750sf low compression readings (was ignition question!)

I can’t even start it now.
Dropped the carb bowls and cleaned out. Some small particles in there and rusty looking fuel.
I had been using the tank which I had cleaned out but obviously not perfectly.
Cleaned main jet pilot jet and cold start jet with carb cleaner.
Fitted aux tank which is spotless inside.
Bought fresh fuel.
It fired a couple of times but since then nothing.
Dropped bowls again.
Nice and clean in there now.
I have spark….
Have tried three sets of plugs, Champion n2 and Bosch.
Spins over great on starter but nothing…
 
The carbs sit on aluminium stubs, not steel, and they are prone to corrosion at the o-rings. It also looks like the original over 50 year old insulators. Have you had these off and checked? Are you sure that the choke plungers are actually opening. It should run on choke virtually regardless of other carb issues. It has good compression in both cylinders?
 
82385
 
Tippie said:
The carbs sit on aluminium stubs, not steel, and they are prone to corrosion at the o-rings. It also looks like the original over 50 year old insulators. Have you had these off and checked? Are you sure that the choke plungers are actually opening. It should run on choke virtually regardless of other carb issues. It has good compression in both cylinders?
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I have just found I have very low almost no compression on the left cylinder… right cylinder shows 125 psi.
 
Ok
Oil in left brings compression up from 0 to 120 ish
Oil in right brings compression up from 125 to 150 / 160 psi.
This bike stood a long time. Sticking rings?
Anyway it’s head and barrel off time I think.
 
Vince said:
Well, that's not good. No simple fix. At least it's fixable without pulling the engine like a triple
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Even on a twin it is far easier with the engine out. Not much fun accessing the lower rear engine mounts with the balance pipe bracket in place though. I took out the swing-arm last time I encountered one.
 
Im thinking this might be a job for a pro. I no longer have my lovely spacious workshop with hydraulic work bench, air tools, space to work… just a lockup with other bikes and stuff in it. No power or light either… I could do it in there but I thought I had left grovelling around on the floor fixing stuff behind.
 
Once you've got the pistons, rings and head off and sorted I think you'll find it purrs. Might even need a rebore. DEFINITELY need to remove those old inlet stubs, check the black composite heat insulators between carbs and head (the perish/crack) and replace the four o-rings. Probably still originals! Once head is off, pour some fuel down each port (with valve closed!) and check for leakage past the seats.
 
Tippie said:
Even on a twin it is far easier with the engine out. Not much fun accessing the lower rear engine mounts with the balance pipe bracket in place though. I took out the swing-arm last time I encountered one.
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A large screwdriver tip between crankcase and nut of lower engine mount allows you to deal with it.
However, if you only plan to tilt the engine to access the top end:
- all 750s don't have a balance pipe, far from it
- you don't need to touch the lower engine mounts to tilt the engine enough to get the top off, engine weight will suffice.

Paul
 
if you get as far as to be able to tilt the engine forwards for top end removal, I recomend undoing those LAST TWO bolts (thats all!) and removeing the motor, use trollety jack underneath or a stack of old car tyres, get an assistant for this 5 minute process, especially so if you are working on the ground, then the motor can be put on a bench, or a box (tea chest with plywood top?) and it will be so, so much easier and safer (broken rings?) to work on.

CLEM
 
As an aside, Bosch spark plug W22 in my experience would be the worst plug you can use. If it does start, it wont idle, absolutely no good reason to use Bosch plugs on a Laverda. On W22 plugs, 6 hours trying to get a new installed iis fitted 3c to start and idle, 3 new NGK plugs later, perfect. Others may have better experience, very cold plug the W22.

Other more pressing issues for this motor, seemed to happen all of a sudden this start idle issue, into a compression issue? HTH j
 
iis_iis said:
As an aside, Bosch spark plug W22 in my experience would be the worst plug you can use. If it does start, it wont idle, absolutely no good reason to use Bosch plugs on a Laverda. On W22 plugs, 6 hours trying to get a new installed iis fitted 3c to start and idle, 3 new NGK plugs later, perfect. Others may have better experience, very cold plug the W22.

Other more pressing issues for this motor, seemed to happen all of a sudden this start idle issue, into a compression issue? HTH j
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It never ran properly and always smoked the few times I was able to start it.
 
CLEMTOG said:
if you get as far as to be able to tilt the engine forwards for top end removal, I recomend undoing those LAST TWO bolts (thats all!) and removeing the motor, use trollety jack underneath or a stack of old car tyres, get an assistant for this 5 minute process, especially so if you are working on the ground, then the motor can be put on a bench, or a box (tea chest with plywood top?) and it will be so, so much easier and safer (broken rings?) to work on.

CLEM
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I disagree. To drop the engine, you need to take the chain off which is a problem if the chain is riveted.
And you need to disconnect the ignition, and the dynamo. More hassle.
I shall be doing it soon and that engine is staying in the frame.

Paul
 
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