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.
 
Had a similar rattle years back, the top rear engine mounting bracket on the frame had cracked through, Both halves were hitting together under load.
 
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
 
Worn valve tops- they dish quite badly and the feeler gauge measures the high points but not the dip that the tappet sits in.
Saying that - a lot of 750 ‘s do make a top end rattle that goes after about 3500 rpm. Mine was vey loud, changed valves, springs, tappets ( checked cam joint and rocker shafts etc) it’s now a lot quieter but I can still hear a little tinny rattle . Remember Laverda altered the head castings on later bikes to quieten them down- they must have been noisey to start with. I reckon you may not be able to get rid of it completely
 
Well I am guessing but, these are things that can make noise:

Get motor hot and accelerate and see if the noise gets louder as the bike gets up to speed and the load on the motor becomes less. If it does, you have a bad small end bushing.

Check camshaft key as it being loose is not uncommon.

Check crank shaft sprocket nut is very tight.

Check starter clutch is tight on the crank.

Check cam bearings are not rusted. As the 750 breaths through the cylinder head, I have seen condensation form in the head on 750's that have sat for a long time.

Check rear sprocket splines are not worn out.

Are you sure you have the cam and primary chains adjusted correctly. If loose the chains can rattle around and make noise.

Check the cylinder head breather rubber ball. I have seen them turn rock hard over time.

Motor mounting bolts all tight?

And a really bad thing, piston pin clip popped out and the pin is tapping on the side of the cylinder.


That's all I can think of for now. The camshaft key is the most common source of noise on a 750.
 
Owned and worked on SFs for over 50 years ...
1. Concave worn valve tips will make a light clicking sound (exactly as you'd expect a loose tappet to sound!), not the deeper thunk you describe. But it's dead easy to check just by removing a tappet cover and inspecting.
2. Loose cam flange key will sound like more like a big end with a very distinctive consistent 'clunk clunk clunk' IMHO unlikely to be your problem
3. Carb slides? Absolutely no way - I wore mine until they have daylight visible through them, no noise
4. If not noisy at idle, it's unlikely to be either the cam chain or the primary chain, but too easy to pull the cam chain tensioner out and check anyway. Refit and adjust correctly (with tension on the front side of the chain when loosening and retightening the plunger. Check and adjust primary tension as a matter of course, turn the adjuster until you feel tension by hand and back off half a turn. Personally, I'd pull the primary cover off and check it out under there ... as mentioned, crank nut could be loose.
5. With the tank off, check the rubber strap that holds the reg under the tank - these soften and become ragged - reg could be vibrating under tank at revs under load. Inspect under the tank for any evidence of marks indicating something has been rattling against it.
6. On the centrestand, back wheel clear of the ground, run the bike through all the gears. See if the noise occurs even though not under load. Could indicate a transmission prob, like a dodgy bearing or a broken mainshaft in the output 5th gear
7. Get the timing sorted pronto - check with a strobe to be sure it's advancing correctly. Don't just rely on static timing marks.

If you can't find anything really obvious, it's no big deal to drop the motor and pull the bottom case off ... new bike (to you), unknown history, can you have absolute trust in its condition inside?
 
Hey Mike. Welcome to the forum.

I've never owned a 750 twin, but I've had several triples. My Jota had a rattle very similar to what you describe - a metallic clanking sound within a particular rev range that sounded like it was coming from the top end of the motor. I spent ages checking all sorts of things in the engine: cam chain/tensioner, the bolts holding the sprockets onto the camshafts, clutch rubbers, primary drive, etc. Couldn't find the problem.

I eventually discovered the source of the noise weeks later while doing a job on the front end. A mudguard stay had broken at the bracket where it bolts to the fork leg. The stay had cracked completely through (presumably a fatigue failure) but hadn't moved out of place, so it was almost impossible to see the crack unless you looked very closely. When the engine revs got close the mudguard's natural frequency, a vibration would set in and the crack would begin to clatter. I welded it up and the noise went away. Such a simple thing that had been bugging me for weeks. Before I discovered the real problem, I would have put money on it being something in the engine.

Something you said made me think your metallic noise might also be associated with the front mudguard. You said it doesn't make the noise when you load up the engine with the front wheel against a wall. Pressing the wheel against a wall will stop the front end vibrating, or at least change the frequency of vibration, so the noise doesn't happen.

After welding it up, I painted the mudguard stay as a temporary measure to stop the weld rusting until I could get it re-chromed. I chose black as the least offensive of the various colours of rattle cans I had. That was probably 20 years ago and my Jota still has a black mudguard stay.

Cheers,
Cam

Clean bike.JPG
 
One last thing I just thought of, does the Boyer use the auto advance mechanism for the points? If so I would look there as it is an ignition part that no one ever looks at until it is completely worn out and rattles around!

Also, do you have a small fuel container you can use in place of the fuel tank? This will allow you to ride the bike without it and it makes hearing motor noises much easier without the fuel tank in the way.

And finally get a motor stethoscope. They are cheap and can be a big help.
 
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Speaking of broken bits rattling around, what about that bulb cover inside the headlight shell that aways snapps off.
 
1200ts said:
Also, do you have a small fuel container you can use in place of the fuel tank? This will allow you to ride the bike without it and it makes hearing motor noises much easier without the fuel tank in the way.

And finally get a motor stethoscope. They are cheap and can be a big help.
Click to expand...
Or pack some foam over the frame under the tank to eliminate the possibility of something under there being the culprit.
 
Thanks for the many suggestions! I’m working my way through them but so far no luck.

Ignition timing set near 38degrees with strobe – with it pulling better I can almost induce the noise with it pushing against the wall. Primary drive cover removed and looks ok, bar some medium scrapes in the tensioner blade, and very slight wobble of chainwheel (I assume that’s normal). Crank nut tight and tab washer intact. Various potential rattles checked – I thought the header pipes may have been it (there’s no balance pipe to brace them) but no luck. I’ll try and run it without the tank, or with foam underneath, but barring that I think the head may need to come off.

Will keep you posted.

cheers
 
It could be a good Idea to change the Oilpump to a bigger one ( 10mm)
and take a look to the LH-Cam and Rocker.

Rainer
 
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