240 General: New (to me) 1980 Volvo 244

[Bob 1967]

I knew I had one in the shed somewhere....


Hadn't even been opened


I was in the habit of buying tools and equipment that I might need someday.

Better to be looking at it, than looking for it!

[Othen]

Originally Posted by Bob 1967

I knew I had one in the shed somewhere...


Hadn't even been opened

I was in the habit of buying tools and equipment that I might need someday.

Better to be looking at it, than looking for it!

Most of us are like that when it comes to tools Bob :-)

[Othen]

Unfortunately my purchase of a new Gunson Gastester digital fell through - the supplier reported it was out of stock (but did credit the cost back to me straight away). Fortunately I found a second-hand unit for sale on eBay for only £82, and the seller worked nearby in Kettering. I collected it today and am pleased to say it works just fine.



... after purging it thoroughly and then calibrating the sensor I have tried it out with the Barge. After my recent changes (disabling the constant air temperature device and changing the thermostat) I now get 3.9% CO content:



... that is a bit higher than I'd like (3.0% is the target), but it would be an MoT pass. In the fullness of time (but not today) I'll strip down the carburettor, change the needle/jet assembly (I have a spare somewhere) - lower the needle one notch and swap the diaphragm.

I'll arrange for the emissions to be checked again at F1 one day next week (and so corroborate my meter's reading).

:-)

[Martin Cox]

I remember buying the earlier analogue version of the Gunsons Gastester.

I never managed to get any sensible readings at all from the thing. I must have wasted hours and a fair amount of fuel trying to get something useful out of it It was relegated to the darker recesses of the garage until it was eventually passed on to an unsuspecting friend and I went back to using the Colortune.

Martin

[Othen]

Originally Posted by Martin Cox

I remember buying the earlier analogue version of the Gunsons Gastester.

I never managed to get any sensible readings at all from the thing. I must have wasted hours and a fair amount of fuel trying to get something useful out of it It was relegated to the darker recesses of the garage until it was eventually passed on to an unsuspecting friend and I went back to using the Colortune.

Martin

That is interesting.

This one seems to report sensible results - but I did purge it thoroughly (10 minutes) first and again afterwards. I'm pretty confident it is working properly, but I will take it to the MoT garage (F1) on Wednesday next week for a confidence check.

Alan

[Laird Scooby]

Originally Posted by Martin Cox

I remember buying the earlier analogue version of the Gunsons Gastester.

I never managed to get any sensible readings at all from the thing. I must have wasted hours and a fair amount of fuel trying to get something useful out of it It was relegated to the darker recesses of the garage until it was eventually passed on to an unsuspecting friend and I went back to using the Colortune.

Martin

If an engine is running excessively lean, this increases the HC level - the gastester reads that as excess CO and reports a rich mixture. Turning the mixture screw towards lean exacerbates the situation. While this is counter-intuitive, turning the mixture towards rich will improve things.

This was one of the primary causes for negative reports on these devices. Also incorrect fitment of the 8mm bore clear plastic hoses on the Airpulse pump on the back of the unit. The one from the exhaust should go to the lower of the two 8mm stubs while the sampling tube (about 4-6" long) goes to the top stub which then goes to the actual part of the unit that measures the CO using a thermistor.

Again, as CO is heavier than air this is counter-intuitive, however the exhaust gases are hot, reducing the Relative Density of the CO content so it actually rises, hence the arrangement of the hoses on the Airpulse pump. It also means any water content is allowed to condensate out of the Airpulse pump, courtesy of the ~3mm pipe.

[Othen]

Originally Posted by Laird Scooby

If an engine is running excessively lean, this increases the HC level - the gastester reads that as excess CO and reports a rich mixture. Turning the mixture screw towards lean exacerbates the situation. While this is counter-intuitive, turning the mixture towards rich will improve things.

This was one of the primary causes for negative reports on these devices. Also incorrect fitment of the 8mm bore clear plastic hoses on the Airpulse pump on the back of the unit. The one from the exhaust should go to the lower of the two 8mm stubs while the sampling tube (about 4-6" long) goes to the top stub which then goes to the actual part of the unit that measures the CO using a thermistor.

