GT 750 top end rattle 3-4000 rpm.

[Exmoor beast]

I’ve bitten the bullet and whipped out the motor of my ‘69 GT750. I’ve got a dimpled valve top that needs adressing, slight smoking after its idled a while and a rattle that seems to be from the left hand that starts at 3-4000rpm.
Rocker side play seems consistant at .20mm across all rockers, bores are glazed ( which I suspected dueto it standing for 30 years..) but apart from that no real disasters, all nice and clean.
I’m going to whip out the valves and check the head out this afternoon.

 

[Paul Marx]

Combustion chambers are rather oily. How are the piston tops?
Probably oil going down the guides. Nothing to worry about.

Paul

 

[Exmoor beast]

Both exhaust valve tops deeply pitted,other two slightly dimpled, inlet manifold insulator blocks are disintergrating ( escapee “O” ring) but all the cams/ followers/ spindles are like new. Spindles came out with hot air gun . Removing Down pipes was biggest headache, still they have been on 53 years. Cam chain wheels seem ok but are old and the rubber buffer that acts on the tensionor is split. I’ve got a feeling I may not completely solve the rattle, valves are going to make things quieter, in truth I only noticed the noise when I rode around the block with no ear plugs!
I had adjusted valve clearances using thread pitch as feeler wouldnt work, but I still wasn’t happy.
Bike has stood for three months but the top end was awash with oil, I was pleasantly surprised.

 

[Exmoor beast]

Combustion chambers are rather oily. How are the piston tops?
Probably oil going down the guides. Nothing to worry about.

Paul

Most of the oil is where I’ve turned head upside down on bench, bit of carbon buildup but not soft oily buildup.
I suspect you are right about the guides- doesn’t use a great deal of oil to be fair, or smoke when I’m riding- just a little if it’s been idling at traffic lights and I look down and blip throttle. Valve tops was main concern and reason for tear down- the noise may be normal for one of these- there’s a lot of over heavy moving parts clattering around in there.

 

[Paul Marx]

Most of the oil is where I’ve turned head upside down on bench, bit of carbon buildup but not soft oily buildup.
I suspect you are right about the guides- doesn’t use a great deal of oil to be fair, or smoke when I’m riding- just a little if it’s been idling at traffic lights and I look down and blip throttle. Valve tops was main concern and reason for tear down- the noise may be normal for one of these- there’s a lot of over heavy moving parts clattering around in there.

Yes, top ends are more or less noisy according to wear.
Check cam coupling though I suppose you have.

Paul

 

[The Sock]

Both exhaust valve tops deeply pitted,other two slightly dimpled, inlet manifold insulator blocks are disintergrating ( escapee “O” ring) but all the cams/ followers/ spindles are like new. I’ve got a feeling I may not completely solve the rattle, valves are going to make things quieter, in truth I only noticed the noise when I rode around the block with no ear plugs!
I had adjusted valve clearances using thread pitch as feeler wouldnt work, but I still wasn’t happy.

You could accurately set the valve clearance with a dial gauge, though with pitting that deep re-facing/re-hardening might be a better option.

 

[Exmoor beast]

Yes, top ends are more or less noisy according to wear.
Check cam coupling though I suppose you have.

Paul

Cam coupling is tight, no signs of movement and bearings are good. Piet recommends .10-.15mm end float on spindles, mine are .20, I don’t think enough to cause rattle. It may be just a natural resonance that sets its self up at certain rpm, it doesn’t get louder with higher revs or on overrun . Maybe why the factory stuck more fins on later model?

 

[Exmoor beast]

You could accurately set the valve clearance with a dial gauge, though with pitting that deep re-facing/re-hardening might be a better option.

Haven’t any experience of refacing/ hardening- you would have to take a considerable amount off to get back to flat which would alter the working angles of the Tappet - probably just put new valves in. Quite why both exhaust valves are so bad and only one inlet has the slightest mark , I don’t know.

 

[MarnixSFC]

Cam coupling is tight, no signs of movement and bearings are good. Piet recommends .10-.15mm end float on spindles, mine are .20, I don’t think enough to cause rattle. It may be just a natural resonance that sets its self up at certain rpm, it doesn’t get louder with higher revs or on overrun . Maybe why the factory stuck more fins on later model?

Later ones had one fin less on both cylinder and head, to reduce ‘singing’ and make them easier to cast.

Marnix

 

[Exmoor beast]

Later ones had one fin less on both cylinder and head, to reduce ‘singing’ and make them easier to cast.

Marnix

Of course, less fins more alloy, makes sense-

 

[MarnixSFC]

Of course, less fins more alloy, makes sense-

The old ones phased out in 1973-1974.

