750 SF2 engine compression numbers

TeddyG

TeddyG

Hero member
Location
Portland Oregon
Greetings,
After 2 compression tests, one cylinder shows 150 lbs and the other shows 125 lbs. After squirting in a little oil the 125 lb cylinder increases to 150 lbs, which I’m assuming indicates piston and rings issues. So what are ideal compression numbers for these motors, and does 125 lbs compression warrant engine rebuilding? While no rocket ship the motor seems to be performing well and will do 80 mph at @ 5K RPM, but I haven’t pushed it any farther.
Thank you
 
TeddyG said:
Greetings,
After 2 compression tests, one cylinder shows 150 lbs and the other shows 125 lbs. After squirting in a little oil the 125 lb cylinder increases to 150 lbs, which I’m assuming indicates piston and rings issues. So what are ideal compression numbers for these motors, and does 125 lbs compression warrant engine rebuilding? While no rocket ship the motor seems to be performing well and will do 80 mph at @ 5K RPM, but I haven’t pushed it any farther.
Thank you
Click to expand...

Chances are it’s been like that for 40 years.


A rough rule of thumb: If it stops pulling wheelies downhill at 60mph in second gear, you know it’s time to freshen it up. 🤣
 
I was once told the time to fix them is when lumps of piston start hitting the spark plug, and its gap closes. The funny thing was, he was right.
 
Thanks guys,
I bought the bike w 15k miles and it had been abused by the PO and barely held an idle and performed poorly. The cylinder walls were scarred and a local machinist experienced w vintage bikes re- bored the out of true cylinders for new Ross Racing .5 over forged pistons and rings, along with re-seating the valves. An initial compression check showed this discrepancy and I was hoping it would improve after breaking in. The motor is performing much better, but not being familiar w vintage bikes or Laverdas, I’m not sure if it’s performing as it should since I,ve read these supposedly can cruise at 90 mph all day long. But I,m very happy w the bikes handling and vintage vibe and it gets constant compliments like “beautiful bike- what is it?”
 
TeddyG said:
Thanks guys,
I bought the bike w 15k miles and it had been abused by the PO and barely held an idle and performed poorly. The cylinder walls were scarred and a local machinist experienced w vintage bikes re- bored the out of true cylinders for new Ross Racing .5 over forged pistons and rings, along with re-seating the valves. An initial compression check showed this discrepancy and I was hoping it would improve after breaking in. The motor is performing much better, but not being familiar w vintage bikes or Laverdas, I’m not sure if it’s performing as it should since I,ve read these supposedly can cruise at 90 mph all day long. But I,m very happy w the bikes handling and vintage vibe and it gets constant compliments like “beautiful bike- what is it?”
Click to expand...
Don't believe what you read.
Cut off date for vintage period is 31/12/1931
How many miles have you put on the engine since rebuild?
Paul
 
Thanks Paul,

I’ve ridden it @ 2k miles and followed the mechanics advice to vary the speed w/o nearing redline for @ 300 miles followed by an oil change, but perhaps I didn’t push it enough. But the motor seems to be responding well and I don’t have a bench mark to compare it to since I’ve owned only modern super bikes.
 
TeddyG said:
I,ve read these supposedly can cruise at 90 mph all day long.
Click to expand...
True that the motors are robust but I think you'd find yourself exhausted if you attempted to cruise at 90mph all day long on the SF2.

I and the SF2 were much happier at 75'ish when covering all-day distances with periodic 75-85 but not sustained.

All day and multi-day rides in recent years have been on a GTL where I and it seem happiest in the 65-75 range.

The twins are really not interstate or super-highway missiles, much more fun on the country roads.
 
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