750SF2 - Strange ?engine? noise when under load

I only hear random acceleration and not much else. How do the plugs look and are the carbs jetted correctly and synched? You could be hearing induction noise during acceleration, unless the noise is metallic.
Before taking the motor apart use a boroscope to see cylinder walls, but removing the head is the definitive solution ( if the noise is indeed from there).
Since the crank and bearings are robustly built, the bottom is end probably good.
How many miles does it have?
 
piranha-bro2 said:
I agree it might be time to pull the top off. Peace of mind if nothing else.
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Someone has to explain, so it settles my non mechanical mind.

In the top end: ‘Apart from pistons’, what is there that works hard under acceleration or load?

Cam, cam chain and valve train only react to the action of the bottom end ….. bike could be towing a coal train, but nothing in the top end would be under any more strain than it is during normal operation.

Or, is my non mechanical logic coming to the fore?
 
I'm impressed but not surprised by the posts on this this topic. The time taken by the various posters to posts very knowledge responses is typical of this forum. The different opinions are not being slagged by others. This is pretty typical of this Forum, at least in the Technical Section! This all just reminded me while I'm thankful for this group it makes me wonder why so much of the other public discourse in the world is such a fricking shit show.

Back to the actual topic....My 750 SFC with the factory two into one exhaust was so loud I couldn't hear any engine noises. Ignorance was bliss but not much help in this case.
 
henrywillms said:
The different opinions are not being slagged by others. This is pretty typical of this Forum, at least in the Technical Section!
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That’s because we get our hands slapped for inappropriate posts in the Tech Section

Slagging off in all other sections is welcome 😉

Back to the topic at hand. Knocking and clattering under load is usually a bottom end warning you it’s about to shit itself. All other symptoms seem to have been exhausted.

Maybe it’s time to split the cases.
 
chrisk said:
That’s because we get our hands slapped for inappropriate posts in the Tech Section

Slagging off in all other sections is welcome 😉

Back to the topic at hand. Knocking and clattering under load is usually a bottom end warning you it’s about to shit itself. All other symptoms seem to have been exhausted.

Maybe it’s time to split the cases.
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A dodgy cam key won't clatter all the time, Chris. Will depend on back-and-forth forces acting on the chain ... and that can be as fundamental as the interval between each cylinder firing. A small end for eg might have a 'stasis' where it doesn't make a racket ... dunno, just hypothesising.

Obviously if we could each ride the bike and hear and feel the noise for ourselves it would be a lot easier to diagnose. Mechanicking on bicycles has taught me that noises can emanate from somewhere quite remote from their causes. Ditto with motos. I once pulled my motor out and apart because it had developed a knock. There was bugger all wrong anywhere. I discovered the retaining bolt on the drive pulley of my Kwaka ND alternator conversion was loose and the knocking was backlash in the splined fit. Fark me! And that's with 40 or 50 years experience in every aspect of these motors.

We wait with bated breath for the cause to be revealed. Don't we just LOVE these little mysteries!
 
Just like the cwack said to me after 3 of them were investigating with really high-tech gadgets the other day, we see a lot of people with that and find no known cause.
 
I agree with all the knowledgable responses, but it’s unclear where this noise is coming from- top end or bottom end, so it’s difficult to access the cause.
Again, were metal shavings or extreme swirl present in the last oil change?
These motors have a robust reputation, but my 750SF2 came w 15k miles and needed valve work and new pistons, and probably due to abuse from the PO.
But they are mechanically noisy motors, and best of luck determining and fixing the problem.
 
piranha-bro2 said:
A dodgy cam key won't clatter all the time, Chris.

We wait with bated breath for the cause to be revealed. Don't we just LOVE these little mysteries!
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FFS …. wasn’t thinking in that regard.

He needs to check for a dodgy cam key, before taking my advice and splitting the cases, where he’ll find a bung mains 😉
 
I couldn't hear the noise through my shitty little computer speaker, so I played it through a set of headphones and it was more audible.
I think you're right Mike. It sounds to me like its frequency is at engine speed, not the half speed of the valve train or clutch. That would tend to indicate it's related to a conrod or piston. Main bearings tend to have a lower frequency rumble because the balls roll around the bearing races at a fraction of engine speed (like planetary gears).

Excessive clearance in big or little end bearing tends to be most audible as you back off the throttle while the engine is at a couple of thousand revs. As the load eases on the engine, the bearings seem to rattle a bit more than when loaded.

Piston slap is another possibility for an engine speed noise. That tends to be louder under load than on the overrun when you back off the throttle.

Looking at the cylinder walls with a borescope won't tell you anything about bearings or valve train. But if the bores are obviously worn and glazed, then that might add weight to the piston slap scenario.

I hope you get to the bottom of this mystery. We're all emotionally invested in it now.
 
