Ported CAG carburetor with pictures!

[Crash Dummy]

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.


CLICK ME FOR THE LARGE VERSION

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

[RD RACING]

well done, crash dummy. great for guys to learn how to ect, we will leave this post as a sticky.

[Dreamah]

Looks good mate, I know of someone Who went a Little to far on their Cag Carby and Ended up with a Split in the Wall, But you have done a Top job bro..

Good work !!

[Pocket Wog]

Done this port job on my carby yesty and i must say,its worth the couple hours of messin about with it,found its made my excelleration nice and responsive,especially up at higher rpms
Heres a piccy,once again sorry for the blurry pic(my jeans are more in focus then the carby itself)haha


Cheers Theo

[serbius4]

great post crash, ported my carby and gave it a run, major improvemants. Took it off again and ported the motor side, manifold and took the lip off my stock reeds, havent got a jet in it yet(just the stocker), put it on my stock bike and had a race with one from the same place (stock) and left it for dead, awesome acceleration, heaps more top speed well and truly worth the time and effort.

Why buy it when you can build it

[barnsey]

is 17.67 big or is it a good size because to big will flood the engin wont it??

[mac90]

top stuff m8 & good diagram on how the internal slide, pin & fuel venturi works

[Crash Dummy]

G'day all

Just an update on the carburetor porting article.
I don't have any pictures of the procedure but I can tell you what to do.

Even after porting the intake that made a pretty good difference but my bike felt like it was hitting a huge wall at about 9500rpm. I thought it was funny. No amount of playing with jets was going to fix it so I put pen to paper and did a quick calculation.

I haven't been able to find any sure fire way of calculating just how much air you can flow through X sized hole. My maths is terrible so we won't go there.

39cc cag
11500rpm = 448.500 per minute 7.4l per second

49cc cag
9600rpm = 470.400L per minute 7.8l per second

Both with a 12.8mm throat. Seeing a pattern yet ?

To add injury to insult the damage ? Fuel consumption at a rate I have never heard of. I recall all those times I ran around for what seemed like forever on half a tank of fuel however on my first outing with the modified carburettor and enlarged jet to get mixture right I found out the hard way my engine was choking from my carb being small. Having some drag races against a buddy I'd filled my tank with 250ml of fuel after a prior run with lots of high speed work. Remember this.. On the prior run I let someone else ride and I watched and listened carefully. I had a beautiful blue haze from the pipe indicating a pretty good mix, a good burn and the acceleration was beautiful but then it hits the top of the rev ceiling and starts belching smoke like someone has lit a bushfire inside the engine.
Now back to the refill. How long does 250ml last you ? I got about 4 minutes after giving my bike a couple of minutes to warm up at low speed. I think what has happened is the mix is great right up until you hit the flow limit then you get the same effect as turning the choke on which absolutely floods the engine stupid with fuel. Normally after this situation occurs the bike with stutter and be cranky for a couple of seconds until it's allowed to clear up then it would come clean again as the spark plug dries up.

I ended up attacking my carby again with a file to open it up even further. I wasn't working with precise tools this time and was limited by the time I had. I used a cone shaped metal grinding bit in a 240v power drill and a course round file (rat tail file) for opening up the throat because I haven't got any drill bits or drill that will go that big. The throttle slide (piston) is 13.98mm so for obvious reasons you can't enlarge the throat further then this through the fuel metering section of the carburettor body. But before and after is all free game provided to don't run out of metal.
In my case I chose to open the bell mouth to the venturi up to 19mm which I don't recommend because you will be coming very close to breaking into the float chamber. I got the sizing for the throttle slide area to about 13.90mm on the horizontal axis and a fraction over 14mm on the vertical because that is less essential. Keeping it all beautifully symmetrical would have been ideal but I couldn't do it under the conditions I was working under. After throttle slide the bore continues from 14mm roughly and expands to about 15.5mm as it enters the standard manifold. The expansion is cut in such a way it should help direct the air downwards into the manifold. I've patched the throat of the carburettor to that of the manifold and at the same time cleaned up the edges within the manifold to allow a smoother transition in air flow.

My theory is probably flawed but I'm hoping by doing this I will have greatly elevated the maximum air flow potential of the carburettor as well as increasing the venturi effect to help draw more fuel up from the emulsion tube.

Thanks for reading.
I hope the update of this modification will help some of you who are finding that performance brick wall.


Gary Hope

[Crash Dummy]

Now for the ugly part

In my haste to get the job done I went overboard with the grinding bit on the drill and chopped a little material out of walls of the area the throttle piston slides and seals against. The end result is if you look through the carburettor at a light source you will see a tiny bit of light peeking through the side near the idle speed screw. If you hadn't gathered already this is really bad.
The reason it is so bad is that air is allowed to bypass the fuel metering component of the carburettor so the airs gets into the engine without any addition fuel being added. The result is a lean idle mixture and will cause all kind of problems the worst one being in aspect of pure ride-ability is that after doing a high speed wide open throttle pass when you snap that throttle shut there won't be enough fuel there to sustain the fire and the engine will stop. It will also likely make for a boggy mid range as well.

My solution to my mistake is going to be to take to my carburettor once again with a dremel or even a cordless drill. On the surface I removed too much material on being the idle speed screw side I intend to remove even more material until it's a big gaping hole. Once there is a genuine gap formed I will use a drill bit or my dremal tool to cut several burrs into the wall of the throttle slide area to really rough it up. At this stage your thinking what am I up to. I'm going to use JB Weld to fill in the area that I messed up and reform it into a workable solution. By removing so much material and roughening up the surface will allow a strong bond. JB Weld is an epoxy based liquid steel putty or cold weld. It's good for 315.C and is resistant to just about everything I'll ever need to worry about in a carburettor.

Once the JB Weld has been mixed and slathered into place I just have to wait for it to harden into a workable state that will allow me to sand and grind it to shape to form a new wall for the throttle piston to seal against.

With any luck it will work and I won't have to scrap my carburettor and try again.


Potential tuning issues

Tuning is going to be tricky. Air flow through the venturi is now reduced so picking up fuel may be more difficult at lower speeds. Raising the tapered needle may be required to the extreme to get satisfactory results. If you have to use the lower most slot on the needle to raise it. If that still isn't far enough consider shimming under the needle with a thin washer. Another thing that may be worth trying is attempting to reduce the thickness of the taper so that more fuel is supplied at a smaller throttle opening while top and is close to the same.

I hope this works. Part of this is pure speculation. If anyone can help me with this one I'd be most appreciative. I'll see what I can do at my end. It's just difficult as I haven't got anywhere to test my bike.

My main goal of these modifications is to provide a genuine alternative to having to upgrade to a Dellorto 14/14, Walbro or other after market carburettors. A easy potential to save a lot of dollars if your good at what you do.

[gtti89]

easy solution .... dellorto