Degreeing in your cams.

[Guest]

Hi there.
This is my first post to this group so please be gentle with me.
I have a 1977 gs 750 which I am in the throws of dialing in the cams.
After checking,rechecking and checking again I have come to these specs.
117 on the intake and 103 on the exhaust. I have read on numerous threads that the magic numbers are 106 on both cams,so my next move would be to slot the cam sprockets and adjust to 106 on each cam.
Would these numbers explain why I couldnt get a very high vacume reading when balancing the carbs and a poor fuel ecconomy?
I have checked valve clearances,stripped and rebuilt carbs, new "o rings" in the manifolds etc and thought I should check the cams while I'm at it.

Iain

[biker66]

you sure about those numbers ?? Sounds like the intake is one tooth off.
Can you show us the opening/closing numbers too?

[SqDancerLynn1]

A 12 deg dif does sound like it is off a tooth which could also cause low vacuum

[kiwisika]

degreeing in your cams

Thanks for the replies.
I have just rechecked my numbers as I was doing this at midnight last night,but the numbers are the same.
I have used a .5mm (.020") and 1mm(.040") lift in case this changed things but end up with the same numbers.
The intake valve lifts at 9.5 BTDC and closes at 62.5 ABDC.
9.5+62.5=72. 72+180=252. 252x.5=126. 126-9.5=116.5
My next check to do would be to count the links between the cams as you have pointed out that it sounds like I'm 1 tooth out on this.
This will have to wait till I get back from work tonight.
I also intend to make a bigger degree wheel to make it easier to read, the one I am using is only the size of a cd.

IAIN

[SqDancerLynn1]

Check this degree wheel

http://www.grimp.net/ryan/degree_wheel.pdf

[biker66]

From memory it sounds like your intake opens too late.
If I recall correctly my cam opens somewhere around 15-25 degrees BTDC and yours at only 9,5degrees BTDC !?!
You check that tooth now !

Nice degree wheel !

[kiwisika]

Well here goes,
I went out and bought a 8 inch diameter 360 degree protractor from a drawing supplies shop, put it in my milling machine and bored a hole dead center. Glued some white paper on the back to help show the markings and made an adjustable mounting so I wouldn't have to try and bend the wire to line up the TDC 0 mark.
I also dug up a brand new DTI in case my first one was sticking.(it wasn't)
I then set up the dti with its pointer resting on the edge of #4 inlet bucket.
I then turned the crank 15 times and made sure the DTI returned to the same point each time.(it did)
I then put my piston stop back in to make sure degree wheel hadn't moved.(it hadn't)
After all this I still end up with 9.5 BTDC and 62 ABDC with .5mm lift.
I then removed the idling sprocket and counted the number of pins and there are 20 just like the book says. So I guess all I can do now is slot the cam sprockets and reset the cams to 106 degrees.

IAIN

[Don Lobacz]

Iain,

I just counted the teeth on the cam sprocket and came up with 30. That means each tooth represents 12 degrees (360/30). I would just move that intake cam 1 tooth and it would be at 105 (117-12) and see how that changes things. You would probably have to move it there anyway to keep from making huge slots in your sprocket.

[Mr. Jiggles]

On my 1100 it's 30 teeth on the sprocket but the sprocket only turns 0.5 degrees for every degree that the crank turns. Wouldn't that translate into 24 crank-degrees per tooth?

It's times like this that I miss 2-strokes.

Jeff

[kiwisika]

Mr Jiggles,
I was thinking along the same lines as you, but didn't know how many teeth were on the sprockets.
I removed the cams, counted the teeth (30) and replaced the inlet cam
with only nineteen pins between marks 2 and 3.
I got a result of 92 degrees this time, 116 - 24=92. As I thought.
This means that I only have to slot the holes approx half a tooth to get what I want.
A bit of simple math and I could work out exactly how much I have to increase the slot arc length and then make a pin this diameter to fill the gap behind the bolt.
Thanks to all the people who took the time to help me out with this,I hope
I can do the same for someone else one day.

IAIN

[biker66]

No need for a pin.
Clean the threads, use loctite and you will be fine.

As you're moving your timing around alot it won't hurt checking piston to valve clearance,
covered on my site under 'modifications/cam timing'.

[Don Lobacz]

True, I missed that - the degrees are measured at the crank and the ratio is 1 to 2.

[Mr. Jiggles]

True, I missed that - the degrees are measured at the crank and the ratio is 1 to 2.

The only reason I knew this is because I spent this past weekend trying to degree my cams. I gave up and ordered the slotted gears from APE. Hopefully I'll have better luck this weekend.....

Jeff