Advice on top end rebuild

[Allyboy]

hello all, I need some help/advice on my top end rebuild. I currenly have a derbi gpr50 with ebe/ebe engine with metrakit 50cc top end,full arrow exhaust and carb rejetted and rolling roaded by my local bike shop. Bike is running at 7.5 bhp and goes ok but I'm after a change. I was looking at a 70cc airsal top end but unsure about changing carburettor. I have seen airsal 50cc tech piston/ mpower kit which claims to be up there with 70cc kit with different timing options. I'm unsure if it will be an upgrade to what I have currently. I'm looking for a bit more acceleration and top end. I have seen 62mph on my speedo (probably more like 50mph in the real world) I currently have a 12t front sprocket and 52 rear. Any advice opinions would be most welcome! Thanks

[toratora]

There are lots of ways you can go. You might be able to use the same carb, and pipe with a different cylinder. The Metrakit that you have might be a ProRace 50cc, and if so it's quite powerful for a 50cc top end. It might out produce some 70cc kits.

You can look on sites like Maxiscoot for top ends. The MetraKit 70cc SP kit is quite nice.

This BariKit top end is nearly identical to the Airsal (it's made in the same foundry), but it has a multi piece head, which provides better cooling.

Stage6 just came out with these BigRacing kits that look awesome, but the 77cc Euro2 kits never seem to be in stock.

ItalKit makes a nice Gilardoni 48cc kit for the Euro2 that Treats usually has in stock (currently out, but you can ask them to get one for you). It's also available as an 80cc with a 50mm bore. I've installed these on a number of bikes and they ore very nice.

Depending on where you ride you should be able to pull a 14 or 15 front sprocket with these kits, and you should see something from 10 to 15hp, and maybe a little more depending on the kit.

For carbs you'd probable want to run at least a 19mm, but you might also go to a 24mm depending on the rest of the setup. The Arrow works well with 19-21mm carbs. Polini makes a really nice CP carb for these ranges. You can get a cable choke adapter for them, and there's also a quick access float bowl available for them too making tuning much easier.

As I said there are actually many ways you can go with all of this stuff. I recommend reading through the project threads to see what sorts of options you find appealing.

[Allyboy]

Excellent thanks for the reply. My top end that I bought from racing planet says it's an alloy cylinder not a performance one(even though it was a metrakit one) anyway I think I will go for the airsal 50 tech piston kit and get bike shop to tune it /upjet it again and put a 13t front sprocket on hopefully see a gain from what I have.

[toratora]

That piston may not work in the MetraKit, it may only be for the stock cylinder, or maybe an Airsal cylinder. You'd be much better off to order a MetraKit piston. Also, the pistons come in different sizes. If you pull the cylinder the size will be stamped on the piston, and on the cylinder. It's usually an A, B, or C.

[Allyboy]

Sorry i realised that I gave a poor explination. I'm going to replace the whole top end barrel, piston, head etc with the airsal tech piston cylinder kit the metrakit top end will all most likely go in the bin as it's been on for over 3 years. Thanks for the replies so far

[toratora]

If the cylinder is in good shape it’s probably fine to just do a piston refresh. Make sure to replace the wrist pin and little end bearing too.