Hey all. I hope these pictures can help any of you budding pocket bike modifiers come to grips on what people mean when they suggest you port your standard CAG carburetor.
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To do this modification you will need to strip the carburetor down completely of all parts possible to avoid damage from metal filings.
All that is really involves is you need to open up the mouth of the carburetor to create a venturi. By doing so you should end up with significantly more air being able to be passed through the carburetor along with fuel and thus creating a bigger bang and making your bike go faster then before.
All I needed for this modification was my digital calipers as you can see so I may measure my work as i progressed. A drill press with a appropriate grinding stone fitted. In this case a cone shaped stone was used and ground down until it was the appropriate shape for this mod. And finally some 200 and 400 grit wet and dry sand paper.
Just some basic measurements now.
In standard form my carburetor had a 12.83mm throat (quite large for a stocker) and had a really nasty looking bell mouth that would have been lucky to be 2mm deep. It really didn't do much at all to help air flow so we had to do something about it. The depth of the opening of the carburetor is approximately 13.26mm before you reach the edges of the throttle slide. You can not grind past the half way point of the throttle slide which is approximately 17mm or else you end up with a gap between the walls of the throttle body and the throttle slide. This is bad! So don't grind too far in.
So now we have all the tools we need. All you need to do is grind down the throat of the carburetor with the grinding stone until you end up with a nicely tapered venturi. It's pretty easy to tell when you've gone as far as you can. Look for where the stone has been grinding based on the top of the throttle body where the slide normally retracts to at full throttle. On mine I have only just gone in as far as the very edge where the stone had just kissed the material. It is essential that you keep the job locked in a vice and straight to prevent injury if the stone grabs and to keep the grinding nice and even.
By now the venturi is looking nice and cone shaped but is rough as well something very rough and there is a metric ton of aluminum filings everywhere. I gave my carburetor a good blowing out then dunked it in some diesel fuel along with my piece of wet and dry sand paper. Wrap the paper around your finger in a cone shape and get stuck into it. Just sand around the insides using your finger until the finish is nice and smooth. A smooth looking finish with no marks that you can feel with your fingernail is the goal. Going for a mirror or polished finish isn't going to help or if anything it will hinder your efforts.
By now your carburetor is looking like it will swallow any nearby low flying aircraft if you were to crack the throttle open with your standard carburetor looking very asthmatic in comparison. *gasp gasp gasp*
Now you can give your carburetor a good wash down to remove all traces of filings and metal dust. Reassemble and take note that your engine is now going to be sucking in a lot more air. You may be required to fit a larger jet so your engine gets enough fuel.
Well that's it for the moment. If I have made and mistakes please PM me and I'll rectify it ASAP.
Gary
