The technique is based on the Stoichiometric Ratio, which is 14.7:1 or 14.7 air is to 1 fuel (read this). That is for every 14.7cc of air only 1cc of fuel is required to attain the highest combustibility of gasoline w/o being lean or rich. Meaning it is the finest tune for your Motorcycle engine EVER. Yup! It sounded as if you need to have a lot of measuring device to achieve this on your carb. In fact, it is the Stoichiometric Ratio that is being maintained in EFI or fuel injection systems where a lot of sensors is used to tell a computer how to attain the proper air-to-fuel ratio while running in different environmental conditions and loads.
Unfortunately with our carbs, we don’t have a computer and sensors to accurately attain this fine A/F ratio. But with the technique that I have mentioned, we are blessed with the simple adjustment screws that we have on our PZ27 carbs: the idle-speed adjust screw and the mixture adjust screw to achieve Stoichiometric Ratio (Pls refer na lang from the z200 parts manual found on the XPhil website for the proper locations of this screws. Anyway, most of the forumers in this thread knows it already.) The mixture adjust screw is also called Air screw in other sites, and this is what we need to focus on. To attain the Stoichiometric ratio, you don’t need to know which way with the mixture screw is going lean and which way is going rich. You don’t even need to turn the mixture screw to full close, where it is risky for the mixture screw gasket and spring to get damaged. Just do the following:
1. START THE ENGINE AND LET IT WARM UP to operational temperature (you may ride it around your barangay or subdivision for a few minutes also). Do not adjust carburetor when engine is cold.
2. With the engine running at idle speed. turn mixture adjust screw slowly clockwise (turning in) by 1/4 increments, until engine falters. Stop at this point. Now turn screw counter-clockwise by 1/4 increments until the engine falters again. But this time, count the number of turns.
Note: The PZ27 mixture screw is quite small and facing downward, that a proper precision flat-head screw is needed. It should be short enough so you don’t get burned by the hot engine.
3. Divide the number of turns by 2, and take note of the quotient.
4. Turn screw clockwise by 1/4 increments again, and count until you reach the quotient. This is exactly halfway between your first and second positions, and the optimum setting on your carburetor.
5. After mixture adjustments have been made, idle speed may need to be adjusted to its user’s manual prescribed rpm for idling.
The main idea of this 5-step procedure is to find the Stoichometric ratio between the leanest and richest operating condition of the carburetor, without using any measuring device. It is based on the fact that, engines run poorly when too lean, and runs poorly when too rich. So the mid-way between this two points (lean and rich) must be the ideal A/F ratio.
But wait… this procedure only works if both your carburetor and engine don’t have damages. It is best to have them fixed first before doing the tuning. Anyway, you can find out during the procedure if there’s something wrong with your carb or engine. Visit this site: http://www.off-road.com/atv/kidskorner/idleadjust.html
FINAL WORD: Adjusting carbs on our own is really not advised by mechanics because of the dangers mentioned above, and they thought we can’t get well informed enough to go around these dangers. Well, let’s understand them. But when you have modifications on your exhaust pipe and airfilter, change to split-fire cable and plug, you will need to re-tune your carb. So it is nice that you know the proper how-to, instead of taking the trouble of going to the mechanic and pay for his services for the simple re-tune. I personally believe, that the procedures I mentioned here are quite safe for your carburetor and engine, as long as it is followed properly to the letter.
BlueKnyght is a forumer of MCP (www.motorcyclephilippines.com) and is based somewhere in the Philippines.