Snr Rotax Setup?

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Permalink Reply by Ross Tait on January 3, 2012 at 13:49

See what drive sprocket you have on and then that will give you a ratio.Also with what you have on what max revs are you getting. On the engine make sure all is clean including carb,jetting and fuel and air filters to start with and make sure the battery is good and charges correctly or the engine can misfire !

A jetting program is great and a weather station so you know what the atmosphere is doing for jetting changes.Also is the engine new or when was the last piston etc done as that will give a idea on condition and hours it has done.

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Permalink Reply by Allan Clough on January 3, 2012 at 21:46

Thanks Ross, Im still real confused....this is what was on the kart when I got it: 11T front, 81T rear, K98, Clip 2, 170 jet. Pulled 14K RPM down the start / fin straight......THEN was told to run 11T / 83T Rear, K54, Clip 1 & 162 jet. the latter seems to get out of corners faster but similar lap times. it all appears to be black magic to me. the engines just done 10hrs since a rebuild. As for the weather, the jetting programs keeps telling me I should be around the 168-170 jet.

 

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Permalink Reply by Ross Tait on January 3, 2012 at 22:14

Ok, 14k is where you want to be and putting 2 extra teeth on may get you out of the corner easier but be flat in the mid/top. Did the revs increase ? From what you say this may not be the case,you can do splits of sections of the track (like straight,corner sections to confirm changes unless you have a datalogger to do this for you.  K54 is better than 98 in my view and jetting in that range but the tuning needs to be set to your carb as they are not all the same and also setup of them,not sure what brand of tuning you have. I would play with the jetting and K54 to suit your setup and handling can effect it all very easily so be carefull on that. Gearing comes down to each track and where you want to be fast/slow.

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Permalink Reply by Allan Clough on January 4, 2012 at 20:56

Thanks Ross, From memory my revs did'nt change a great deal.No I have'nt got a logger, im just going off my mychron. I have tried the Jet-Plus program.....only the free trail version though,along with a weather station & a basic Jetting chart. The other thing is tyre pressures, some say high, some say low.........I was always going out cold around 14psi in my KT. I think my little KT is a heap easier to tune,one gear......Flat out! Im just getting too old "n" lazy to push it these days.

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Permalink Reply by Colin Lewis on January 4, 2012 at 21:37

Hi Allan

for what it is worth, there are so many variables in the carb set ups that weather stations and jetting programs will only get you somewhere close (within 3 jet sizes is my experience).  Even the specific gravity of the fuel makes a difference.  I have used Jetmax for several years and would recommend you get to understand the carb without a jetting program.  Once you know the carb then the program is a tool that can be used for quick setting up but only if all the variables understood and are set correctly.

When you are at the track, don't ask anyone else what they are running as you will need to know all the variables, jet size, needle, ring position, float level, float weight, sg of fuel.......  Even if all these are the same your engine will be slightly different to theirs.

Recommend you get to the track on practice day, run the kart at good race temp (55 -65 degrees) with a 168 needle and write down the performance, plug condition and weather conditions. Temp, humidity, altitude.  If you have the gauge then the "air density" will cover the weather variables.

Once you have done your first run, change to the next jet down and run again.  Record the performance, plug condition and weather conditions.

Keep changing to the next jet down.  if it is too lean the engine will pop (backfire) between 10 and 12,000 rpm.  Stop racing and increase jet.  Warning that continued running with lean mixture may cause damage.  Your notes now should give you a starting point when weather conditions are similar.

You must realise here though that the main jet is only influencing 3/4 to full throttle.  Once you have full throttle sorted then start playing with the needle type needle ring position.

A tip for you, when you are going down a straight at full throttle try lifting the choke slightly.  This will let a little more fuel in.  If the engine bogs down then you are too rich.  If the engine picks up then you are too lean.

I do regret using the jetting program as I failed to understand the carby.  I am more confident since I have been using the above method and it has shown results with my daughter suddenly moving from the rear of the field to be competitive.

