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Diff. between GS750 and GS850 cams

  • Thread starter Thread starter gssuzukixxx
  • Start date Start date
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gssuzukixxx

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Can anyone tell me the difference between the cams from a '79/80 GS850 and the cams from a '77 GS750.

I am upgrading my '77 GS750 to 850 and have the choice of either cams. Which would work best?:confused:

Thanks
Brian
 
GS750: Intake opens 30 degrees BTDC and closes 70 degrees ABDC (280 degrees duration).
Exhaust opens 70 degrees BBDC and closes 30 degrees ATDC (280 degrees duration).

GS850: Intake opens 28 degrees BTDC and closes 68 degrees ABDC (276 degrees duration).
Exhaust opens 66 degrees BBDC and closes 26 degrees ATDC (272 degrees duration).

It depends what you want from the motor. If more torque from lower rpm then the 850 cams would be best. If you want the valves open longer and more power in the upper ranges of the motor then go for the 750 cams.
 
Suzuki_Don said:
GS750: Intake opens 30 degrees BTDC and closes 70 degrees ABDC (280 degrees duration).
Exhaust opens 70 degrees BBDC and closes 30 degrees ATDC (280 degrees duration).

GS850: Intake opens 28 degrees BTDC and closes 68 degrees ABDC (276 degrees duration).
Exhaust opens 66 degrees BBDC and closes 26 degrees ATDC (272 degrees duration).

It depends what you want from the motor. If more torque from lower rpm then the 850 cams would be best. If you want the valves open longer and more power in the upper ranges of the motor then go for the 750 cams.
Click to expand...

Thanks Don, that is exactly what I was looking for.:D

Brian
 
Or, intake cam from the 850 and exhaust cam from the 750.. gives a little more overlap and duration, gives you a bit of both worlds there.
 
Suzuki_Don said:
GS750: Intake opens 30 degrees BTDC and closes 70 degrees ABDC (280 degrees duration).
Exhaust opens 70 degrees BBDC and closes 30 degrees ATDC (280 degrees duration).

GS850: Intake opens 28 degrees BTDC and closes 68 degrees ABDC (276 degrees duration).
Exhaust opens 66 degrees BBDC and closes 26 degrees ATDC (272 degrees duration).

It depends what you want from the motor. If more torque from lower rpm then the 850 cams would be best. If you want the valves open longer and more power in the upper ranges of the motor then go for the 750 cams.
Click to expand...

Are these degree measurements made at a standard valve lift such as .050" or 1mm?
 
gssuzukixxx said:
Are these degree measurements made at a standard valve lift such as .050" or 1mm?
Click to expand...

It's from the Suzuki factory service manual(s). Suzuki doesn't reference how the cam duration is measured but I think it has to be at valve lash considering the numbers.
 
I have a degree wheel mounted on my GS750 and I have TDC established using a positive stop. When I advance the crank it is hard to tell exactly when the exhaust valve starts to open but it appears to be between 100? and 110? ATDC (110? is the spec) so it is in the ball park.

But when I keep rotating until the exhaust valve closes it looks like about 50? ATDC (30? is spec). Which is quite a ways off.

I have a feeling it has something to do with my valve clearance.

Any thoughts?

Haven't even gotten to intake.

Brian
 
I thought degreeing the cams involved setting the lobe centers, not trying to set the open and close points?
 
Suzuki_Don said:
GS850: Intake opens 28 degrees BTDC and closes 68 degrees ABDC (276 degrees duration).
Exhaust opens 66 degrees BBDC and closes 26 degrees ATDC (272 degrees duration).
Click to expand...
Those are numbers for the '79 cams. The '80 850s had the intake cam advanced 10 degrees.

Here is a clip from the factory 850 manual:
GS850camtiming.jpg


.
 
Nessism said:
I thought degreeing the cams involved setting the lobe centers, not trying to set the open and close points?
Click to expand...

Yes, you are right. The openning and closing points are only used to calculate the lobe centres. It is easier to pick the exact point when a certain lift is reached vs when the lift starts.

So my next attempts will be picking say .02" lift on the way up and on the way down and then figuring out lobe centres.

Interesting stuff.

Brian
 
OK here are my lobe centre readings. I used .020" lift as my reference.

Intake lobe centre 122.5? ATDC (supposed to be 110?)
Exhaust lobe centre 104? BTDC (supposed to be 110?)

I took out the intake cam and moved it to 19 links instead of 20 from the #2 mark but that makes too big a difference. I was thinking that with 30 teeth on a cam sprocket that it would make a difference of 360?/30=12? but it actually make twice that difference because the crank sprocket moves twice as far as a cam sprocket.

So now what? I figure 104? on the exhaust wouldn't be too bad because I have seen it suggested that reducing the lobe centre angle will actually improve low and midrange torque. But the intake at 122.5? needs to be addressed. But how do I do this?

Brian
 
Brian, you need to slot your camshaft sprockets so you can adjust the cams to each other and the crank. I have done it on my bike and can give helpful hints if required.
 
Suzuki_Don said:
Brian, you need to slot your camshaft sprockets so you can adjust the cams to each other and the crank. I have done it on my bike and can give helpful hints if required.
Click to expand...

Thanks Don, I will definately contact you if I get in over my head.
 
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