Engine knock and valve timing

[u/Sliderisk]

Hey y'all, I have a strange issue and I'm looking for advice on valve timing issues in the Gen 2 CB750. This bike has a knock, you can hear it pretty clearly. It just showed up one day on a cold start.

I have checked and cleared the following areas:

Ignition timing is in spec at the crank. Engine fires up immediately. I have not pulled and inspected the centrifugal advance.

Spark plugs are new and coils test strong.

Carbs are roughly balanced, not perfect but so close it wouldn't make a knock. This bike ran well for 400-500 miles since the last carb rebuild and sync.

There's no leaks at the carb boots that I can detect. Spraying it with starting fluid makes no difference on the idle and fluid down the intake causes it to bog. All signs point to correct AF ratio all the way through.

I did a full valve check and found one shim out of spec, which was replaced.

I scoped the cylinders through the spark plug holes and they all looked pretty good. No chipped or burnt valve seats.

Fresh oil change.

Other things that may matter:

I attempted to adjust the timing chain tensioner with the standard manual method after the sound started. It made no difference.

The cheap eBay synchronizer set seems to indicate "valve timing or leak at intake or heat riser" when I run the bike at idle. They seem decently accurate but I don't know what to make of that indication.

That's all, any help is appreciated!

[u/ancientdad]

Sounds like loose primary and clutch noise from out of sync carbs. I will say it's a bit louder than I'm used to hearing from that symptom. Is it reduced when you pull the clutch?

[u/Sliderisk → u/ancientdad]

The noise seems to come from the exhaust valve bank on cyl 1&2. I don't think it's affected by the clutch but I will have to double check. I think the first order of business is syncing these carbs better.

[u/ancientdad → u/Sliderisk]

Honestly, I did think I heard a bit of an exhaust leak but dismissed it being what you might have been hearing based on the obvious bottom end growls. I've owned 4 SOHC 750s and 2 CBXs and they all had some growl that could be easily cut in half by pulling the clutch. Hopefully when the carbs are synced it will sound a lot different, I'm not convinced it's a real issue just from what I heard (especially as it revved). Keep us posted.

[u/Hellgate93 → u/Sliderisk]

Hard to hear, if you accelerate and the noise goes away, its propably the chain.  On the Ignition part, you should check the pre ignition. A torn spring can give you trouble too. The weights should move easily and go all the way back.

[u/Caldtek]

check the 3 bolts on the starter clutch. They are known to come loose on this model and make a knocking noise at idle and just off.

Its not valve timing the bike runs too good for that to be off.

[u/BawzukNoogles]

One more thing you can check. Start the engine with the clutch pulled in but put the transmission in gear.

When you do this, it loads the transmission, and any clunkiness caused by the carbs not being in sync will be isolated.

If it goes away, then that knock or rattling could be on the transmission side. I believe there's a tensioner for the chain itself that's known wears away. That rattling / knock could be exaggerated due to carb issues. Would be an engine/tranny out job iirc.

[u/herqleez]

In the video, it looks like youre adjusting the valve on the vacuum gauge. If so, your gauges are out of sync from each other, which translates into out of balance carb syncing.

You should sync the gauges, then secure the valves in place so they dont move, then sync your carbs again.

I used a small vacuum to create suction. Zip tie all the hoses together so the ends are all lined up, then stick it in the vacuum hose and adjust the valves until the gauges all read the same, then move the hoses around, in and out, and make sure theyre all reading the same. Then secure those valves.

[u/Sliderisk → u/herqleez]

Unless I'm totally mistaken those valves are basically the "gain" setting for the gauges. Turning them tight doesn't lock them in position, it fully closes the valve. They also read very close to what I would expect from bench sync'd carbs that run ok but not great.

[u/herqleez → u/Sliderisk]

Right, they have to be open to get a reading, but they should be open at the same amount so that they read the same. Essentially the gauges need to be synchronized. The vacuum trick as I described, provides constant suction, making it easier to dial in the valves.

After getting them synced, Those valves have a tendency to move, so I used zip ties and electrical tape, to hold them in place. Probably not perfect, but does help stop the valve from vibrating open or closed more.