[ace.cafe]

I ran the bike last year with the VM34 fitted and just made a different bellmouth and tuned the motor. Im pretty sure I was on a 210 main jet but will check. IVe just made a UFO insert for the base slide and fitted a very small pilot jet. It really helps the motor pick up really quickly out of corners. Its a tip my brother gave me as he races a Gold Star and uses the same carb type. The UFO basically makes the VM more like a flat slide as it smooths the air flow at the bottom of the slide, increases vacuum (hence the smller pilot jet) and gives a better intake shape at full throttle.
I plan to dyno the bike now the mods are done, and lockdown is easing so will share any interesitng results!

I believe you will find that a leaner main jet, like maybe a 185, will give more power and respond better. From my experiences with Mikunis and the 535, the 185 seems to be the ticket.

Maybe take a couple of leaner mains like 185, 190, 195 to the dyno with you to see if they help. I think they will.

[gizzo]

So, I have the head off my CGT while I wait for an order from H's. I'll be fitting a new 1 piece exhaust valve. Not that my original was ever a problem but might as well, while the bike's having some down time.

Anyway, with the valves out I see a few areas in the ports that might be improved on. The ports on my head don't have the core shift displayed in the head of another forum member recently. But, the exhaust port has a ring of material, like a donut, raised into the exhaust port downstream of the valve seat, 1, 1.5mm high. That can't be optimal. I'm dremelling that out to make the port smooth. The inlet, there's a sharp transition between the cast port after the valve and where the port has been bored out for the valve seat. So I'll smooth that transition as well.

The big one is shown in the photo here: The inlet port and insulator are badly mismatched. The port is a mm smaller than the insulator, and that mm is offset to one side. The picture will show you what I mean. Before I get stuck in there and remove a load of material to match the port to the insulator, is there any good reason to not do this? THB I've never had any experience porting. I did a 6 cyl head once that worked out well but i was only copying someone else's work.

What do you think, Tom, is it worth the trouble? I can't imagine it doing any harm but thought it's better asking before I ruin it. Thanks

[ace.cafe]

Simon,
It depends on the diameter of the port entry, vs the diameter of the injector body leading to the port.

What you don't want is to have smaller going into larger and then smaller again. It is bad for induction like tbat.
So, I would measure the throttle body exit, which should be 34mm.
Then measure the injector body, which should also be 34mm.
And the port entry should be 34mm.

If that all works out, then I would move the insulator so that it lines up correctly with the port. You can do that by elongating the stud holes in the proper direction to get the insulator moved over to where it matches the port.
If the injector body is also shifted like the insulator, then you will need to do the same to it.

The idea is to get it all aligned and matched up. Then if you need to do a little minor grinding to get a final match-up of a few tenths of a mm, that is fine. Strive for a straight and consistent port diameter from throttle body all the way into the port entry of the head. Making any part of the passage larger than the 34mm throttle body diameter is counterproductive.

Also, make sure that the insulator and gasket notches give clear alignment for the injector to squirt unimpeded into the port. I have seen them misaligned where the injector was partially occluded to a point where the fuel squirt would be deflected by the gasket or insulator.

If you wish, you can polish the passage walls as far in as the insulator. But no polishing in the port inside the head.

[gizzo]

Thanks Tom. I had a bit more of a measure up:

The throttle body exit is close enough to 35mm (actually 34.8mm)
Injector body exit = 35mm
Insulator = 35mm
Inlet port entry = 33mm

So the entry to the cylinder head is 2mm smaller than the rest of the induction tract. In addition, it looks like the bolt holes for the injector body flange are drilled offset to one side. So the port matches on one side of the port but the mismatch gets worse the further around to the other bolt hole.

I can't tell whether the inlet port opens up again after the port opening. All I have is the caliper, no inside measuring tools. But I guess what I could do is stretch the holes to get the injector body central to the port like you described and then grind out the hard ledge presented by the undersized port opening to get a smoother transition into the port. The port is still going to be smaller than the inlet tract but what can you do? Other than spending $$ having a proper port job. I don't think that's on the cards.

See if you can understand what I'm getting at with the photo. Looking down the injector body into the inlet port. You can see the mismatch.

Thanks again mate.

