Hi Bruce, I'd get an Aerco. Cost is less & they work very good. Here's why I feel this way.
I have a few thoughts. I learn new things all the time. Just the other day I learned Emgo steel plates are extra thin to compensate for their extra thick friction plates. So I may change my thoughts later date... I doubt it though.
One thing I certainly do not know is why some owners clutches are so hard to free cold after bike sits overnight or longer. . Oil type seems to play a part. Exactly what oil are you using. 1970 Bonnie clutch shares motor oil. The level will find its own level very close to 150cc which is about 1/2" deep using dip stick behind chain through filler hole.
Obviously the rod adjustment, lever free play must be correctly set. The #1 problem is owner doesn't have enough free play in cable during rod adjustment so 3 ball cam is not in the zero position. This cannot be overstated. If/as needed run cable deeper into trans or on early bikes different spacer for cable or whatever needs to be done. The cable must have enough play to zero the cam.
Wobble of pressure plate doesn't need to be perfect, but reasonably close.
Back to you. This I do know for certain. Unless springs are overtighten so much it's causing spring bind, the type or tension of springs will not effect clutch release. AGAIN THAT IS IF NOT SPRING BOUND! If spring bound the binding of the coils prevents the pressure plate from lifting further so the plates cannot properly separate as they should.
Extra strong sprigs or normal springs tightened deeper (but not coil bound lever pulled) will increase lever effort, but not effect release. Lifting of pressure plate is important. Measured at adjuster screw, a good lift is about .115-.120". More will not cause problem but can change where friction point starts during take off.
I've done several experiments with this. With the exact same correct clutch adjustment. Think about what happens. You let lever out & pressure plate starts pressing plates together. As you release lever further spring tension increases to the point the bike moves forwards, finally the lever is all the way out.
So again spring tension presses enough to move bike. So if you have very strong springs, the press harder sooner so lever is closer to grip at take off.
Weaker springs you need to let lever out further. Again the same rod, lever adjustment.
Turning springs tighter or looser will have similar effect as stronger or weaker springs. I learned a lot about this trying to reduce lever effort. Was very repeatable.
Turning to 7 plate clutches, the small friction pads give more PSI in a concentrated area. Indeed the contact patch is very small compared to stock size friction pad. However the concentrated pressure in fact creates more friction than the normal size friction pad. So it will slip less with the same spring pressure as a normal friction pad would. The down side is if the 7 plate is allowed to slip at all, it will wear very quickly. Much quicker than normal plates. Many times quicker?? I think so.
Regarding freeing, I really don't know why the 7 plate clutches free so well. Maybe the small friction pads let go easier?? Or is it the friction material??
The only two 7 plate kits currently on market is Norman Hyde & Aerco. I have lot of experience with Hyde, a certain amount of experience with Aerco.
I have ridden both extensively now. I have installed both. I've examined the plates side by side new & used. Hyde has a smaller friction pad circle than Areco. That seems to be the only difference. They look to have exact same friction material & overall construction of the steel plate part of friction plate. The inside diameter if Aerco friction circle is smaller. Aerco has less "grooves" in the friction surface between the pads.
On the road both feel very similar in normal riding & take off. I feel the smaller friction pads on Hyde gives it more overall grip with same springs/tension. There's more though...
It also depends on pressure plate. This cannot be overlooked if maximum performance is wanted. On normal riding not so important.
The friction pads are in a circle. The factory steel pressure plate has about an 1/8" pressure face, with pressure circle of this face in about the center of the friction pads. Two factors. The width of the face & the placement of pressure circle in relation to size, position of friction pads.
All the friction pads have about the same outside diameter. But the inner diameter of the friction pads is greatly different. This is what makes the 7 plate pads so small. The inner diameter of the pad circle is larger.
How does this relate to the pressure plate? What difference does it make. The steel plates are fairly thin & somewhat flexible. The flexing of the steel plates becomes important. Remember the original pressure plate pushed pretty much in center, but not with 7 plate kits...
