Fitting an air fuel ratio meter

[Derek UK]

Plug cuts with modern fuels are very unreliable. You no longer get them looking like the pictures in the Haynes. Your plug images don't look very nice. Looking at the numbers on your plugs I might guess they are rather old. Maybe freshly used but still old. I don't think Bosch have used that numbering system for a long time. Yes I do remember it and it may be listed in the Volvo documentation but that was printed 50+ years ago. I would consider a nice set of NGK BP6HS. They may get it running sweeter.

[142 Guy]

The hot cut method is necessary if you are trying to read the spark plugs to estimate full to mid load engine output and then you have to look at the insulator farther down the insulator (closer to where it contacts the metal plug body). The tip of the insulator is the indicator for idle mixture conditions. Drive the car around enough so that it has been up to normal temperature for a bit, stop, let it idle for a couple of minutes and then turn it off. That should get you a reasonable 'plug read' for idle conditions.

The color of the tips of your insulators suggest to me that your idle fuel mixture is very close to 14.7 or maybe a little bit higher (leaner). I run my B20E with aftermarket digital fuel injection and closed loop fuel control right around 14.7 in the idle region and last time I looked at the plugs (probably 2 years ago) the color (not the condition) would have been very similar to yours. Very much like the color of the second plug in your photos which doesn't look too bad. If you really are running an AFR of 13 at idle I would expect the plugs to be more on the tan side.

What is notable about your plugs compared to mine (last time I looked) is that you seem to have a huge amount of deposits on the plug insulator (all that grainy stuff). The last plug in your photos looks quite nasty. Ash deposits can come from the fuel (millers additive?) and can come from the oil. The oil may be a notable contributor on a B18 because they have those umbrella valve stem things instead of the more modern seals. You might need to do a little research; but, if you are using one of the oils with a lot of ZDDP in it, the lower effectiveness of the umbrella seals combined with a high additive oil may be causing the deposits. A very slow leak of glycol engine coolant into the combustion chamber can also cause deposits to form on the plugs.

Whitish deposits can also form if the plug is running hot There is some reaction with iron and oxygen which can take place at higher plug surface temperatures. These kind of deposits run from 'ash like - you can brush them off' which is OK to '' hard and sometimes glassy' which is an indication of way too hot. Are you using the recommended heat range for your plugs?

The deposits make getting a good read on your idle fuel mixture more challenging. As a test, I would suggest ditching the fuel additive and reviewing what type of motor oil you are using. If it is a high additive oil perhaps switch to something more conventional for a while. Drive around for a while to make sure your fuel is additive free and then insert a new set of plugs of the recommended heat range. Drive around and then pull the new plugs to examine the tip color. Of course, if your front and back carbs are still doing mysterious different things this is going to make consistent reads a challenge.

I live in North America, so different fuels. The only carb equipped engines I currently have are Honda GX and GC engines on lawnmowers, pressure washers and the like. Whenever I replace the plugs on these engines they all have the tan - brown insulator color of an engine running with an AFR of less than 14.7 so modern regular grade pump gasoline where I live continues to display the brown = rich color skew.

[Rustinmotion]

Originally Posted by Derek UK

Plug cuts with modern fuels are very unreliable. You no longer get them looking like the pictures in the Haynes. Your plug images don't look very nice. Looking at the numbers on your plugs I might guess they are rather old. Maybe freshly used but still old. I don't think Bosch have used that numbering system for a long time. Yes I do remember it and it may be listed in the Volvo documentation but that was printed 50+ years ago. I would consider a nice set of NGK BP6HS. They may get it running sweeter.

Thanks Derek
Have ordered plugs it won’t hurt to change them

[Rustinmotion]

Originally Posted by 142 Guy

The hot cut method is necessary if you are trying to read the spark plugs to estimate full to mid load engine output and then you have to look at the insulator farther down the insulator (closer to where it contacts the metal plug body). The tip of the insulator is the indicator for idle mixture conditions. Drive the car around enough so that it has been up to normal temperature for a bit, stop, let it idle for a couple of minutes and then turn it off. That should get you a reasonable 'plug read' for idle conditions.

