Lambda probes – jet your scooter on the road! | TECH
We always welcome input at SLUK especially where the input provided offers a real alternative to some of the more woeful chores we’re confronted with. One of the most tedious and often frustrating is getting the carburation right on your beloved two stroke. Marc Zeltner from Southern Germany looks at an alternative that won’t cost the earth and can be used time and time again.
Quite few of us have happy memories when it comes to Chemistry and I don’t mean the non academic kind you “experienced” in abandoned warehouses during the nineties. I mean the boring classroom type with all the funny short names assigned to the elements on the Periodic Table.
Chemistry depicts the specific formula which makes the difference between your engine running well or welding that expensive high-tech forged Wossner piston into your even more expensively tuned Cylinder Barrel. Of course the obvious way to reach the optimum set up is by manually setting your carburetor e.g. with different jets and alternative slide and needle set ups. That usually happens while riding out on the road, listening to the engine and getting a feeling of whether it is boggy (too rich) or equally “Pinking” which means you’re engine is running hotter than a Dragon’s Nostril and you’re heading for even more trouble.
If you are like me, this is one of the hardest parts of an engine rebuild or, indeed, engine tuning. I am, on the one hand, not patient enough and on the other hand, too quickly satisfied if the engine seems to be running “safe”. Mostly, a safe engine is actually running much too rich and I tend to side up with the bodger in my head saying; “we’ll fix that soon, don’t waste your time right now.”
Day of Reckoning
That system worked, until the day I built a touring engine for my wife. She was not that happy, spluttering around town and regularly changing spark plugs which were darker than Bavarian ham beer!
Sort your AFR
On scooter-related websites and forums, I read about measuring the air/fuel ratio of two stroke engines. I’ve seen Lambda probes used many times on dyno’s and most of us know that they are also used from the catalytic converter in your car to govern emissions. But a simple “plug-and-play” solution for two-stroke engines was new to me.
So, I decided to give it a try…….
Spontaneous Internet Purchase
Two clicks of the mouse later and I had bought an AEM Lambda Controller Kit for approximately 200€ (£170 GBP). The actual Kit comes with almost everything required and includes a Bosch LMU 4.9 wideband probe. There are other kits available and you can bet, there are equally enough pages of pros and cons in the online forums regarding Zeitronix, Innovative, Koso…etc.
The controller for the kit needs 12V DC. So if you are running a battery, just plug it in behind a 10A fuse for protection, after checking that your stator makes enough output to charge the battery whilst running the probe. If like me, you are running the controller temporarily to set up your scooter, use any small sized 12V battery (A borrowed one from a Vespa PX is fine) and put it somewhere on your scooter. A leg shield bag seems ideal for the job: You can easily put everything in it and place the supplied Air Fuel Ratio (AFR) Gauge securely but visibly, whilst allowing you to change the kit to other scooters, as required.
The probe required a M18 x 1.5 threaded adaptor welding to the exhaust. Because the probe should fit approx. 30-40cm after the outlet port and ideally in a 45-90° angle (probes don’t like condensation), there are not a wealth of positions for you to weld. Accordingly I decided that it was fine to run without floorboards whilst I was setting the scooter up. So I found a position on the top of the exhaust, just in front of the frame loop (see pic) which seemed far enough from the outlet port to do the job. You should also bear in mind that these probes are tougher than you think. They have been developed for heavy duty automotive use, with no real ongoing maintenance requirements.
Mark your throttle openings then ride
Once you have installed everything, which really is as easy as I’ve described, mark your throttle positions on the twist grip with a bit of masking tape (0; 1/4; 1/2; 3/4 throttle openings as well as full throttle). Now find yourself a nice road at a time where there is little traffic, with a long incline. Hold a specific throttle position and at the same time read what the AFR gauge is recording. When you reach the maximum peak revs according your throttle position, you should be reading around AFR 13.1. Less than that is too rich, more than that is too lean. Following most people’s experience, a safe AFR measuring at full throttle is about 12.5.
(The stoichiometric fuel rate = 14.7:1 (Lambda 1), but two-stroke engines should run more on the rich side)
Uphill under load
The readings are much more continuous if you ride uphill, and the engine is under load.
Keep your eye out
My personal recommendation is – keep your eyes on the road! There have been more than a couple of occasions where I stared at the display for too long, and ended up heading towards a corn field or for a second class Lance Armstrong copy in his tiny bicycle shorts! There is no need to watch the AFR reading jumping up and down like sugar doped kids at a birthday party, unless you have the revs up riding under load.
Got the Needle?
What impressed me most, was the influence of different carb needles. They had an incredible effect on the AFR reading between 1/4 and 3/4 throttle, depending on the needle diameter and taper.
If you are happy with the newly set up carb, remove everything, close up the welded adaptor sleeve (a special plug is provided with this kit) and enjoy riding.
Anyway, if you have a good selection of jets and needles with you and maybe a couple of slides, the job should be done in 1 to 3 hours.
Out with the old in with the New?
Some people will argue that this whole exercise is waste of money and carbs have been jetted the traditional “road test / plug chop” way since Kaiser Bill was a lad. Others will say jetting on a Dyno is fine and less dangerous due to the absence of traffic. Both are right. But I don’t really have access to a Dyno and I’m not really patient enough for the alternative.
Share with your mates
So for me this is the perfect tool. You don’t have to be Dr. Emmet L. Brown and the whole set-up is much cheaper than replating your alloy barrel if anything goes wrong. Give it a try, especially if you can share the overhead with your scooter club mates or the local garage community.
Words and Pics Marc Z
EXPLAIN THE TERMS!
Air Fuel Ratio (AFR): this literally means the ratio of fuel compared to that of air as defined by weight.
Stoichometric: this is the ratio for optimum burning efficiency. For petrol it is around 14.7 parts air to 1 part fuel.
Richer for 2-strokes: In a 2-stroke engine the incoming fuel also served the purpose of keeping the engine cool through evaporation, so in real life you can’t safely run at 14.7 (Stoichometric) ratio, and often need to be closer to 12:1.
Rich or lean: Rich mixtures are too many parts fuel to too few parts of air. If the ratio drops below 10:1 then the engine will become smoky and misfire.
The art of carb setting is finding the happy-spot. In that way it’s a lot like making love to a beautiful woman…