750 GT timing Marks

[ducat]

Please forgive me if this subject has been covered countless times, although I can't seem to find the answer.
I have a 1973 750GT that I'm rebuildind as necessary. I understand that when properly timed, all the dots should line up. This makes sense and is eaily accomplished. My question is this: I thought this corresponded to the vertical cylinder being at TDC. All my dots align, but neither cylinder is at TDC. When the dots align, the crank pin seems to be equally between both cyllinders. I've confirmed that the woodruff key is intact. What gives?
Thank you for your time.

[Rick F]

I may be wrong about this, and I am not a mechanic and have never rebuilt one of these,

But I thought than one of the strange and great things about the Duke V-twin is that Both cylinders fire at the same time.

Therefore, that would lead me to think that both pistons would be at TDC at the same time on the same stroke.......

perhaps a more knowledgeable person may be able to agree or disagree

I also thought that that is why carby tuning and synchronisation is so important....and tricky

I have a GT and I have noticed this whenever I adjust the valves clearances

Rick

[wdietz186]

They don't fire at the same time.The horiz. fires 270deg. after the vert.

[DesmoDog]

I think what you are thinking of are engines that use a "wasted spark". They fire both cylinders at the same time but that doesn't mean both cylinders are on their power stroke. Harley does this.

It's been a long time since I did the math but I seem to recall that on a Harley twin, the wasted spark occurs at 15 degrees into it's intake stroke for one cylinder and somewhere in the exhaust stroke for the other.

As mentioned, a stock roundcase isn't set up this way...

[Rick F]

Like I said, someone more knowledgeable would comment!



RF

[wdietz186]

The Rita ign. for the Squarecases use a wasted spark setup. The system uses only one pickup and has a two bladed rotor. And to think about it all the engines with crank mounted ign. triggers waste a spark or two every revolution.

[Chris Bushell]

Rita on a roundcase also uses a waste spark set up!

[Bomber]

Ducat,I'm in the middle of a 74 GT rebuild myself and yes you are correct about the bevel gear dots and the rods at half mast.The dots are there to determine the valve timing is correct.I know most singles and some twins cams are punched to align with tdc but the ign. timing is usually determined by marks on flywheel or degree wheel mounted on crank.The 750 gt has seperate points,condensers and coils and geared away from crank,why would there be a wasted spark?