Mixture Problems (too rich)

[svenedin]

The car started to misbehave in traffic yesterday. The engine revs would drop and the engine repeatedly sounded as if it was just about to stall. Increasing the revs with the accelerator and it would run fine. Very odd I thought but maybe fuel vaporisation (although this car has never done it).

When I got home I investigated. It seemed like the fuel pump was not doing anything until it was too late and the float bowl was almost completely out of fuel. This seemed odd as no changes have been made recently. I found a very poor condition earth connection to the base of the pump, replaced this and the problem has gone. As well as the connection hardly being connected at all, the insulation was discoloured where it had overheated.

Anyway, one problem solved but another found. Over the last few months, the spark plugs have been sooty (not oily) suggesting the mixture is too rich. I moved up one heat step in spark plug but this has made no difference at all. I use a "Colortune" to adjust the mixture but I do also use the carburettor lifting pin and of course my ears (for engine speed) and nose (for petrol stink from exhaust) and eyes (for smoky exhaust). The spark plugs being sooty suggest the mixture is too rich, this is all the plugs not just one. BUT, when I adjust the mixture weaker, closer to "correct" according to the Colortune the engine revs drop and it falters. At that time lifting the carburettor lifting pin also causes the engine to immediately try to stall, agreeing with the Colortune.

Compressions across all cylinder is consistent and good at 150 psi.

The only thing I can think of is that the engine is burning oil. Burning oil confuses the Colortune as it makes the mixture burn yellowish. The exhaust does smell like it is burning oil to some extent but there are no clouds of blue smoke. This engine has a Smiths PCV valve which is very effective at drawing in crankcase blow-by gases. It should not be drawing in much oil though as I replaced the wire mesh in the tappet chest oil separator recently.

The cylinder head was professionally refurbished in September 2023 and I changed the heads gasket (copper) at that time. The carburettor was professionally refurbished in January 2024. I have checked the float adjustment and it is correct.

I am puzzled. Perhaps this is just a sign that the bottom end of the engine is overdue a major overhaul.

Stephen

IMG_9826.jpeg (1.36 MiB) Viewed 456 times

[philthehill]

Forget about using the Colortune.
If the fuel level in the float chamber is correct use the piston lift pin to determine if the mixture is correct or not.
If when lifting the lift pin the engine speed rises and then drops back the mixture is right.

[svenedin]

Forget about using the Colortune.
If the fuel level in the float chamber is correct use the piston lift pin to determine if the mixture is correct or not.
If when lifting the lift pin the engine speed rises and then drops back the mixture is right.

OK. I will try but if have always found this method very difficult. I wonder whether I am doing it right? I lift the pin and take up the slack (nothing happens). Then I feel the pin start to lift the piston and lift a very small distance. Whenever I have done this (for years) the engine always tries to stall. Not once have the revs ever risen. I think I am lifting the pin too far. I will take the air filter off so I can see the piston and try to lift only the 1/32” book figure

Stephen

[svenedin]

I took the air filter off, disconnected the choke cable and had bright light shining so that I could see the carburettor piston. Eventually I think I got the hang of it. A tiny lift on the lifting pin does cause the engine revs to momentarily rise and then return. It's very subtle though.

Using the lifting pin to adjust the mixture is one of those things I never got the hang of but I may finally be getting there. Don't mention beehive springs though as I am unable to fit those despite numerous attempts over the years!

Stephen

I made a short video. The engine revs are 850 rpm (as measured with a strobe-tacho). Incidentally, with a rebuilt dynamo, my dynamo will charge the battery at those revs (charging light out completely). The engine does not like idling much lower than this.

https://youtu.be/8zF748Oax_E

[Chief]

The engine does not like idling much lower than this.

As an inexperienced mechanic...

Seemed to have a slight stutter between the 6-7 second mark of your video almost like it lost fuel momentarily and then caught itself again.

