CG250 Head - A Closer Look.
 

Yes, I have nothing else to do with my money, so I buy bike parts. To be fair, this was 50 bucks to my door, so not a huge investment lol.

I bought this head as a port and polish project for my Hawk (which arrives Thursday thanks to the Holiday and weekend combined with my work schedule).

Before I get into a lengthy report, let's just get on with the pictures.

Intake port and combustion chamber side first.

Exhaust port.

So right away, these images alone give a great idea as to why we only get a massive 14hp from a 230cc engine.

The intake side of things really isn't too bad, but there are a few things I observed right away with just my eyes. The intake port is smaller than 30mm. Without going and digging out my calipers to measure, I would estimate it to be around 27-28mm. The casting around the valve guide is rather obtrusive into the port, but surprisingly, there is very little in the way of flow restriction from the short side radius. There is a little bit there, but way less than many other heads I have worked with. On the right side of the port is also a slight intrusion into the port that causes a small narrowing of the ports cross section about halfway down the port. It's nothing big, but another flow restriction.

The exhaust side of things is by far the worst. The pictures really don't do it justice. There has to be at least a 30% reduction in cross sectional area at the valve guides greatest protrusion into the port as compared to the exhaust valve area and port size. The exhaust is literally leaving the valve to slam into a huge bottleneck halfway out of the head.

I would honestly not be surprised that proper porting to simply even out and open up the restrictions in the ports, as well as a port match with a tapered reduction through the port could easily pick up a 20-25% increase in power without altering the lovely powerband this engine possesses. I may not even bother decking the head at all.

Oddly, I have never been more excited to do head porting in my life. Now I just need to find the time lol.

There's a lot of room for reshaping and improvement in those ports. It may flow enough to be able to step up to a 34mm or 36mm carb.

That was the plan. I want to use a PWK/KOSO style 34mm flat slide carb with this head. The trick will be reshaping the ports without killing velocity off, which is where the tapering of the port "bore" from the opening down to the valve is crucial. I want to improve flow without killing charge velocity so it maintains a good swirl and atomization at low engine speeds.

One thing this head does have going for it is the limited amount of short side radius bending and valve shrouding. The angle of the valves is fairly wide, so it does have that going for it.

Do you plan to polish the exhaust side?

Mega... Many have threatened to go thru with this but never reported back here. I Guess you are one of the few of us that are not so tight fisted. This looks interesting.

I plan to port both intske and exhaust and polish the exhaust side, yes. I also plan to smooth the edges of the quench ring and polish the combustion chamber,.

I have been accused of having more dollars than sense... but yes, I plan to follow through with this. Having 2 bikes affords me the luxury of being able to do this kind of work while still being able to ride. Plus, I like to tinker.

Well so far, your toy signature seems pretty sensible.

It only appears that way. :)

I'll be following with great interest. :tup: I've often considered polishing the surface of the piston as well. I've noticed that stock pistons are typically quite a rough casting on the combustion surface, whereas the rest of the piston is nicely machined. I've also noticed that aftermarket pistons are generally machined on the combustion surface as well as the skirt, etc. This leads me to wonder if there might be two benefits to polishing the combustion surface; one is the theoretical efficiency increase in moving fuel/air and exhaust, and the other is resistance to carbon build-up. What do you think?

From my personal experience, polishing does help resist large carbon deposits forming, but you will still get a thin layer no matter what you do, because even the best polishing job possible still has microscopic grooves for carbon to cling to and build up on. I did consider pulling the piston and doing that kind of work, but that may be for a later date. I will do what I can with the head, and put on a new head gasket.

That said, it would be kind of fun to go all out on one of these motors, but even I have my limits on how much I want to spend. What I would personally do is send the piston and head off to Swaintech, have their ceramic coating applied to the piston top, combustion chamber, valves, and exhaust port, and their low friction skirt coating done as well. Since it's a one cylinder engine, the cost wouldn't be too horrible. This does amazing things for keeping aircooled motors run a little bit cooler, since more heat is retained in the chamber during the combustion process. Current prices for that treatment would be $52.50 for the piston, and $54.50 to do everything to the head. So...$107 bucks.

I did consider seeing if I can find some undercut stainless steel valves that would work with this motor and have a shop install and hone some bronze guides for it. Again, not entirely necessary, but an idea. I would definitely consider this more if I were to bump the compression up a point to around 10:1. I really don't want to do that personally, at least not without some way to control the ignition timing.

With a full P-N-P, valve work, larger carb, better exhaust, and higher comp, I think the cam will start to be the limiting factor. I'm not sure if it would be possible to run a different ratio rocker arm on these engines. You could always try degreeing the OE cam.

It's tough with only one cam lobe; neither event is optimized. That said, you could get a cam grinder to shave a little off of the base circle for a little more lift. :D