That Dropped Valve


The chap from whom I bought little Suzi had been most insistent that she would run at 70mph all day. I have to say that I wasn’t convinced.  Revving the poor little thing to within 500rpm of the red line constantly just didn’t feel right. So instead I would back it off to 60 or 65 and trundle along at that. The exception on that day was the sweeping and diving turn down the slip road from the A14 to the A1 where I overtook a truck and allowed her to run up to 70 on the downhill stretch and for a little while on the flat. Then I backed it off again and carried on at 65.  Just as I pulled onto the slip road to leave the A1 there was a very loud BRRRRRRR and a loss of power. By the time I’d registered it, I had the clutch in and the noise had gone, so it wasn’t the chain. I blipped the throttle a couple of times to no effect, but the noise was a letting go noise, not fuel starvation. Once down to a low speed I gingerly let the clutch out again. Same noise, no noticeable retardation, and the back wheel was still free.

The engine clearly hadn’t converted itself into a bag of bits, but it didn’t sound at all happy either.  Once stopped I switched onto reserve and cranked it over, we always hope for the best don’t we.  There were no particularly nasty noises and it cranked over freely, but it obviously wasn’t going to start.

After pushing it off the exit ramp I tried giving it a bump on a downhill stretch. The same noise. The same lack of starting. My old Rotax powered Harley had always made a noise a bit like that when bump started. I think it was the sprag clutch clonking over, but it really didn’t feel right coming out of this bike.  Then it was time for the mile and a half push home.

Once home, it was time to go through the standard routine. Personally, I always do. So out comes the plug, it’s wet, so we have fuel. Next, hook it up to the plug lead and turn it over. OK, we know it’s not an electrical failure, they don’t make nasty noises, but a hall effect sensor bouncing about in the generator casing might. All clues help. As an incidental aside, some people are of the opinion that you should never turn over an injured motor, for fear of doing more damage. I see it this way. That exciting barn find may well have dry seized rings which will break when you turn it over and go everywhere, so don’t. Similarly, if the engine makes horrid noises but keeps going. Stop it quickly, that may be the difference between a collapsed bearing and a rod through the side of the crankcase. But… this engine failed at about 6500rpm. Let’s say I have lightening reactions and got to the clutch in a tenth of a second (honest mister) the engine would still take a second or so to wind down, so by the time it stops anything making contact in the combustion chamber has done so a hundred or more times and anything in the cam box fifty or so. Either way, the chances of me doing much more harm with the starter motor should be pretty slim.

Anyway, back to the story. Sparks were there aplenty and I’m now convinced that it’s either a dropped valve or a broken piston. Why? Because it cranks over at the same speed with the plug in as it does with it out, so there’s clearly no compression. Placing a careful finger over the plughole (not in it!) and winding it over confirms as much.

Next move is to whip off the rocker covers and turn it over again. Four valves, all going up and down as prescribed. Rats, it looks like a piston problem.  So, clinging to the clearly vain hope that it’s really only a head gasket (they don’t make those noises) I adjourn for dinner and return at the weekend to take the head off. I am then rewarded with the sight of a valve head, seemingly growing out of the cylinder head like some sort of metallic mushroom.


The piston seemed to come off quite well as the valve clearly hit it square on…


…but that valve stem went an eighth of an inch into the head and raised a nice little mound around the crater. That’s one dead head. Looking at the bits was intriguing. The valve had not been bent, so it hadn’t been sheared off. The position of the break and a bit of a look at the crystal structure on the end of it looked like a brittle fracture at the weld. A lot of engines are made with welded valves nowadays, it saves material in the manufacture. Basically, the valve head had just dropped off,  presumably when it hit the seat whilst closing.

The question was why?

I would have expected the valves to bounce, if ever, on the fast bit, not slowing down afterwards. There was oil everywhere there was supposed to be, so it didn’t look like anything had seized. It didn’t even go on the overrun. There wasn’t massive clearance on the tappets, but they were clearing. There was no step, either of wear, erosion or carbon build up on the stem that could catch on the valve guide.       The only thing I could imagine was that the valve stem was fatigued anyway, had started to crack on the fast stretch and then hung on until we hit the slip road. Not exactly definitive, but all I could see.  Since then I have realised that I missed the painfully bleedin’ obvious,  but we’ll catch up with that later. So for now I had a headless bike that was the wrong shape.

And that’s where the story really begins.




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