Mixture Setting Problems

[svenedin]

I am having difficulties setting my fuel mixture. This is a job I have done many many times before and usually I have no trouble.

The problem is, I can set the idle mixture so that the idle is smooth and even and the mixture is correct according to the lifting pin test (lift the piston 0.8mm and the engine revs should slightly rise and then return to original speed). Now I know that spark plugs are not normally examined after tuning up at idle, but if I look at a plug with tuning at idling or fast idle (1,000 rpm) it is a good biscuit colour. The idle is smooth and the exhaust is even.

Then if I go for a drive the engine behaves well. Pulls well and behaves as expected. BUT on examining a spark plug on returning home after such a drive the spark plug is pale and suggests the moisture is too weak.

It's all very odd and this hasn't happened to me before. The engine is a standard 1098cc with all the standard bits. The HS2 carburettor was professionally rebuilt about 2 years ago. Yes the dashpot has oil and the correct oil. The needle is standard AN.

Back in May 2025 I drove significant distances in France and the plugs were a good biscuit colour. What has changed? Nothing I have done. I am very puzzled. One possibility is an air leak on the induction side. I have meticulously checked and I cannot find one. All the plugs are the same and consistent. They are standard spark plugs (NGK BP6ES) gapped to 30 thou (as I have a high output coil and Lumention Optronic ignition).

Any thoughts?

For now all I can do is monitor and tweak the mixture according to the spark plug colour after a decent drive. The trouble is this is probably going to lead to a rough idle which is too rich.

The only other thing I can think of is to check the fuel level in the float bowl.


Stephen

[King Kenny]

If the car runs well then I can't see why you need to check the plugs every trip. The colour of the plugs is not an exact science, there are so many variables that it would take forever to rule each one out. I used to check my Traveller's ignition and carburettor settings by driving up and down the hill at Bradford on Avon. If she ran up the hill and the exhaust burped all the way down I knew every thing was alright. I would just leave the plugs alone and enjoy the ride.

[svenedin]

Thank you. I was just sitting down and thinking exactly the same thing! If it runs well, which it does, who cares!

It does not misfire, hesitate, stutter or anything that would suggest the mixture is actually incorrect.


Stephen

[Bill_qaz]

Over thinking, if it's not broken don't fix it

[svenedin]

Over thinking, if it's not broken don't fix it

Yes you're right. It hadn't been running right. The plugs were white and the engine did run rough. I did a lot of fiddling and found an air leak at the oil filler cap (I wrote a separate thread about this and the effect on the PCV valve). I replaced the oil filler cap with a new one and immediately the idle was better. I did some minor mixture adjustment and the car ran well, very well in fact. The only thing is the plugs are still a bit pale but not as pale as they were and the car runs fine so now it's time to leave it alone!

I had a bit of a day of it today. My day off and lovely weather so I wanted to go out for a hike with my dog, naturally in the Morris. Tried to start the car and just a click. Tested battery, 12.87 V. Connected battery charger (which diagnoses battery), charger says battery charged and fine. Removed battery terminal clamps, cleaned terminals and clamps with emery, replaced and tightened. No improvement. Tried to crank engine by pushing solenoid button. No cranking. Changed solenoid. No cranking. Checked gearbox earth cable (because gearbox replaced last week). Earth cable fine, resistance test fine. Felt dismayed and put back the original solenoid, reconnected and bingo off we go!!!

I think it must have been a poor connection at the solenoid but why it did not work with a different solenoid I have no idea. Possibly the different solenoid is faulty (it's has never been used but is very old). Maybe the solenoid is on its way out. That's another possibility. I've ordered a new one as my spare doesn't work. I must just throw it away or I'm bound to accidentally use it again and get confused when it doesn't work.

Stephen

[Bill_qaz]

A bit confusing as your oil cap post was Wednesday and your mixture post Thursday.

Have you carried out a continuity test on the solenoid before you bin it?

[svenedin]

A bit confusing as your oil cap post was Wednesday and your mixture post Thursday.

Have you carried out a continuity test on the solenoid before you bin it?

Ah yes. Sorry about that. I just had the time yesterday to write the PCV valve post. It was a few weeks ago.

