I think I figured it out - I got the buggy on an incline where I could replicate the issue by only rolling a few feet forward. I marked the chain/sprockets and it appears that the chain is slipping on the driven sprocket despite being fairly tight. This wasn't an issue before the engine swap, with the same sprocket. You can see the white mark on the sprocket has shifted forward 2 teeth in the pic. My finger is pushing hard on the chain. There isn't hardly any slack at all.
Picture was taken before I tightened it a little more - I believe I need a new drive sprocket. I'm also wondering if the swingarm flexing is partly to blame. I drove a couple miles then did a short speed run.
This is from the original engine after I did the head port matching. The speed during the flat straight varied between 34 and 35mph.
This is from today with the terribly cast 61mm cylinder and head, 30mm pumper carb with out-the-box jetting. Stock exhaust. Speed during the flat straight fluctuated around 31mph.
I've got plans for header/exhaust but I want to figure out why my top speed dropped despite the 10% increase in displacement. I want to eliminate the carb jetting first. If that isn't the issue then I'll swap the variator/clutch from the other engine. If that isn't the issue I'll buy a large valve head and admit I suck at head porting.
