Hello,
I looking to get more top end power out of my GS with stock cams, Should I adavance or retard the cams? and how much? thanks

I don't have first hand knowledge of doing this on the GS so others may correct me....but I do this a lot on V8's I build.  For more top end power you want to retard the cam.  Notice I said cam (singular).  On the V8's I build, the intake and exhaust are on the same cam.  Speaking in the most general of terms, you will lose bottom end and gain some top end.

Without knowing piston to valve clearances on the GS motor, I'd say a safe bet would be to retard both cams 2 degrees and "just see".  That's what I'd do anyway..  You should be able to accomplish this by egg shaping the cam sprocket holes.

Srinath or Bob B. might know more.

Just two degree's? I was thinking somewhere more along the lines of 5 to 10?

the valve positioning,proximity and opening height suck on the GS. both between intake and exhaust but the piston aswell.

Retarding the cam would make the valves open later right? as in, for example, the intake valve would open at 5 degrees BTDC when retarded versus 10 degrees BTDC when not?

Quote from: Chris_BRetarding the cam would make the valves open later right? as in, for example, the intake valve would open at 5 degrees BTDC when retarded versus 10 degrees BTDC when not?

First, to answer your last question, YES, retarding the intake cam will make the intake valve open later, but why do you want to move the powerband HIGHER in the rev range? Is this going to be a racetrack / dragstrip engine, or a street engine? If it is for the racetrack only, I can understand why you'd want to do that, but if this engine is for the street, you'll just make the bike slower off the line and less responsive to the throttle. If you move ONLY the intake cam, the valves are probably going to be fighting for room inside the combustion chamber, and BOTH are going to get damaged, which means expensive head work in your future.

To be honest, degreeing the cams (which will require slotting the cam sprockets) is a complicated subject, and it gets even MORE complicated in a hurry. Judging by your questions here, I don't think you're aware of just how involved this can get. If you simply slot the cam sprockets and move a cam (or both) just to see what happens, the odds are excellent that you'll be looking for a new top end pretty quickly. Valve timing is a critical issue, and things get REALLY serious as the RPMs increase; doing it wrong will pretty much ruin your entire top end. You can have valves hitting the piston, or valves hitting each other (especially at high RPM), and other calamities you really don't want to deal with. Believe me, this isn't something to do on a whim; OTOH, if you have a spare engine handy, and don't mind the risk...

You'll need a degree wheel (obviously), and a good amount of free time to devote to this. It also would help tremendously to have access to a rear-wheel dyno, in order to establish a baseline reading, and then to make adjustments as needed as you dial-in the carbs and cams. Degreeing cams can be a bit of a crapshoot; since you're working with both an intake and an exhaust cam, you'll want to set them so that they complement each other, which will (hopefully) diminish the effects of any carburetion flat spots throughout the powerband. You'll also need to calculate the lobe centers of both cams, measuring from a certain point of lift (.050 inch is the common starting point) and working from there (on the intake) BTDC around to the same lift point (.050) ATDC. That will give you the duration of the (intake) cam (in degrees), and with a little math, you'll be able to figure out the lobe center of each cam, and (eventually) where to set each cam for your particular engine.

Don't forget that unless you're building a track bike, you'll want to set the cams to move the powerband LOWER, so you can utilize the power sooner in the rev range. That way, instead of waiting for the engine to "come up on the pipe" at 7K - 8K RPM, you can start making usable power lower in the rev range, which is where you want it on the street.

Think about this; opening the intake valve SOONER will allow the cylinder to begin filling with fresh fuel/air sooner; setting the valve overlap correctly will allow the exhaust valve to open sooner, which should improve the intake flow, since the open exhaust valve will cause a negative pressure around the exhaust valve, and that negative pressure will suck more fresh charge into the cylinder (through the still-open intake valve) than would otherwise occur. In essence, degreeing the cams and setting proper valve overlap (within the physical limitations of the cylinder head) will allow for a "poor man's supercharger" effect, and that will almost undoubtedly require a fair amount of carb rejetting to take advantage of. Of course, we haven't even considered effective cylinder scavenging yet, keeping the cam chain happy, or half-a-dozen other factors yet, so as I mentioned before, this gets REALLY complicated in a hurry.

