Carbs adjustment with combustion analyser 2

[fjbalaux]

I want to try to set the carburators air/fuel ratio on my motorcycle with a combustion analyser. Does the combustion gaz varies a lot with the load on the engine? Did any of you ever did this without a dyno? Are there any special tricks I need to know before I try this?
Thanks for your help
Francois

 

[evelrod]

Francios---

The small portable exhaust gas analizers available to the hobbyist are OK, but a little on the expensive side considering what you get for your money. I had a small unit , from Sears, I think and it worked well for small stuff, but when we decided to put it in the race car and test at Willow Springs, it failed after only a couple of laps. Not ment for that type of use. In your case it would be cheaper and faster to take your bike to a chassis dyno operator and have your jetting and timing done there. If your bike is stock I would stick to OEM specs. If you have added or changed parts in the intake , ignition or, exhaust, take it to a competant dyno operator for your fastest fix. If you can read plugs and don't mind a lot of tedious jetting changes and timing changes and plug changes and so on and so forth you may be able to get your jetting pretty close. At least close enough to shorten the time necessary on the dyno.
Either way, you still need a starting point. Check with other owners of the same bike if available. Check with dyno operators or speed shops in your area. Try performance books if available for you type of bike.
Always try to start a little on the rich side and work your way to the correct jetting in small steps. Make sure you have the right heat range spark plug. If you are not sure, start with a set a couple of steps colder than specified stock. Do the small stuff, the tedious stuff, first and then check with an analizer will get you where you want to go. Just keep in mind they are not intended for over the road use. Especially on a bike, you can get hurt trying to drive fast and read that darn thing!!!


Rod

 

[fjbalaux]

You have good points Rod but there are some things I should have said. I have a very good gaz analyser I can borrow from a forklifts repair shop. I can't go to a dyno because it's now winter here so I will not be able to ride for a couple of months. What I was thinking to do was to lift the rear wheel of my bike and use the rear brake if I need to simulate a load on the engine, then to set my carbs until I have a good gaz ratio. I have a self made exhaust and a jet kit installed. I think I am close to a good setting now. Do you know what values of CO, CO2 and O2 I should look for?
Francois

 

[franzh]

I agree with Rod above. One item to consider when using an exhaust analyzer with smaller engines and relatively short exhausts is exhaust pulse reversion. When this happens, a slug of raw air enters the exhaust pipe and dilutes the sample exhaust entering the analyzer. This makes it very difficult to obtain a true reading. You will either need to sample all cylinders at once for an engine-out average, or individual cylinders for specific tuning. Each has its advantages.

As a rough rule of thumb, You want HC and O2 as low as possible, indicating complete combustion. CO will hover around .5 to 1.5%. CO2 should be high, in the 12 to 14% range, also indicating the quality of combustion.

If you have the capability, install an automotive O2 sensor in a direct exhaust stream (NOT NEAR THE EXIT!) and connect a digital volt meter. You should find readings below .450 volts and above .200 volts (lean) for economy, and above .650 to .850 volts (rich) for power. You cannot reach .500, dont even try. The O2 sensor is not designed for this resolution. At light load cruise (120 KPH) you might see O2 readings fluctuate from .450 to .700 volts. Linear O2 sensors require some electronics to translate the signals.

Help any?

Franz

 

[evelrod]

I didn't think of welding in an adapter for an o2 sensor, sorry I wasn't thinking along those lines. If you can accomplish this addition to yout exhaust system (relatively simple) hardware to translate the voltage readings is available at most any speed shop in the US and probably elsewhere. The unit I saw came from Australia and was very inexpensive. Also in your tuning you will find the use of a pyrometer useful in checking exhaust gas temp at each port.
Good luck and don't go too lean!


Rod