ESE's works engine tuner

**Chambers** · 19th December 2009, 16:15

Hopefully no dickhead will be in my grid position this year and I manage to finish the race this time, its not fun sitting on the infield watching.

**TZ350** · 19th December 2009, 17:43

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Another interesting clue was the carb with the power jet. As the motor started getting onto the pipe you could see the petrol drawn up the clear plastic tube to the power jet. Then as the motor was still pulling hard and sounding ok, you could see the petrol in the power jet tube pulsing at maybe 3-5hz quite slow but marked. Then as the motor topped out the petrol retreated from the power jet, so at over rev the vacuum at the bell mouth must have fallen right away.

So over 9,000 odd rpm I think that induction is being blocked by some sort of wave action, or the clash of two waves that is stopping the motor from revving on and making real power. With the pipes, the motor should have revved higher but something is getting out of step, I suspect the power peak being seen is from the combination of a second order and a primary resonant peak and the motor and pipe have not fallen into step together properly on matching primary resonant peaks of their own.

But what to change?

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**bucketracer** · 19th December 2009, 19:57

Originally Posted by Chambers

Hopefully no dickhead will be in my grid position this year

There is always a slow as, dickhead that pushes to the front and nabs someones allocated grid position. When the flag drops I expect most front rower's can always start faster than a dickhead and it must be tempting to give them a shoulder barge on the way past......

**bucketracer** · 19th December 2009, 22:49

Two Stroke Tuners Hand-Book http://www.datafan.com/TunersHandboo...efiltered.html

**sonic_v** · 20th December 2009, 00:07

Originally Posted by TZ350

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Another interesting clue was the carb with the power jet. As the motor started getting onto the pipe you could see the petrol drawn up the clear plastic tube to the power jet. Then as the motor was still pulling hard and sounding ok, you could see the petrol in the power jet tube pulsing at maybe 3-5hz quite slow but marked. Then as the motor topped out the petrol retreated from the power jet, so at over rev the vacuum at the bell mouth must have fallen right away.

So over 9,000 odd rpm I think that induction is being blocked by some sort of wave action, or the clash of two waves that is stopping the motor from revving on and making real power. With the pipes, the motor should have revved higher but something is getting out of step, I suspect the power peak being seen is from the combination of a second order and a primary resonant peak and the motor and pipe have not fallen into step together properly on matching primary resonant peaks of their own.

But what to change?

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I have seen a similar affect on reed valved engines. On a 80cc motocross engine tuned for roadracing that produces 25bhp at 13000 revs/min using a 30mm carb, fitting of a 32mm carb resulted in a violent drop in power at 12,000 revs/min. Violent enough to almost launch me over the handlebars. Everything pointed to some sort of resonance effect in the carb floatbowl. I tried different things to counteract it such as varying the length of plastic tube going to the float bowl vents, plus drilling of the vent apertures but to no avail.

Any chance you could show the power curve even if against road speed? I am particularly interested to see if you actually have a double hump to your curve which would indicate some sort of recovery.

Certainly at this moment in time, everything points to your inlet as you have tried so many exhausts and have seen this problem with them all. Just maybe earlier closing of disc valve might help you.

**sonic_v** · 20th December 2009, 00:21

Originally Posted by TZ350

It beats me how my new engine which has more port/time/area than the old one, and with the same RM pipe as my previous 11,000rpm engine now makes slightly more max power but at only 9,000rpm.

I have tried pipes optimized for 10,11 and 12,000rpm on it but it always made much the same 21 rwhp at about 9,000 rpm, with the high power/rpm RS pipe being the best.

Although the new motor is making a little more power than before, its as if, just as its getting well into its stride something chokes its potential, the suspects are the transfers or the exhaust port.

The plan is to reduce the inlet closing point, make some spacers for the RS pipe and re-measure the port/time/areas, make some changes and hopefully back to the dyno next week.

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It is neither your exhaust port or transfers. That exhaust port is good for 26 bhp. Going for even more time area is likely to send you backwards. This is an inlet problem. Any chance you are running your float bowl dry? Could you modify your previous carb to work as a diffuser?

**TZ350** · 20th December 2009, 06:58

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Sonic_V thanks for your ideas, saved me from screwing up the exhaustport.

