Broken rocker arms - 1990 GPX250

[Sparrowhawk]

So... I was riding along the motorway, when the bike lost power. At first it felt like I needed to switch to reserve, but had plenty of fuel. Managed to pull over safely, and got the bike home on a trailer.

Bike wouldn't start, engine wouldn't even turn, so I thought I'd seized it, but further investivagation made me think something was mechanically jamming it.

Took the head cover off, and I have found 7 broken rocker arms, the 8th must be there somewhere, it's AWOL at the moment. I removed all the broken bits, but the engine is still jammed.

Any idea what could have caused the rocker arms to break?

The timing chain is still there, seems to be ok. Haven't taken the bottom end apart yet though.

Help!

[FJRider]

Sounds like stuck valves ... and ... $$$$$$$$$

[Sparrowhawk]

Sounds like stuck valves ... and ... $$$$$$$$$

Bugger. Is there any way to... unstick... valves?

I guess the first step is to get the head off. The great thing about working on bikes vs. cars is that I just lifted the engine out of the bike, and put it on my workbench. No hoist required!

[FJRider]

Bugger. Is there any way to... unstick... valves?

I guess the first step is to get the head off. The great thing about working on bikes vs. cars is that I just lifted the engine out of the bike, and put it on my workbench. No hoist required!

Getting the head off is the easy (read cheap part) ...

The damage may not have stopped there ... (or started there) ... if the (some) valve(s) was stuck down ...

It should be able to be dismantled down to the base gasket ... without removing the engine from the frame.

[Sparrowhawk]

OK, turns out one of the rockers pieces fell down the gap for the timing chain, and got wedged between the chain & the crankshaft sprocket.

Since I removed it, the engine is no longer jammed

I think the timing chain is stretched though, which may have caused the problem, or may have been caused by the rocker piece jamming. Still gonna check out the valves though.

What's the best way to check if the valves are opening or not?

[pete376403]

A little engine like that will have pretty soft valve springs - you should be able to press the valves open by hand. You can see how well the valves are seating by pouring a bit of solvent (petrol) down the ports and seeing if it leaks into the combustion chamber (stand the head with the inlet ports upright so you can see) then turn it over and do the exhausts)

[Taz]

I'm going to guess the rockers broke after the pistons hit the valves when the "loose" cam chain skipped and put the valve timing out. Possibly a cam chain tensioner problem. But that's just my guess. Let us know when you get the head off.

[Sparrowhawk]

A little engine like that will have pretty soft valve springs - you should be able to press the valves open by hand. You can see how well the valves are seating by pouring a bit of solvent (petrol) down the ports and seeing if it leaks into the combustion chamber (stand the head with the inlet ports upright so you can see) then turn it over and do the exhausts)

I'm going to guess the rockers broke after the pistons hit the valves when the "loose" cam chain skipped and put the valve timing out. Possibly a cam chain tensioner problem. But that's just my guess. Let us know when you get the head off.

Pete, tried what you said, all the Valves seem to be opening and seating fine. A couple of them were quite stiff to open, maybe they'd got gunked up and that started the problem, broke one rocker, which fell down the cam chain hole, got caught in the chain, and threw the timing out, which meant all the others broke?

Taz, you could be right about the tensioner, the chain seemed quite loose with the tensioner in place. It did look like there was some impact between the piston & the exhaust valves. I measure the chain over 20 links like the workshop manual said, it seemed to be right on the "service limit", and there looks to be some damage caused from the rocker bit getting caught in it. Should I replace it?

[FJRider]

Pete, tried what you said, all the Valves seem to be opening and seating fine. A couple of them were quite stiff to open, maybe they'd got gunked up and that started the problem, broke one rocker, which fell down the cam chain hole, got caught in the chain, and threw the timing out, which meant all the others broke?

Taz, you could be right about the tensioner, the chain seemed quite loose with the tensioner in place. It did look like there was some impact between the piston & the exhaust valves. I measure the chain over 20 links like the workshop manual said, it seemed to be right on the "service limit", and there looks to be some damage caused from the rocker bit getting caught in it. Should I replace it?

Any damage to the pistons around the gudgeon pins ... if the top of the piston showed impact damage ... ???

As for the timing chain ... there seems to be a few bits needing replacement ...

[Sparrowhawk]

Any damage to the pistons around the gudgeon pins ... if the top of the piston showed impact damage ... ???

As for the timing chain ... there seems to be a few bits needing replacement ...

Where abouts are the gudgeon pins? Are they underneath the pistons?

[FJRider]

Where abouts are the gudgeon pins? Are they underneath the pistons?

THEY hold (connect) the piston onto the con-rod.