Topic: 1978 CB750, oil/ water boiling sound when turned off

[Can550]

Very strange sound like a liquid cooled bike. Runs good, no smoke or extra noise. When turned off, sound of boiling oil/ liquid comes from motor top ??

Removed tappet cover, nothing. Removed spark plug, tad bit rich.

Could it be leaky exhaust gaskets (crush rings)?

Thanks


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[754]

When you check the oil was there any mayonnaise  at top of dipstick from cold weather riding, and only occasional  starting... that might cause a bit of steam till it gets cooked out.
.how long does it do it, if it's head gasket it would not take long for pressure to drop..

[BenelliSEI]

Any chance there is water (condensation) in the oil? Get it HOT and do an oil change......?

[Can550]

Hmm
You got me thinking now

Got this bike only few days ago. Before that it sat for about 4-5 years.

Oil in tank was towards bottom, so I topped it up.

Sound lasts a minute or so


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[ekpent]

Hmm
You got me thinking now

Got this bike only few days ago. Before that it sat for about 4-5 years.

Oil in tank was towards bottom, so I topped it up.

Sound lasts a minute or so


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If your new to 750's always check the oil after it has ran for a little bit. When parked for a couple days or stored oil can run out of the tank and collect in the engine until started again.

[mrbreeze]

A crackling sound when you shut it down sounds like a lean condition but you said your plugs show more rich and a gurgling sound? What ever's going on you've only had it a few days so do as suggested and change the oil. Couldn't hurt and you'll know for sure your oil is fresh. If you've never changed oil in these bikes read up on it. There are 3 places to drain and you gotta be careful on the oil filter bolt. Have fun with it man! They're great bikes!

[754]

 Oh man , not good, crack the drain plug , let out a few tablespoons,, then tighten....water will be at the bottom.
 Ig0f they been sprinklers they can get a lit of water in them.
 I had a nighthawk, fave the motor away.l every hearing in lower end was suspect..
 Had a KO motor in a bike , had a bit of oil in tank looked fine.. was going to pull motor and maybe use it... opened drain plug, cups of water came out.... granny and bearing maybe useable .. maybe.l may not lastvtho..
 Main bearings, materialwas there but looked a bit like crepe paper.. my thought was they would crap out in short order.
 Crank was not that rusty , journals pretty good...

 So drop a cup or less of oil , you get water.... don't be planning a trip.

[754]

No matter how hot oil gets, never seen it build pressure or make noise.. but water,,oh yeah. .

[Can550]

Ok so update

Removed headers, replaced crush gaskets, tightened them back, fired up and waited even longer than yesterday, no abnormal sound or gurgling.

Now oil is draining and no water but oil seems a bit thinner so either gas in it or it’s just very old oil.

Will put new oil + filter and report back

Thanks for all suggestions and information


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[754]

Is oil greenish or whitish ?

[Can550]

Blackish
Just like a regular overdue oil change colour and viscosity


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[754]

Well that sounds normal .. if its a small head gasket leak I think it would only do it a minute or two.

[lash]

If its not very loud might be just the tank vent at the gas cap.

[mrbreeze]

If its not very loud might be just the tank vent at the gas cap.

That's a possibility and very easy to check.