GS1000/GS750/GS850 camshaft specs???

[Chuck78]

Anyone have any idea where I can find specifications on lift, duration, etc on cams the factory cams for the gs750 & gs1000? Curious about the 850 also. I'm considering upsizing my 77 GS750 to 850 cylinders with os pistons or aftermarketto make an 870cc or 920cc engine. I figured upgrading cams to aftermarket would be very pricey ($400-500 new ), so I wondered about running a gs1000 cam with the 750/850 head on a 750 crankcase displacing probably 920cc's, running vm26ss carbs with gs1000 jet sizes. I think in 1980+ the 1000 started using a larger carb than the 750/850, but earlier models used the same but with larger jets.

[hampshirehog]

The 850 and 1000 run with exactly the same cams. The 750 has slightly 'hotter' cams - a tiny bit more duration. I don't think you would notice anything in the real world.

The early bikes all share VM26 carbs. The 850 has 102.5 mains, 15 pilots, the 750 105 mains / 22.5 pilots or 97.5 / 27.5 or 100 / 15 (depending on engine number) and the 1000 95 mains / 15 pilots. Over here some of our 1000s got VM28s - same jets.

[Chuck78]

Are any of the larger bikes going to have a hotter cam with more duration and lift than the 750? Such as the 1100 & 1150? Or are those all 16 valve models only and not a compatible swap with the older 8 valve cams?

[hampshirehog]

You've got it - the 16 valve cams won't work with the 8 valve heads.

[Steve]

You've got it - the 16 valve cams won't work with the 8 valve heads.

True enough, but the 1100G was an 8-valve head.

Here is cam timing info for an 850. '80 and up is on the left, '79 is on the right.




Looking in the 1000 manual, I see no difference for the E and G models.

And here is the 1100G


Looks like all of the '80-and-up 850s are different, the 1000 and 1100 have the same timing as the '79 850.

.

[49er]

Looks like all of the '80-and-up 850s are different, the 1000 and 1100 have the same timing as the '79 850.

The lift and profiles on all the 850 cams are identical.

The factory advanced the opening of the intake on the '80 onward cams, 10 deg by re-timing the mounting point of the sprocket. You will notice that the intake closes 10 deg earlier on those '80 cams too.

The experiment was abandoned for the 1000 and 1100 models which reverted back to the '79 specs.

[Chuck78]

I have a few questions on this cam timing.

Does the factory specify how the timing is calculated? Is this actual "advertised duration," or is it duration at 1mm/.040" lift?

I'm trying to compare the low end carachteristics of some 260 degree @ 1mm lift Yoshimura cams that I have vs the duration on stock cams. The gs850 comes put at 276 degrees intake 272 degrees exhaust. I assume that is over all lift from the instant the tappet starts moving off of the base circle?

or are the Yoshimura cams a tighter duration with a much faster ramping lobe?

[Chuck78]

also, advancing the intake cam 10 degrees as they did on the 850 models 80+, that looks too possibly build more compression, so would this increase the low and mid-range response? Or does this increase the high-end response? I should refer to the lengthy webcams article on degreeing cams, but figured I might as well tack at this question on as well.


I absolutely love the exhaust sound of the early 750 8v cams, and I was considering altering the timing to boost low end torque & mid range torque, but now I'm considering looking at putting a GS1000 intake cam in to do the same, as if I recall correctly from previous manual references, the timing is about 4 degrees less duration but the lift was around .310 inches, whereas the gs750 lift was around .302"-.304" or somewhere near there.

Thoughts?

Lastly, I had read specs saying that the GS1000's models recieved cams with much higher loft. If I recall it was in the .320-.330"-ish range. Is this correct? That'd make a nice street cam! Megacycle grinds new run $450&up, so the 1000s cams aren't some hot ticket high dollar item aside from scarcity, but worth looking into if this is true. I will have to compare fiches for it sometime.

[Chuck78]

GS750 8v cam specs:



280 degrees duration intake & exhaust
36.28mm intake lobe height
35.75mm exhaust lobe height