Topic: Engine Noise

[M_Stone76?]

Hello, I just got my bike out of storage this spring n started it to this noise. Could anyone possibly have a clue , sound the the whole bottom end is about to implode .
Bike is a 76 SOHC with 14,500k miles , all maintenance has been done aswell
The Noise is more apparent towards the end of the video
Not sure if I’m at the end of the life of this engine or not. Thanks

https://drive.google.com/file/d/1XAt0b-IM_gLEB76NXxXXeQRe_GMN0OjF/view?pli=

[denward17]

Cold start?
Good oil pressure and level?

Carb sync been done recently?

Sounds like clutch basket rattle to me, but I could be mistaken, wait for experts to chime in.

[M_Stone76?]

Everything is in perfect working order for maintenance n performance wise. It sounds like either the main/can chain slapping, it gets quieter when it’s fully warm but still there.

[newday777]

Hello, I just got my bike out of storage this spring n started it to this noise. Could anyone possibly have a clue , sound the the whole bottom end is about to implode .
Bike is a 76 SOHC with 14,500k miles , all maintenance has been done aswell
The Noise is more apparent towards the end of the video
Not sure if I’m at the end of the life of this engine or not. Thanks

https://drive.google.com/file/d/1XAt0b-IM_gLEB76NXxXXeQRe_GMN0OjF/view?pli=

Your carbs are out of balance in the video, that puts unequal pressure on the pistons and crank causing the primary chains to slap the cases.
Vacuum sync the carbs.

[newday777]

Everything is in perfect working order for maintenance n performance wise.

Nope. The carbs are out of balance.

[M_Stone76?]

Ok, I have heard that can be a cause. I have the Murray dual carbs on the bike .

[HondaMan]

In the clutch of the 1976 SOHC4 750 bikes, there is one clutch plate that is called the "dual-steel-sprung-plate" (at least by those who don't cuss it...). This consists of 2 steel plates with tiny flat springs in between them, the plates being riveted together with brass rivets. Somewhere around 8000 miles the rivets got a little bit loose.

After that, every time the engine became slightly uneven, due to any cause like:
1. Carb imbalance (rare with stock carbs that haven't been removed)
2. Spark timing not being dead-on between the 1-4 and 2-3 points (or installation of the Dyna S system)
3. Engine parked a long time with old sparkplugs aboard, so they don't fire evenly come the Spring reawakening
4. Old gas in the tank

then the engine 'rattles' because of the slightly loose rivets. Once the engine speed is raised to about 2000 RPM it quiets back down, if the cylinders are at least within 80% of each other in power output.

Certainly those carbs aren't helping, but I'd suggest riding it a while to see if it smoothes back out. These engine last 80k miles or more with decent oils and halfway decent tuning. I've seen many pass 100k miles in stock trim.

[PeWe]

My CB750 K2 had a harder sounding idle when clutch stopper screw was too far out.

I changed from +1/2 turn out from tightened stop to 1/4 turn out.
That made a big difference.

I have noticed that ignition can contribute too. If too advanced.

I had to cut advancer springs on both my CB750 to keep ignition to not advance too early. Idle more stable which will end up in less rattle.

[HondaMan]

I had to cut advancer springs on both my CB750 to keep ignition to not advance too early. Idle more stable which will end up in less rattle.

Yep, Per, I've often seen where the advancer weights are loose at stop (even sitting on the bench!), due to the springs being heat-sacked and self-stretched too far.

[PeWe]

I had to cut advancer springs on both my CB750 to keep ignition to not advance too early. Idle more stable which will end up in less rattle.

Yep, Per, I've often seen where the advancer weights are loose at stop (even sitting on the bench!), due to the springs being heat-sacked and self-stretched too far.

That improvement thanks to your earlier posts about the advancers with their springs to cut, Mark

[seanbarney41]

Everything is in perfect working order for maintenance n performance wise.

Nope. The carbs are out of balance.

Nope again, I will say its not running on all four cylinders.  The noise is perfectly normal when one or more cylinders are missing.  Warm it up, spray water on individual header pipes.  The one that does not instantly vaporize the water is the cylinder not firing

[M_Stone76?]

The engine is firing on all 4 cylinders, I checked the plugs n they seem fine to me. Went on a long ride and the bike was ripping, so the engine is performing

As for the clutch, I replaced them with the upgraded one from Cycle X (5 plate I believe) so no spring clutch plate inside

I believe the carbs are definitely the culprit, I am gonna get rid of these Murray carbs, they are troublesome and I know for a fact the bike ran smoother before I put them on. So, I will order a refreshed pair and see if it helps

Thanks for all the advice.