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Started by raueda1, 15 July 2018, 02:15 PM
Quote from: s class on 18 July 2018, 04:33 AMOK, the first thing to decide when using the Gunson is what actual CO figure you are targetting. Spec for the K jet 116 and 107 cars was usually 0.5 to 1.5%, but that is at sea level. K jet self compensates for altitude, but is limited in its scope to about 1200m. I'm in Johannesburg at about 1700m, and it is generally accepted practice to aim for 1.5 to 2.0% here for two reasons : a) to deal with being outside of the altitude range that Kjet can cope withb) these engines are old now with stretched timing chains, carboned up ports etc etc. The Gunson works by comparing the exhaust gas to ambient air. Ambient air has a typical background CO level of 2.0%. The Gunson unit is extremely sensitive. It takes about 30 mins powered up before it achieves stable readings. If you try before that you will be frustrated. After about 15 mins of being powered up, adjust the calibration knob to 2.0% (ie the background air). Its easy too see when its stabilised, as if you wait another 5 mins, it should still be at 1.9 to 2.0%. If it has dropped below that, it means that the initial warmup phase was not completed before you calibrated for background air. The gunson is intended to be powered off the car's battery. This does NOT work, because the battery voltage fluctuates as the engine is revved., buggering up the calibration of the Gunson. Much, much better is to power the gunson off another car battery, loose on the bench. We go a step further, and run it off a dedicated, stable 12V supply unit. Also, bear in mind that when you stick the Gunson sniffer up the tail pipe, it slowly gets polluted with exhaust gasses and so the test duration is limited to a few minutes. If you carry out a test on a properly warmed up car, properly set up a 1.5%, you would see the following : a) initial reading 2.0% (due to calibration in background air)b) within 15 secs, reading will climb to about 2.2 or 2.3% as the measurement cell acclimatises to the hot exhaust gassesc) after about 30 secs, the reading will slowly drop, in steps of 0.1% until it reaches 1.5% (or whatever the car is set at)d) after about a minute, the readings will climb slowly as the Gunson's sensor cell gets polluted. The minimum measured ie in c) above, is your reading. My procedure is as follows : 1) power up the Gunson for about 15 mins2) drive car hard preferably on highway for about 15 mins to burn chambers and plugs clean3) perform test as per a) to d) above4) adjust CO screw as required5) go for another 15 min drive, during which time the Gunson cell will burn clean and it will settle back to 2.0% calibrated setting6) perform test as per a) to d) above7) repeat this until you get your setting. My experience is that if you fail to do proper highway speed driving, the plugs will be a bit foulled, and one will get a false high reading. The difference it makes too the result is about 0.5%. In other words, if you set the CO level by only idling the car and driving around the block a few times, and you set it at 1.7%, then after a proper highway burn, if you retest, you will find it is actually about 1.2%. Setting the CO below about 1.5% (notwithstanding my comments above about altitude) will result in a noticeable 'seat of the pants dyno' drop in performance. Going down from 1.5 to 1.2% will cost you about 10% in perceived pickup. Conversely, going richer, above 2.0% will have a similar loss of pickup. Going to 3% will feel noticeably 'stodgy' and lazy in comparison to 1.5% to 1.7%. A final comment, since the Gunson works by comparing the exhaust gas to the ambient air (its reference), it follows that you cannot do this in a closed garage or workshop where the background air is contaminated with exhaust gases. We always do the test outside the workshop in open air.
Quote from: adamb on 19 July 2018, 05:25 PMI got one last year. It's good as an amateur tool but doesn't beat garage analysers. The main limitation is the absence of HC measurement.
Quote from: Feather535 on 03 August 2023, 11:56 AMReviving this old thread. I just acquired one of these instruments, but after unpacking and assembling it and going through the set-up procedure, it won't calibrate correctly. After waiting 10 minutes with the unit connected to 12v power, the display showed negative numbers (like -2.0). Turning the adjustment knob all the way clockwise and waiting another 10 minutes only brought it to 0.7%, nowhere near the 2.0% the instructions call for. Has anyone else encountered this issue?
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