Again, as CO is heavier than air this is counter-intuitive, however the exhaust gases are hot, reducing the Relative Density of the CO content so it actually rises, hence the arrangement of the hoses on the Airpulse pump. It also means any water content is allowed to condensate out of the Airpulse pump, courtesy of the ~3mm pipe.

This is a different meter from the one you describe Dave (I suspect you mean the previous model, the yellow one with the analogue gauge). This one has three 8mm plastic pipes. The short one is for the lowest port on the pulse pump (water drain), the longest one fits on the middle port of the pulse pump and runs to the aluminium probe that fits in the exhaust pipe, the remaining tube goes from the outlet of the pulse pump to the collector box.

The HC level is fine, that isn't the issue.

Alan

[Laird Scooby]

Originally Posted by Othen

This is a different meter from the one you describe Dave (I suspect you mean the previous model, the yellow one with the analogue gauge). This one has three 8mm plastic pipes. The short one is for the lowest port on the pulse pump (water drain), the longest one fits on the middle port of the pulse pump and runs to the aluminium probe that fits in the exhaust pipe, the remaining tube goes from the outlet of the pulse pump to the collector box.

The HC level is fine, that isn't the issue.

Alan

I daresay either they changed the water drain tube or it was lost and the previous got a piece of 8mm hose to go over rather than inside the water drain stub Alan.

However, the sampling circuit works the same, whether it's an anlogue display or digital.

It's essentially a Wheatstone Bridge, the lower legs are thermistors, upper legs fixed resistors fed by a potentiometer (wiper on +ve, each end goes to the fiexed resistors) and the meter is obviously in the usual place between the junctions of the fixed and thermistors.

They are NTC thermistors and are both heated to a constant level by the fixed resistors/potentiometer. One thermistor is the control value and is isolated from the exhaust gases. The other is fed by the sampling tube. The CO content cools the thermistor increasing resistance, therefore increasing the +ve potential on the joint between the smapling thermistor and fixed resistor in that leg of the bridge. This results in increased deflection of the analogue meter or higher numbers dispalyed on the digital display.

I was not suggesting the Barge had high HC content Alan, merely pointing out that many negative reports on these devices were mainly down to users trying to tune engines with high HC and/or excessively weak mixtures causing high HC content.

[Martin Cox]

I did indeed mean the earliest model; yellow with an conventional analogue meter.

The sensor was a flat black plastic affair mounted on the end of a rigid black plastic tube with a clip on the side that was supposed to retain it in the end of the exhaust pipe. It didn't have a pulse pump and must have relied purely on the exhaust gases diffusing through it.

The sensor operated as Dave described except that I don't think it used thermistors. In deperation I opened it up and it consisted of two wire coils, one of which was exposed to the exhaust gas and the other wasn't. It's likely the wire had a positive temperature coefficient of resistance unless it was something very fancy.

Anyway I think it drifted badly, was very slow to respond and was soon superceded by the digital model.

Moral; Be careful buying version one of anything!

[Othen]

Originally Posted by Martin Cox

I did indeed mean the earliest model; yellow with an conventional analogue meter.

The sensor was a flat black plastic affair mounted on the end of a rigid black plastic tube with a clip on the side that was supposed to retain it in the end of the exhaust pipe. It didn't have a pulse pump and must have relied purely on the exhaust gases diffusing through it.

The sensor operated as Dave described except that I don't think it used thermistors. In deperation I opened it up and it consisted of two wire coils, one of which was exposed to the exhaust gas and the other wasn't. It's likely the wire had a positive temperature coefficient of resistance unless it was something very fancy.

Anyway I think it drifted badly, was very slow to respond and was soon superceded by the digital model.

Moral; Be careful buying version one of anything!

Thank you for that clarification Martin, I think Dave was referring to the yellow box version also. I'm quite happy with the black box digital model, it seems to do exactly what it says on the tin. It will be interesting to see how close the results are to a professional meter when I take it back to F1 next Wednesday.

Alan