Hey, new guy here. got lot's of questions

welcome to the forum! Deacon has several designs for these things.

I use 2 pulleys, a long brake cable and a brake lever for my "gravity" clutch, it pulls hard but does the job If I get out of work early enough I'll get some pictures of it. It's a pretty simple design, and works well now. I tried with only one pulley and it wasn't working for me so 2 was the trick.

The hard part is in the first couple of days the cable stretches a LOT and I had to adjust after every ride no mater how long it was. I haven't rode much this week but I haven't had to adjust it at all.

Weedeater friction drives are the best- simple, easy and fun.

I am all about simplicity. Simple to build, Simple to maintain, Simple to pay for.

The first decision is where you are going to mount your engine. Front or rear.

My thinking the ideal bike for these friction drive is a coaster rear wheel and a front hand brake. The second brake lever is good for a gravity clutch cable and the shifter from on old mountain bike is a best throttle in my opinion.

Front mount engine and you can use the brake cable right to the engine mount to lift and lower it for your clutch action. If you go with rear mount I use a long lever to raise and lower the engine and a brake lever and cable to pull the clutch lever down. It ain't purdy but I like it for it ugliness as well.

Instead of the robin 40cc give some thought to a 31cc from a weed Whacker. I have a used one on the way from ebay that I paid 20bucks for. Craigs list has a couple if you want to make sure it runs before you buy it. Those run about thirty five or so. You might want to start with something like that and refine it before you purchase and expensive engine.

The 25cc you have would be really good to begin with. Work out your kinks then go to a bigger engine. AS shoes said there are always kinks.