SOHC.co.uk Forums > CB500/550

New head gasket blowing and lumpy as hell ...

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madoc:
Ok.

I done the strip enough to get a good look inside:

http://www.sohc.co.uk/index.php/topic,4615.15.html

I could not find where that piece has come from. It's conceivable that the damage was already fixed as it looks like some bits have been apart before.
I changed the camchain, re-ringed (the rings were about .4/.45mm) and honed the bores as they were ok.
All the valves came out for grinding and non-bent ones went back where required.
I found an OEM gasket kit at an autoumble for a tenner. In it's original sealed bag.

Took a while to fire up but immediately the head gasket was blowing everywhere and then settled down to a very small blow at the front only (and I mean tiny). I torqued it all down in the order, and to the numbers, in the book and even waited 48 hours and did it again. I am thinking a 20 year old head gaskets might not have been the best idea.
More worrying is that the engine however sounds sick. Really sick. I guess the blow is upsetting compression but I can check that with my tester at some point.
It has the original exhaust (4-1) and it's now leaking badly where it joins the silencer, in one rusty lump, which isn't helping. Not sure this fixable as it's tough to get in there with sealant or the welder.
It will hold an idle but it won't rev at all.
On the plus side, I cannot hear the camchain now  ;)

I am thinking that I could lift the rocker and re-torque or am I kidding myself and I need to take it all apart AGAIN and do the whole top end AGAIN.
I am also thinking that it could be a misfire or carb synch causing the lumpy running  ...

For the amount of time and money, so far, I could have completely built three old british bikes from scratch.

hairygit:
It's possible that the cam timing could be out by a tooth, that would let it idle but not rev properly, also the head gasket may well have been damaged by even a short period of running whilst blowing. Did you make absolutely certain that all the studs and dowels were clean and had no sign of binding? and the gasket face of the head was not damaged/warped?  Sadly, if the gasket is still blowing, no amount of tightening is going to stop it, all that will happen is things will distort, or may even rip a stud out of the crankcases ( to be avoided at all costs!) In your position, I would take the top end off again after checking the cam timing, but it looks like a new head gasket will be needed (I don't think age of the gasket has any bearing on this problem) I know it's disheartening, but I really cant see any other satisfactory solution :( :'( Look on the bright side, at least you can leave the engine in the frame unlike those of us with 750s ???

madoc:

I thought I was 100% certain on the cam timing.
I lined it all up and drew a little picture before taking it off and matched the manual and picture on putting it back together.
I also thought I had removed all dirt as there were no gasket traces and I was almost polishing with meths .. :D

everything went together fairly cleanly although the barrels were an utter faff, even with three of us.

I'll do a compression check, balance the carbs, check for a misfire, try and seal the exhaust and then I'll curse and dismantle it all again.

Bryanj:
Did all the head nuts spin freely and did you have the proper thick washers under the nuts?

Also are you sure it's the head gasket leaking and not one of the exhaust port gaskets

kendo57:
Could the silencer have blocked causing back pressure and blowing the head gasket.
Just seems strange the head gasket and exhaust have both blown.

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