Raising compression ratio

[TommoT]

Folks, I have just completed a cylinder head swap on my G80. I have had a "new" head on the shelf for a couple of years, waiting for the right time to be fitted. The old head had a few stripped threads, a worn guide and a broken fin, so a swap was justified. Before fitting the new head, I took great care cleaning up the cylinder spigot, and gasket surfaces to ensure a tight joint. When checking the condition of the gasket surface on the new head, I noticed that it was recessed deeper into the head than on the old one, so I concluded it had been skimmed. When fitting the pushrod tubes this was confirmed, because I had to re-arrange the washers around the seals to achieve correct seal pressure at both ends, and allow the head to be securely clamped down on the gasket. I used an annealed copper gasket, and measured the distance between head and cylinder, once again to ensure good clamping action. So all in all I was pleased with myself and the new setup. I used a "new" rocker box as well, mostly for costemetic reasons.

The engine started started after the 2nd kick, so I was well pleased. It took 2:30 for the return flow to appear at the tank, but this is normal, so I was not worried. I rolled out of the driveway, and took it easy. I immediately noticed that the engine was noticably noisier than before. I returned to the garage and checked oil flow in the places possible. All was normal. I went for another test drive. No change, the engine sounds much "harder", the noise coming from the cylinder area, its not a deep rumble, its more like a hard ringing metallic noise, following rpms. Once revs rise, the noise seems to level out, and on the open road I can't hear it, but this could also be due to wind and other factors cancelling it out. In fact the engine has a lot more punch and when reaching 60mph, it really flys, and I have a feeling that if I keep winding the throttle, 80-90mph is easily within reach, if all the other bits stay on the bike! When stopping, allowing the engine to idle, it sounds really awful. Clangin and'a ringin'. I have checked everything else again, valve clearances, oil flow, etc., and everything still seems ok.

So this has got me thinking. Could it be that the skimmed head has raised compression enough to cause this change in engine behaviour? It makes sense to me anyway, the noise combined with a slightly heftier kickback and the extra punch could easily be caused be a raised compression. Unfortunately I didn't take any measurements, to establish the amount of skimming, but I would estimeate between 1 and 2 milimeters.

Comments anyone? Has anyone tried raising compression, and experienced these effects? If this turns out to be the case, I will probably fit a compression plate under the cylinder, just to ease things up a bit and not put too much strain on bigend, valves, etc.. So please feel free to comment and make suggestions?!?



TommoT

[clive]

what sort of piston? I recently had trouble with a barrel that I had fitted following a rebore on it. Been on the shelf years and I forgot it had to have a decompression plate fitted. Anyway it problems fitting the pushrod tubes but when finally sorted it ran fine but a knocking noise, fine high pitched ring rising with engine speed as yours. cut a long story short I eventually stripped it and found the piston was just touching the valve or cylinder head not sure which causing it to rock slightly and wear a slight ring groove in the side of the barrel.
my advice is to take the head off and see if there are any witness marks as the head skim may have the same effect as the missing decompression plate. good luck

[TommoT]

Thanks Clive, I think I will do at least that, just lift the head off and check for possible damage. I am sure the piston, standard flat crown, does not touch the valves, I checked with my finger tips, turning the engine over. But then again its not the same as a running engine, when metal is flying around at higher velocity. Thanks for the suggestion.

[1608]

1 or 2 mm is a lot to skim off! As Clive suggests, your tolerances are probably too close. The noise is probably exacerbated by the timing being out now too, pinking or knocking. Definitely try the compression plate.