Home » Features » Formula TKM Column » Formula TKM News

Formula TKM News

carb jets

The two jets on the Walbro carb

This month it is time to go back to some basics to clear up some ill-founded paddock gossip regarding carburettor settings for the TKM BT82 used in our 2-stroke classes.

For the simple fact is that by not paying attention to the simple guidance Tal-Ko give on carb set up, quite a few drivers are losing out big time on performance and also reducing the life of their engine.

There are a couple of fundamental reasons for this and it seems to be a trend that goes in cycles as new people move into the class so let’s do our best to stamp this one out promptly.

The first basic that people get hold of is that allegedly 2-stroke engines go faster with a lean (weaker) air fuel mix. It can be true of some types of 2-strokes, but it depends on the design of the engine and in the case of the BT82 it certainly does not apply.

So while obviously the engine doesn’t want to be flooded with fuel, it will not get faster and faster the more you weaken off the mixture by screwing in the jets mostly because by running it too lean you are also causing the overall engine temperature to rise which is not good for engine power!  All air-cooled 2 stroke engines run faster when cooler!

In fact the opposite is true. If you put an engine on an accurate dyno, you discover that in reality it likes to be quite juicy and rich for best power, especially in the very important mid-range which is where most running is done.

The second fundamental which confuses people is that the Walbro carb has two jets which can be adjusted which effectively interact with each other. On many carbs the low speed jet is non adjustable which leaves it simple as to which one to adjust.

By getting the matching of the pair wrong you can seriously compromise not only the power from your engine, but also its life because too little fuel mix means too little oil lubrication and therefore potential engine death!

It is perhaps worth adding a third fundamental which is that while years ago many people had a basic understanding of air-fuel mix because it was something you had to set correctly on your car, these days it is all done by fuel injection and on-board computers.

So right back to the start. The fuel mix which goes through your engine has two basic functions. The petrol is the stuff that goes bang and gives you power. The oil mixed in with the fuel gives you lubrication. The amount of both of those going through is controlled by the jets – screwed in means less (weaker) and out means more (richer).

The other element in there is air which is what is sucked in through the carb air cleaner and obviously you want as free a flow as possible so that means a nice clean air filter element.

Now the slow speed jet, which has a screw head on it, is a small diameter jet aimed at supplying the engine with fuel used at low revs. The size of the jet is geared to this low level of fuel quantity required.

The high speed jet – the one with a bar across it for easy adjustment on the move – is the one that has the size to cope with the greater volumes at high revs. Note both jets are marked with an L and H respectively.

The two jets must be balanced to give the correct overall performance through the range and the starting point suggestions for the jets are given freely by Tal-Ko in their books of regs available to buy or on their website in the 2-stroke running section. Go to www.tal-ko.com

Now the fundamental mistake that people make here is to listen to the paddock gossip which says that you should screw the low jet right out and leave the high speed one only open a fraction. No, No, No!!

By doing that you will end up with an engine that has holes in its power curve and be much more critical on having the mixture setting just right. Most fundamentally, at high revs it will starve your engine of its life giving fuel and oil mix, causing overheating and drastically reducing its life.

Let me just repeat – the jets must be correctly balanced. A tad either side is no problem. A large chuck out of sync and the whole thing turns into a problem.

So to start from scratch, the low jet should be turned right in gently with a small screwdriver until it feels shut. Do not force it too far as damage will occur. Then turn it back out to two full turns. Screw the main jet in and then bring that out to just less than half a turn. Now those settings will give you a ‘rich’ setting which is a good safe starting point.

If you have a clutched engine the first thing you need to do at this point is start the engine and see if it will tickover smoothly with these settings. If it does then great. If not then trying screwing the low jet in a small amount to see if the tickover speeds up and smoothes out. Once you have that setting then you are ready to go on track. If you have to turn in the low jet 1/8 of a turn then simply turn out the high the same amount so that the sum of the 2 jet settings add up to 2 1/2  (2 on Low + 1/2 on High = 2 1/2 Total)

Whether clutched or direct drive you now need to assess the feel of the engine. And the good thing here is that even if you are a Dad watching from trackside you will be able to tell. Ideally you want the engine to be giving out blue smoke for the first lap or so, reducing down to maybe just a small puff when going back on power after a corner.

The thing you are more likely to get on those fairly rich settings is something called 4-stroking. We won’t bother to go into explaining the techy reason, but it is basically a point when the engine will not rev any more and goes flat sounding at high revs on a straight. That is a sign of being too rich.

Now it is very important to get your engine to initially 4 stroke so that you know where you are with your settings especially when you change carbs, engines etc and this will aid you to then adjust the main jet in a small amount at a time each lap until you just lose the 4 stroke as you are about to brake at the end of the longest straight!

Once you feel/hear a nice crisp note all the way down the straight you have got the right setting. Then you should have an engine ready to race and win!

It is all very simple stuff but you would be amazed how many people do the wrong thing and listen to gossip which leaves them seriously underpowered. So be rich and powerful!

Sidney Sprocket