All Things CR250

[Ian Harrison]

Hi Steve

Thanks for your input and I can see the logic behind routing the airstream directly over the idle jet drillings. However, although I appreciate that this will increase the "draw" on available fuel, I believe, that it is actually it's contribution to "engine side vacuum" that will achieve better operational conditions at idle.

On the Super SBN44 what you call the fuel discharge hole is in fact a series of 1 large (low speed outlet) and 4 small drillings (by-pass holes). Of these 4 are line-astern at 6 O'clock on the "atmosphere" side of the closed butterfly. The other larger one is closer to 5 O'clock and is on the engine side of the closed butterfly.

As I understand it, they are arranged in that way so as the butterfly opens and exposes the holes in turn across the zero to quarter throttle range, more fuel is added to the increased flow of air, in a proggressive manner to give a smooth transition. I'm not sure how your notch would affect this progression, but perhaps you do? I'm in a steep learning curve with these carbs.

The "trick" (I believe) is to reduce the pop-off so that the engine side vacuum will allow atmospheric pressure to move the diaphragm enough to open the needle and seat via the engine side hole. By making your notch hopefully you can keep a higher engine side depression, while increasing the fuel draw from the low speed outlet and passing enough air to maintain an idle. Your mod will no doubt help, but it is still "beating the pop-off pressure" that is the holy grail and the simplest way (I now believe) is to reduce the pop-off pressure.

Out of interest, I have tried with the return line blocked and the needle and seat still remains closed, unless the carb is almost inverted and run above 4,000rpm. Fuel pressue is a pretty constant 5-6psi at around 2,000 rpm running on sprayed fuel, but the needle and seat are of course closed, so once I stop spraying the motor stops running.

Once I have the thing running, I may well try your "notch" idea, but as I said, my concern would be affecting the smooth progression as the butterfly is opened.

Just awaiting those damn springs!!

Best Regards

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012

[Sam Zavaglia]

[steve kemp]

Hi Ian, sorry to hear you are having so much trouble with the bn44 carb. Maybe the 44mm carb is just to big, like putting a 1050 cfm dominator carb on a 350 chevy. I did notice the 41mm Tillotson did not want to idle as well as the 38mm Tillotson when I made that change. The 38mm will idle, has awesome throttle response, pulls well through the low and mid range but dropped of at 10,500 rpm's. The 41mm is a little soft on the bottom but pulls all the way through the rpm range, but it won't idle, but who cares. Let me know if I can help with your project in any way.
Good luck Steve

[steve kemp]

Hey Mark, I have carbs available. Let me know if you want to try one.

[Ian Harrison]

[Ian Harrison]

Hi Guys

The pop-off springs arrived today. Unfortunately they were the wrong ones!!

The 65 gram springs I ordered are both stronger and substantially longer than the 80 gram spring that was supplied with the carb. I guess the supplier found he had no stock and just sent me what he had.

So I chopped a couple of coils off my 80 gram spring and stretched it back to the original length.

This has given me a nice consistent pop-off of 9psi, and with a 6psi pressure applied to the fuel inlet with my leakdown tester and the return blocked, I can now quite easily open the fuel cut-off valve by sucking on the engine side of the throttle body with the butterfly almost closed (i.e. simulating idle position)!!

Unfortunately, the dyno is occupied with more pressing work for the next week, but I should at least be able to fit the carb to a kart tomorrow and see if it will start/idle. If that looks promising, I may make the effort to put it on the dyno between other jobs over the weekend, just for a quick "look-see".

Best Regards

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012

[Sam Zavaglia]

[Ian Harrison]

Hi Sam

Lots of things on the go at the moment and this is the CR250 thread!!

Anyway, back on topic . . . . . and I did get chance to give the Jet Ski carb a run today.

So it now runs and idles (sort of) with it's 8/9 psi pop-off. So at least it's a step in the right direction.

What I have found is that the slow running screw has to be well out (rich) to get a reliable (if somewhat lumpy) idle. I am showing an idle EGT of around 85 degrees C, whereas I would normally be showing around 110 deg C with the TMXX. If you then gradually turn the screw inwards the mixture is obviously leaned off and the motor begins to race. You then turn the throttle stop screw down to reduce the idle speed and I believe that what then happens is that the vacuum on the fuel chamber reduces (due to the increased restriction of the idle passage) to the point that the fuel cut-off dosen't open, the fuel chamber empties and the motor stops.

