750SF2 - Strange ?engine? noise when under load

M

mikeb

New member
Location
new zealand
Hi. First post and another ‘strange noise’ question. I’ve just bought a ’74 SF2, fixed a couple of minor things, new oil, adjust tappets, cam chain etc, and had some test runs. It runs ok, ignition still a bit retarded (has a boyer 3). But there a strange noise when under load from about 2000rpm. A kind of metallic loud light-ish clattery thonking. Not a heavy noise. Doesn’t sound like pinking – too harsh/loud. Only when under acceleration and only on-road. When I push the front wheel against a wall and load it up on the clutch I can’t hear it. My first impression is it’s from the head – ie high, forward, tho hard to tell with an alu motor.
  • No such noise when stationary at idle or revs when cold so I doubt its piston slap. The noise is present when hot or cold – and only when the bike is on the road, as above.
  • I’ve checked for the 2 cam halves movement, but can’t feel any difference in movement when moving the rear wheel in gear etc. but first laverda so I could be missing something.
  • I’ve checked for loose bolts etc and held the tank and various bits while riding – ie in case a harmonic vibration.
  • Occasionally its not there, or I can’t reproduce it. But it is 90% of the time.
  • I’ve started to wonder if could be rattling in the PFH36 carb slides, but I doubt that would be so loud.
So… any ideas? Or diagnostic steps?

Thanks!

Mike
 
My advice is worth exactly what you are paying for it.......... If it were me I would be verifying the cam timing using a dial indicator and degree wheel and verify BOTH cylinders cam timing. I would then move on to ignition timing. Once those were verified I would move on to possible mechanical noise ie cam tensioner etc.

There are lots of folks here with SF experience and hopefully they can narrow it down for you.

I hope it turns out to be something simple.
 
I own a 74 SF2 so familiar w them after much work done to it. You,ve checked the valve clearances but do a leak down and compression check and also verify that the timing is good, both static and strobe. What do the plugs look like and how is the engine performing? Check both cam chain and lower end chain tension per the Green Book. Check that all engine mount bolts are snug and that the mufflers and headlight bucket are rubber mounted. These motors do vibrate a bit-
 
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Pinking can be really loud. You are unsure of the ignition timing so get it sorted. With fuels today it is ever more critical. Other than that I would be thinking hardware, something cracked or just loose enough to contact a neighbouring piece. Virtually all of the mechanical engine stuff would show up with what you have investigated. I have seen totally worn out motors that don´t rattle like you describe.
 
Wasn't there a sound produced on the standard tapit adjusters that has a worn step in them and removed by replacing them with what? Apilia ones? Q or Marty, didnt you do that? I dont own a Twin, other than my Atlas.
 
Repeat the ride and pull the clutch at the rev where the noise appears. If it goes away transmission is the culprit
 
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