SOHC.co.uk Forums > CB750

Low Compression

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Dazza215:
Been struggling to start my 750 K5, refurbed the carbs, changed leads, plugs, points, timing is spot on, etc etc. Thought I would check the compression. only 65 PSI per cylinder.
I suppose thats what you get for buying a bike that hasnt turned a wheel since the 80's. (USA Import)
So engine out, valve relap, stem seals, piston rings and honing I suppose.

hairygit:
Don't drag the engine out yet! Try a a teaspoon or so of oil down each cylinder, then re test. I assume you tested correctly, with the chokes OFF and the throttle held WIDE OPEN? If it shows any improvement, pour a little "Plus Gas", diesel or a good squirt of WD40 down each cylinder, and walk away for a couple of days, then test again, the rings may be gummed in the grooves giving low compression. Got to be worth a try, as it may improve it enough to get it started, and if it does start, run loads of Redex or similar through the system and the rings may free off, saving an awful lot of hassle! Good luck.

Dazza215:
Thanks Graham,
Ive done most of that.
I cant do the redex thing as it wont start, or it only runs on one cylinder. it flattens two batterys trying to start it sometimes and still wont start.
Judging by the rest of the seals in the oil pump and all around the rest of the bike it would benefit from valve stem seals etc so I think I'm just going to go for it.
If i'm honest, I am sort of looking forward to getting into it.
Thanks for the advice though buddy.

hairygit:
I'm not Graham, you're confusing me with Trigger! You could try Redex straight down the bores and leave for a couple of days. You say it sometimes starts on one cylinder, is it always the same cylinder? What are the compressions like on each individual cylinder, as if they were all as low as 65psi I doubt even one would run!

Trigger:
Did someone mention my name  ;D
If it is as low as 65psi and from the states, don't be surprised to find it looking like this.

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