CG250 Head - A Closer Look.

[BlackBike]

didn't know whether that plan was nixed or not

[Megadan]

Originally Posted by BlackBike

didn't know whether that plan was nixed or not

Oh, believe me... I want nothing more than to finish this head and just throw it on. I just know that there are quite a few people curious to see the results of this kind of work, including myself if I am honest. It's easy to speculate a couple HP bump from intake and exhaust work, but I can imagine a few more people may go down the head porting route if it can prove a decent power increase, especially if it is more than simple bolt-ons provide.

[JerryHawk250]

Originally Posted by BlackBike

Meg...can you get an app that will measure g force or some kind of hp simulation so you can take road measurements? Saying this since it seems you indicated previously that the dino fell thru. I think this would be a good way to at least quantify what you have done. No, it would not be accurate/ perfect and all that but just like a inaccurate bathroom scale, as long as you use the same one you know what you have gained or lost.

BB I never thought of that. I have a G-tech that I used to use when I raced. I forgot I had one. I wish I would of thought of that and did a before and after.

[BlackBike]

Yes, well I figured you folks could use one as a cheap way to prove good or bad. Anyway, at lest you guys are jumping in and finding out what's possible.

[Megadan]

I had to order in a longer, slightly more expensive burr, to achieve what I wanted on the intake side. Not too terribly expensive. https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

It allowed me to finally get rid of the more pronounced ridge around the sides of the valve guide and blend it into the bowl more smoothly as well as open up the airflow around that pinch point.

Admittedly, there is more that could be done if you wanted to go over the top. Cutting back the valve guide would give you much better access to cut around the guide deeper and smooth and tail out the area just past the guide at the bowl to blend it in better.

This is the intake side now completed. All I need to do now is get this head milled and painted black.

[Megadan]

Comparison photo (no, I didn't get the angles matched up exactly, sorry). I figured this would show exactly where almost all of the work went into the intake port side. So many comments about "Doing it wrong can kill power." etc. that get thrown around. Maybe if I show just how you can open up a port and increase port volume without harming velocity.

The reduced ramp angle leading up to the valve guide, and the clearancing around the guides to flow smoothly into the bowl will not only increase airflow, but will also increase velocity. Making the center of the ramp more pronounced should also help keep the air focused around the sides of the guide, which is where much of the reduction in this pinch point was achieved.

I did some careful measuing of the area at a few points leading up to the bowl. At no point does the area ever increase. In fact the cross section has a much more gradual decrease all the way up to the bowl.

The lack of that pronounced ridge right at the guide also reduces turbulence right before the valve seat on the roof of the bowl, which will increase airflow at the higher ranges of valve lift, as well as charge velocity in general.

I do expect to have shifted my power curve a little bit, but the biggest shift should be with the peak power numbers. I should retain the mid range that I love so much with this engine.

[JerryHawk250]

Nice work Dan I guess I'll do some touching up and polishing on mine since the USPS held my keepers in Azusa for a couple of days. It should of arrived today or tomorrow but now they won't be here till Jan 2nd. Maybe the one that flew will mysteriously show up.

[Megadan]

Originally Posted by JerryHawk250

Nice work Dan I guess I'll do some touching up and polishing on mine since the USPS held my keepers in Azusa for a couple of days. It should of arrived today or tomorrow but now they won't be here till Jan 2nd. Maybe the one that flew will mysteriously show up.

I am waiting on a couple of things as well thanks to USPS. They are still overwhelmed from the Holidays thanks to all of the returns and exchanges.

I have been slowly picking away at smoothing up the finish on my exhaust side, but realistically it's about as good as it gets. People tend to obsess over trying to make their exhaust ports and chambers look like a mirror, but anything beyond a 320 to 400 grit finish is just for looks. Carbon will still stick to it even with a mirror finish.

[Megadan]

Yeah, so I got bored and decided to try a little harder around the valve guide on the exhaust. I used a cone dual cut burr, and spent probably 20 minutes with the tiny little tip to get rid of that ridge that was right in front of the guide, and then a tree and flame burr to round out the edges as best as I could to help flow around the sides of the guide at the roof. I also danced my flame bits along the back side of the guide on the intake and exhaust to blend the seat into the bowl, so now there is zero ridges on the transition between the seat and bowl. Minor little detail, but nothing like chasing CFMs.