I need to test the new but old solenoid that didn't appear to work. I was in a hurry to go out and needed the car to start. I suppose I could have just used the starting handle but the engine was stone cold and now it has been rebuilt and has excellent compression it's rather hard work to turn over. The solenoid on my car is a Lucas round solenoid SRB316 with the push button underneath. This is not the original type of solenoid fitted to my car's year. Originally it had a square Lucas 4ST solenoid with small red push button. That car's original square solenoid was replaced in the 1990's but by that time the type with push button was no longer available. That always annoyed me so some years ago I fitted the SRB316. These push button types can be activated by pushing the button which is quite useful for testing. Anyway, it is possible the brand new (but old) solenoid (without push button) I found in the garage did not work because the coil earths through the body and although my engine bay has been completely refurbished I kept the rusty machine screws that attach the solenoid because I couldn't find any suitable replacements. I forget now but I think they are BA. I rather sloppily fitted it for testing purposes and maybe it had such a poor earth it could not operate. Putting the SRB316 back and everything magically worked because I assume, it was a bad connection at the solenoid all along. By that I mean a bad battery to solenoid connection. I have a battery quick disconnect, I took that apart as well and cleaned it all up.

I'll test the solenoid and apply some dielectric grease to the battery terminals and solenoid terminals. I could do without this hassle again when I need to go somewhere!

Stephen

[Bill_qaz]

You can always just bridge the two main terminals with pliers or screwdriver to start the engine if you needed to drive it.
Or clip jumper cable to starter side of solenoid and touch other end on batt neg in your case( + earth )
Remembering to have key on and check in neutral.

[svenedin]

You can always just bridge the two main terminals with pliers or screwdriver to start the engine if you needed to drive it.
Or clip jumper cable to starter side of solenoid and touch other end on batt neg in your case( + earth )
Remembering to have key on and check in neutral.

Thanks yes. I did consider doing this but I'm not a fan of the alarming big sparks! Definitely worth knowing if stranded somewhere.

Stephen

[Bill_qaz]

The quicker and more positive you make the connection the less sparking, its only what happens inside the solenoid

[svenedin]

Yes I’m sure. I’m just a bit scared of big amps.

Stephen

[mobylette]

To allay your fear of "Big Amps" just grab hold of the positive and negative terminals of your fully charged battery and observe the results

[geoberni]

To allay your fear of "Big Amps" just grab hold of the positive and negative terminals of your fully charged battery and observe the results

Except that doing that won't allow any 'Big Amps', putting a human across the terminals is far too much resistance.
'Big Amps' occur when very low resistance is placed across the battery, like accidentally shorting the battery with a misplaced spanner

That's why (if it's possible with some cars these days, it's not with my Ford ..) when disconnecting the battery, you should always do the Earth first, and when connecting it Earth last.
Accidental connecting of terminal to chassis doesn't matter if it's the Earth.
Then when doing the Supply terminal afterwards, should you accidentally connect to chassis, there's no effect because Earth is already removed.

[svenedin]

Exactly. The resistance of my body is far too much for puny 12 v of electromotive force but the resistance of a screwdriver across the solenoid terminals is very low and big amps flow to the starter motor which needs a lot of power to turn over the engine. I don't really like doing things like that due to the sparks, nasty sparky noise and the possibility of unintended spot welds! Actually I bypassed the solenoid to test the starter with a heavy duty jump lead with big insulated crocodile clips (connect non-earthed battery terminal to the terminal post of the starter, do not touch the body of the starter because that is earthed). There was a very healthy spark from the battery terminal as I touched the crocodile clip showing the battery was indeed fully charged and lively and a healthy kick from the starter.

My starter, almost unbelievably, is the original starter to the car and is dated 1/69. To my knowledge it has never had any maintenance at all and it still works perfectly. It is overdue service though and so I am going to take it to Robson and Francis Rewinds for a full overhaul. They can do the car's original dynamo at the same time. Then I have a set on the car of original starter, dynamo, carburettor, radiator, engine, distributor, wiper motor all of which will have been fully overhauled. I cannot tell whether the fuel pump is original. They're not date stamped. The dynamo was rebuilt once before in 1990. This is why I don't want to swap to an alternator. I doubt an alternator is going to work from 1969-2025 with one rebuild!!!


Stephen

[geoberni]

I cannot tell whether the fuel pump is original. They're not date stamped.

Stephen

Some earlier SU Pumps seem to have odd hand stamp digits around the base but I've never been able to make sense of them.

The aluminium body replaced the brass body in 1948, but they never changed the overall part number

I get the feeling that Burlen don't have complete records of changes that SU made; they know the changes happened, like from brass rings as guides to the plastic/nylon guide, but the production records of actual change points seem to have been lost...

