340 1.4 rough ans slow idling

[Two340'sman]

I think you may be right, stupid me, I'll check!

[Two340'sman]

Yes, stupid me, I had the switch in the check position.

Switch in the other position the meter is fully to the left, no adjustment possible with the control knob.

[Laird Scooby]

Originally Posted by Two340'sman

Yes, stupid me, I had the switch in the check position.

Switch in the other position the meter is fully to the left, no adjustment possible with the control knob.

Check the physical mounting of the cal pot, i found on mine it was mounted skew-whiff so the shaft had been bent for so long, the pot wiper was no longer making contact with the track inside the pot!

[Two340'sman]

Originally Posted by Laird Scooby

Check the physical mounting of the cal pot, i found on mine it was mounted skew-whiff so the shaft had been bent for so long, the pot wiper was no longer making contact with the track inside the pot!

Will do, good tip. Thank you.

[Two340'sman]

Slider of pot was contacting track OK. Still meter needle hard up to left side of scale.

[Laird Scooby]

Originally Posted by Two340'sman

Slider of pot was contacting track OK. Still meter needle hard up to left side of scale.

In that case, i think you'll have to trace the circuits through. The thermistors should be either 2.5 or 5k @ room temp, depending which they used then it's just the 4th leg of the Wheatstone Bridge you'll have to find and measure.

Probably something silly somewhere...........

[Two340'sman]

Originally Posted by Laird Scooby

In that case, i think you'll have to trace the circuits through. The thermistors should be either 2.5 or 5k @ room temp, depending which they used then it's just the 4th leg of the Wheatstone Bridge you'll have to find and measure.

Probably something silly somewhere...........

Thanks,

I'll get the other gas tester from my friend, check that one, and perhaps compare component readings.

[Laird Scooby]

Originally Posted by Two340'sman

Thanks,

I get the other gas tester from my friend, check that one, and perhaps compare component readings.

Good idea - been trying to find the basic circuit for them online with no success, i have it on an old hard drive but can't manage to access it at the moment.

I'm sure it was Practical Electronics from 1967 or 1972 that did a constructional project using the same idea and gave details of calibration etc. Tried searching for back copies online and only found one from January 1977 but the meter for it only shows "Good-OK-Bad" and seems to read "backwards" - the higher the deflection of the meter, the better the emissions.

[Two340'sman]

Originally Posted by Laird Scooby

Good idea - been trying to find the basic circuit for them online with no success, i have it on an old hard drive but can't manage to access it at the moment.

I'm sure it was Practical Electronics from 1967 or 1972 that did a constructional project using the same idea and gave details of calibration etc. Tried searching for back copies online and only found one from January 1977 but the meter for it only shows "Good-OK-Bad" and seems to read "backwards" - the higher the deflection of the meter, the better the emissions.

Good of you to think of all this. I used to get Practical Wireless from about 1964 to 1969, I think there was a circuit in there for a car gas analyser. I still have most of the magazines.

[Laird Scooby]

Originally Posted by Two340'sman

Good of you to think of all this. I used to get Practical Wireless from about 1964 to 1969, I think there was a circuit in there for a car gas analyser. I still have most of the magazines.

I may have the publication wrong, it might well have been Practical Wireless, i have however found a similar circuit :

https://archive.org/details/Heathkit...lyzerschematic

That is along the lines of the "Good-OK-Bad" meter as mentioned further up but with a bit of thought, could easily be changed so it would be calibrated at 2% in free air (i believe the reason for this is there is naturally 2% of CO in free air) which would just be a different meter and different resistors in the bridge. The test switch would short out one of a pair of resistors in one leg of the bridge where the resistors are to simulate FSD, the calibration pot would be in the other leg, likely with another resistor in series to prevent overloading the meter.

Might have a look at that tomorrow, see if i can come up with something. Note that circuit uses a 5V (or 4.7V) Zener to generate the stabilised 5V supply that the LM7805 in the Gunsons uses. I suppose when it was first designed, LEDs were too new and expensive to "waste" by reverse-biasing one to give 5V............