Forum whore
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A rotary valve cover that has been opened out at an angle of 6deg I plan on boring the carb from just behind the 24mm venture at an angle of 6deg and carry it on all the way to the rotary valve port in one continuous taper.
Pic-1 shows the plan.
Pic-2 the standard cover
Pic-3 the modified cover
Pic-4 inside of both covers
Pic-5 outside of both covers
Absolutely no idea if this will work but it sure opens the inlet tract up.
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Highway Biker
What equivalent diameter have you went for at the port?
Is your intake length being lengthened and if so by how much?
Forum whore
'D' bored 28mm flatside carbs from the older RG250 and RG400's work just fine. I got more than 22hp out of my KE/RG100 with a nice mid range too
Forum whore
C'mon Green we all know TF100 cranka are bullit proof especially at Taupo...Originally Posted by speedpro
Forum whore
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“equivalent diameter” = 32mm…..“intake length” = std GP125 at 90mm from bell mouth entry to disk face.
The port window is 50deg wide like an RG500 the std GP is 40deg wide. Inlet opens 145deg BTDC and closes 85deg ATDC. I figure if it was 32mm straight through then I could close the inlet timing of a std GP125 which is 145/55 and still have good port/time/area.
The inlet tract starts with a bell mouth. The first part of the inlet tract converges from 26 to 24mm at the needle jet and is 15mm long. The second part diverges from 24 to 32mm from the needle jet to the rotary valve face and is 75mm long.
The overall length of the inlet tract to the rotary valve is 90mm and past the rotary valve the inlet tract diverges bell mouth like into the crankcase.
I have a new OKO 24mm flat slide carb. It is bored 24mm parallel right through. There is plenty of metal to turn a 6 deg taper starting from just behind the 24mm choke at the needle jet diverging out to 28mm at the flange.
Then the rubber manifold and the rest of the inlet tract can then be ported, diverging out from the 28mm manifold rubber to 32mm at the rotary valve face.
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Forum whore
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Flow tested some carbs today. Thomas made up a manometer, which proved to be very effective at measuring flow through the carburetors. We found that the shape of the standard GP125 carbs concave bell mouth is not the restriction I thought it was.
Thomas made the manometer from some clear tube an old tape measure and some din rail and filled it with anti freeze and it worked a treat. He's a clever fellow is Thomas.
Tests show that the modified 24mm carb did not flow much if any more than the standard carb. The various obstructions in the carb like the size of pilot jet nozzle and idle screw made more difference than the modified bell mouth did.
The 24mm OKO flat slide flowed as much as an ordinary 26mm round slide carb.
Pic-1 the setup, first time it was switched on the vacuum cleaner sucked all the anti-freeze out of the manometer. Thomas then fitted a restrictive washer with a 22mm hole into the white tube.
Pic-2 shows how the restriction of the carb allows the vacuum to suck one side up the tube. The more restrictive the carb the greater the imbalance is and you measure the distance between the high and low side.
Pic-3 Of the three carbs being tested. The OKO flat slide was the best by far.
Pic-4 Measuring the difference between the columns. The greater the distance the more restrictive the carb.
Pic-5 Obstructions in the carbs that made a real difference to how they flowed.
Pic-6 Thomas also turned up a standard 24mm orifice so future measurements can be compared to a known standard.
The needle, neddle jet and the sharp edges around where the slide goes are really disruptive to air flow. The old English TT GP racing carb had the needle out of the throat and in a chamber on the side of the carb so that there was no obstructions in the air stream.
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Highway Biker
If you used a settling box between the carb and vacuum source it would allow you to test the whole inlet system as one. The rotary valve cover could be bolted directly to the box with the various carbs and rubbers also attached. You would then test the inlet systems at the same selected box vacuum. Essentialy the box is your crankcase.
Downstream of the box you can use a oriface plate machined to the british standard for oriface plates. By measuring the pressure each side of oriface plate you can calculate the mass flow. This will allow you to put some numbers on the various inlets i.e x flows 10 % better than y etc. You can even test for varying disc openings.
Forum whore
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Thanks for the ideas Sonic.
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Forum whore
More flow does not always mean more useful HP.
Forum whore
Yes I know bucketracer.I could get more peak hp but less total area under the torque curve due to poor inlet tract shape and gas velocity.
I have been playing some more with Thomas’s flow rig. And all the while there was a tape playing in my head reminding me that big flow test numbers do not necessarily mean improved hp, but I can't resist.
Inside carbs where the slide goes there is some sharp edges that I think must be very disruptive to airflow.
On my OKO carb I turned a 6 deg taper from the rear of the carb to just before the needle jet and radiused the sharp edges around the slide, now when I push my finger through the carb the edges feel smooth and less disruptive.
The carb is still 24mm at the choke but flows significantly better.
But because I changed two things at once I cant say for sure which contributes the most. I will experiment with just radiusing the sharp edges in another carb later to see.
Pic-1 I figure that if just these sharp edges were shaped then the net flow through the carb should be improved without a down side on the torque curve, its the diverging taper bit I am not sure about as most anything you read (4-stroke) has the inlet tract converging there is very little about 2-stroke inlet tracts and even less about rotary valve inlets.
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L-Plate Rider
Good read, just registered and up to page 7
Before I get to page 89 and forget, back in 1976 or 77 we went to a 125 national mx and Marty Smiths bike had a copper cylinder head. It looked like it was all machined not cast. It was vary humid 100 deg F day and i'm sure it helped, psyched them out too.
Can you guys only use parts from bikes legal for the class? I'm looking at an old sachs 125 6b radial head thats got loads of fins, could easily be welded up and cut for another motor. Some were sold as street legal bikes over here.
Luther
now where's some dirt!
This is for a class called buckets. Max 2 stroke capacity is 100cc water cooled or 125cc air cooled (max 24mm carb too), 4 stroke is 150cc max. After that pretty much the only rules are no competition engine parts allowed (so no MX or GP bike engine parts). There are a few exceptions but those are the basic rules. So most of the bikes are commuter type bikes highly modified. Chassis is totally free so you can do whatever you want there. It's a great starting class, you can have a bike for around $300 and go racing!
Forum whore
Hi Luther, glad you like the thread. Near the top off the page is a tool bar "Thread Tools" which opens to "View All Thread Images". A quick way to see all the pictures.
Thanks for the tip about Marty Smiths copper head, I found picks of the internals of one of Martys water cooled engines here http://www.mxworksbike.com/Honda%20works%20parts.htm I would love a pic of the copper head if you ever come across one.
Our engines/gearboxes are restricted to road legal stuff except ignition/carb/piston but you can hand make anything you like for the engine/gearbox. And anything goes for the rolling chasis. There are a few other class restrictions, 125 2-stroke must be air cooled and 24mm carb. And a class formula 100cc 4-strokes run with 50cc 2-strokes. 150cc 4-stroke run with 125cc 2-stroke.
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Forum whore
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You have just got to go and look at these goodies. a works 2-stroke magnesium cylinder:- http://www.mxworksbike.com/Yamaha%20works%20parts.htm
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No8 Wire Extraordinaire
yeah if the carb is just firing unatomised fuel at the engine there will not be efficient running. but i'd assume it would be a given with such high velocity that better flow would only be of benefit. good idea with test jig.
the shroud many 2 strokes have over the needle area on larger carbs help atomisation, but the velocity in smaller carb prob doesn't worry it as much.
Don't you look at my accountant.
He's the only one I've got.
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