CB750 Sound of Engine Seizure?

[Gengle]

I’m new to motorcycles and recently picked up a 1974 CB750 SOHC. Got it running and it sounded great. I accidentally left the petcock on overnight and now after it warms up it has a chirping coming from the engine and eventually stalls and dies as it heats up. Definitely metal on metal. I’ve done a complete oil change in case gasoline spilled in through the carbs and the oil reservoir is full.

Is there a way to check if oil is getting to the top end? I’ve ordered an oil pressure gauge to install in the pressure port but will this tell me if oil is getting to the top? Hard to tell where the sound is coming from and I don’t want to run it long enough to find out.

Do I just go ahead and plan on an engine rebuild? How do I know if I need to do top/bottom or both?

I have a video of the sound but can’t figure out how to upload it here.

Any thought appreciated.

 

[BAHNST]

The chirping sound shouldnt b associated with leaving the petcock on.....
Fuel taps get shut off as a cautious measure in case crud stops a needle from seating/floats are poorly adjusted....to stop fuel overflow...

If you feel the engine may have been siezed...rings a possibility of the sound you hear.....though rings are more rattle like..
Is the sound there with clutch engaged and not engaged...??

A compression test and leakdown test will let u know the health of the engine..(top end)

 

[hinksy]

These carbys do not overflow into the engine , they have external overflows below the float bowls and will spew fuel all over the ground under the bike . I had a cam chew out in my original F2 and it was a high pitched squeaking noise . It was caused by the restrictor/oil jets being blocked in the head . If you undo the caps used for the valve adjustment , you will be able to see if you have oil or not , but it wont be exact . I have had a bike , once , make noises on start up and I tracked it down to lose engine mounts . If it is starting , but cutting out once warm , it could be either a stuck float needle or a stator problem . So unfortunately lots of options

 

[Gengle]

Thanks for the replies. The carbs have been swapped out for dual Mikuni’s from Murray’s Carbs. The bike starts easy and sounds good for the first ten seconds or so and as it heats up there is a tapping and then the chirping gets progressively louder until it slows down (sounds like due to friction) until it shudders with a few long metallic screeches. If I give it throttle I can keep it alive a bit longer but certainly feels like it’s trying to overcome more and more friction.

I did a compression test and got exactly 90psi across all four cylinders. 100-120psi seems to be the goal but because there’s no outlier it doesn’t appear to be cylinder specific. though because it is chirping as opposed to a continuous screech or squeal it seems like it might be from a single cylinder.

I just don’t want to tear into the engine without a bit of knowledge on what I’m looking for.

 

[dirtdigger]

Bike on center stand, pull a tappet cover on the far right and far left see if you are getting oil on both ends. if no oil then cam and cam towers and rocker arms are junk more than likely. oil orifice plugs from silicone and other debris and blocks oil to one side or the other of the cam. Also just for the heck of it see if it goes away with the clutch pulled.

I suspect your compression gauge isnt good as it would barely if even run on 90psi. Good compression is 150 plus any lower and engine is tired.

 

[Biebs]

Could be the clutch making noise - pull in clutch to see if it stops.

 

[Gengle]

Bike on center stand, pull a tappet cover on the far right and far left see if you are getting oil on both ends. if no oil then cam and cam towers and rocker arms are junk more than likely. oil orifice plugs from silicone and other debris and blocks oil to one side or the other of the cam. Also just for the heck of it see if it goes away with the clutch pulled.

I suspect your compression gauge isnt good as it would barely if even run on 90psi. Good compression is 150 plus any lower and engine is tired.

Thanks for the tip. I pulled tapper covers and turned the engine over without ignition for 15 seconds or so. valves on cylinders 3&4 are dry. 1&2 look ok. I installed a pressure gauge at the oil port and reading about 25psi.

Assuming clogged oil journal, is there any way to clean it without pulling the engine in hopes I caught it in time? My next step was to pull the valve cover and see what I can see, maybe add oil manually and see if sound goes away?

Also my compression tester is from harbor freight so likely wrong.

 

[BAHNST]

If the compression tester hasnt got a check valve where it contacts your motor, and a long hose attached it will read low .....

 

[dirtdigger]

The oil orfices are under the cam towers in the head. If its a stock frame you have to pull the engine to get the valve cover off and pull the cam and towers out. Once you pull the valve cover pull the two cam bearing caps and you will more than likely see destroyed bearings.

 

[6adan]

When you do the compression test make sure your throttle is wide open or you will get a low reading.

 

[Gengle]

Bike on center stand, pull a tappet cover on the far right and far left see if you are getting oil on both ends. if no oil then cam and cam towers and rocker arms are junk more than likely. oil orifice plugs from silicone and other debris and blocks oil to one side or the other of the cam. Also just for the heck of it see if it goes away with the clutch pulled.

I suspect your compression gauge isnt good as it would barely if even run on 90psi. Good compression is 150 plus any lower and engine is tired.

Okay. Is there any cleaner I can pump through the oil system to flush the oil galleries?

 

[hinksy]

Should have remembered , but it was in the eighties . If you read dirtdiggers post , I think he has it . I siezed a cam journal back in the eighties and it made a chirping noise . The oil restrictors had been blocked and the cam ran dry