Porting the ATC200 for the dumb (like me!)

[Gabriel]

Something things (it seems) can be VERY difficult to learn the easy way. Seems the more off path you go, the less those who know it want to share. I'm new here so I am NOT saying that's the case here mind you.
Google porting the Honda 200cc ATC's and you won't find much.

So I have this head. The cam journal was chewed up and in the process spun the rear bushing, tearing out the locating dowel and ruining the head. I suppose one could re-machine it for needle bearing but I have neither the tooling nor the inclination. Instead I want to learn what NOT to do when trying to port at home. This will prove invaluable later when I do it for real. AND on the off chance someone else as inept as myself happens upon this thread, maybe I can save them from my mistakes by sharing what I learn.

Those of you who actually KNOW what you're doing, feel free to be brutally critical, my skin is thicker than a big block Chevy head.

I picked up a long round nose carbide burr from Amazon for less than $15. I prefer to use a straight line air drill as opposed to a die grinder. Slower speed, less chatter, more control. I cut a piece of stainless steel cloth i had and wrapped the shank. A drop of oil inside and I can hold on to it for increased control.

Holding the head can be a pain but I use a plastic handle screw driver slide through the head, locked in the vise and tap it with a . It'll snug up and hold it in position. There are better ways no doubt.

Go easy and there's only one side of the burr that will cut without chatter and jumping like a jack rabbit. You'll find that out QUICK. A light coat of spray oil makes it more free cutting and use an air hose to CONSTANTLY keep the chips blown out.


First thing that will ruin your day are the humps where the valve guides pass through. I kinda figured these were thin. I found out. They are VERY thin. Don't touch them with anything unless its just a lick from a sanding drum to polish.

Not sure if my lousy phone captured it but you can see what I found under that hump VERY quickly. Again, I would stay off that at all costs.




Looking at the port from this view, the most benefit seems to be gained from opening up the top. It doesn't take much and you'll see a pretty dramatic increase in the size. Ain't much sense in getting too carried away, there's only a limited amount we can do because of the tiny valve here so I would guess we're toying with the law of diminishing returns here.




I'm going to play with the intake side and I will post more as I go further into ruining this ruined head. Afterwards I will play with using a sanding drum of some sort to smooth things out.
It's always been my (limited) understanding that the intake port should be smooth but NOT polished. The exhaust side can be made into a mirror if you feel the need but the exhaust needs some rough edge to it to create turbulence for a better air fuel mix.
Again, if you know more, please say so. I am wide open to accepting error so long as I learn something in the process.

[Gabriel]

Got back on this finally. For the life of me I can't get a picture worth posting.

One thing I found to be a good idea, get an old spark plug and grind the electrode off so you can tighten it without anything entering the combustion chamber. I didn't do "much" in the combustion chamber, that's not a good idea but I did use some Cratex to clean it up. The ground off plug let me get around everything without junk getting in the threads, and rubberized abrasive gets EVERYWHERE.

I worked over my good head today. Got the ports opened up slightly and if you sight through the port, it's more of a straight shot now. The intake I used a 1/4" OD 60 grit drum to finish. The exhaust I worked all the way up to a felt bob and some jewelers rouge. It looks like it was chrome plated.

Kinda proud, I only nicked one valve guide and it was rather insignificant.

I hope to find an XR200 cam to go with this head. Add that to the 65.5mm cylinder & 10.5:1 Wiseco I have and this thing should boogie. (for a 200 anyway) Now to attempt to fab up a 1.5" exhaust. I really don't have the $$$ for a DG exhaust but I may have to FIND it.

[El Camexican]

I love this topic, but hadn’t seen your thread till now.

When it comes to 4 strokes I’ve had good luck following these guidelines:

- Decide what it is you want to achieve before you start. Is this for racing, or trail riding? If racing, are you willing to invest in a larger carb, cams, higher compression, pipe etc.? If its for trail riding, then don’t plan on doing anything more than smoothing out the stock shapes and correcting any factory shortcomings
- Less is more if you’re not sure what to do. Measure your carburetor, then measure your intake port diameter. As an example, if the carb is 28mm and the port is 26mm, I would try to open it up to 27mm. It might not be the best it can be, but it sure beats what will happen if you accidently make the port larger than the carburetor throat.
- Unless you have a flow bench and a lot of time to experiment most of your power gains are going to be in the valve pocket area. The factory just tosses the casting into a mill and drills the holes where the program tells it to. I’ve seen as much as 5mm of aluminum blocking a 24mm valve seat on a stock engine. Just cleaning these up and blending the edges can add huge power everywhere. Honda employees women to hand port the CBR heads in this area before they are assembled
- Always shape your ports so that starting from the valve openings they remain the same size (sectionally), or get slightly larger as you approach the carburetor
- Never make a port with a cross section larger than that of your carburetor
- Don’t remove material from the floor of your intake port and don’t make the intake port floors round. The floors should be as close to flat as possible. Remember, the velocity of air flow on the top of the port can be almost double that of the bottom due to the distance the air has to travel *** I made a note below.
- On a street engine you want to shape the valve guides and the bosses they protrude from like ovals, or diamonds. If you’re not sure you have the tools or skill for this, just leave them alone. There is performance to be gained in this area, but rough edges, or bad angles will decrease flow
- On a race engine you MIGHT be able to completely cut off the section of the valve guide that protrudes into the port (use caution, not all engines have long enough guides for this)
- Exhaust ports should be matched to the exhaust pipe diameter and tapered in towards the exhaust valve. Smooth is best. Don’t remove any obvious lumps in the roof or floor of the exhaust port. They are there for a reason.
- Don’t overthink the polish, or not polished intake ports. It won’t make a noticeable difference to any work we do. If it did, we’d never feel it. Do whatever pleases you aesthetically.

*** You mentioned “Got the ports opened up slightly and if you sight through the port, it's more of a straight shot now”. That’s probably not a good thing on the intake side. As I understand it he intake port should appear as “long” as possible. This is why some builders build up the intake floors with epoxy. It exaggerates the length to the floor making it closer to the flow of the air at the roof of the port. The same applies to the section of the port floor that turns into the valve. You want this area to be as straight as possible when looking into the hole from the combustion chamber and then giving it a small, but smooth transition as it turns towards the carburetor. On the roof side you want the same thing, as long a straight surface as possible from the valve seat and then a smooth radius as it turns towards the carburetor. These two areas are where the guys with time, money and flow benches find those last few elusive world record setting horsepower.

If you look at a modern sport bike engine, or modern F engine dirt bike, you’ll see that the intake ports are almost vertical. The Holy Grail of 4 stroke engine design would be to have the intake port running parallel with the valve stem, but that isn’t possible (that I know of), so they try to get as close as possible. Unfortunately these older engines aren’t built like the new ones, so we’ve stuck with less than optimal valve angles to work with.

I started porting with a 3/8" drill, then a small Dermel and a straight shank die grinder (air and electric) Finally stepped up and got a CC Porting kit now that I want to do more with 2 strokes. I prefer sandpaper over carbide in most applications, so much less likely to make a mess. The CC tool is the way to do, but it's not cheap.

I’m not an expert on this topic, nor have I ever used a flow bench, but the work I’ve done has been measured on drag strips and most of it has been positive. My biggest let down was probably my YTM 200 that I way over-ported for the carb, cam and use of the machine. Based on that I suggest you just clean everything up nicely and don’t get creative on the shape of anything if you do this to an engine you plan to use.