[jez]

Thanks but with slide carbs the main jet mainly deals with the last quarter of throttle opening. This is understood on SR500 forums rather better than here. To replicate the VM34  the sizes of idle jet, the needle jet, jet needle and slide cutaway are needed in addition to the main jet. There are literally hundreds of needles for Mikunis which is why some tuners prefer Amals. If you've just bought a carb and are only fiddling with the main jet don't worry, unless the carb came with the jetting specs written down, in which case by all means enlighten us.   

[ace.cafe]

Simon,
Your plan is good.

I would suggest stretching the bolt holes on the injector body laterally only, to centralize.
Then, grind the roof of the port entry to match. It is best to avoid grinding the port floor because a high port floor flows better due to more gradual turning down to the valve. Port roof and sides are fair game to match with the grinder.
You can grind a bit deeper into the port to blend the transition to be less abrupt inside the head. Maybe about three-eighths to a half-inch in.

The factory port will narrow a bit near the valve guide, and that is fine because you want air speed to pick up there to atomize fuel on the way into the cylinder. The port will enlarge in the bowl behind the valve to help the transition around the valve. That is normal and proper.

[gizzo]

Cheers Tom, sounds like a plan. I don't think I'll go to town on the port roof or sides,  I'll see how it goes. I'm thinking it'll be good to at least blend that sharp edge into a nice transition to start with and see how it goes from there. And centre the inlet to the port, of course.  Thanks for your advice.

[ace.cafe]

Just FYI, Otto bored his throttle body out to 36mm and made a new throttle plate to fit, and enlarges as I described, but out to 36mm.
He picked up 2 hp @6500rpm at the rear wheel on the dyno, after a tune.
Our hottest result yet, 37.79 rwhp.

[gizzo]

I remember. That would be a beast compared with the stock one.

[gizzo]

So, I fooled around with that head today. Only took about 30 seconds with a rat tail file to get the injector body and insulator centered on the port. That left about 1mm all around to widen and blend in. That took a bit longer. I have no porting tools, but made do with the dremel. The little sanding drums do a pretty good job. They can't reach right in there but were fine for the job I was doing. I opened up and blended that inlet port opening, took the sharp edges off the short radius where it meets the bowl and got rid of the donut in the exhaust port, and some casting flashing from both ports. I didn't polish anything, or try to open the ports any more than just the clean up. I don't imagine it's going to make any difference I can feel, but you never know. At least I'll know that my port matches my inlet. Once I saw that mismatch, I couldn't unsee it and I's always wonder, what if? Hopefully my package from H will show up tomorrow and I can throw it back together.

[ace.cafe]

Sounds good.
Just be sure that the notch in the insulator gives a clear squirt path for the injector nozzle.

If you could take that head to a race shop for a 5 angle valve job, that would be a really good idea. Between what you have done, and a good valve job, it would probably be good for about 3hp.
75/65/55/45/35
About 1.25mm seat width on the intake 45 for flow, 1.75mm seat width on exhaust seat for cooling.
Throat 89% of valve diameter.

[gizzo]

Perhaps I'll call the race shop tomorrow, but I think it'll have to wait. Thanks for taking the time to list those specs though. I'll keep them handy. 3 extra HP's would be fun to have.

The notch in my insulator does line up fine, but I'm wondering how it works? The notch doesn't go all the way through the insulator but has a dead end and even if it wasn't, the fuel spray would hit the machined face of the head, not go down the port. Is the spray supposed to go into that notch and get deflected down into the port by the wall at the end of the notch?

It looks like a bodge they came up with at the last minute.

[gizzo]

Hooray! A care package from H!

[gizzo]

So, I have my new exhaust valve in, bike is all back together again. Today I picked up the stainless header from the local exhaust shop, who welded an O2 sensor bung in for $30. Pretty happy with that. The bike exhaust guy here wanted $70-80 and he's a grumpy old shit anyway. Good fabricator though... The guys at the exhaust shop usually have something cool on the hoist: a big block Torana drag car, some American muscle, a hot rod. Today it was a Suzuki Mighty Boy (google it) with some kind of big Jap 4 cylinder bike engine engine swap. That'll be exciting.

[gizzo]

Stainless header with O2 bung and Ace spec reducer.