SF750 timing

C

Cowboy

New member
All of the info I have gleaned from this forum and a workshop manual suggest that at 3000rpm my timing should be at 40deg advance or at least align with the mark on the starter clutch wheel. I have set the timing to this with the aid of a timing light but it will advznce even more above 3000 rpm. Is this correct or should that be max advance above 3000rpm?
Still running points ignition at present.

Cheers
 
Hi, Cowboy.
Shouldn't advance more than 40?!
Afaik the advance unit has a limiter.
Seems the static timing is already advanced,
has to be at TDC, have you checked that (the other
mark at the hole)? Did you check the points gap, 0,4mm?

hth,
Ernesto
 
40? sounds a bit excessive. If the centrifugal advance mechanism isn't fully advanced at 3000 RPM then it will continue to advance beyond 40? at higher revs.

I'd have thought a maximum advance of around 30? - 35? would be closer to optimum, depending on engine tune, fuel octane rating, etc.

But what would I know? I'm not a mechanic.  ;)

Cam
 
You are on topic, Cam. However the book says 0deg static (the PM mark), and full advance is 40deg. I agree with you that it is excessive but they run ok with it. There is quite a distance between the marks so it is a good idea to mount a pointer so that true TDC can be set. The amount of advance is set by the travel of the pin on the back of the points cam. It is easily evident when the advance unit is removed. The advance system on these SFs makes it easy to alter amount of advance (by altering the travel for said pin) or changing spring tension (bending the the spring tabs or replacing the springs). It could be that Cowdy?s advance needs quicker advancing if he wants 40deg at 3000rpm. It could just be that the points cam is not lubed on it?s shaft and thus reluctant to rotate when the bobweights spin up. Could be also that the felt pad isn?t lubing the points cam, thus making it harder to advance, too.
 
Ernesto said:
Hi, Cowboy.
Shouldn't advance more than 40?!
Afaik the advance unit has a limiter.
Seems the static timing is already advanced,
has to be at TDC, have you checked that (the other
mark at the hole)? Did you check the points gap, 0,4mm?

hth,
Ernesto
Click to expand...

Thanks Ernesto,

If I set the timing statically at 0deg I get way more than 40deg advance. Have set the timing with aid of light at the 40 deg mark at 3000 rpm but it will advance more! Sounds like I need to pull the mechanism apart and see what is happening.

Cheers
 
Hi, Cowboy.
Just another idea: have you checked if the advance mark
is really at 40? (f.e. with a tape measure)?
If so, advance units often come up cheap in fleabay, from
the friends who mounted electronic ignitions.
Might have one over myself, must check.
Never heard of one advancing too much.

Ernesto
 
Follow Tippie's advice. Shouldn't be poss to go beyond 40 deg if it's timed statically at TDC as the pin in the adv unit limits maximum advance. You can even use a cigarette (reefer?) paper b/w the points to get it roughly close at TDC then recheck that full adv (put the paper between the points somewhere before TDC and wind motor forward to TDC firing on that set of points - cig paper should come out as soon as points open = firing). Make sure the points gap at max open is 0.4mm. Pretty simple system so hard to get it wrong.
 
My custom elec ignition setup still uses the Bosch Advance unit anyway. But twin points worked a treat for the first 10 years I owned the bike. But yep, one of the new bewt conversion kits seem to be the go.
 
Thanks all for the input. Will look at electronic but I'm waiting on an Aussie guy who is developing one at the moment. This is more a short term fix as I don't want to do a short term blow up like hole in piston. Looks like I'm going to have to pull the ignition plate off and do a number on the advance mechanism first.

Cheers
 
If there's no obvious damage or mods, you can check the full advance curve of the std unit once it's off, using a degree wheel it should swing 20 deg (turns half crank speed, so half of 40 deg at crank).

That should be good starting point.
 
If there is more advance occuring after 3000 revs then for the safety of your pistons you need to set the timing to take account of that. Make sure that at max advance (at whatever revs that occurs) that the ignition is no more advanced than the AT mark. This may throw out the static timing but that's not so harmful.
You can fiddle and adjust the springs on the auto advance but I for one can't tell you what you need to do. If the advance has levelled out by 4000-4500 revs it may be okay anyway.
 
Normski said:
If there is more advance occuring after 3000 revs then for the safety of your pistons you need to set the timing to take account of that. Make sure that at max advance (at whatever revs that occurs) that the ignition is no more advanced than the AT mark. This may throw out the static timing but that's not so harmful.
You can fiddle and adjust the springs on the auto advance but I for one can't tell you what you need to do. If the advance has levelled out by 4000-45000 revs it may be okay anyway.
Click to expand...

:o :o :o

            Dave
 
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