lowering compression/timing

I am about to fit a spacer ((30 thou/inch) under the cylinders of my 750 Commando as the compression is too high..(don't know if it's high comp pistons or skimmed head but will find out soon enough)..does anyone have experience of this procedure and what might I need to do regarding ignition timing adjustments?

I am NOT fond of the spacer option!! as you have to remove the cylinder I would rather deck .030 off the top of the pistons. why would you want to add another joint where it could leak and a greater possibility of the cylinder moving about at it's mounting point. I would not retard the timing more then 2 degrees IE set it at 26.

I have run an 030" spacer for 7 years on my Combat headed 750. The added joint is not a problem, same no of joints as if using a gasket which I don't, just a thin smear of the dreaded hi modulus clear domestic silicone ( i'll wait for the inevitable gets in the oilways! not if you're sensible it doesn't). No change to ignition timing ever considered as it's still about 9.3:1 now. As to removing material from the pistons my guess is the cylinder and pistons have to come off anyway :lol:

bill said:

I would not retard the timing more then 2 degrees IE set it at 26.

Click to expand...

I'd advance it, if anything.

If RGM (UK) where I got the spacer say it goes under the barrels then I'll put it there..not keen on the idea of shaving down the pistons...to advance or retard timing? that is the question..keith1069 is your spacer at the head joint or crankcase? PS pistons don't have to be removed to fit the spacer either way.

best to just put in a thicker head gasket

see it here:

http://users.gotsky.com/jimschmidt/nortonrods2.html

page down to "adjusting compression"

In general, Commando engines need less advance at higher CR and more at lower, but it's not linear. At high CRs, the optimum advance doesn't change as much with CR changes. As a guideline, 28 degrees full advance is about right for 10:1 and higher, 29 degrees for 9:1, and 30 degrees for 8.5:1. Above 10.5:1 you might run as little as 27 degrees, or you might not. At 11.3:1 my 750 didn't show much difference on the dyno between 27 and 28 degrees. I've never experimented with lower CRs, but Dunstall used to recommend 31 degrees with 7.4:1. Other engine mods don't have nearly as much effect on optimum advance as CR does, but the kind of gas you have available might have an impact.

All these numbers are for mechanical advance or fixed advance ignitions, where the advance reaches a max and doesn't keep changing at higher rpm. With systems like the old Boyer, where the advance keeps increasing with rpm, you might have to experiment to find the best setting for your bike and riding style.

Ken

Either pistons or base plate require the same tear down efforts.
Head gasket can also be increased - for sense of labor saving
way to avoid detonation.
Do compression tests first.
Then I vote mill pistons if PSI found too hi for the octane.
One, this lightens the jerking mass and avoids the weep prone
and less stable base plate method.
Two, any change in head gasket or base plate thickness alters
the geometry of rocker tips on valve stems.
Depending on current geometry base or gasket increase may
or may not improve this.
Putting in a hotter cam also lowers effective CR until
really reving up. Hotter cams prefer higher CR for idle quality.

hobot

I use a gasket on both sides of the plate. This procedure also restores rocker geometry and no change in timing was needed. The cylinder base is a low compression area (crankcase pressure limited) where as a thick head gasket is not advised.

That being said, CAUTION! The ridge wear in the cylinder can raise havac with piston rings having to interfere with its new position. Honing is needed to nutralize the bore throughout. If the ridge is really bad, whether high or low, boring to the next size may be needed.

Have gone through this, the compression plate is still the right way to go. Your compression may still be up a little and you will have the benefits of the 32mm ports.
I am running a 34mm TM Flatside carb and 1 1/2" bigbore exhaust. Superbike.

Good points and important bore lip alert, filed it in my mind forever
more now, thanks Mr. Big Pipe pvisseriii.
Just be sure to re-torque as the softer gaskets tend to crush.

If you love to run engine into red line glory zones under load, the
elastic nature of metals and Norton engine is more obvious
with base plate installed is all.

hobot

If RGM (UK) where I got the spacer say it goes under the barrels then I'll put it there..not keen on the idea of shaving down the pistons...to advance or retard timing? that is the question..keith1069 is your spacer at the head joint or crankcase?

Click to expand...

Mines the RGM, under the barrels in place of gasket. It's nice to have metal to metal wherever you can, even with a little sealant in the way admittedly.

Thank you for all the advice. Though if the barrels are raised by a spacer under them then the bore ridge will be above the reach of piston travel. Or do you mean wear at the bottom of the bore?

paulcali3 said:

Thank you for all the advice. Though if the barrels are raised by a spacer under them then the bore ridge will be above the reach of piston travel. Or do you mean wear at the bottom of the bore?

Click to expand...

Yes, the bottom of the bore. If your cylinders are fairly fresh, very little blade (not ball) honing will be needed. Give it a good cross hatch and a fresh set of ring goes without saying.