Overheating outboard - Honda B100 4 stroke 2 cyl

[sjchilde]

Hi,

I have recently bought a secondhand Honda 4 stroke 2 cylinder outboard. I haven't used it on the boat yet, just in a bin of water. It starts and runs well but I was concerned there was hardly any water from the tell-tale.

I have separated the lower leg and I found the impeller was new, and the gaskets. I was able to blow through the pipe up into the engine so I don't think there is a blockage there. Then I noticed the shaft seal under the impeller was not pushed down properly, so it was blocking the cut out where the water finds its way into the impeller. I have pushed the seal down to where it seats properly. This didn't solve the problem.

There is plenty of water coming up the pipe that comes out near the oil filler and goes into the manifold. So I have now removed the manifold. It was quite nice and clean and although the thermostat was stuck open I didn't find any problem. I replaced the thermostat with one that works and ran the engine for a while. It had a reasonable flow from the tell tale for a while, then it stopped and the engine got hotter and hotter until it stopped. (I kept turning it over so it didn't seize.)

I am guessing there is a blockage somewhere but I can't think where. Does anyone have a diagram of the water flow in this engine? What is the role of the venturi (orifice) in the water pipe going into the inlet? Is it possible for the thermostat to sit in an airlock and not open? These engines have been about for thirty years, someone must know!!

Many thanks for your help!

Steve

 

[Bobobolinsky]

Had this problem on a 75, eventually found that the water jacket was totally blocked with white rust. The only way to get it out is to cook it out with hot phosphoric acid.

 

[sjchilde]

Hi Bob,
That's not what I wanted to hear! I'm still hoping I don't have to strip down the engine this year. I have had the manifold off and looking into the head it looked ok as far as I could see in. But you are probably right....

 

[the_wanderer]

I had this problem with my B75 which is a very similar engine. I could blow through the water intake tube but passing enough water to cool the power head was another matter. the problem was rectified by removing the limescale deposits where the water enters the power head with a phillips screwdriver. Also remove the bypass pipe and have a give the orifice a good poking too. One thing you could also try is to put the thermostat into very hot water and see if it opens. Mine consistantly sticks open but yours may not be opening at all. Hope this information is of use.

Regards.

Alan.

 

[andrewdingram]

cleaning blocked waterway

I was guilty of not flushing and had this on my 3 yr old 4hp.

I ran it in a bucket or hot (not boiling) water for 3-5 mins revving up and down, and putting my finger over tell tale to get a bit of back pressure, then releasing.

Mine is fine now, as new.

Hope it helps yours.

I did find a product called salt-x which may work for you.

Good luck.

 

[Tommyrot]

Must be a good week for hot Honda's!
Mine was due to a knackered thermostat, opened in boiling water, but not if it was any cooler....
Put it back together without thermostat, will be OK until I can get another, any recommendations for Honda spares?

 

[sjchilde]

Hi, Since that post I have taken of the cylinder head, and cleaned out lots of scale, though I don't think any waterway was fully blocked. I made sure all the waterways were clear. Then I ran it and it was no better.

My brainwave was that maybe the water was leaking in the leg. So I took off the leg at the top, and turned the shaft with a cranked socket driver in a bucket of water. This showed that water was leaking from the top of the pump, and in fact the seal had escaped up the pipe. I pushed it back down, re-inserted the pipe, put the leg back on the engine, and hey presto it was a bit better. It idled happily for 10 minutes staying nice and cool, and with lovely water coming out of the telltale. So I put it in gear and ran it at a moderate speed. It overheated in about two minutes, maybe less.
Next, I removed the lower unit to examine the pump and the seal properly. The impeller is new. In the pump body I found an old impeller blade, partly blocking the water channel. I fitted a new seal to the top of the pump, and put the leg back together, and carried the engine back to the bin. I am sure it is getting heavier. Anyway I fired it up and hey presto no water at all came out, I couldn't believe it and waited for the water to come through, but it just steamed and smoked and I stopped it before it stopped itself. Somewhat disappointed, I checked that there is no impeller on the garage floor, not any impeller locating pin, etc. All the bits are in there. I left it for a few days until now I feel prepared to think about the b....... again.

 

[the_wanderer]

Well, this has certainly got me stumped!

It looks as if you have covered everything. There is only one other thing, running the engine in a bin does seem to restrict the water flow especially in reverse. Have you tried it on the boat, probably tied to a stout mooring?Just a thought.


