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BF 90 hitting Spark plug

R

Rkoek

New member
Hello all, I hope that you might be able to help me with a problem that I have on my Honda BF 90 fitted to my mirrorcraft 1744 dual impact.

I bought this boat in may and had a great month of boating.... untill I went on a new lake following a friend that "knows" it.... well as you can imagine they hit a rock, and so did I.

Yes I know it sucks! Anyway the hit snapped my skeg off but nothing on my prop.

I had the skeg fixed and after putting it back in the water noticed that the engine was missing and running rough. Initially I suspected fuel but after pulling the spark plugs I noticed no 4 as having the porcelain broken and end "missing".

I tried cleaning it by removing all spark plugs and cranking the engine, also tried a thin hose taped to the vacuum cleaner (wife still upset!). Nothing came out so I installed new spark plug and started motor up. Started fine and then started running rough again.:(

I pulled the plugs and no 4 had the gap closed again.

I am ok with my hands and can fix most small issues (qualified as a diesel and motor mechanic a long time ago but have been tied to a desk for too long)

I have attached some photos of the spark plugs the first one is the initial one the second one is the new one fitted and idled.

I have tried a borescope but cant see much ( camera seems to be too big) and the compression gauge I do have do not fit this sparkplug thread size.

My questions are: is this a bent crank,spun bearing or something else? And can I fix this myself (where to go for parts and instructions?)

Also any helpful hints will appreciated as I am allready being asked by the wife why we bought a boat that now needs more money....:p
 
OK, here's what is likely a dumb question. About the new plugs, have you confirmed, beyond ANY reasonable doubt, that you asked for and received the correct plugs?

Explain to the wife that accidents happen, just like when you're in a car.
 
Alan Hicks said:
OK, here's what is likely a dumb question. About the new plugs, have you confirmed, beyond ANY reasonable doubt, that you asked for and received the correct plugs?

Explain to the wife that accidents happen, just like when you're in a car.
Click to expand...

Oh trust me I have used that line before! Yes they are the right plugs, definitely not a dumb question at all!

I only wish it would be that...
 
This doesn't sound good at all.

When you ran over the rock, a very large amount of energy probably traveled up the main shaft and may have distorted the crankshaft in some way.

Since the #4 rod journal is closest to the "smack zone", damage there may be why you are experiencing the piston contacting the plug tip.

You might have gotten lucky, in a relative way, and perhaps just the rod cap or bolts got distorted or "stretched". But that's a big maybe.

The only other thing that comes to mind would be a chunk of carbon or small piece of valve being knocked loose and rattling around in there.

As Alan asks, can you see contact marks on the piston crown? I know you said you couldn't see much with your borescope but there are some inexpensive flex cams you can use with your phone that have pretty tiny heads.

You're probably looking at head removal, at a minimum, to asses the engine. I would not try running it anymore to prevent further damage.

You might want to verify that the valve timing hasn't been altered. Look the belt over very carefully for any sign of slippage taking place during the event.

Good luck.
 
Further thoughts-
Regarding running, when you ran it after replacing the spark plug, the little bit of time the motor ran prior to it ruining the second plug, were there any obvious mechanical noises like a LOUD rod knock for instance? It would seem to me like anything that happened, that might allow the piston to hit the spark plug, would be rattling violently. As in you wouldn't WANT to run it again!

Even if the crank twisted out of index, that wouldn't let the piston travel any further than normal - unless the crank was broken - but that would screw up the cylinders above it as well I think. Twisting COULD mess up the valve train timing to the point where the piston and the valves were no longer cooperating with each other though....

I'm curious as hell about what's going on here. It's a good one for sure. One thing of note maybe, regarding the big shock of hitting a rock like that, these 90's have a big flywheel on the BOTTOM of the motor, that may have shielded the upper part of the crank from damage. There's also the curious factor regarding lack of prop damage one might expect when considering internal engine damage....

It doesn't seem to be adding up right.....
 
I agree Alan, now all I need is a usable manual for both parts and a workshop manual. Any recommendations?

It's a 2003 BF90. As soon as I have those two crucial parts I will pull the head and update with photos.
 
I learned the less expensive aftermarket overhaul/workshop manuals can leave you high and dry with missing, incomplete, or just plain bad info. If I need a manual, I get the Honda manual. That lesson learned the hard way, as usual, here.

You do see OEM Honda manuals on e-bay for about half of what Honda gets for them. Some are brand new. Some not so much, but as long as they are complete and legible, who cares? -Al
 
From my experience there is only one way a piston can possibly hit a spark plug and that is if the big end bearing has failed causing the piston to travel higher than normal.
 
Here's the HONDA MARINE PUBLICATIONS link. Pricy but priceless when exploring the innards of your outboard for the first time. As Alan has already said, don't WASTE your money on any other publisher. They can do you more harm than good.

https://publications.powerequipment.honda.com/marine

If you can get used to using boats.net pictorial parts searches, IE: your part not always being in the view you would think it should be....
....it's not much harder to use than the convoluted Honda Parts manuals. AND it's FREE.

boats.net lists EVERY part for AMERICAN Honda outboards that Honda still supports. So, that isn't a concern for using them as your parts source.

Good luck and looking forward to what you find with your engine.
 
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