GS550 to 673 conversion.

[Chancho196]

Hey folks, I'm about to embark on this conversion because, here in Oz, it seems far easier to find a complete 650 motor than a 550 head (the cam journals are worn out on mine).

Sooo... I'm after a few tips on which cams to use? I've read on here previously that the 550 cams will make it rev, whereas the 650 cams will produce more torque (which is probably what I'd prefer).

Aside from the relieving of the crankcases to fit the barrels, is there anything else I should be aware of?

Thanks in advance.

[dorkburger]

Maybe some info here.

https://www.thegsresources.com/_forum/showthread.php?201784-Finally-getting-around-to-the-550-699-project&highlight=

[Steve]

Personally, I would stick with the cams that you already have.

The 650 came in two variations, based on the final drive (chain or shaft). I don't know if the cam profile was different between the two, but they had different number of teeth on the sprockets. I don't remember the exact number (or which was which), but one had 15 teeth on the crank and 30 on the cams, while the other had 16 teeth on the crank and 32 teeth on the cams.

I did a head transplant on a 650 shafty, the donor was a 650 chain model. I thought the cam lobes on the donor looked a bit better, so I installed them. With everything put back together, I did the obligatory two turns of the crank to check everything. Only made it about 1 1/2 turns and it bound to a HARD stop. Disassembled, reassembled, checked everything twice (or more), got the same results. Then I noticed that the sprockets were a different size and check the number of teeth. After installing the proper cams, it went back together smoothly and ran well.

Not sure which cams would match your 550 cams, so just play it safe and use what you already have, knowing that the number of teeth will match. There will be enough more grunt from the larger engine that you won't miss not having the 650 cams.

.

[Chancho196]

Thank you guys, much appreciated.