The only other change point that has a known date is the change of Earth Screw from 2BA to 4BA in 1985

[svenedin]

That's a good point Berni. By the BA earth screw I know that the pump that was on my car when I got it in 1989 is pre-1985 but that's as far as it goes. Back then people needed their cars for daily use and pumps were just swapped out when they caused too much trouble. Now of course they're very expensive and absolutely worth a full overhaul when they can be picked up very cheaply in "spares only" condition.

I have a bit of a winter queue forming of branch members who want me to fix their pumps!

Stephen

[svenedin]

I tested the new (old) solenoid. There is continuity in the coil with a very low resistance (tested resistance between points A and B). Whether it actually works I will have to determine by connecting A and B up to a test battery and then measuring the resistance across the switched contacts (C and D).

This is a Lucas 4ST solenoid. Later type with black plastic dome in place of the red rubber push switch. Wish they still had the switch.

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[svenedin]

I did a bit more research on sparking plug colour. Looked at lots of charts. My conclusion is that an electrode that is very pale or white is definitely showing a mixture that is too weak. This will be no surprise because the engine will be running rough and will not have a good steady idle.

It seems that with modern ethanol containing fuels, the digestive biscuit colour is not the only colour that shows a good fuel/air mixture. A grey or grey/tan electrode is also acceptable. I have found over the years that the standard Minor sparking plugs (Champion N9YC or NGK BP6ES) are a bit cool for my particular engine. Due to the Smiths PCV valve system that I have, the standard plugs can get a bit too much carbon build up from the oily fumes sucked in to the engine. Not on the electrode itself but on the end of the threaded part. I have found a small tweak to NGK BP5ES which are slightly hotter offsets this. Confusingly, NGK plugs with a lower number run hotter but Champion plugs with a higher number run hotter. I found a chart that shows that sparking plugs actually have a wide temperature range at which they will operate properly.

Here are some resources that I found. The SU mixture setting advice is very good. Not just the use of the carburettor lifting pin but looking at the exhaust colour and listening to the exhaust rhythm. The difference between a small irregularity of the exhaust rhythm and a very even exhaust rhythm might just be half a flat of the mixture nut. The other thing to notice is to look at the engine. When the mixture is spot on the engine runs with very little vibration. When the mixture is wrong, the engine vibrates unhappily.

Some tips for using the lifting pin to set the mixture. Run the engine up to temperature, ideally by going for a drive. Remove the air filter so that you can look in to the carburettor air intake and see the piston. Slacken the choke pinch bolt so that the jet can move up and down without restriction. Remember that the lifting pin method relies on a very small lift of the piston. When you lift the piston look at how much it rises; it is a tiny rise that is needed. It takes patience to get this right. Take your time. Before adjusting the mixture, get everything else correct: ignition timing, points gap, clean spark plugs correctly gapped etc. There is no point getting the mixture spot on and then putting a dirty clogged air filter back on. The mixture will immediately be wrong.

Stephen

Mixture setting (SU advice, single HS carburettor): https://sucarb.co.uk/technical/technica ... ing-single

Sparking plug temperature guide: https://www.ngkspark.co.nz/technical_in ... mperature/

Sparking plug temperature cross reference table: https://www.opieoils.co.uk/t-spark-plug ... 4QU71pYGNx

Electrode colour chart: https://ngksparkplugs.com/en/resources/read-spark-plug
https://www.championautoparts.com/Parts ... plugs.html

[svenedin]

I had an interesting morning at Sanspeed tuning in Bexley.

The Morris was put on the rolling road dyno and the results were interesting. Before adjusting anything the result was 40 BHP. The ignition timing was tweaked slightly to give more mechanical advance and the mixture checked. At idle the fuel mixture was fine but at higher revs it was too weak.

The engine is a standard 1098cc with HS2 carburettor, standard air filter etc. Standard AN needle. Compressions checked and 175 psi in all cylinders.

Peter, the tuner concluded that when the engine was rebuilt it had a camshaft fitted which is not exactly the same profile as the original. The new camshaft has 16 thou rocker clearances whereas the standard has 12.

The SU needle profile charts were consulted but there was no obvious ideal needle in stock so Peter custom profiled the AN needle to give more fuel at higher rpm. Perhaps an H6 would have been suitable.

The result a useful of gain of 5 BPH to give 45 BHP (a 12.5% power increase) and a more responsive engine. Very pleasant 18 mile drive home.

Well worth getting a rolling road tune done if there has been a major change such as an engine rebuild. There is a limit to what can be done at home. You can get the idle fuel mixture spot on but you can't really judge whether the fuel mixture is correct up through the rev range.

Stephen

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[mobylette]

45bhp at the wheels is very good, 1098 only rated at 48bhp