The color of the tips of your insulators suggest to me that your idle fuel mixture is very close to 14.7 or maybe a little bit higher (leaner). I run my B20E with aftermarket digital fuel injection and closed loop fuel control right around 14.7 in the idle region and last time I looked at the plugs (probably 2 years ago) the color (not the condition) would have been very similar to yours. Very much like the color of the second plug in your photos which doesn't look too bad. If you really are running an AFR of 13 at idle I would expect the plugs to be more on the tan side.

What is notable about your plugs compared to mine (last time I looked) is that you seem to have a huge amount of deposits on the plug insulator (all that grainy stuff). The last plug in your photos looks quite nasty. Ash deposits can come from the fuel (millers additive?) and can come from the oil. The oil may be a notable contributor on a B18 because they have those umbrella valve stem things instead of the more modern seals. You might need to do a little research; but, if you are using one of the oils with a lot of ZDDP in it, the lower effectiveness of the umbrella seals combined with a high additive oil may be causing the deposits. A very slow leak of glycol engine coolant into the combustion chamber can also cause deposits to form on the plugs.

Whitish deposits can also form if the plug is running hot There is some reaction with iron and oxygen which can take place at higher plug surface temperatures. These kind of deposits run from 'ash like - you can brush them off' which is OK to '' hard and sometimes glassy' which is an indication of way too hot. Are you using the recommended heat range for your plugs?

The deposits make getting a good read on your idle fuel mixture more challenging. As a test, I would suggest ditching the fuel additive and reviewing what type of motor oil you are using. If it is a high additive oil perhaps switch to something more conventional for a while. Drive around for a while to make sure your fuel is additive free and then insert a new set of plugs of the recommended heat range. Drive around and then pull the new plugs to examine the tip color. Of course, if your front and back carbs are still doing mysterious different things this is going to make consistent reads a challenge.

I live in North America, so different fuels. The only carb equipped engines I currently have are Honda GX and GC engines on lawnmowers, pressure washers and the like. Whenever I replace the plugs on these engines they all have the tan - brown insulator color of an engine running with an AFR of less than 14.7 so modern regular grade pump gasoline where I live continues to display the brown = rich color skew.

Il fit new plugs at the next fill up and leave out the additive
I’m not burning oil that I can see and have not needed to top up the coolant so might just be the fuel or the additives
I almost always use Shell V power as it’s an E5 fuel and Millers VSPe as I don’t know if the exhaust seats are hard to cope with unleaded

DESCRIPTION:
An ‘all in one’, multi shot, petrol fuel treatment providing ethanol protection, lead replacement and an octane improver dependent on selected fuel quality.

APPLICATION:
Recommended for all classic engines previously requiring leaded fuel and now running on unleaded fuel. One bottle treats 10 tanks, based on 50L tank size.

USER BENEFITS:
Endorsed by the FBHVC as a fuel treatment for protection against corrosion in metals
Octane enhancement of up to 2 octane numbers (20 points) dependent on fuel quality
Prevents valve seat recession when used with unhardened valve seats
Prevents detonation, hot spots & running on
Prevents fuel system corrosion and helps protect components
Combats the adverse effects of ethanol in fuel (up to E10)
Carburettor anti-icing formula
Contains anti oxidant to improve fuel life

[142 Guy]

The transition to unleaded fuel probably had as much to do with extended spark plug life as the switch to closed loop fuel injection systems. Crappy looking sparkplugs were kind of the norm in the good old days. If you have a lead replacement in the fuel additive package that may be responsible for the deposits. It has been 35+ years since I looked at a plug from an engine that was running leaded gas so that possibility never occurred to me.

I still find it odd that the #2 plug looks so much cleaner than the other three. Do you have a non contact IR thermometer? If you could measure the individual exhaust runner temperatures just beyond the exhaust flange that might tell you whether you have an uneven fuel mixture issue. At idle, a lean cylinder exit temperature will be lower than a rich cylinder. I recognize that measurement can be rather difficult / impossible with some of the B18 manifolds.

