TOPIC:

I've built a cafe racer from a 79 kz650 donor that was not running when I bought it due to failed ignition. I have it running now, but it's slow to accelerate and peaks at about 4K rmp where it sputters and chugs. It has new coils, condensers and points and the timing is set correctly. The mechanical ignition advancer was frozen and after much soaking in WD-40 works much better, but I'm not sure it's 100%. Regardless, as I understand it, the advancer really should not be a factor once the bike gets to 4K rmp. So I suspect the carbs (don't we all?). I cleaned them during the cafe build, but since I replaced the stock filter box with pod filters, I'm sure I need larger main jets in the carbs (it idles fine). Two questions: 1) Would new larger jets usually remedy the problem described, or am I barking up the wrong tree?; and 2) What size jets should I try (I've heard to try two sizes larger). Many thanks!

I rebuilt some GS1000 yesterday. The owner had #95 main jets installed. The bike fired right up and took off great but the tiny main jets wouldn't allow the thing to run smoothly over about 4K rpm. I could eventually get the rpms up over that and had it up to 80 on the small mains but the bike wasn't happy about the situation.

I swapped to some #105s and the bike ran perfectly. #102.5 mains would have been fine but I didn't feel like pulling the carbs a 3rd time and a bit rich; rather than lean at the very top of the rpm range is just fine.

VM24s started life with a variety of main jets ranging from #90s to #102.5s so I would go up 3 sizes (2.5 per step/size)... I believe that might just fix your problems.