Unfortunately I don’t have a graph, I had some saved to a memory stick, but can’t open them on my home computer, computers!!!!!!

I intend changing the inlet closing point to 75, first thing and will take a 65 with me on my next trip to the dyno.

I tried two distinct styles of diffuser carb, I could make something with a clear tube so I can keep an eye on whats happening in the float bowl and I will also try the original carb on my next dyno trip.

Taupo is a week away, so I am quickly running out of time, just have to be philosophical I guess and enjoy the day as is.

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**TZ350** · 20th December 2009, 07:18

Originally Posted by bucketracer

Two Stroke Tuners Hand-Book http://www.datafan.com/TunersHandboo...efiltered.html

Cheeky bugger

minime just for that, you can go out and mowe the lawns.

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**TZ350** · 20th December 2009, 09:31

With my so called diffuser carburetors the idea was to move the 24mm high velocity area away from the disruptive metering area around the slide.

As I understand it, a carburetors size is measured at its smallest point, traditionally the choke is at the metering point as this gives the strongest signal to the jets but it doesn’t have to be. Carbs with throttle plates like on older cars have the air flow control and metering areas in different places, motorcycles traditionally use a system where air flow control and metering are done together in the same place.

The way I see it there is no rule about what a carb should look like. The only requirement of a carburetor is that it controls the air flow and meters fuel and in the case of F4 125 2-strokes that it has a choke no bigger than 24mm. If someone else sees the rules differently now's a good time to let me know so I don't make a complete ass of myself.

Pic-01 has the 24mm choke well clear of the disruptive area of the slide and flows 50% more than the original 24mm carb and is the one I have been using. It was made by permanently inserting a 24mm choke into a 28mm Keihin flatslide.

Pic-02 is a 24mm OKO (Keihin copy) flatslide carb that has been taper bored so that only a small area in the bell mouth is 24mm and the disruptive metering area is larger, this carb flowed about 30% more than the original 24 on our test rig.

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**sonic_v** · 20th December 2009, 09:57

Originally Posted by TZ350

Pic-02 is a 24mm OKO (Keihin copy) flatslide carb that has been taper bored so that only a small area in the bell mouth is 24mm and the disruptive metering area is larger, this carb flowed about 30% more than the original 24 on our test rig.

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Was any dyno testing done with this carb and if so what was the result?

**TZ350** · 20th December 2009, 10:34

Originally Posted by sonic_v

Was any dyno testing done with this carb and if so what was the result?

No, the bulk of testing was done with carb-01 the 02 carb had the power jet, at first the jetting was not good and when we had mostly dialed it in, we could see it mimicked the shape of the first carb's power curve, maybe slightly lower but with all the same problems.

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**TZ350** · 20th December 2009, 14:56

Here are pictures of the actual carbs themselves.

Pic-01 and 02 the 28mm Keihin with a permanently fitted 24mm choke.

Pic-03 and 04 The taper bored power jetted OKO (Keihin Copy) 24mm at the bell mouth 28mm at the engine side.

I think this inventiveness is in the spirit of buckets but if anyone thinks its not within the rules and can explain why please tell me now before I hit the track at Taupo.

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**TZ350** · 20th December 2009, 15:42

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Ok...just re-checked everything and the numbers are:-

Ignition timing 16 deg BTDC
Ex opens 86 deg ATDC 73% bore width
Transfers open 116.5 deg ATDC
Inlet opens 145 deg BTDC and closes 85 deg ATDC
Inlet port at the valve is 30mm dia equivalent.

Nothing totaly out of kilter there, its all as was planed but its now thought that the inlet timing is wrong. I didn't realy know what size equivelent to use for the inlet port, 24mm carb choke, 28 or even 30 at the port window when calculating the port/time/area.

I will change the rotary valve closing point from wild to something mild. A quick re-calc of the port/time/area required for 9000 rpm and 28mm dia equ and settle on an inlet timing of 145/60 and thats handy as its also a disk I allready have.

As it had the most over-run I am going to refit the RS pipe and try a 50mm spacer in the header. When I get back to the dyno I will take a series of spacers from 15mm to 100mm to try.

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