You are also interested in how EFFICIENTLY it compresses. On an engine that is in really good condition, you should see the cylinder pressure rise to around 60% of the final reading ON THE FIRST PULL and it should only take a maximum of four pulls to achieve ultimate pressure. I know this probably isn't all that important to you now but it is part of a good evaluation and since you're there, why not practice good procedure?

Also, for MAXIMUM accuracy, the engine should be hanging in a test tank that simulates operating conditions. That means with the water level in the tank above the cavitation plate as it would be while on the water. The reason for this is that it immerses the engine exhaust and provides the backpressure resistance that is normally there during operation. Not a big deal but it's part of the "hunt" when you are trying to diagnose a sick outboard. You would certainly be forgiven if you omitted this part of the test.

So, there it is, my version of the old DRY compression test. There is also a WET test but we can discuss that later. Try it this way and I'm betting you will get much different results. You will also have an accurate baseline, if you are taking shop notes, to refer to any time she starts giving you trouble in the future. Let me know what you think.

Regards,
jimmyd

Just to follow up on this I finally tested the motor on the dinghy. Runs like an absolute champ. And did the compression test as jimmyd said. Compression up to 120 now, which is maybe still a little low, but I probably need to give it time to bed in. At any rate it seems to run perfectly. Thanks jimmyd for you advice and encouragement!

bigd14,
I know it's a shopworn phrase and some folks may get tired of hearing it but I'm going to use it anyway;
I love it when a plan comes together! Yes, it may actually gain some compression the more you run it. As long as it starts easily and idles smoothly, I wouldn't be too concerned at this point. You might consider doing a leak down test sometime in the future to see if you can pinpoint any leakage. but, for now, just run it and and enjoy it. I know from our back and forth here that you can pretty much handle yourself if anything needs attention later.

It's actually been a while since I've been out on the water myself and had that little smile come on my face that says "no better place to be than right here". When your outboard is tickin over and pushing you across the water with the bow wake sizzlein off the prow....now that just don't get old!
I'm glad to hear from you and that you're happy with the all the work you've done. And I'm grateful that I could take a small part even from a long ways away!
Fair winds,
jimmyd