Honda-SOHC
SOHC.co.uk Forums => CB750 => Topic started by: Martin6 on October 17, 2023, 09:17:52 AM
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I'm in the process of going through the top of my motor. I will be replacing the cylinder head (incl. valves) and cover, due to bad damage on the gasket surfaces of both (previous postings!).
As the engine is looking shabby, I've cleaned it up, down to bare alloy and am re-painting it. Before i masked off the old head, which is still on the engine, I checked the cam timing. It looks to be at least 1 tooth out. When it is at TDC, keyway at 12-oclock, the points are only at the 'F' Mark for 1 and 2. I think that means I've been running the bike at about 20 degrees(?) retarded?
It ran OK. No strange noises. Smooth enough, but nowhere near as powerful as my 1975 850 Commando. Is this the reason why?
I will check the pistons for any sign of valve contact. Could I have done other damage? What should I check?
Thanks.
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I'm thinking it wouldn't run at all well at 20° retarded 🤔🤔🤔
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I'm thinking it wouldn't run at all well at 20° retarded 🤔🤔🤔
Just my guess of the difference between 'F' and TDC. I know a Norton Commando is timed to fire at 22 degrees BTDC, at tickover. The Honda may be different. As currently set up, it definitely sits on 'F', rather than 'T' at the points plate, when the cam end marks are horizontal, keyway at the top. It looks to be at least one tooth out on the cam chain sprocket. In other words, at tickover, the points are opening / engine firing at TDC at the valve train, rather than ahead of it.
Just wondering what damage this could do?
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Should be half error at camshaft timing (from crank markings) some history of people moving it one tooth out, which suggests it's possible without causing valve damage.
Expected affect would be smooth and possibly increase in low rpm torque, but compromised higher rpm performance.
Agree to inspect piston valve potential contact, borescope available ? Then set it back to correct and timed at the "T" mark on crank for running.
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Should be half error at camshaft timing (from crank markings) some history of people moving it one tooth out, which suggests it's possible without causing valve damage.
Expected affect would be smooth and possibly increase in low rpm torque, but compromised higher rpm performance.
Agree to inspect piston valve potential contact, borescope available ? Then set it back to correct and timed at the "T" mark on crank for running.
Thanks.
The head is coming off anyway, to deal with my leak, so I'll get a good view of the pistons.
Thinking about it, the ignition timing was OK, as it was accurately set to 'F' on the crank. It's the valve timing that's out. It was opening and closing the valves slightly too early. I guess that might impact compression and optimum flow in and out. My experience tallies with your view: Tickover was lumpy below 1200rpm, but I get a sense that's not uncommon. I set it to about 1200-1300rpm and it seemed happy. It was smooth above that. Best acceleration was around 3-4500rpm. It pulled above that, but didn't romp away, like other 4 cylinder bikes I've owned.
I had sooty plugs on 1 and 2, less so 3 and 4 was fine. Carb adjustment made no difference. I'd put that down to the head leaking. I have a new (2nd hand) head c/w valves, to go on. However, I will look closely at the original head, to see if that suggests any additional problems, particularly cyls 1 and 2. I'm expecting a lot of carbon build up. :(