Sport moped exhaust on 66cc (Build thread)

[gary55]

Stock NT's are 14mm carbs. Speed carbs are 14.8.

 

[JerboaJohn]

there is product variation in anything made in China. I use +/-5mm as a rule of thumb for stuff. this is just my experience. I don't trust anyone or anything so I measure and weigh lots of stuff. I like when there is enough material to mill things to close as I can get to a certain spec, but this carb was yanked after 4 or 5 days from new. just can't do much with a stock carb just basic jet it,,

 

[gary55]

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I see what your saying, but that is not where you measure to determine the size. You have to go in farther to the smallest portion of the intake. Their are also carbs that use 2 size cal outs. The nt is a 14/14. Meaning that both the filter side and the intake man. side are 14mm at the smallest points. Some Dellorto's will be 14/12 which means filter side is 14 intake side is 12.

 

[Vikingimike01]

Is a 16mm carb good? Or is a 17mm one a better option for more airflow at higher RPM's? And like an all around RPM as well?

Remember this will be with a scooter pipe (Giannelli Next 31623X)

I also have sanded the cylinder head like a milimeter so a higher compression as well, port widened a bit too.

 

[gary55]

Looks like my memory failed me. The Speed and NT both seem to have the same size throat size and the speed's slide is 1/2 mm wider. It may be that the older speeds had a larger throat, but the new ones I have are 14.

 

[gary55]

Is a 16mm carb good? Or is a 17mm one a better option for more airflow at higher RPM's? And like an all around RPM as well?

Remember this will be with a scooter pipe (Giannelli Next 31623X)

I also have sanded the cylinder head like a milimeter so a higher compression as well, port widened a bit too.

Unless your Planning on changing your intake you are going to be limited in how large you can go with changing your carb. size. The stock intake is 16mm ID 19mmOD so your stuck with a carb that will clamp to that size. Anything over a 16mm carb on the stock intake isn't going to do as much as it should because it is still restricted by the intake. I use a Dellorto 16/16. It's the only one I found that is a direct bolt on (using the provided sleeve) for the stock intake. It works well, but is a bit pricey form Dellorto Direct.

Removing that much material from the head is never a good way to up the compression on these bikes. It should be taken from the top of the jug. The crush ring on the stock head already encroaches into the cylinder and the more you sand off of it the farther it moves inward. If it moves to far inward it can actually slap the top of the piston. You can see in the pic. how much smaller the crush ring id of the stock head is compared to a aftermarket head. You want the id of the crush ring to be 47mm just like the inside of the cylinder.
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[Vikingimike01]

Unless your Planning on changing your intake you are going to be limited in how large you can go with changing your carb. size. The stock intake is 16mm ID 19mmOD so your stuck with a carb that will clamp to that size. Anything over a 16mm carb on the stock intake isn't going to do as much as it should because it is still restricted by the intake. I use a Dellorto 16/16. It's the only one I found that is a direct bolt on (using the provided sleeve) for the stock intake. It works well, but is a bit pricey form Dellorto Direct.

Removing that much material from the head is never a good way to up the compression on these bikes. It should be taken from the top of the jug. The crush ring on the stock head already encroaches into the cylinder and the more you sand off of it the farther it moves inward. If it moves to far inward it can actually slap the top of the piston. You can see in the pic. how much smaller the crush ring id of the stock head is compared to a aftermarket head. You want the id of the crush ring to be 47mm just like the inside of the cylinder.
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Sorry, I actually took the material from the cylinder, not the head.

So, what carb? 16mm max?

 

[gary55]

Sorry, I actually took the material from the cylinder, not the head.

So, what carb? 16mm max?

For stock intake yes. if your thinking about the 16mm Keihin carb. it won't fit the stock intake. The only 16mm carb I know of that does fit the stock intake is the Dellorto SHA 16/16.

 

[gary55]

Sorry, I actually took the material from the cylinder, not the head.

So, what carb? 16mm max?

So I'm not trying to be a downer here, and engines are different so you may get away with it. I have found thet 1mm is right at the limit of how much can be removed from a cylinder before getting into problems with piston clearances. when I first started screwing with decking cylinders I had pistons bounce with .040"/1mm decking. If you get away with it your compression is going to increase by about 45 lbs. Your stock head is going to have a real hard time holding up to that. It will be interesting though to see haw it turns out. Might be a screamer, might be a grenade.

 

[Vikingimike01]

I had like a weird style of crank, and the piston is acutually like 2 milimeters lower than normal ones. That's the main reason why I took a bit off. Now it just about doesn't touch. I tested.

That Dellorto SHA 16/16 carb is pretty expensive. I may just try and fix this one, but the bend... Ok I'm getting the hammer.