About as good as it gets. I cant get to the little rough spot on the sides too well, but I can live with that. The rest of it smoothed out and shined up pretty nice. A couple of small imperfections here or there, but i am calling it good finally. Head is getting decked tomorrow.

[Megadan]

On a side note, I have been having a bugger of a time trying to figure out jetting for the VM30. I finally decided that the closest analog I can get is a CB450 with a dual VM30 setup. Similar displacement and (hopefully) power.

The standard VM30 jetting is a 250 main, 40 pilot, 2.0 Air, 6F5 Needle, and 159-P5 needle jet.

The VM30 "pre jetting" for the 450, per carb, is a 150 main, 50 pilot, 2.0 Air, 6F4 needle, and 159-P0 needle jet.

So, since I have a UFO for the slide which recommends a 50% reduction in pilot jet size, I am going to start with a 150 main, 25 pilot 6F4 needle and P0 needle jet on the way. I also have a couple of sizes up and down for the main and pilot on order, because I know better.

Hopefully between those parts, and the "standard" needle and needle jet, I will have enough to find a good combination for a VM30 with a ported head.

I also ordered a few main jets for the VM26 clone as well. Basically a 120-127.5. I figure that Silva's 1 1/2 up on the main is a good analog, so I ordered up to 2 full steps up just in case.

[JerryHawk250]

Nice job . I knew i could pressure you into it. enough of this bench racing. Lol I'm going to test out my work today at the spillway. I took a few rides around the yard last night. I sure does feel better. I'll find out today how the mid to upper rpm's perform.

[Megadan]

Originally Posted by JerryHawk250

Nice job . I knew i could pressure you into it. enough of this bench racing. Lol I'm going to test out my work today at the spillway. I took a few rides around the yard last night. I sure does feel better. I'll find out today how the mid to upper rpm's perform.

All I can do is tinker and bench race my ideas until I can install this head.
Unless I just stop caring about the dyno results, which wouldn't be too hard for me to do. My aim with that was more for the benefit of others than any of my own.

Looking forward to your results though. I will just live vicariously through you until the time comes.

[Megadan]

Last bit of work done. I milled the head down .95mm at work, and finished it with 120grit on a machined plate sander (My own creation for our mill) to correct the surface. Mainly because our bits at work suck and don't leave the best surface finish for such a large area. Final measurement .9902mm, or .3898" because ten thousandths matter. I decided to go a bit more than I intended to make up for the small increase in chamber volume from my efforts in unshrouding the valves.

Taking the head down this far definitely removes a good portion of the "ring" machined into the outer edges of the combustion chamber. I went back and smoothed those out with some 320 grit emery and then lightly touched up/polished up the chamber with 800 and 1500 grit just because I could. If you look hard you can see the three little nicks from my die grinder chuck that I smoothed out. Even when being careful...be careful!

One thing I am very pleased about is my efforts to unshroud the valves paid off. Taking the ridges back a little from the outer edge of the chamber, and then decking it, left that path completely open on both valves. Now they should have free flow in every direction.

All I need to do now is lap those new valves in, check the seating, and then paint and assemble it, or vice versa. Once I know my valves actually seal I can CC the head and calculate what my actual compression will be.

[JerryHawk250]

Looks good kind of tempted to bolt that baby up aren't you? Lol

[Megadan]

One little note I forgot to mention. When I went to mill the head I actually discovered that the factory machining of said head was not actually level, and canted to the right (as if sitting on the bike) away from the pushrods 1 or 2 degrees. I doubt this would cause any issues, but it was something interesting to note. My first light pass (removing a scant .1mm) with the mill revealed this small imperfection, as the bit lost contact with the right side of the deck surface in the last little bit of travel.

I am sure it's not the only flaw to be found for sure, as these things are cranked out of casting molds that have been used countless times and machined with machines that probably haven't been calibrated or adjusted very much in their lives.