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[stinkwheel]

Riding along on my old faithful 2007 350 bullet last weekend thinking "This is running really nicely, I was going to de-coke it after this run but maybe I'll leave it." Purring along at about 60mph on a sunny day when it cut out. No funny noises, just total silence. Ammeter flicking away merrily but like I was coasting.



Pulled into the side of the road to have a look at it. Nothing doing. Turning over on the kickstart but no compression. Checked the tappets and the exhaust one was tight. Backed it off all the way, still tight. Hmmm. Ok, I'll take a look at the sparkplug. Ah, the end is missing and hammered like the end of a fencepost. Best call recovery.



Just whipped the head off this afternoon. I don't think this is a very good sign!















Now this bike has done a LOT of miles. I'd estimate between 70 and 80k and this is the first time it's left me stuck at the side of the road for anything other than a puncture. Although as is usual with these things, it's a bit like triggers broom. So that's one of our hosts tuned heads with the competition valves. the bore looks fine. The remainder of the piston that isn't full of valve is intact and still attached to the conrod. No detectable vertical play on the conrod. Bore looks fine, no scoring. Rocker gear is all fine, pushrods are straight, plenty of oil about in the top end. I think this must be a primary valve failure. I think the end just came off and drama ensued.



I'm going to have to split it because there's a fair bit of piston unaccounted-for so I'll replace the mains and I had been toying with the idea of a forged piston for it. The big question is, what about the crank? It's got a lot of miles on BUT it also seems fine. I've already spent a fortune on my 612 bullet so I'd rather not be throwing a new crank/big end at it unecessarily. Am I correct in my understanding that a floating bush is generally either perfectly ok or has failed, there's not much middle ground in that respect?



I'm hoping the head can be rescued. There's a few dings in it but if I tidy it up with a burr tool so there are no sharp edges/deep pockets, it should be ok?



On the plus side, that was going to be my last ride out before a major refit as a road trial machine so most of it was coming apart anyway. I hadn't intended to take the engine out though. Mind you, in some ways, tinkering with the chassis is much easier without the engine because I can get the frame in my heavy duty bicycle stand.

[stinkwheel]

Well. I've got it apart and located the rest of the piston.



The conrod/big end seems fine BUT there is a fair bit of play on the crank drive shaft. Both rotation around the axis (about 5 degrees) and vertical play (say 5mm measured at the end of the shaft). Now this wasn't noticeable when it was assembled and you tried to move the end, presumably because it's sat on bearings and was being pulled hard against the flywheel.



I see from the parts diagrams it's keyed in, I suspect the key is all that's been preventing this spinning up. I presume it's supposed to be an interference fit in the flywheel?



That being the case, the next question is is it likely to be the flywheel or the shaft that's worn/a bad fit. A replacement drive shaft isn't too pricey and I have a mate who has all the tools to fettle cranks but if it's the flywheel itself, it's a bigger/more expensive job and I presume I'd need to have bought the shaft anyway to see. Or maybe it's both?



I don't think this is what caused the initial problem though.

[Adrian]

Ouch.



How many miles on that valve before the head snapped off?



A.

[Norm]

Purring along at 60mph, I can't get my head around that, I don't think they were ever designed to do that

[stinkwheel]

Heads probably done 30k miles. ALthough now I think on it, I had to change that valve because the original one landed up all "nibbled" round the edge from carbon scoring. That was before I sorted out the crankcase breathing and oil burning by drilling a hole in the crankcase for a proper breather valve (it's one of the later ones that's supposed to breathe through a 6mm hole into the oil tank).



Someone has suggested that a similar thing may have happened here. If a lump of coke got jammed between the valve and seat causing the stem to bend very slightly at the "neck". Every subsequent revolution would see it flexing slightly as it opens then re-seats. We're talking minimal amounts here but after hundreds of thousands of cycles, it'll eventually let go. I went and had a closer look at the remaining piece of valve and it's sheared cleanly across the stem. The sheared surface appears to be slightly blued suggesting very intense localised heat (such as you'd get by bending a piece of metal repeatedly thousands of times a minute). I'll have a proper look when I dismantle the head.



It'll sit pretty happily at 60 providing it's on the flat and there's no headwind. I didn't have a whole lot of luggage or a trailer on either which works wonders for the cruising speed.

[Bullet Whisperer]

If a lump of coke, however small, got jammed between the valve and seat, it would probably be blindingly obvious straight away, with a change in exhaust note and loss of most of the power or probably stopping the engine, due to loss / lack of compression - your bike wouldn't be purring along at 60 mph, that's for sure! Certainly, the engine would not run under load for long enough to cause the valve any damage. That said, I have had valve failures just like yours in some incarnations of the 350 racer's engines and I think they break where the stem and head of the valves are friction welded together.

[Wheaters]

The deposits on the sealing face of the valve head looks like it's been blowing for some time. When this happens, the valve overheats.

On my then recently acquired 2004 350 the exhaust valve leaked and the edge burred up in a few places. The seat then recessed like crazy, as if it was made of soft metal. I was having to reset the tappet every fifty miles to get some compression back to get me home.

I had a new seat insert fitted and rebuilt the head with a replacement valve from Mr. H. Since then all - OK so far.

[Bullet Whisperer]

I don't think they are deposits on the seating area of the valve head, just marks and dings, where it has been battered by the piston after it broke off!

[Wheaters]

Paul, possibly so, but the back of the valve has similar whitish, hard deposits baked on, as does the piston - looks like it's been running very hot. Possibly the mixture was too weak? I think the standard 90 main jet might have been part of the reason why my bike's valve problem occurred - the plug centre was running bone white even though everything was standard when I bought it. Maybe that's why it has a "non-polluting vehicle" sticker from India inside the left hand toolbox

I wonder if they would accept it in central London...doubt it!

I've recently fitted a 105 main jet to my bike and judging by the plug centre it seems to be running about right and "safer"; i.e. slightly rich at the top end. I have modified it with a cone air filter, ported it and fitted a Gold Star "silencer". I'd tried a 95, and a 100 before that, along with a richer slide and raised the needle a notch.

The pilot jet is standard - I did try a bigger one but found it's not needed.

[stinkwheel]

It's got a 105 main and #2 slide in. It was previously running with an amal with an horrifically worn slide and body which I eventually took off and replaced with the original mikcarb.



I did manage to find a manifold to fit a 500 bullet mikcarb to it but it wouldn't run properly in the midrange and I didn't have time to fiddle with it.