The Hyde has a friction circle so large that the original pressure plate just covers the inner area of the friction pad. This allows the steel plates and friction plates to flex slightly which reduces effective pressure on friction pads. This can be seen by an uneven wear pattern on the top 3 plates or so. Again not an issue at all for normal riding. Higher horse power or higher gearing can cause slip.
Aerco has a larger friction pad. The pad is large enough where the steel pressure plate presses on the friction circle about 1/3 way up pad. In real life this works quite well with the normal steel pressure plate.
Special large diameter pressure plates were made specifically for the friction circle of 7 plate clutches. These work very well with Hyde clutch. I strongly recommend purchasing one if you choose Hyde. These also have a wide pressure face so they fully press on friction pad, giving maximum grip with the least spring pressure. Of course if you go higher power on motor, you'll need both the special pressure plate & strong springs.
The only pressure plate currently on the market that has this large CORRECT pressure circle is MAP Cycles. They have a special rod & ball, but I think you can use with normal rod & screw. Britech used to sell a pressure plate for Hyde. Out of production now. I don't know if Jay is going to actually make more or not. I have one on my bike, Dave has one on his. They work really good. Both these work perfectly with Aerco. I'm currently using Britech with Aerco.
I just did a search a few weeks ago, yet again. To be clear no other alloy plate other than MAP is an real improvement over the steel plate with either of these clutches. In fact all the others are smaller diameter than the factory steel plate. In my experience if you get any alloy pressure plate other than MAP you will be wasting $70, but more importantly going backwards for good pressure circle.
I experimented back to back with Britech & factory steel on exact same road test, same springs, same adjustments. Trust me, the steel pressure plate works perfectly good with Aerco plates.
End of day, if I had your bike I'd install Aerco 7 plate kit & use 650 springs. That is a very winning combination. If your springs are factory original they may be still good. The only aftermarket 650 springs I'll use are JRC Engineering. They are to original 650 spec & excellent quality.
Regarding 65/750 springs, 750 springs are substantially stiffer. Way stiffer. You can feel this most decidedly in lever effort. Regarding slip, I got set of JRC 650 springs. Adjusted nuts stud flush with dome. Experienced slight clutch slip. Not all, but many I've spoken with that installed 650 springs also experienced clutch slip. In all cases reinstalling 750 springs cured slip.
The lever effort of 750 springs is a killer to old hands like I have. Using 7 plate with 650 springs allows the easier lever effort without slip. 650 springs are much easier on clutch cable as well. I will only use Barnett cables as they are the very strongest due to the swaged, not soldered steel ends. I lube with motor oil. Same easy pull as Featherlite cable, with much greater strength.
Moving onto steel plates.... The 7 plate kit comes with a smooth steel plate. The friction material on 7 plate kits wear more quickly on etched original plates, yet doesn't seem to grip better either. The friction pads also wear the etching off rather quickly. If short on money, reuse your 6 steel plates so long as they are flat & good. But if you can afford it, I strongly recommend getting all new steel plates.
I you reuse your old steel, put the new steel under pressure plate, so the last steel installed.
How do they get 14 plates of the 7 plate kit in space of 12 normal plates? The friction pads are paper thin when new. That's how. All the other adjustments remain the same. Rod, lever play, wobble adjust etc.
All the 7 plate kits I've installed are thinner than original stack height of the 12 plates. The effect is you have reduced spring tension with dome of nuts flush with studs. Measure all 14 plates stacked together. Nominal is 1.400". If your stack is thinner, set nuts flush, then go deeper by amount the stack is thinner. This compensates the spring tension. Coil binding is not factor as the spring is not overtightened. Just brought to spec. This works really well for worn plates also.
Since the friction pads are so thin, the first friction plate may hit/bind on back corner of basket. Check for this carefully. Chamfer backside of the steel part of
friction plate as needed to insure plate sets flat into basket. This is very important. A cause of 7 plate problems. If it binds, clutch will work horrible & slip. LF Harris baskets will usually bind with 7 plate. The back corner is rounded, not sharp like original basket. Not hard to chamfer, just use a file.
If your basket, hub are worn, grooved etc. no clutch plates will work properly.
On the bikes that clutch is hard to free, with all adjusted correctly, the owners report the 7 plate kits cures it & it doesn't seem to start sticking again.