Topic: CB750 shrieking noise

[turboguzzi]

Hello 750'ers

I mostly into CB500's so need some help here.

A friend just bought a CB750 cafe job at ebay, supposedly with a 78' motor with 6300 miles on the clock. Bike arrived, friends starts motor and it sounds rather healthy. Four-five minutes into the ride a shrieking noise comes from the motor. Friend pushes back home, lets the thing cool, starts it up and again, sounds good but after 3-4 minutes shrieking noise appears again.

Was told the story over the phone so cant give much more details but asked him if there's oil and if it flows and he said yes, of course.

Sounds to me like a gearbox ball bearing which wouldnt be so nice, as he'll have to open the bottom end, unless someone has another idea for an easy fix or thing to check. Was thinking also about the generator brushes but it doesnt make much sense

Cheers and thanks for your help on behalf of my friend

TG

[Gordon]

That doesn't sound good.

Without knowing anything more, the first thing that comes to my mind is the camshaft.  Possibly a clogged oil passage causing the shaft to start seizing and screeching when it gets nice and hot.

[dakeddie]

I have got a serious hate-on for ebay.  For every 1 honest seller, there is a dozen dishonest ones.

[333]

And check if oil is reaching the top of the motor by removing the inspection caps and starting the engine to look for oil squirting in there.

[MRieck]

That sounds like a cam tower being taken out.

[mlinder]

That sounds like a cam tower being taken out.

+1

Was going to say, if it does it without moving, it's probably not anything in the transmission.

[mustangcar]

 if it does turn out to be no oil to the top of engine can you fix the cam tower?, you have to replace the cam but can the tower be fixed,or do you usually look for another head?

[MRieck]

if it does turn out to be no oil to the top of engine can you fix the cam tower?, you have to replace the cam but can the tower be fixed,or do you usually look for another head?

You can usually replace the cam and towers/shafts. Rocker arms need a good looking at too. That is one of the good things about the CB head. Early Z's were good about that too as they used cam bearings

[turboguzzi]

thanks for your help and it look like its indeed oil not reaching the top end.

As we say here in italy:

Merda

Luckily its a 750, on a 500 you'd be throwing the head.... we'll see, might need some help with parts here

cheers

TG

[eurban]

thanks for your help and it look like its indeed oil not reaching the top end.

As we say here in italy:

Merda

Luckily its a 750, on a 500 you'd be throwing the head.... we'll see, might need some help with parts here

cheers

TG

Lucky?  At least on the 500s one wouldn't have to pull the engine from the frame.

[754]

Yes lucky..
 I would rather pull the engine & save the head, than to lose a head. moreso if it is ported.

[turboguzzi]

I'm with you on this one, 754....

Mike, never knew Z's had separate cam bearings, I learn something new everyday. Makes you miss the days when everything was over-engineered!

Big question now is if the ebay seller should be grilled. He's kind of known in the 750 community (and been told of the problem by now). Will wait till there's a final veredict on the failure. IMHO, the guy should cover the new parts.

Cheers
TG

[bryanj]

You will need a minimum of














You will need a minimum of:-
Cam
1 or 2 towers
4-8 rockers
Top end set.

Maybe
oil restrictor jet(s)
cam chain
tensioners

[mustangcar]

they say the later 750's had pressed in oil restrictors in the head?,,do you run a wire through the oil holes to clean them out,how will he assure it won't happen again when he reassembles the top end?

[upperlake04]

they say the later 750's had pressed in oil restrictors in the head?,,do you run a wire through the oil holes to clean them out,how will he assure it won't happen again when he reassembles the top end?

 The oil restrictors on my engine were more like drop in than pressed in and probably would have fallen out on the floor if the head was turned upside down. No problem inspecting and cleaning them - mine were partly crudded with an amber coloured substance  which had caused a drop in oil flow to the towers - not good.

[kaysystems]

That sounds like a cam tower being taken out.

I'll second that one.

Not a nice noise when you are miles from home

messed up my sprin riding season

David      

[turboguzzi]

thanks bryan, will pass over the info

sounds like a hefty bill.

TG