 

[Paul Marx]

Haven’t any experience of refacing/ hardening- you would have to take a considerable amount off to get back to flat which would alter the working angles of the Tappet - probably just put new valves in. Quite why both exhaust valves are so bad and only one inlet has the slightest mark , I don’t know.

I've seen 0.25 exhaust clearance noted for the early models. I think the Green Book quotes that figure.
I use 0.20 on my S but haven't put tens of thousands on it yet.

Paul

Paul

 

[sfcpiet]

Quite why both exhaust valves are so bad and only one inlet has the slightest mark , I don’t know.

Probably general neglect.

Once the valve tips start to dish, clearance cannot be accurately adjusted, it just gets worse and worse. With a mm of clearance, the adjuster screw is hitting the valve at considerable speed when the cam accelerates the rocker. I'd suspect the screws to be completely knackered too.

That's a tired old dog, top ring grooves need to be inspected, loose rings act like an oil pump. Piston skirts generally look quite OK, but bores are usually worn. Rare is the case that a new set of rings will do the trick. The damaged valve tips can cause excessive guide wear, the valves are knackered anyway, waste of time to measure those...

Engines like this can rattle from a multitude of causes, piston slap being the most common. Small end play, big end play, out-of-true cranks, bent rods. You name it, I've found it.

Even most "well-running" cranks are on the verge of (self)destruction...

piet

 

[Exmoor beast]

Cheers for that- I’ll give everything a thorough check tommorow-first motor I’ve stripped since wearing glasses- I need better light than just strip lights in garage. The rattle is not like piston slap but a tinny sound coming from top left- it starts at 3-4000 rpm then goes higher up-almost like pinking , but I had just set ignition timing / new coils/ condensers…anyway. He might be a tired old dog- but he’s my tired old dog!

 

[Shajota]

Just on a bit of a tangent away from the tech aspects, your comment that "rattle" is around 3 to 4k revs peaked my interest.
I was getting that from time to time on my AE which was driving me nuts as it sounded very "tappety" at around cruising speed and over where the engine felt strongest.
In my case fixed by tightening the myriad little bolts holding the battery tray, tool tray, air filter. This is an absolute pia area with the number of fixings creating potential for rattles caused by vibration.
No doubt you had done this however just thought I'd mention it!!
No doubt your engine tear down is best course of action anyway and probably would be for mine to, particularly after reading Piet's post

 

[Exmoor beast]

Just on a bit of a tangent away from the tech aspects, your comment that "rattle" is around 3 to 4k revs peaked my interest.
I was getting that from time to time on my AE which was driving me nuts as it sounded very "tappety" at around cruising speed and over where the engine felt strongest.
In my case fixed by tightening the myriad little bolts holding the battery tray, tool tray, air filter. This is an absolute pia area with the number of fixings creating potential for rattles caused by vibration.
No doubt you had done this however just thought I'd mention it!!
No doubt your engine tear down is best course of action anyway and probably would be for mine to, particularly after reading Piet's post

Must admit I will now check everything “under the tank” that could be rattling..

 

[arancia]

While I don't want to turn this into another eternal debate about which oil etc and I don't know how many miles since "it stood for 30 years", have you been using a high-zinc oil during your ownership to protect the rubbing surfaces that you would not need to use in your more modern engines?
I know we hear all the time that we don't need special oils but I'm a firm believer that high-zinc oil or an additive is valuable on the sliding surfaces of the rockers just as on the cams and followers of older american V8s which can quickly be ruined by a modern synthetic which has to be without zinc to protect catalytic converters.

 

[chrisk]

Must admit I will now check everything “under the tank” that could be rattling..

Terry?

 

[Exmoor beast]

While I don't want to turn this into another eternal debate about which oil etc and I don't know how many miles since "it stood for 30 years", have you been using a high-zinc oil during your ownership to protect the rubbing surfaces that you would not need to use in your more modern engines?
I know we hear all the time that we don't need special oils but I'm a firm believer that high-zinc oil or an additive is valuable on the sliding surfaces of the rockers just as on the cams and followers of older american V8s which can quickly be ruined by a modern synthetic which has to be without zinc to protect catalytic converters.

Totally agree- I was using Silkolube Classic 20/50w which has high levels of ZZDP but have now found one with higher zinc / low detergent , low dispersion . Rockers/cams ,pistons are all fine it’s just the exhaust valve tops, bike has done 23,000 km, and is clean inside .
Reading about big V8 yank motors they also suffer from exhaust valve tip wear ( similar size and layout) and it’s put down to weak valve springs, mine have compressed by 2mm from standard, may be just initial compression i don’t know. I have a feeling it may be just aggressive rocker geometry, guide wear , tip wear- which comes first I don’t know!

 

[Exmoor beast]

Terry?

Knock- Knock.. Who’s there ? Terry…….?