OK, now that you know all of that, use the search feature and look for the terms "degree cam". Search for ALL the terms; you should see about a dozen pages listed, and as you start reading those, you'll find that they link to OTHER pages with more information, some of it helpful, some not. IOW, you've got a LOT of research to do on this subject, and once you've learned all you can from this website, you should let Google do the walking and do MORE reading and studying. From what I found on the 'net, this seems to be a fairly cut-and-dried subject, but that is only in theory. In practice, on a motorcycle cylinder head you aren't intimately familiar with, things get fuzzy. For example, you might have problems with valve-to-piston clearance once you degree the cams properly. Will performing a three-angle valve job recess the valves deep enough into the head to provide the proper clearance? I really don't know, and I don't know if anyone else here has those answers readily available. Will a FIVE-angle valve job do the trick? Beats the hell out of me...

I get the feeling that you want a quick-and-dirty answer, but there really doesn't seem to be one readily available here (not that I located with a quick search on this site, anyway). I'll admit, I'm considering writing a general how-to on this very subject, but finding the time to do it is a factor. There are other gear-heads on this site (too numerous to mention, though starwalt, dgyver, Bob Broussard, and Srinath come readily to mind), so don't be too surprised if you see a new cam-timing how-to sometime in the future. My GS is on loan to someone right now, so I can't use it for an example, but maybe another member will want to attempt this. I can write a general step-by-step on how to find the lobe centers and other required calculations (though I'd only be reinventing the wheel; the same information is available with a quick Google search), but someone else will have to actually perform the work on his own GS, and write the specific details that apply to the GS engine.

Since I don't have a GS here to work on, I have no idea whether or not this is even feasible. I don't know how much clearance the head and combustion chamber provide, so this might be a complete waste of time, which is another reason why I'm reluctant to even begin writing a how-to for the GS engine. If it can't in fact be done, then there'd be no point in writing anything more than, "Forget it; it can't be done".

Anyway, now you have some idea of just how involved this can get. I'm not trying to discourage you, but if you don't have money and time to burn, along with a spare engine and MORE money to burn, you might want to proceed cautiously.

Quote from: Chris_BJust two degree's? I was thinking somewhere more along the lines of 5 to 10?

Yes, just two degrees.  I would go two degrees and see how you like it.  This is of course assuming you have puttied up the valve-piston clearance so everything is cool.  If you don't like two degrees and you have the clearance, go another two.  I doubt you'd have to go farther than 4 degrees before the results were not to your liking but again - my experience is V8 based.

I would shy away from moving the cams independently as you then begin playing with about a hundred other variables - and I doubt any of them would be good with a stock cam.

Keep in mind that cam advance/retard is usually done to accomodate a poor cam choice.  Go too big on the cam and you have to advance it to get some bottom end back and vice versa.

Let me know how it turns out.  I am always looking for a free 10th.

Here are links to some good articles on cam timing, especially as how it applies to DOHC engines:

http://sportrider.com/tech/146_9604_hand/

http://www.starracing.com/Cam%20Lobe%20Center%20Explained.htm

http://mail.symuli.com/vw/camp1.html

http://mail.symuli.com/vw/camp2.html

http://mail.symuli.com/vw/camp3.html

Those should give you a real good idea about the basics of cam timing.


V8Pinto:

I have friends and relatives all over California, and I expect to visit them next month; where are you located? What company do you work for?

Thanks for all the help guys. Ive degreed the cam on my cb360, which is a single cam motor. I am a bit worried about doing it on the DOHC motor. I got really good results in advancing the 360 motor about 5 degree's though. Ill have to think about whether its worth the risk. Thanks again for all the help.

single cam engine is easyer as the valves wont interfear with eachother timing wise.  your advancing/retarding both at the same time so its mainly looking out for the valve to piston clearence.   with twin cams running both valves in same cyl, you now have to watch the relationship between all 3 at same time,   IN, EX, Piston.