750 SF2 Engines - Mods & Fixes?

ksoholm

ksoholm

Senior member
Location
Bellingham, WA, USA
Friends,

As I start thinking about restoring my 30K KM garage-find SF2 that has sat for 35 years, I want to ask the hive-mind if there are light performance or longevity measures that are commonly taken with these engines. Mr. Todd mentions they are amenable to tuning; I do not want a highly tuned race bike with shortened life, but, am just interested in known light modifications that allow the engine to work better. If any.

Thanks -

Kristian
 
It's a bullet proof engine.
Apart from checking cam timing, 2 cams remember, not much without going for heavy mods.
If you open the engine, you can check if your crank is at 360?. They aren't always.

Paul
 
Paul Marx said:
It's a bullet proof engine
Click to expand...

Hmmm, in my 43 years worth of motorcycling, only one motor has ever completely broken down on me - 1972 SF 750. That cam-centre joint on the 750s is a definate and serious weakness.
 
The cam flanges are not a weakness, from my experience they loosen after being off and on again. Should be replaced not refitted.
The only thing I would do with an SF1-3 motor is ensure it has correct compression and 1mm squish as per spec. Most I have checked have mid 8 to one comp and 2mm squish. Cam degreeing, maybe with an SFC cam and proper exhaust you could advance a few degrees from std for more midrange, but they go great with std cam. There is absolutely no point in going for a top end power motor with one of these.
 
Tippie said:
The cam flanges are not a weakness, from my experience they loosen after being off and on again. Should be replaced not refitted.
The only thing I would do with an SF1-3 motor is ensure it has correct compression and 1mm squish as per spec. Most I have checked have mid 8 to one comp and 2mm squish. Cam degreeing, maybe with an SFC cam and proper exhaust you could advance a few degrees from std for more midrange, but they go great with std cam. There is absolutely no point in going for a top end power motor with one of these.
Click to expand...

Cam flanges loosening after on/off is DPO intervention.

Remember Phil Todd's post about nickel plating the cam ends in his washing basin to ensure a tight fit?

Paul
 
the factory racers very often had three runs of weld across the cam flanges at 120 degree intervals of about 12mm length, so they knew more than we do! don't fit SFC cams as the valves will very likely touch the pistons, varies with type of SFC cam and type of pistons, but I once saw an eletronnica that had been fitted with Flat top 1200 Mondials and on one side only, the valves were kissing the piston top, which doesn't say much about the standardisation and matching of both cylinders but there you go, some SFC cams (as in 6C) will cause the standard valve springs to be coil bound, and that busts things badly.
CLEM
 
I completely get the 'DPO' effect on the cam flange guys, however when I put the shout out about my failure, there was an instant flood of in depth knowledge on the subject. So, it certainly isn't uncommon. A trawl of this very forum will show as much. Therefore the area surely has to be considered a weakness, IMHO.

Reasonable advice to newby 750 owners (as I was) might be 'When you get a new bike, as a matter of course, droop your motor and attend the the cam flange before running the bike hard'. None of us know DPO history with our bikes.

Maybe we can say the cam joint is a weakness 'after 40-ish years and untold DPO spanner work' - but I don't think we can say this area is robust. If it were robust, it would be 'maintenance by normal folk' proof. Too many failures indicate it is not.

Hence my response to Kristian's direct question about longenity measures. I suggest Kristian that this area is worth a good check and refurbishment if necessary.
 
It's like doctors Hag, you only hear about the failures.

Yes, cam keys do break, far from unheard of. I could even tell you about a friend who changed his cam key on an SFC in a campsite on the IOM during the TT.

I wouldn't even think about checking for play between cam halves that have never been apart. Checking that the TDC marks are accurate however is a must.
Cranks anywhere between 356 and 364? however are common.  :D

Paul
 
Making sure the oil can get up the studs to the head is important.  If the stud isn't centralized in the hole, it can block the oil getting to the head.  I have seen a peice of bent wire used to run down the side of the stud, ensuring oil supply. 
 
I would also question the need to pull the crank apart to check it out for corrosion in a machine that has sat that long bathing in corrosive oil; unless the crankcase was filled to the brim with nice clean oil before storing.

Paul
 
Also agree with Paul on cam flanges - if it's never been apart, HIGHLY unlikely it will loosen. Once pressed off and on again , it's compromised. My SF1 did 160,000+km on its original cam and rockers before I converted to SFC cam, valves and later-model rockers. BTW, the key in the cams does NOT secure it - it's a locating device only; the press fit of the flanges is what holds it together.

The factory could afford to weld stuff together - chuck it away as required. A guarantee whether there's risk or not, and plenty of replacements available to the factory race team, so why not? Probably welded SFC crank webs up as well!!

No idea how you could correct an asymmetrical '360' SF crank - they are splined centres. If the plan is simply to set timing as per the L-R discrepancy, that certainly makes sense.
 
Regardless, as Haggis points out, with a stinking old bike that you have no idea of what has been fucking with it's head, it is a good idea to make sure you have correct interference fit of the flanges, amongst other things. Paradies used to make different spec ones, and with a bit of googling and youtubing you could plate your cam stubs (or flanges) to achieve it.
 
Well Quentin, I don't do cranks, but the splines in the webs which are press formed at the factory allow a total 8? discrepancy, 4 either side of 360?.
It can be, and is corrected by people who do cranks.

And yes Tippie, as I mentioned above, Phil Todd informed us as to how he gained thickness on his cam ends so as to ensure a perfect interference fit with a do it yourself nickelling kit at home.

Paul
 
Haggis said:
Hmmm, in my 43 years worth of motorcycling, only one motor has ever completely broken down on me - 1972 SF 750. That cam-centre joint on the 750s is a definate and serious weakness.
Click to expand...

Seeing the condition of some flanges and the mating surfaces of the sprocket, it really is no wonder the joint sometimes gives up.  Not only do the camshafts need to be a rather tight fit in the flanges, the flanges and the sprocket surfaces have to be dead flat to be able to stay assembled in use.  Any high points on either surface that cause the mating surfaces not to contact over the entire surface will compromise the strength of the joint.  Once movement sets in, everything else starts moving around, not long to go until the final death rattle chimes in.

Heating the flanges before re-fitting helps to preserve the interference fit.  Care should also be taken to ensure the camshaft ends are properly de-burred and smoothed so that they actually pass through the flange without taking metal with them.  Pressing the flanges on cold will most likely remove a sliver from the flange bore.

Using star or spring washers under the small nuts at the connecting studs cannot be recommended.  Schnoor-type washers are a far better option.  Using Loctite would also be an option, if it wasn't for the pita it causes involving future dis-assembly.

The entire camshaft assembly relies on the interference fit of the shaft/flange, as well as reliable cohesion between flanges and sprocket.  Neither the key nor the studs are able to transmit the forces present.  The key is only for location, the studs afford the tension required for the flange/sprocket cohesion.

piet
 
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