Good racing.

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Permalink Reply by Allan Clough on January 4, 2012 at 22:51

Thanks Colin for the great advice, like you said the best thing is for me to keep on doing laps & recording my data huh?. So are you saying start on say a168 jet with a K98 Needle, clip 2??? Guess if it were too easy everybody would race a rotax....

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Permalink Reply by Colin Lewis on January 5, 2012 at 7:48

Well the 168 jet is reasonably safe as this is what is supplied by rotax in the standard carb, same with the K98 needle.  If you keep the needle ring at 1 or 2 you won't end up with it being too lean out of the corner at 1/4 to 3/4 throttle.  Play with the needle settings once you have the main set and are more familiar with the carb.

Another point as you jet down through testing.  If you haven't reached the popping stage but the engine temp is raising over the 165 - 170 degree c range, consider going up one jet size as you are possible close the the correct mixture.  Rotax's loose power once you go over these temps so nothing more to be gained looking for the pop's.

Also, if it is a wet day you might lean it out more, Because the engine will operate more at lower rpm and less full throttle,  this will over come tendency to foul plugs and bog down.  The weather station used to tell me to run 170-172 jet but we run as low as 162.

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Permalink Reply by Ross Tait on January 5, 2012 at 8:08

Yes some tuning programs are unadjustable and will not be telling you the correct jetting for your carb ! I used Jet-Tech and that is adjustable for your settings,carb,fuel etc and once you fine tune it to your setup it is totally accurate and I have used this for various carbs not just Rotax (need the pro version) and has given excellant results.

Like Colin has said testing is the only way to find out things to how your carb,engine is setup and also make sure it is cleaned regulary and also battery is good and voltage.

Mychron is fine and you can do splits with that as well. If revs did not change then that tells a story as it would if been slower down the straight but like you said better off turns. Tyres you only want a couple of psi rise from cold to hot or then they start to go off so check hot and cold.Start on 168 and then move from there and the 98 can feel good but was not as strong as 54 in my testing,datalogging in certain areas.

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Permalink Reply by Allan Clough on January 5, 2012 at 18:11

Thanks Colin & Ross for all your advice, Guess I'm off to the track this wknd for another test day......We have first rnd of Twilights on the 14th so it would be nice to have a good starting point without being too far back. The long range forcast is for rain on the 14th, so that will be a whole new ball game huh? Just one question Colin, I hope the temp you quoted "165-170 deg c" should have been "degree F"  as degree C might be a tad hot?

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Permalink Reply by Ross Tait on January 5, 2012 at 18:39

Yes testing is the way and write things down and the the jetting and weather station readings so you get a history to work from, and do not have the weather station shut in a confined space as you will get silly readings as well.

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Permalink Reply by Colin Lewis on January 5, 2012 at 19:04

yes correct "degree F", sorry about the error.  This is a good example of the need to consider everything you are told at the track and from others.

Have fun, rotax is a great class.

Colin

 

Allan Clough said:

Thanks Colin & Ross for all your advice, Guess I'm off to the track this wknd for another test day......We have first rnd of Twilights on the 14th so it would be nice to have a good starting point without being too far back. The long range forcast is for rain on the 14th, so that will be a whole new ball game huh? Just one question Colin, I hope the temp you quoted "165-170 deg c" should have been "degree F"  as degree C might be a tad hot?

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Permalink Reply by Allan Clough on January 7, 2012 at 20:33

Well, interesting days testing....

Started with the K54 Needle & 162 Jet, avearge lap times low 43.....blahhh!  spark plug appeared a tad lean, then changed to the K98 & 165 jet using same rear sprocket....biff bam whollop....down in the 42's!! engine temp round the 65-67 deg C The main diff I felt was with the K98 it appeared to bog a little out of a few slower corners, might be where I had the needle clip? Any thoughts guys?

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