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What have I done

whats wrong with my engine

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Martin Levy22/04/2019 21:44:26
56 forum posts
30 photos

Just after posting in another thread about fixing engines I started my OS 30FS today and the rubber cone in my starter flew out, jolted the engine and now the prop turns either way half a turn and then jams solid.

I bought it unused only 6 months ago

What could I have done?


Simon Chaddock22/04/2019 21:58:16
5405 forum posts
2824 photos

I fear something is seriously amiss that will require some disassemble to identify the problem.

Do not do this unless you really know what you are doing and most definitely do not try to force it to turn!

Martin Harris22/04/2019 22:21:39
8669 forum posts
214 photos

I'm afraid your crankpin has probably sheared. It's a bit like bolting the door after the horse has bolted but did you turn the engine by hand before trying to start it? If the cylinder has flooded with fuel, a hydraulic lock can occur and the starter can then do serious damage. The cone coming out was probably due to the engine locking, not the cause of the problem. It's also possible that a manufacturing defect, normal wear and tear or corrosion during storage has started a tiny crack which has caused the failure - this can sometimes happen while an engine is in use.

Starting small engines with a starter is something that must be done with particular care - the reverse flick technique is often the best way if the engine is well set up.

The good news is that often the only damage is to the pin when this happens during a start and a replacement crankshaft may be the only part needed if you're lucky.

Steve Hargreaves - Moderator22/04/2019 22:22:11
6711 forum posts
189 photos

Yes indeed....I fear a sheared crank pin.....same thing happened to my ASP91 a while ago....sad

Removing the back plate is a fairly simple task & should confirm (or hopefully deny) my diagnosis...

Peter Miller23/04/2019 08:27:57
10011 forum posts
1177 photos
10 articles

If the crank pin has sheared I would expect the shaft to turn more than half way as there is no pin on the crank web to stop it.

A broken big end is more likely but even that would allow the crank to turn more than half way

A valve dropping into the cylinder could do it. A quick look under the rocker cover would confim confirm that.

Paul Marsh23/04/2019 08:49:15
3645 forum posts
1017 photos

If you can get one, a Crankcase if £39 plus postage from JE

Con Rod: £12

ken anderson.23/04/2019 09:04:07
8415 forum posts
772 photos

hard lines there martin, probably a broken c/shaft or hopefully something a bit more has been mentioned take of the back plate and have a look, get ready to spend some dosh......

ken dosh dept.

Doc Marten23/04/2019 09:45:31
353 forum posts
4 photos

By the very brief description I'm thinking there's a broken valve that's slipped into the head, jamming the piston.

Jon - Laser Engines23/04/2019 09:46:35
4723 forum posts
174 photos

a busted rod or dropped valve are the most likely candidates but i wouldnt rule out the crank pin as they are relatively easy to bust.

Martin Levy23/04/2019 21:43:23
56 forum posts
30 photos

I took the engine apart and indeed the valve was bent and had dropped into the cylinder. However notice even after removing there was still a jam. I will check the crank pin as there was a loose ball bearing in there from the end of the pin.

Crank shaft seems fine though


Martin Levy23/04/2019 22:03:28
56 forum posts
30 photos

And another question. why was there a ball bearing loose under the camshaft cover. Is this supposed to be attached to the end of the camshaft? there is one fixed in the other end and looks like a recess where it had fallen out of the other. Does this mean I need a new camshaft as well?

Jon - Laser Engines23/04/2019 22:03:54
4723 forum posts
174 photos

im not sure i follow. if the crank shaft is undamaged, ie it still has its crank pin, there should not be much to check. A loose ball bearing, and by that i mean one loose steel ball floating around, is usually pretty terminal so i assume its a different issue.

For the valve, i would imagine the valve spring broke and this allowed the retention hardware to fall out and the valve to drop. if the spring is intact then it likely got stuck due to oil or rust....but then there would have been no compression from the get go.

If in doubt, post some photos so we can see the situation

Martin Levy23/04/2019 22:10:02
56 forum posts
30 photos

so to try and explain better. I opened the canshaft cover and there was a loose single bearing. On the innermost end there is a visible beaqring set into the end of the camshaft and on the outer end (the one you see when opeing and before removing there looks like a recess where that bearing came from. I cannot see how else a bearing got in there.

I hope this makes sense but if not I could post some photos

Martin Harris23/04/2019 22:10:08
8669 forum posts
214 photos

Is it a single ball? If its an 1/8 of an inch in diameter or larger it sounds like the main bearing has broken up. Have you removed the backplate yet? It's just a case of removing the carburettor and the backplate - in normal circumstances there's nothing loose under it so you can remove it without fear of things exploding outwards!

Martin Levy23/04/2019 22:11:49
56 forum posts
30 photos

yes Martin. I have now stripped it right down with the exception of the main bearings

Jon - Laser Engines23/04/2019 22:22:58
4723 forum posts
174 photos

i think i understand. It sounds like the bearing (the whole thing, not just a ball) has come out of the cam chest cover and is stuck on the end of the camshaft. This is not the end of the world and not really any sort of problem as long as it isnt shot. If it is, its a stroke of luck as its one less bearing to remove!

Martin Levy23/04/2019 22:28:51
56 forum posts
30 photos

os 30 3.jpgos 30 2.jpgos 30 1.jpg

Martin Levy23/04/2019 22:32:50
56 forum posts
30 photos

1st pic is the crankshaft - looks ok to me (I hope)

2nd pic - one nd of the cam shaft and next to it a bearinf that was loose under te4 cam shalft cover

3rd pic - the other end of the cam shaft with a bearing fixed in it and the pic below which is the cam shaft recess and that bearing was loose in there. New camshaft as well or drop that bearing back in?

os 30 4.jpg

Martin Harris23/04/2019 23:13:38
8669 forum posts
214 photos

I've found an exploded view of this engine as I've never stripped a 30 but incredibly, the camshaft IS carried by single ball bearings in the ends, not ball races!


Edited By Martin Harris on 23/04/2019 23:18:41

Martin Levy23/04/2019 23:15:54
56 forum posts
30 photos

thanks Martin. So I should be ok just placing that bearing back without having to buy a new camshaft?

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