SOHC.co.uk Forums > CB500/550

CB 500 Four - engine problems

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McCabe-Thiele (Ted):
The only question I feel qualified to answer is that  the compression  test should be good right away.

Have you tried squirting some oil down the plug hole then test immediately afterwards. If you then get 140 psi or more that points to pistons. Can't remember if you have gone deeper than the top end. Low compression tends to give poor starting from cold particularly overnight, then once it's fired it will re-start immediately for the next few hours.

Bryanj:
Is this a 500K3 with the godawfull PD carbs?

Seabeowner:
Don't think so Bryan as he mentions choke lever.
Did you do the compression test with the choke flap open and the throttle wide open (wot)?
Has this compression tester given good results on motorcycles in the past?

Siloxious:
I believe they are the 069A carbs from Keihin.

The compression tester is good, it measured my last compression readings quite okay without problems.

The choke flap was in the open position and the throttle was fully open. I’ll do the compression test again, now that the engine has run for some minutes. I’ll post my findings here. 

Murf:
Assuming all the valves and seats are reground and good then lack of compression would be due to worn piston rings or bores or both. Was there much bore wear, indicated by ridge at the top of the bore.
Did you change the piston rings, if you did, did you hone the bores to remove the glaze and give a cross hatch to allow rings to bed in.
After a ring change or rebore it can take a bit of running for the rings to bed in and give best compression readings.

Does the cam show signs of wear on any of the lobes, if so is the wear mostly on one side of the cam lobe.
Did you check the rocker shafts in the rocker cover for any up and down movement. These shafts do wear their mounting points oval, as already mentioned, it’s a problem and I doubt that it is possible to get the engine to run well once this has happened.  Cam wear on one side of the cam is a good indication that the rocket shaft mountings have worn on one side and the shafts and rocker are at an angel to the cam face.


Did you use good quality replacement carb bits, there is a lot of cheap useless crap on sale.  It’s often far better to clean up the original jets etc.  Same with spark plugs lots of fake ones around, use a trusted supplier for them, not online auction sites.
Good luck

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