Setting the Carbs

Iame Gazelle Engines are pretty close once they have done some time and had a rebuild or two. We had 4 engines and there was a maximum difference between them of about   .4 of a horse power at any time.  To compete at top level you will need to manage the time on each engine so that you always have an engine close to going off one ready to come on  and one at its best. We used four so that we could test on the 4th all the time.

So why did we always seem to have loads of power even when we joined the best cadet team in the paddock we seemed to have more power than they did. To state a fact when we were doing the carbs not the team we were fastest on track.

It was all to do with the Carb set up and now that we are out of cadet I will share some of what we found with you so you can implement this yourselves. If you feel you are unable to do this then I will be happy to set them for you.


Don't get to confused with this but it is critical that you get each and every carb you own to have the same pop off pressure. 

If you increase the pop of pressure it will make the carb run leaner if you decrease pop off pressure it will run richer so pop off pressure will effect your jetting settings. What you are looking for with pop off pressure is that it pops off and re-sits as close as you can get.  The possible problem we found was that yes my original thoughts were to try and pop it of at 14 and re-sit at 11or 12. what we found is that it makes the carb un-reliable and you are forever re-kiting the carb as it tends to dump a lot at low pop off.

Not good if you are hoping to finish the race, so what was the answer? well the ideal pop off is at 17 then you are aiming for  it to re-seat at just over 10. We found this setting to be the best for performance and of course reliability. How then can you achieve this. It is difficult but it can be done with patience and patience is what you need here i spent hours and hours on one carb at a time. Buy all the springs available and keep changing springs no matter until you find this magic pop off number. You will find it easy to get a spring to pop off at 17 but they will most definitely re-seat under 10 this is too low. you will find lots of springs say 42 grams that will hold at just above 10 perfect but will pop off at 19 0r 20 which of course is to high. Springs are not all the same and the answer is not necessary that the pop off is the magic formula, it is that by using that gauge 17-just above 10 you will find a spring that performs the best, i.e the best springs available. I am sure that if you used a different gauge say 19-12 you would keep trying and again find the best performing spring for that range and have the same result a more reliable and better performing spring. So what am I saying, it's not the Pop off re-seating that is the most important it is using that gauge to find the best performing spring since they are all so different. I say to you stick to 17 and 10 that works.

Ok next lets look at the stupid assumption everyone thinks is correct. Chasing bottom by using the jets. Oh my god seriously what is all that about. Driver comes in says dad I have no bottom end!! Dad heard someone say go richer on the jet to get more bottom so that's what they do. Yes it will work but what will happen is that you will loose top if you are to do that then you need to keep the gap the same between low and high. For instance if your carb is set 60 high 75 low they come to a total of 135 if you go richer on the high 2 minute and stay the same on low it adds up to 137 (62+75=137), therefore you will need to go 2 minutes leaner on low jet which keeps it at 135 (62 +73=135) all you have done is move the power band that is is. Trust me if you are chasing bottom with the jets you really do not have a clue so stop it leave it to the idiots out there who think they know what they are doing. You want more bottom use the gearing not the carb.

What does a carb do? 

well it mixes fuel and air 

what does an engine need to run at it's very best? 

The correct fuel to air ratio that's what, 

guess what when you go richer that alters the mixture so obviously no one knows how to get the correct mixture they are guessing all the time, you can dyno the carbs over and over again but that will never get the correct mixture. It will get you close if the person knows what they are doing but never bang on, bang on is what you want bang on every track every day with as little grief as possible so that the driver can do their bit. Not guessing with the carb.

For best results on setting mixture ratio you need to do it on track not the dyno. Here is why, the difference between the air molecules on any given day makes a difference to the ratio (fuel to air) i.e on a day when the air pressure is 1024mb  and the next day it is 995mb the later day shows less air in a measured area i.e less molecules so that will alter the mixture. You could look at it like altitude the lower the millibars the higher you are above sea level again less air molecules available. That is why small single engine aircraft have a maximum altitude. They simply are not powerful enough to generate enough forward thrust to create lift once they hit the ceiling. to create lift you need dynamic pressure over the wings. So think about that a minute have you had your carbs on a dyno that replicates the dynamic pressure that will occur on the track. I have been to most and not one replicates that they, put a fan on that is it. does that fan replicate more or less dynamic air pressure. My guess is a lot more which would result in a power curve set on an air to fuel mixture which is to rich for the track. But hey great keep doing it was my opinion. so you need to do the following:

Once pop off is achieved my suggestion would be to find someone who can produce a power curve from data taken from the mycron. Then set the low Jet at 75 an high jet 59. Start engine on stand, and use the mycron as a dyno to set the starting point. You need to rev the engine for a count of 3 seconds then apply brake. You are looking to acheive a top rev of 14750 once you do that you are close going leaner on high will give you more revs and vice versa.At this stage just use high jet.

Put kart on track and concentrate on low jet Very important step. send driver out for ten laps on stand setting. then ten laps richer and leaner i minute at a time, you wont need to go more than 3 minutes in either direction. Compare power curve on software looking in particular at the transition at around 7,000 rpm it needs to be smooth and show that it does not effect the top end i.e stall its performance. This is the most important part for sure it is the transition from low to high that gives the best tuning. People set the low jet far to high which in turns slaughters the true performance of the carb. The reason is that it is done on a dyno with far to much dynamic pressure as explained earlier. That is why when the air pressure is high they get better performance out of the engines. but when it is standard i.e 1010 they do not. once that is done concentrate on the high repeating the process do not worry about the low at this time. Look for the maximum power compared to the drop off you are looking to hold the power for as long as you can before it starts to drop. Now go back to the low and in half minute increments go richer and leaner for 3 laps at a time you will not need to go more than a minute each way. See how that effects the power curve choose the one that shows the best transition again. Then from there repeat the same with high choose the one that again is best. Carb is then as good as it can be. If you were to put that on the dyno they will tell you it is too lean. Fantastic that is what you want to hear from them.

Now record that days air pressure you then will have something along the lines of the following 

Carb no db 1     high 58.5   low 74   set at  1008mb

once you have that every time you turn up to a track check the millibars. If it is 1008 leave the same you only need to go richer or leaner if the millabars change by 15 in either directions.

I.e  993mb you will need to go leaner by half a minute. 1022mb half minute richer. the more air the more richer to keep same ratio and vice versa.

Just on more thing, wetbox take daily reading of air pressure first then always 2 minutes leaner on high from that days settings.

Always remember to put back carb to the settings you done on track don't leave them or you will get lost. dont chase bottom with carb always gearing. Hope that helps.