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How accurate is a Gunson Gastester?

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I always have problems with the MOT emissions idle test for my 1986 Lotus Esprit S3. It sometimes fails the idle test on high HC but passes the test at 2000rpm. The MOT numbers for CO at 2000rpm is too low at 0.124% to 0.265% which could be causing the high HC.

 

Firstly I used the Carbtune Pro manometer to test balance of the carbs but none was needed (very simple to use). 

 

I decided to use a digital Gunson Gastester (some years old) to see if I could increase CO to within the factory range. I calibrated the Gunson in the air at 2.0% and followed all the instructions.

 

I then tested my BMW E36 which had 0.02% CO at MOT idle test but Gunson gave 0.9%. Two years ago the Gunson was 0.9% out from the Lotus MOT results. So both out by 0.9% from MOT.

 

I adjusted the idle mixtures to give a Gunson result of 1.9% which is within the factory range of 0.7% to 1.5% if I deduct 0.9%.

 

Gunson claim accuracy for the Gunson of +/-0.5% CO but mine seems to be 0.9% out. Anyone compared their Gunson to MOT result?

 

 

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One issue with Gunson's and others of a similar nature is that presence of HC also gives the same effect on the sensor (hot wire type) as CO, so if you have unburned fuel as well as CO, the tester simply reacts as though it's all CO.

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The problem with exhaust gas testers is that they read output from all for inlets together.

For example 2 could be very lean, and the other 2 v rich, but still give you a correct CO reading.

You maybe better off getting the carbs tuned on a rolling road, by a known carb expert.

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I was concerned about the issue above as my car pops and bangs a lot on overdrive. My local MOT tester has been years in the car trade and has experience of carbs (most garages dont have a clue) and I asked him whether I could be running with two overich and two on the weak side.  but overaul producing the right emissions. He reckoned that if the carbs were balanced properly in the first instance and that the mixture screws were all turned to stop and then turned by exactly the same amount (my manual recommends 6 half turns), then unless there was some other problem with the carbs they should all be fairly even. 

 

The reading on his exhaust analyser was spot on so he reckoned leave it alone. 

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Mike, opening idle mixture screws the same number of turns is just a basic 'get you started' setting.

There are several reasons why they'd be different, but the obvious one is that the carbs receive different amounts of air/air flow rates. Especially when using a standard air box.

I'm not using a standard air box but this was made clear to me during a recent dyno session.

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Not sure I buy that reasoning with the airbox. On my carb turbo the pressurised air comes into the plenum dead centre at the top and being a fairly spacious unit I doubt the air entering each cylinder will differ much.

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Hi John

 

Gosh, Gunson Colortune.

 

Much loved by those of us with twin-cam engined Elans, +2s and Europas.  Apart, that is, from burned fingers and occasionally melted plastic shroud tubes.

 

While we're at it, 25kV up our arms as we adjust ignition timing by rotating the distributor with hands trapped under the Weber or Dellorto carburettors, is an experience.

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I've never had any success with the Colourtune, it's resulted in too much violent back-firing.

 

According to the manufacturers this is due to the adaptor being too short, and not extending far enough into the combustion chambers - as would a normal Spark Plug, and causing irregular ignition (hence the unburnt fuel igniting in the exhaust).

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I'm setting up my carbs after a rebuild. I used Carbtune II to get all 4 balanced, then used a pair of Gunson Colortune plugs for first time. I found them easy enough to use and no real issues other than it being a struggle to get sight of the colour flame (awkward angle and bright daylight) but overall it was very accurate - carbs had been far too lean, need at least 5 full turns - possibly 6. Now running sweet. I've got a Gunson exhaust analyser too, never used that before either but that's next.

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Why not use a Gunson Colortune?

 

It's what I always used as you can set each cylinder individually & the car always passed the MOT.  :thumbup:

Is it hard to see the colour with the slant of the engine and the cam towers?  

Cheers,

Simon

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Regarding setting the idle mixture screws the same as a 'get you started' and a rolling road/dyno session and tech fine tuning each carb choke.  How is this achieved?  How would a good tech know which choke is rich or lean?  I am guessing that its by the vacuum drawn through each choke using a manometer?

I am wanting a session on a rolling road but having trouble getting my head around how the tech will know what to adjust and how much.

This last weekend I managed to balance my carbs evenly with a 4 tube manometer and the car runs better but not 100% smooth; it still has a stumble and if you turn off the engine too quickly there is an almighty delayed bang sometimes.

I am going to try a Colourtune next then rebalance.  I'd really like to try a rolling road/dyno too, but I want to be able to advise the tech on the specifics of the carbs as they haven't worked on an Esprit before.

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Like others have said above once the carbs are balanced you can use the colourtune to fine tune each choke. Its a bit of to and fro adjusting the mixture and flow until all are running the same.

 

Im sure any tech worth his salt will be able to tune the carbs. They are only simple carbs and found on many older carbs. Basically the same as Webbers. 

 

A chap with a good feel can do great things...will all the money in the world watch this guy tune a mura with no computer equipment. Just a good ear and some practice. First he feels the pulses are even with his hand.

 

https://www.youtube.com/watch?v=FlGfcN-QVg8

 

 

Buddsy

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"Belief is the enemy of knowing" - Crrow777

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My Colourtune plug came with a long black tube with a tilting mirror at the top so you could see the colour whatever the angle.


Margate Exotics.

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Carburetter tuning is a black art. You just sort of stand there...allow your senses to be receptive to what the machinery is trying to tell you. A bit "Star Wars"....."Feel the force, young Skywalker"... Instrumentation can help, and it can also confuse. A length of rubber tube so you can listen to each choke seperately is very useful. So is short circuiting each spark plug in turn to get the same rev drop....

GREAT fun......

What ever you do...DON'T just twiddle things at random(!)


Scientists investigate that which already is; Engineers create that which has never been." - Albert Einstein

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Had My garage set up my twin su's on my old spitfire using an analyser - might be good on emissions but wasn't great....

A mate of mine whom was a mechanic before taking up a clean trade - he had a two minute fiddle - and oh my god - really is a black art - but the car was transformed........

It's an art it really is........and I'm crap at drawing and painting......so leave it to some good eyes, ears and fingers....


Only here once

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