I know these engines inside out, I have had a B75 for about thirty years and still going. Hope you resolve the problem as they are a lovely piece of kit, except for the corrosion.

Regards.

Alan

 

[sjchilde]

New idea
Now I am thinking maybe there might be more remains of old impeller blades in there. In the inlet channel would be most likely (as there is plenty of space for rubbish to lurk in the water jacket without causing a blockage). So I am thinking of back flushing with pressure form a hosepipe if I can connect at a suitable point. Maybe where the thermostat fits. That's the plan for this evening, I am feeling a little bit more optimistic, or maybe it is just that I don't want to be beaten by a simple outboard.

 

[fergie_mac66]

I had a problem with my which took ages to solve. It was caused by a small amount of corrosion on the water pump casing where the pump housing is seated on the gsaket I discovered it leaked on the inlet side of the pump .As soon as the engine revs up a little exhaust pressure pushes the water level down in the leg and the water pump started to suck exhaust into the pump(through the inlet side of the gasket) as well as water I could leave it on tick over ad it was ok but any revs and it started to overheat .. I added some chemical metal after a good cleaning to the thinning part of the housing and it has been ok for since for 8 years

 

[sjchilde]

could be it....

Hi Fergie,

What you describe sounds plausible, especially as I have had a lot of gas from the tell-tale, which may have been exhaust getting drawn in (although some was steam).

What I don't understand is exactly where the leak would be.

On mine the pump housing is plastic and not damaged. However it doesn't seal onto the cover next to it. This little cover sits over two holes in the gasket plate. One is for water coming up from the inlet, and the other is for the water to go down under the plate again to be drawn into the pump. I don't know why this strange arrangement is used and I didn't worry about it not being sealed, because it is all under water. But as you say the exhaust will depress the water level. In fact it could empty it of water completely as the exhaust outlet is below the cavitation plate.

So now I'm thinking that little cover has to be sealed onto the pump body, but it so obviously doesn't. If this is the problem then it was designed that way....

Can you have a look at the picture of my water pump and see if yours is the same? Can you explain how it is ever supposed to work? There is definitely no seal between the pump body (2) and the little cover thing (5).

Very many thanks

Steve

ps hope the picture is clear enough. I don't know how people get photos on here...

 

[fergie_mac66]

part 5

Hi steve

That is the same pump that caused me the problems the part number was number 5 I had bought a compleat pump kit first which was £64 from a place in wroxham norfolk I was suprised at the amount of wear on the metal liner in part 2 which was included in the pump kit but after fitting I was still getting steam from the tell tail but I noticed under the part 5 with (part 3 under) small paper gasket the aluminim that it bolted to bolt 15 was really thin the water gets sucked up to the elongated hole from the fin in fount of the prop .. the exhaust gets in through gasket 4 near bolt 15 then goes under the casting and part 3 and into a small hole under part 2
the thin wall near where bolt 15 screws home was the problem I really cleaned the alumin then built it up with chemical metal carefull placed part 5 and 3(no gasket) in situ with a tiny smear of grease to stop it sticking then took it off when the chemical metal had set , I built the ali casting up at the sides . resealed part 5 bolted down with gasket coated in silka then to be absolutely certain put some silka on each side of the of the casting and over the sides of part 5 It only took about an hr I did the same to the other side near bolt 19 while I was at it the ali walls are so thin !! .... If I remember there some drain holes down the side of the gearbox housing which I cleaned out I think that water wasn't draining from around the gear housing when the leg was lifted from the water ..

The pump kit I got was well worth the £64 even though that wasn't the problem because when I saw that the walls of the new impella liner were more than a sixteenth of an inch and my old one was paper thin I realised it would have let me down later

Careful you don't over tighten the bolts holding part 5 down

I had that pump to pieces a dozen times before I solved the problem !! It drove me up the wall for 2 weeks I cursed the git who designed the thin walls all that summer

edit : the first time I took it apart I had no drawing and I did not realise that part 3 came off It was well and truly stuck on . I could not believe the dried salt under part 3! I think that part 3 stainless next to the thin ali walls set up galvanic corrosion to the ali

 

[the_wanderer]

Just a thought.

Have you tried poking out the waterway in the stainless steel exhaust pipe that connects the head to the leg. Due to the extreme heat there the lime deposits can build considerably.

Regards.

Alan.