Tomayo Posted December 30, 2013 Report Share Posted December 30, 2013 Hi All, OK, this isn't really a performance bike question, but I'm baffled. I bought a little Suzuki GN250 to commute on while rebuilding my ST1100. After about two weeks it dropped a valve. The previous owner had told me that it was happy at seventy all day (I wasn't convinced) and I had been cruising at about sixty. Opened it up to seventy briefly, to overtake a truck, dropped back down to sixty and after about a mile an exhaust valve snapped, destroying the head. Since I hated the riding position anyway, I took the opportunity to break out the grinder and welding set and rebuilt it into what I think is quite a cute baby café racer (the rear sets were the fun part) complete with a replacement head from a quad bike, a 300cc conversion and slightly higher gearing. All seemed well and a after a longish running in sortie with my girlfriend following in the car I deemed it working. On the way home from work, on exactly the same stretch of road I took it up to about sixty to overtake a truck (still running in, so nothing strenuous) dropped back to fifty or so and after half a mile it dropped an exhaust valve, destroying the head! The head, valves and camshaft were all "new". I did re-use the cam cover and cam followers/rockers, but couldn't see or feel any play anywhere or see any other sort of problem, and still can't. It had about eighty miles on post rebuild, there was lots of oil in all the right places and it was far from being thrashed. Has anybody got any bright ideas as to why? Link to comment Share on other sites More sharing options...
porter_jamie Posted December 30, 2013 Report Share Posted December 30, 2013 collets and retainers probably Link to comment Share on other sites More sharing options...
Tomayo Posted December 30, 2013 Author Report Share Posted December 30, 2013 Sorry to be thick but can you explain the mechanism for failure. The collets and retainers were different on each head (I used the ones that came with the replacement head) and all were happily and firmly in place after the failure. The valve broke off just below the valve guide and left everything else in place. Link to comment Share on other sites More sharing options...
porter_jamie Posted December 30, 2013 Report Share Posted December 30, 2013 any pictures? Link to comment Share on other sites More sharing options...
Rich Posted December 30, 2013 Report Share Posted December 30, 2013 Clearances too tight? Link to comment Share on other sites More sharing options...
Tomayo Posted December 30, 2013 Author Report Share Posted December 30, 2013 I'll try to take some pictures tomorrow. It was certainly running lean the second time (I hadn't increased the jet size enough for the bigger bore) but only for eighty miles or so and never for more than 45mins at a time. The first time it went the mixture was spot on, so it wasn't that then. I did read a few minutes ago that Suzuki sometimes use cheese for the springs in their cam chain tensioners, but, if it had slipped a tooth, I'd have expected it to go at peak revs or on over run, not half a mile later. Link to comment Share on other sites More sharing options...
wesifume Posted December 30, 2013 Report Share Posted December 30, 2013 any pictures? Of your girlfriend ? Link to comment Share on other sites More sharing options...
Tomayo Posted December 30, 2013 Author Report Share Posted December 30, 2013 OK, spot on may be a bit strong, but the plug was the right colour, there were no undue deposits anywhere and it was on standard jetting (it was still a 250 then) so I'm fairly confident on that one. After the second failure the plug was decidedly white/grey, so it was clearly a bit lean then. Somewhat annoyingly I was thinking that I'd check it over when I got home just before it stopped. Clearances are possible, but I think unlikely. I'd set them up when I rebuilt it 80 miles beforehand and I checked the surviving three before I took the top off and they were still OK, so I guess one could have been out, but I wouldn't have thought enough to clatter the piston. Unfortunately I didn't think to check the valve timing before taking the top off (engine, not girlfriend) so I can't be sure that the timing chain hadn't jumped a tooth. They do say that beauty is in the eye of the beholder, but my girlfriend refuses point blank to test your collective eyes, so it'll have to be just the head/valves I'm afraid. Link to comment Share on other sites More sharing options...
Tomayo Posted December 31, 2013 Author Report Share Posted December 31, 2013 I seem to be struggling to get pictures to work (I put the bbcode in the box and click "OK" then nothing seems to happen for a a very long time) so this post may end up somewhat strange. Link to comment Share on other sites More sharing options...
porter_jamie Posted December 31, 2013 Report Share Posted December 31, 2013 you need to get a photo hosting account sorted out, i use photobucket. upload your pic(s) then you click the link for posting in forums and paste it in the reply box and a piccy appears there should be a how to somewhere in the forum, take a look http://pbmagforum.co.uk/index.php?/topic/4-how-to-post-pictures/ Link to comment Share on other sites More sharing options...
Tomayo Posted December 31, 2013 Author Report Share Posted December 31, 2013 Aha! that seemed to work and is the cam cover, which was the only common top end component in the two failures.Now for two pics of head number one.Two of head number twoand one of the rest of it. Link to comment Share on other sites More sharing options...
Tomayo Posted December 31, 2013 Author Report Share Posted December 31, 2013 Thanks, I'd set up a Flickr account and followed the destructions in the how to, but it didn't work that way, switching it into BBCode mode seemed to fix it, but isn't in the "how to". Link to comment Share on other sites More sharing options...
porter_jamie Posted December 31, 2013 Report Share Posted December 31, 2013 how does the oil get to the valve stems? via the carrier? is the oilway bunged up with that black snot? Link to comment Share on other sites More sharing options...
