M750 Scrambler Project

[Speeddog]

Anyone know what the stock cranking cylinder pressures would be?

Official service manual for 3ph carbie 750;

Engine warm, throttle wide open, crank til it stops going up.
8 bar minimum, 9-11 bar standard, max difference between cylinders 2 bar.

So 118 psi minimum, 132-162 standard, max difference 30 psi.

[Speeddog]

I'd be a bit concerned with a difference of 30 on cranking pressure.

My 750 clatters quite noticeably, but it's run ages making the same racket.

My thumbnail conclusion is they don't run hard enough to hurt themselves as long as basic maintenance standards are kept.

[Howie]

I'd be a bit concerned with a difference of 30 on cranking pressure.

My 750 clatters quite noticeably, but it's run ages making the same racket.

My thumbnail conclusion is they don't run hard enough to hurt themselves as long as basic maintenance standards are kept.

Mine had  piston slap on a cold start.  The bike probably has about 100K miles on it.  I do remember somebody (speeddog maybe?) said some 750s had forged pistons.  The manual does indeed say a 2 bar difference between cylinders.  Yeah, I'd be concerned too.

[Speeddog]

I've put 45k on top of the original 11k, if that perilously 2nd hand speedo was to be believed.

It's less clattery when it gets hot.
Allegedly forged pistons, IIRC Brad Black had the info.

[ducpainter]

My injected 750 sounded like it was coming apart. It did eventually.

A lot of the noise is 'fin ring'. Put some hockey pucks back in.

[Speeddog]

~~~SNIP~~~
A lot of the noise is 'fin ring'. Put some hockey pucks back in.

But they keep falling out.
Should I use new ones?

I wonder how many of us have felt the rear tire twitch running one of those over.

[ducpainter]

But they keep falling out.
Should I use new ones?

I wonder how many of us have felt the rear tire twitch running one of those over.

New ones will stay in longer. The rubber does dry out causing shrinkage and subsequent disappearance.

[koko64]

I can happily report that both cylinders are a healthy stock 160+psi allowing regular (cheap) gas. They were within a PSI of each other. Who'd a thought I would be chuffed about stock compression

[Howie]

I can happily report that both cylinders are a healthy stock 160+psi allowing regular (cheap) gas. They were within a PSI of each other. Who'd a thought I would be chuffed about stock compression

Nice!  If you want a stock flywheel I'm sure someone would want to trade.  I would keep the high comps as long as regular gas is not a problem.

[ducpainter]

I hope the noise is fin ring as DP said and not a chewed out alloy flywheel spline. The low comp oem pistons are good as I'm planning some rural adventures.

Did you see any flakes on the screen, or in the oil, when you serviced it?

Fin ring tends to diminish with engine temp. It never completely goes away, but it's noticeably worse when cold.

[koko64]

Clean as a whistle .
Cranking comp numbers are quite healthy and consistent. It's also an oil tight and very clean running motor which  backs up the view that the later castings, pistons and rings were an improvement.

[koko64]

Picked up some bars with more sweep/pull back to the rider and a half to one inch more height. They were used but like new for 50 bucks off a Ducati Scrambler. I think these bars are common to the Sixty-2 and Full Throttle models. Close to perfect. I gave my Pro Taper bars to my buddy to try out on his Hyper 1100 Evo. An expensive misjudgement on my part, but bars are like that.

[koko64]

What I was expecting behind the alternator
20181013_220618 by Tony Kokonis, on Flickr



What was actually in there, (but a polished version). Edit: looks like a Kamna or Yoyodyne version.
images (1) by Tony Kokonis, on Flickr

Hoping it's a machined down stocker or alloy part with a steel centre insert.

[koko64]

20181020_095731 by Tony Kokonis, on Flickr

More rise

[koko64]

20181020_095801 by Tony Kokonis, on Flickr

More pull back/sweep