Dellortoman said:
I couldn't hear the noise through my shitty little computer speaker, so I played it through a set of headphones and it was more audible.
I think you're right Mike. It sounds to me like its frequency is at engine speed, not the half speed of the valve train or clutch. That would tend to indicate it's related to a conrod or piston. Main bearings tend to have a lower frequency rumble because the balls roll around the bearing races at a fraction of engine speed (like planetary gears).

Excessive clearance in big or little end bearing tends to be most audible as you back off the throttle while the engine is at a couple of thousand revs. As the load eases on the engine, the bearings seem to rattle a bit more than when loaded.

Piston slap is another possibility for an engine speed noise. That tends to be louder under load than on the overrun when you back off the throttle.

Looking at the cylinder walls with a borescope won't tell you anything about bearings or valve train. But if the bores are obviously worn and glazed, then that might add weight to the piston slap scenario.

I hope you get to the bottom of this mystery. We're all emotionally invested in it now.
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Oh arr…….. we could liven up the emotional investment and have a little ‘virtual’ (no money to change hands, it’s pretend) flutter on the outcome.

Wanna bet?

I’ve got US$500 it being the bottom end.
 
Initial findings: Both little end bushes have visible/feelable sideways/twisting rocking movement (not up-down movement), like the bush ID is barrel shaped. The pistons look recent, +.4mm oversize, have about .07 – 0.08mm clearance, appear stamped JE (see pic). They have shorter slipper skirts ie are cut away near the pins. The bores show hone marks tho some mild scuffing front and back. There is no detectable movement in the big ends and there was no metal in the oil (tho its new oil with only few kms on it).

My current theory is the noise is the pistons rattling side to side on the wobbly pins/bushes when under additional load. (But there are no obvious witness marks on the side of the bores...) I assume the PO replaced the pistons, likely bored it, certainly honed, but the modern piston design may yield that noise due to the wobby pins and the pistons lack of side support, whereas an older full skirt may not have. Hence the slightly uncharacteristic noise pattern for a little end.

I guess the big ends could still have damage without detectable movement – but no evidence atm.

Apart from that, there’s quite a bit of carbon in the chamber and some dampness around in inlet valves. Some movement in the valves/guides but I need to get a gauge to measure that properly. And quite a bit of end play (0.65mm) on one rocker/shaft.

The speedo (ND) shows 75K miles – but has a blueish coloured face where rev counter is slightly more greenish, so one or both may be off another bike. The bike was originally from South Australia and I don’t have long term records. I assume this engine has done some miles.

So… does this theory sound plausible????

cheers
 

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mikeb said:
Initial findings: Both little end bushes have visible/feelable sideways/twisting rocking movement (not up-down movement), like the bush ID is barrel shaped. The pistons look recent, +.4mm oversize, have about .07 – 0.08mm clearance, appear stamped JE (see pic). They have shorter slipper skirts ie are cut away near the pins. The bores show hone marks tho some mild scuffing front and back. There is no detectable movement in the big ends and there was no metal in the oil (tho its new oil with only few kms on it).

My current theory is the noise is the pistons rattling side to side on the wobbly pins/bushes when under additional load. (But there are no obvious witness marks on the side of the bores...) I assume the PO replaced the pistons, likely bored it, certainly honed, but the modern piston design may yield that noise due to lack of side support where an older full skirt may not have. Hence the slightly uncharacteristic noise pattern for a little end.

I guess the big ends could still have damage without detectable movement – but no evidence atm.

Apart from that, there’s quite a bit of carbon in the chamber and some dampness around in inlet valves. Some movement in the valves/guides but I need to get a gauge to measure that properly. And quite a bit of end play (0.65mm) on one rocker/shaft.

The speedo (ND) shows 75K miles – but has a blueish coloured face where rev counter is slightly more greenish, so one or both may be off another bike. The bike was originally from South Australia and I don’t have long term records. I assume this engine has done some miles.

So… does this theory sound plausible????

cheers
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Those skirts are so short….. but what would I know, I’m not a mechanic.
 
I have fitted the same JE pistons (if I recall correctly) to an SF without issue. They came in the box with a dismantled engine for assembly, I do remember that they were quite tall compared to as SF piston and I was concerned about compression ratio. Cannot even remember the details if I needed an extra base gasket.
 
mikeb said:
The speedo (ND) shows 75K miles – but has a blueish coloured face where rev counter is slightly more greenish, so one or both may be off another bike. The bike was originally from South Australia and I don’t have long term records. I assume this engine has done some miles.

So… does this theory sound plausible????

cheers
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In sunshine, not uncommon in Oz, the faces do fade. I've seen a few gain a blueish tint, as they do.
Without documents, it's difficult to know the mileage. Unless abused, Laverdas last well.
In my experience, carbs, on filtered bikes, are the best indicator of prolonged use.
 
From Wolfgang I installed Ross Racing forged pistons .5 over and had the cylinders trued and rebored to fit properly. At 15 k miles the cylinders were scored and out of true, so it must,ve been previously abused.
 
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