750 GT Timing Marks

ducat · Post by **ducat** » Mon Aug 31, 2009 4:13 pm

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. What gives?
Thank you for your time.

ducat · Post by **ducat** » Tue Sep 01, 2009 2:42 am

By the way, I thought maybe the associated woodruff key may have sheared and changed the crank to cam timing, but it's still in good shape.

DesmoDog · Post by **DesmoDog** » Tue Sep 01, 2009 9:08 pm

On a properly assembled engine the dots only line up once every six revolutions. Did you have the engine apart or at least have the heads off?

EDIT: I just saw a couple other posts and it seems you have had the top end off. Did you follow the instructions in the manual to time it or just line up the dots when you put it back together?

As I'm guessing you just discovered, you can't just line up the dots, you have to follow the procedure in the manual. If you haven't got a shop manual, get one. It's pure folly to think you can work on one of these engines without one.

Did you remove any of the shims when you pulled everything apart?

ducat · Post by **ducat** » Wed Sep 02, 2009 2:53 am

Craig, thanks for the response. I think I may not be making myself clear. As I understand it, upon correct assembly and the dots are lined up, the vertical cylinder will be at TDC. Let's not even talk about the cams, let's strictly talk about the dot on the bevel grear that slides onto the crank and the dot on the gear that it meshes with (the central bevel gear). This is the first step/baseline of getting all the dots aligned. This does not involve either lower tower bevel gear nor any of the cams (yet). so let's stop right there. When these two dots are aligned, the various manuals I've read say the vertical cylinder should be at TDC. Mine is not. It is coming up to TDC and it's probably close to a good 15 degrees or so from reaching actual TDC. I hope I've explained myself a little better.
Thanks again.

Chris Bushell · Post by **Chris Bushell** » Wed Sep 02, 2009 5:10 am

When putting my 74GT together last year, I had a lot of trouble with the ignition timing and getting the dots to match up.

In the end with help from Tony Brancato, we found TDC on the compression stroke of the rear cyclinder, marked that off on a degree wheel, then set the ignition timing on that cyl by winding back the crank. Then we went to the front cyl and we repeated that and shared the difference between the two pots. The bike has Lucas Rita fitted.

I well remember the fact that I couldn't get the dots on the top bevels to match up, but the valves weren't hitting the pistons! Tony advised ignoring those dots and working off the pistons.

Now the ignition timing marks were way out (20deg advanced from memory) which explained why it was kicking back like a pregnant dog and I thought I had broken my foot! Having found that out I am going to leave well alone.

The bike has now done over 2000 miles and is a pussy cat to start

Chris

wdietz186 · Post by **wdietz186** » Thu Sep 03, 2009 8:54 am

It has been ages since I've had a 750 apart but the last one I did I seem to remember that the vert. piston was not quite at TDC when the dots were aligned.This was on a 74 with only about 4k miles and the engine appeared to be untouched.The owners manual I have shows the dot line up but doesn't mention any thing about being at TDC like the Squarecase does.

Chris Bushell · Post by **Chris Bushell** » Fri Sep 04, 2009 2:49 am

Have to agree, I checked mine over last weekend and it would appear that though everything works fine, you cant get the dots to line up with TDC or anything else!

Mine is a late 74.

I suppose there is always the possibility that Louigi had been on teh wine lunchtime when he started doing the dots!

ducat · Post by **ducat** » Fri Sep 04, 2009 4:54 am

Ok, I found the australian club's website that confirms-the roundcase motors will not the vertical cylinder at TDC when the dots line up. It will be about 10 degrees BTDC. I believe the square case motors will in fact be at actual TDC when the dots align.
Thank you all for your input.

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