Exhaust Gas Analyser

[mgcjag]

Hi All......anyone with an ayalyser that they use and are they worth using to set carb mixture....have searched the forum and the only info i can find is on the ?1500 units...Im looking at the other end of the market the ?150 ones.....will they do the job or a waste of money... Steve

[christopher storey]

I have had a Gunson one for many years, and can recommend it highly, but you need to keep a clear head! On a 3 carburetter car, what you need to do is to set the 3 jets at the same height using a depth gauge ( and previously the float chambers should have been set to give an identical fuel level ) . I use about 65 thou as an initial datum . The idle screws must then be set to an identical point on each carb . Then put the probe ( the device having been warmed up electrically by connection to the battery for 5 minutes, and calibrated in clear air ) alternately in the left and right pipes, and alter the mixture screws for the appropriate carbs for each pipe ( I can't now remember which pipe serves the front of the engine and which the rear ) i.e. front and middle serve one pipe, and rear and middle the other - the middle carb is shared between the two - until each pipe shows the same CO reading . Clear the device every 2 to 3 minutes by 20 seconds running at about 2000 rpm . I aim for about 4.5% to 5% at idle of say 800 rpm . When you have an apparently satisfactory setup, take the bells off and again measure the depths of the jets, average the 3 , and set each to the average e.g. if after setting the depths are 69 thou, 48 thou and 75 thou, the average is 64 thou, so set each depth to that, and see how the car feels on the road

[JagWaugh]

I have a Gunsons, from Halfords, about 15 years old now. Worked for me. Here in Switzerland cars had to be tested and a printout of the values in the logbook, which you had to present at the MOT. About once a year I used that info from our and neighbors cars to check that the Gunson wasn't wildly innacurate, and that the calibration procedure was working correctly. It was pretty damned close.

I have access to a Bosch professional unit now, so I haven't used it in about 5 years.

Andrew

[abowie]

The Gunson unit is good; I find it the best way to tune the car.

As Christopher has said tuning 3 carbs from 2 exhausts requires a bit of discipline and mental gymnastics.

If you want a good outcome you can't just shove the probe up the exhaust and twiddle the mixtures a bit. You really do need to start from scratch making sure needles are set to the perfect depth in the pistons and jets centred, float heights are perfect and that all 3 butterflies are synced properly.

I do the same as Christopher and set the jet heights with a vernier caliper although on my cars I tend to start a bit richer; for me around 75 thou seems to correspond to the 2 1/2 turns in the manual.

Following the calibration and warm up instructions for the Gunson unit is vital otherwise you're wasting your time as the measured numbers are garbage. Sometimes the unit will not calibrate properly. If you search here you will find that I have posted the advanced calibration instructions for this eventuality.

I put the probe in the pipe and then adjust the front and middle carbs the same amount until I get the desired CO%. These days I tend to aim a bit richer than Christopher; about 5.5% to 6%. Then I go to the other pipe and adjust ONLY the rear carb until I get the same reading. Hopefully, this means that the jet heights on all 3 carbs stay the same. Sometimes it works, sometimes it doesn't... What is important is trying to keep track of where the centre carb is in relation to the other 2 by number of 1/4 turns on the screw.

With patience you will end up the 3 carbs set the same and with an acceptable CO%.

I like Christopher's final "jet averaging" manouvre. I will try this next time.

[mgcjag]

Thanks all.......much appreciated......next on the toy list.. Steve