The trouble with running an over rich idle is that coming down off a dyno run the idle is initially very fast (perhaps 4,000 rpm), and then gradually decreases as the rich mixture builds up and stifles the motor. Also when you have idled for a while it takes a full run up through the gears at full throttle and under load until the motor fully clears. In fact I found myself having to come back off the throttle in 1st and 2nd as the motor came onto the pipe to aid the clearing. Once it's clear everything is fine and you can link succesive "through the gear runs" (like on a racetrack), without a hint of a missfire.

The obvious answer is to reduce the pop-off still further, but of course once it hits around 6 psi there is the danger that the cut-off will cease to become effective against the pump pressure.

The addition of the bell mouth and airbox may well improve things a little without any other changes, so this will be the next "port-of-call". I am also considering notching the butterfly in-line with Steve O'Hara's suggestions.

If neccessary, I will then tweak the pop-off to 6/7psi and try again.

I did record a dyno run and you can see it displayed against a run using the Mikuni 41.5 TMXX from an earlier run with this engine.


THE SBN curves are the red ones and the TMXX are blue - torque and power are the lower and upper curves on the top graph and and EGT's are on the bottom graph.

Strange thing is that although the curves are very similar in shape, that the SBN curve, (which is down around 4hp at peak on the TMXX curve), has a higher EGT, which is the opposite to what I would expect . I can only conclude that the difference in power is due to the SBN not passing as much mixture. Both runs are taken at WOT in top gear.

To consider using the SBN, not only must the power graphs be equal to the TMXX, but we must establish a reliable and consistent idle and a smooth progression from idle to WOT. However, we are without doubt a lot closer to that than we were 48 hours ago

If I was to draw conclusions at this stage, it would be that perhaps an SBN46 will be required to equal the TMXX performance, but until I have truly optimised the SBN44 in all respects, I'm not even thinking along those lines.

On the positive side, there appears to be none of the "dribbling fuel" that others have reported and I am very happy with the cleanliness of the installation aginst the Keihin and TMXX which spew fuel out of the float bowl overflows.

Anyway, more later, but perhaps a week away, once "other things" are completed and shipped out.

Best Regards

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012

[Jeff Metter]

Ian Harrison.... I may need one of those double top secret inline twins for my ride in South Africa running Avgas of course could you please PM me some details ....
_________________
Zanardi/ TM K-9 C 125cc shifter
Anderson /GasGas
PVP / GasGas
" We never really grow up " !!
" We only learn how to act in public" !!

[Ian Harrison]

Hi Jeff

You have mail

Best Regards

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012

[Sam Zavaglia]

I was on Google street view at Viper Racing address and the front door was open. Managed to zoom in and get this out of the picture.
The jet ski with kart wheels, no sign of the wet suit.......



Zoomed in a little further and found this for you Jeff.
It is the inline twin feed with AVGAS bound for Australia......


_________________
www.samzavaglia.com

[Ian Harrison]

Sam

Did you vist the Mighty Bite, while you were "snooping"

Haven't you got more pressing things to do with your valuable time??

F**ing Facebook

I feel a cancellation of staff bonus coming on. . . . . . Now where did I put that confidentiality agreement.

Surprised you didn't comment on the nice new design of the Anderson front seatpost though or the lovely stack of washers between the change lever and the gear rod

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012

[Jeff DeMello]

[ted dale]

While i was on google street a saw this. I think its Ian's new shop helper. We could not get Ian in a wet suit so he made his mech wear one, I just hope he brings his helper to a track some day,
http://www.youtube.com/watch?v=RdNEEzlR4pk
_________________
Anderson gas gas

[Ian Harrison]

Much too tidy for my workshop and way too big (the workshop I mean!!)!!

Ian
_________________
Ian Harrison
Viper Racing UK
www.viper-racinguk.co.uk
sales@viper-racinguk.co.uk
+44 7984 225 564
+44 161 343 2009
Championship winning Superkart race team 1997-2012