I think the year before the ppm HC was slightly lower on my car, not significantly though.

I stupidly bust my Gunsons CO meter when checking a friend's 340. Connected the power leads in reverse. One might think that they'd put a diode in the circuit to protect the thing.

Was it the digital or the analogue Gastester? :thinking:

It is, or was, a digital one!

I have another, an analogue one that I put in the garage for 10 years, that one does not work either!

The analogue one will be easier to fix than the digital one. Chances are it's the calibration pot needs a bit of contact cleaner and exercise as they're pretty sturdy animals or maybe the needle has stuck - mine does from time to time. Never had a digital one but i know in the analogue one there is an LM7805 voltage regulator IC to drop the voltage to 5V for the meter and deteecting circuits. Other than that and obviously the meter itself, there are no polarity conscious components in the anaogue one.

The analogue one operates on the Wheatstone Bridge principle, +ve and -ve supply to the bridge, calibration pot and reference thermistor in one leg with one side of the meter between them, sensing thermistor and a resistor in the other leg with the other side of the meter between them.

Both thermistors heat up due to current flow in them, the meter deflects to 2% in free air under calibration as (to the best of my knowledge) there is 2% CO in free air.

The reference sensor is hidden away from gas flow so it just gets up to temperature and maintains it, like wise the sensing thermistor will achieve a certain temperature and maintain it - or it will until CO passes over it.

As the CO passes over the sensing thermistor, the CO cools the thermistor, increasing the resistance (should have said before, they are both NTC thermistors) and hence the voltage developed across it. This causes the meter to deflect higher, conversely if there is less CO than 2% in the gas sample, it allows the temperature to rise on the sensing thermistor reducing the resistance in it and hence the voltage developed across it causing the meter to deflect towards zero/lower.

In the analogue onees, there is once central screw underneath if memory serves, remove it and you can separate the top and the bottom. If you have a basic understanding of electronics and can drive a soldering iron, you should be able to fix it now. :thumbs_up:

That is very helpful, I did take apart the digital one, also held together by a central screw, looked complicated, but I did see no series polarity protection diode.

Will have a look at the analogue one. Thanks again.

The basic digital one uses the same case as the analogue one with a plain plated/bezel in place of the clear meter screen so the basic circuitry is likely to be the same with an ADC/voltmeter chip set up to read the voltage across the Wheatstone Bridge and hence display the CO content.

The Gastester Professional i think it was called also has battery volts, rpm, dwell(?) as well as the CO readings. Oviously a bit more involved but i suspect on the simple digital one, it probably uses generic components so replacing the 7805 and possibly the ADC and/or display driver chip will get it up and running for you. The display being LED is of course protected anyway!

On both of them, adding a 1N4007 as a serie polarity protection diode will do the job, cheap, simple add-on to prevent further accidents! The 4007 is rated at 1A but then so is the 7805 so will be up to the job of protecting it happily.

Well did a 10 mile round trip today, still got an intermittently poor idle.

The fuel was down to about 2 gallons, so put the 2 litres of methylated spirts in.

Drove around a bit, don't think I noticed any improvement.

Took air filter off, found the slotted head screws under it that I assume hold ithe carb on. They were not loose, but tweaked them slightly. ran engine, no difference.

Removed the idle valve with idle jet, put in another idle valve that may have been doctored and does not have the idle jet and the car idled better!

Cleaned the idle jet and refitted the original idle valve, idle perhaps slightly better, but not right.

Given up for the moment. MOT at the beginning of July.

Got a bigger problem now, just received a party wall notice from neighbours surveyor. The are doing massive building work next door (it's a semi) including removing the chimney breasts, extensions everywhere. I've having my own surveyor, not using theirs.

That's sounding suspiciously like blocked jets, see how it goes over the next few days which should give the meths time to work as well. Usually works fairly quick on injection engines but they have a fuel return so it gets mixed into the fuel more quickly.

Thanks, I'll do that.

This carb, well fuel pump has a fuel return.

But if the jet is dirty / blocked, I cleaned that when I too out the idle valve.

By the way, funny sweet smell from the exhaust and a little bit of vapour.

Even with a fuel return on a carb engine, the pressure and flow is pretty low so it doesn't mix very quickly. With an injection engine, the pumps are generally rated at about 60psi and 30-40 galls/hr (that's a generalisation, specific models can have specific fuel delivery numbers) so if you use say a gallon an hour driving, 29-39 gallons gets circulated round from the tank, through the injection rail and out of the fuel pressure regulator (usually set ~40psi static, again depending on the engine) so as you can see, the pumps are over-specced on injection engines.

The worrying bit there is some vapour from the tailpipe and a sweet smell - is it a bit like you might imagine burned hazelnuts to be but pretty sweet? That's the nearest i can describe, your nose might smell something different though.