[philthehill]

The revs of the engine shown in the video to my ears are too high.
To set the idling revs of the Minor 'A' Series the ignition warning light should just start to glow when the clutch pedal is depressed with the engine running.

[svenedin]

The revs of the engine shown in the video to my ears are too high.
To set the idling revs of the Minor 'A' Series the ignition warning light should just start to glow when the clutch pedal is depressed with the engine running.

I can bring the idle revs down a bit. It sounds fine at 800 even 750 but at 700 it starts to go a bit lumpy.

Stephen

[mowogg]

Did you change the needle in the carb to suit the pcv valve? My understanding is you are likely to require a leaner needle over the operating range with a pcv? I am still experimenting but found my van was better once I changed the needle post pcv valve being fitted.

[svenedin]

Did you change the needle in the carb to suit the pcv valve? My understanding is you are likely to require a leaner needle over the operating range with a pcv? I am still experimenting but found my van was better once I changed the needle post pcv valve being fitted.

I didn't actually and I did not know anything about that. It is just a standard needle and as far as I know that is what the carb has always had. I have always had a PCV. I can adjust the mixture lean but the engine will not tolerate it so there is enough adjustment. I think I made progress today and the engine seems to run sweetly. I will not know until I have had some good runs and have inspected the spark plugs. They were sooty but they cleaned up easily with a spark plug brush and carb cleaner. They did not have any encrusted coke on them.

Stephen

[svenedin]

Another tinker with the mixture setting.

Chief you were correct that there was a slight misfire in my previous video. Well spotted. I had dropped a spark plug when I was cleaning it. I checked that the insulator was not cracked but I did not re-check the gap. It must have landed on its nose because the spark plug gap was too small on cylinder 1.

Using the carburettor lifting pin I refined the mixture setting a bit further today and it needed to be weakened a bit more (about 1 flat of the nut).

Idle I have slowed down to 770 rpm.

No black smoke on revving. Seems about right.

In preparation for a trip to Europe this Summer (and anticipating traffic jams in heat) I have fitted a stainless steel carburettor heat shield and Grumpy rising fuel pipe heat shield. I have actually never experienced fuel vaporisation in this car but I thought I would take precautionary measures because I have never taken the car overseas before.

Stephen

https://youtube.com/shorts/RP3hIiVVamg?feature=share

[Nickol]

In preparation for a trip to Europe this Summer (and anticipating traffic jams in heat) I have fitted a stainless steel carburettor heat shield and Grumpy rising fuel pipe heat shield. I have actually never experienced fuel vaporisation in this car but I thought I would take precautionary measures because I have never taken the car overseas before.

Stephen

"overseas" is always nice - where will you be off to as a matter of interest? .......and the tuned motor sounds really nice.

[svenedin]

In preparation for a trip to Europe this Summer (and anticipating traffic jams in heat) I have fitted a stainless steel carburettor heat shield and Grumpy rising fuel pipe heat shield. I have actually never experienced fuel vaporisation in this car but I thought I would take precautionary measures because I have never taken the car overseas before.

Stephen

"overseas" is always nice - where will you be off to as a matter of interest? .......and the tuned motor sounds really nice.

Hello Nickol.

Thank you for the compliment. I am going on Minors on Tour to Eastern Holland near the German border in late May/early June and then probably the Loire area of France later in the Summer. I would not think it will be hot in May/June but you never know. August is usually quite hot.

I did quite a lot of research on fuel vaporisation even though it has never really affected my car. It is a complicated subject. I learned that the petrol on sale at petrol stations is different in different seasons of the year. Winter petrol has a lower boiling point and is more volatile than petrol sold in Summer. I knew that diesel fuel was different in Winter and Summer but I did not know that petrol varied as well.

This article tells you probably more than you ever wanted to know about petrol and old cars (see the section 'fuel volatility").

https://www.fbhvc.co.uk/fuels#vapourisation


Stephen