[Rustinmotion]

Originally Posted by 142 Guy

The transition to unleaded fuel probably had as much to do with extended spark plug life as the switch to closed loop fuel injection systems. Crappy looking sparkplugs were kind of the norm in the good old days. If you have a lead replacement in the fuel additive package that may be responsible for the deposits. It has been 35+ years since I looked at a plug from an engine that was running leaded gas so that possibility never occurred to me.

I still find it odd that the #2 plug looks so much cleaner than the other three. Do you have a non contact IR thermometer? If you could measure the individual exhaust runner temperatures just beyond the exhaust flange that might tell you whether you have an uneven fuel mixture issue. At idle, a lean cylinder exit temperature will be lower than a rich cylinder. I recognize that measurement can be rather difficult / impossible with some of the B18 manifolds.

Il see if I can borrow one but the header is wrapped and difficult to get a reading
My main concern is the engine running lean as that can melt pistons, to me the plugs look white so my old brain says lean, gauge says slightly rich at around 13 on the photo I’m getting a fairly constant light at the max power mark sometimes one light weaker and leans off momentarily to lambda when I step off the gas
Il run new plugs and a tank of gas without additives just to see what I get and report back

[142 Guy]

The precise way to measure exhaust temperatures is a thermocouple stuck right in the exhaust stream. Piston engine aircraft operators have been using this method for fuel mixture control for decades. Measuring the outside of the header / manifold runner is a distinct second best. A wrap on the header / manifold probably drops this into the don't even bother trying category.

Running lean fuel mixtures at lower engine load (particularly idle) is a non issue in terms of melting things. Lean AFRs are an issue at high engine loads; but, not because they directly create high combustion temperatures. At high engine loads the combustion temperatures are naturally higher (more heat = more expansion = more power). The problem with a high AFR ratio at high engine output is that the it leaves excess O2 hanging around after the fuel has burned up. Get the temperature of the crown of the aluminum piston high enough and it will react with the free O2 remaining in the combustion chamber. This is an oxidization reaction. The piston literally catches on fire which then releases more heat and can cause melting around the ignition point. In the good old days school science teachers used to demonstrate how you a ball of steel wool could burn up in an O2 rich environment. Aluminum does the same thing; but, at even lower temperatures which is why the piston goes first.

At idle or low engine loads you can safely run AFRs at or above 14.7 because combustion temperatures are generally not high enough to initiate the violent oxidization reaction. But, most older pre emission control carburetted engines do not idle very well at higher AFRs so they typically run below 14.7 with the slightly refinery like smell in the exhaust. If you are happy with the way your engine idles and the insulator right around the center electrode is on the white side you are probably OK.

In terms of high engine load you do not want to be operating close to 14.7 for two reasons
- Air / fuel mixtures, particularly on older engines, are non homogenous. Since most engines are limited by the amount of air they can flow it is beneficial from a power maximization point of view to make sure that all the precious oxygen gets used up to produce heat. You do this by ensuring that there is more than enough fuel which typically means running an AFR around 13. In this case, too much is not a good thing. Liquid fuel requires heat to vaporize and when the AFRs get into the 12 range power can drop because of heat loss due to vaporization of fuel which does not get used up in the combustion process.
- Running an AFR closer to 13 helps insure that all the O2 gets used up which avoids the nasty problem of having free O2 available to react with your hot aluminum piston crown.

In order to assess what is happening at high engine load you need to be looking way down into the spark plug at the color of the porcelain close to where it contacts the outer metal shell of the spark plug. This really requires a bright light and a jewelers loupe to see what is going on down there. A lot of experience helps because this does seem to fall into the practise of the 'dark arts' . Personally, I kneel at the alter of the Bosch LSU 4.9. I prefer to trust in a wide band O2 sensor as opposed to what I imagine I am seeing when staring into a spark plug.

[Rustinmotion]

Just a quick update, fitted new plugs and over a few runs adjusted the fuel to get this on cyl 2 and 3, still not a perfect idle but I think I’m close