Finding TDC on compression stroke on cylinder 1?

[The Jimbonaut]

Hi one and all,

Need guy here so hello and high fives to one and all =)

I'm working on my '81 CB750K, total novice (bought the bike a few weeks ago - cleaned out the carbs and got it running a few days ago but it's a long way from purring like a kitten), and going through the owners manual's Inspection chapter. I'm checking the Ignition Timing and need to find TDC on cylinder 1. I've had a look at a few videos but can't find one that describes the work involved on the dohc, but I'm assuming it's pretty similar.

Looking at the manual, I have to rotate the crankshaft until the "1.4 S.F" mark aligns with the index mark. But I also have to make sure cylinder 1 is at TDC. It was my understanding that rotating the crankshaft will move both the timing disc (is that what it's called?) and the pistons - how do I get TDC on cylinder one at the same time as making sure I get the right alignment on the timing disc?

Whoa.

 

[amc49]

Timing disc is bolted to end of crankshaft, meaning you cannot separate the two. When the 1.4 S.F mark is on you are at TDC......................................BUT, WHICH ONE???

There are TWO, a 4 stroke has to rotate TWO complete revolutions (720 degrees total) to finish an entire cycle, why it's called a 4 stroke...........2 strokes up and 2 down..........intake, compression, power stroke and exhaust, in that order. Two TDCs happen then, one at the end of exhaust stroke and beginning of intake, and the other at end of compression and beginning of power.

Easiest way is to yank #1 plug and with finger over the hole turn the engine over until you feel the compression beginning to push your finger off, the next TDC (1.4 S.F) you roll to will be the correct one if you don't go too far. Engine MUST be turned counterclockwise on that side (left).

 

[The Jimbonaut]

Thanks again amc - really appreciate your help (with this and the helicoil!)

This totally helps in finding TDC, man I'm grateful. Will let you know how I get on.

Thanks man,

- J

 

[SurferD]

This is the timing disc but no index mark. Where do I line anything up?