BBK opinions

declan

Right I’ve got a carb set up for a 70cc but the bikes a 50cc another “but” is I want a cheap 90cc big bore kit will the carb be able to meet the demands of the 90 kit with jetting or am I better off going with a 70 kit and has anyone tried a cheap 70 kit and can tell me if there decent on power

Calum

Literally no idea what you're on about, or why you would waste money on such things.

You've got a DT haven't you? Why mess around with cheap pit bikes when you've got the real deal.

So the carb is setup for a 70cc, but the engine is 50cc and you want to go to 90cc.

Common seizure of engines is incorrect fueling.

When you overfuel, you wash the oil from the bore, known as bore wash, you then get metal on metal contact and the engine seizes.

When you underfuel, the combustion temperatures rapidly increase, igniting the fuel before ignition, causing the upwards stroking piston to meet the explosive gases, shaking the engine to pieces, known as detonation.

Either way, both conditions cause the engine to destroy itself.

Fueling has to be spot on for reliability to sustained.

Will the bike work setup as a 70 for a 90, probably. To be fair, the cc increase is so small that the jets might not make a difference. But for the sake of a few quid, just buy some more jets and needles. It's not hard once you know what you're looking for.

declan

@calum it’s not a pitbike it’s just a little project of mine nothing special money wise so I don’t mind it blowing up or setting a light but I’d prefer it not obviously, I was thinking more will the carb supply the right air I’m not looking for a race machine just something to thrash

Calum

@declan Whilst the carb does meter the air, that's not really what they mean when they say "Set up for a 70cc". They're talking about the fueling, since that is the part you can alter. You alter the air, by adjusting the throttle. The air is fixed by the cut away. It can be permantely bored out with a dremel if you so wish.

So Air wise, yes that should be fine, although potentially limiting the power.

Fuel wise, I would still jet it accordingly.

declan

@calum okay I’ll probably be building it soon I’m gonna try get it running on the 50 first need to set needle back to stock

Minia

As Calum said, you'll have to Jet it accordingly to try and get the best amount of mixture possible. But in my opinion it really just depends how much time you're willing to perfect it. If it's just something you want to thrash and aren't worried about the results you can probably skip a couple steps... I've certainly done that.