Gy6 50 camshaft(s)

[KSR Moto]

Does anyone know if there are several types of stock camshafts that can be found in gy6 engines, I have a long and a short case version, on the long case it seems that the cam differs from a short case engine,Also I have replaced everything on a long case engine but it never revvs as high as short case engine, and I tried to swap everything except cam, any opinions?

[90GTVert]

I'd suspect there's a higher likelihood of variations from one manufacturer to the next or even between manufacturing runs or model years than any standard difference between short and long case camshafts. Could also be related to tire diameter and gearing or even the difference in the CVT from different belts.

[GrumpyUnk]

I would think tire diameter would be the most influential effector. If the gears in the final drive box are the same, then the short & long cases will try to rotate the wheel at the same rate for a given engine rpm. The limited power of the engine will tend to cause slower acceleration in the long case machines, but the final terminal velocity reached might be higher if the engine rpm limit is not reached.
I have a long case that seemed to rev like a turbocharger, getting to 8900 seemingly any time I goosed it when not moving at top speed. I replaced the belt, and bumped the slider weight to 8 grams as I thought it 'rev happy'. I have yet to try it out as it's been cold and rainy, and it does not want to start since I put the new belt on... It'll fire if I goose the throttle a few times to squirt fuel from the accelerator pump, but once that is gone, it bumbles to a stop. It was kind of cold that day, so I rolled it back in after 1/2 hour of trying to adjust the carburetor to get it to stay running.
tom

added:  From reading and web fiddling, I have seen somewhere that there are multiple versions of camshafts for the 139QMB/GY6.  All I read of were labeled along the A8, A9, A10 series.  Add in the 'performance' cams sold by some sites, and you have a case for full confusion.
tom

[ratdog]

I have measured a large number of both stock and after market cams in the last few months. I terms of the two most important measurements, duration and lift, there is no real world difference in any of them. While we normaly assume a “preformance” cam will have more duration and often more lift, this has not been the case on any of the cams I have measured. In fact the so called preformance cams I have measured, have had less duration and the same lift. The major difference seems to be that the “ preformance “ cams delay the opening, and create more overlap.,

So a cam is not the answer to your issues.

[GrumpyUnk]

ratdog, I installed a BBK, and a 'performance cam', along with a 20mm carb. All at once. Could not get it to idle down, it had zero power at any rpm close to reasonable idle speed. But. It sure would move when it was goosed. I tried another 18mm carb that had worked fine in the past. Still poor idle. I removed the cam, replaced with original, and found the cam timing was advanced at least one tooth. Not thinking, I still replaced with the factory cam. Finally got it to start readily and run. Seems it lost its high rpm effectiveness, or mid-range, all I know is that the power seemed to be less than it had been there for a while when it didn't want to run. FWIW, and full of technical information, almost scientific in exacness. Yeah, I know I exceed expectations in that area... Ha.
tom

[ratdog]

It’s very common for us to slightly alter the cam timing. Of course we are running raceing engines and don’t really care where they idle. I don’t recall ever purposely changing it as much as 6 degrees ( one tooth) but we often will make multiple Dino run with the same came andslight offsets to try advancing and retarding the timing. Many of th e cams we have set up are slightly advanced to get the last few RPMs out of a set up. I had a wonderful 68 Firebird with the OHC 6 that at one point jumped a tooth due to an old belt. Suddenly I had very little low end power, but she pulled like a deacon at high RPMs! I was very lucky that I didn’t put any valves in to pistons. Of course I pulled the engine down and replaced the cam belt. Well that’s not totally true. I used it as an excuse to completely rebuild the engine. It had 144,000 miles on it at the time, and , I mean as long as it had the cam belt off anyway... I went thirty over, line bored, decked the block, lightened and balanced for the new rods and pistons, milled the head, multi angle valve job, cc the chambers, port and polish, big cam and heavy springs, replaced the 4 barrel with 3 Webber’s, headers, electronic ignition,... you know, just freshened it up a bit.