'81 GS650G: Jetting advice for oversize pistons

[Guest]

Hi everyone,

I'm currently rebuilding a 650 from scratch. I now have 65mm pistons which are 2mm oversize making it a 740ccm.
Is there anywhere a reference out there that could help setting up my carbs somewhat right to have a starting point?

All I found was guidance for different airfilters/exhausts...

Any help is much appreciated!

 

[Nessism]

You shouldn't need to change much if anything in the way of jetting. It's pods that through the carbs all out of whack.

 

[Who Dat?]

I now have 65mm pistons which are 2mm oversize making it a 740ccm.

I don't want to burst your bubble, but 2mm oversize pistons give you 718cc.
If you want 740ccm, you need 3mm oversize pistons.

I agree with Nessism, stock jetting would be a good starting point.

 

[zed1015]

If the pistons are stock compression and the overbore is within stock OEM over sizes then the original jetting will suffice.
If the pistons are high compression then you may have to go up a size on the mains.

 

[Guest]

Sorry, my bad. Original is 62mm. Oversize is 65mm.

As far as I know there was a 0.5 and 1 oversize. This is the 'bigbore' kit.

My concern is that I need the bike somewhat running to get the oil everywhere as I have completly torn apart the engine. I used assembly oil on all critical parts while putting everything back together, but my concern is, that it won't last forever. So I kind of don't want to crank for too long until it should fire up. Once it's somewhat running I can figure out proper jetting/syncing and all that.

 

[pdqford]

You could keep the ignition key off and the sparkplugs out, andkick it over multiple times until you have oil pressure. Then install the sparkplugs and hit the starter.