Topic: Something just blew in the top end of my cb750. Thoughts?

[Bailgang]

When I had my 77 750F2 at the dragstrip for the very first time I blew a shift so bad it floated the valves and I think at least 2 of those jam nuts came off because of it. That would've been the end of my day except I had one of Frank's (754) frame kits installed which allowed me to pull the valve cover off. I got lucky because those nuts hadn't fallen into the engine and were easy to retrieve once the valve cover was off and it was up and running again in no time. I'm sure over revving like it did didn't do the engine any favors but but it still runs good.

[Terry in Australia]

I've never had one come loose after I've adjusted them, but I do them up tight. I bought a K4 engine last week and the seller told me that he imported it from the US 6 years ago and it ran fine, "with the usual rattles".

I took the cam cover off this afternoon and was happy that the cam and rockers looked really clean, but surprised to see the jam nut on the #4 inlet valve was loose to the point where it was almost off, the adjuster had backed out, and there was a massive gap. I checked the rest, and was able to tighten most of them further.

I think that's why the seller might have decided to sell the bike for parts, but maybe I'm just being over-cynical.....

Redneck Saturday 3 by Terry Prendergast, on Flickr

[USMC5811]

What?!?! THIS is a blown engine! LOL


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[Knightro]

Ouch!!

[flybox1]

OK.   I've fished out nuts. 

Adjusted tappets. Valve clearance. 

Adjusted cam chain tentionerguide

And I'm sad to say it does sound a little rougher than it used to...

Still sounds pretty good...

Sounds fine.  vacuum sync it yet?

[markreimer]

Hey Knightro - I just saw you're from Winnipeg, so am I! Not that common to meet another SOHC4 owner here from my home town. I actually had a '78 cb750K 6 years ago but sold it when I got my '74 750K (still have it, runs great!). What colour is it? The one I sold was black, with turn-out 4-2 pipes. Maybe you have it haha!?

If you even need a hand with your bike shoot me a PM. I've rebuilt everything on my 750 a few times except the transmission.

[Knightro]

Dead link on that one.

[Stev-o]

Rebuilt, right?

[markreimer]

I agree, that engine sounds really good to me. Mine makes WAY more rattling noises than that, on account of the clutch basket chatter. I've had three 750's and they all kinda sounded like a cutlery drawer being dumped on the ground. Now and then I'll get everything just bang on, carb sync/timing/valves etc and it sounds like a little sewing machine, but it doesn't take much to make these engines rattle a bit in my experience. All the SOHC4 bikes I've heard are a bit noisy. Go for a ride!

[Terry in Australia]

Use thicker oil.....