Tomayo Posted December 31, 2013 Author Report Share Posted December 31, 2013 Good question, I did wonder about it, because there was quite a lot of blue snot in the original one when I stripped it as well. The workshop manual schematic doesn't even mention oil getting to the valve stems, only the oil way to the camshaft journal and that the cam face, rocker arm shaft and cam chain are splash fed. I presume the valve stems must be splash fed too, as there's no sign of an oil way anywhere. Incidentally the manual also specifies liquid gasket for that joint, so they must be expecting it to be around. I also wondered whether it might have suffered from oil starvation on the overrun, leading to failure shortly afterwards, but there seemed to be plenty of it about when I stripped it. It is interesting that neither valve stem appears to have bent, which I would have expected if it had hit the piston, so your theory is sounding better than the jumping cam chain one on that score. Link to comment Share on other sites More sharing options...
porter_jamie Posted December 31, 2013 Report Share Posted December 31, 2013 it has snapped off below the valve guide - looks like a clean break. dont valves normally bend not break if the cam timing slips? did the other one break in the same way? Link to comment Share on other sites More sharing options...
Superdunc Posted December 31, 2013 Report Share Posted December 31, 2013 The cam followers appear to be completely fubar'd. Where they like that when you rebuilt it? Link to comment Share on other sites More sharing options...
BudgetBoy Posted December 31, 2013 Report Share Posted December 31, 2013 . I did re-use the cam cover and cam followers/rockers, Id start by looking at these. One looks concaved to me in the pic.(Top one), you can see a ridge on the left side.. Well it looks like it to me. And there would be your problem. Link to comment Share on other sites More sharing options...
Tomayo Posted December 31, 2013 Author Report Share Posted December 31, 2013 I think you may well be right. They're both concave, and were when I stripped it after the first failure, which is probably (allied to appalling stupidity) why I missed it. It fits with it being the only common component too. What I don't quite understand, though, is that if the tappets are set right, how does it make a big difference. To my mind it just means that more of the cam follower is in contact with the cam. Perhaps if it's allied to wear on the shaft (which it doesn't seem to be) then I could see it moving and increasing the valve travel, but then I'd expect the valve to meet the piston and bend before it snaps. Both were clean, straight breaks. Perhaps I'm missing the bleedin' obvious or over thinking the whole thing, but I still don't have a clear sequence of events in my head for the failure. If the cam follower is very concave, could alter the effective cam profile (by which I mean the combined interaction of cam and follower) and cause something strange that way, maybe a collision with the inlet valve, though I couldn't see any signs of damage on the inlets the first time? Link to comment Share on other sites More sharing options...
BudgetBoy Posted December 31, 2013 Report Share Posted December 31, 2013 They have a radius for a reason,, to push the valve open.By not having that radius and having a concave you will NEVER get the same lift/push no matter how far you adjust it. The lobes on the cam push on the lobe of the cam follower. Those parts are fit for the bin only. Edit:- someone like Mille or Dunc will be able to explain it better than me.. Link to comment Share on other sites More sharing options...
Tomayo Posted December 31, 2013 Author Report Share Posted December 31, 2013 I entirely agree that the cam followers are stuffed and will be binned, but... 1) If one assumes that a convex cam follower gives a point contact on the cam lobe, then a concave follower (given that the radius is as great as the widest radius of the cam, which is logically how it will wear) will have a greater contact patch, hence more friction, but will only be out by the difference between the max and min radii, which one would think the adjusters could still handle. Further, since the radius will be greater than the crown of the cam lobe, it will still effectively have point contact at the greatest opening. 2) If the valves are failing to open fully, that will clearly affect the power output, but I don't see why it should snap valves. How about if the radii of the worn cam follower and the cam lobe interact in such a way as to slam the valve shut faster than normal? That would create extra strain on the valve stem and could explain a break with no bend. Link to comment Share on other sites More sharing options...
Superdunc Posted December 31, 2013 Report Share Posted December 31, 2013 It's a guess, but I suspect a combination of worn followers and/or incorrect valve clearances mean that the valves haven't been closing properly or only been shut for a short amount of time. This means the valve over heats, as it doesn't transfer it's heat to the cylinder head via the valve seat. The valves look like they are probably welded 2 piece items, which are pretty common, and have failed around the weld because they are running too hot. I have just returned from the pub so usual caveats apply. Link to comment Share on other sites More sharing options...
Egg on Leggs Posted December 31, 2013 Report Share Posted December 31, 2013 IIRC valve heads popping off like that is due to the valve hitting the bottom of the valve guide. Link to comment Share on other sites More sharing options...
Tomayo Posted January 1, 2014 Author Report Share Posted January 1, 2014 I like the not closing for long enough and overheating theory. I'm fairly sure that they're welded valves and both seem to have gone in the same place, which would fit for the weld. The crystal structure is quite coarse at the break too, again commensurate with a weld. Thinking about it, with the concave cam followers, if the clearance is set up right at TDC the valve is going to open early, as the leading edge of the curve hits the lobe and close late for the same reason. It's unlikely to be from hitting the valve guides (at least in the second set) because the valve stems had just been cleaned so there was no possibility of a ridge or carbon build up there. Thank you all for the suggestions and help. It's very greatly appreciated, even if it does reveal me to be a grade A pillock for not having checked the cam followers properly! We live and we learn Cheers all Link to comment Share on other sites More sharing options...
Damnthistinleg Posted January 5, 2014 Report Share Posted January 5, 2014 With a worn follower, the adjusters have to be wound much further in to keep the valve clearances correct. This changes the angle from which the valve is being pushed and gives a side loading to the stem which would cause the valve to rub hard on the guide. From that point, localised heating could cause a problem or perhaps even a bit of 'picking up' in the guide. Whatever the full explanation, I'll guarantee that the root cause is the follower. When rebuilding, check oilways like they are your own arteries! Link to comment Share on other sites More sharing options...
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