Morris Minor Owners Club

hi again,

I've recently checked my sparkplugs after changing the head gasket, and they're quite cokey. I believe this to be a sign of 'running rich', but I'm not sure how to fix the problem. I believe that there's multiple things that can leed to this, but I'm not exactly sure what, or in what order to check.

Cheers for any advice,
Chris

Hello Chris,
I would replace the plugs, or have them cleaned then have another look. A healthy engine has a tan colour, black and sooty is rich and if they are wet and black there could be some oil fouling.
If it is running rich, then the first check is the air filter, then ensure that the choke is coming off when the knob is pushed back in.
If those are OK I would take off the carburettor dash pot and clean the piston and the chamber inside using paint thinners or carburettor cleaner (don't use a metal polish or anything abrasive at all).
Then screw the mixture adjusting nut fully up level with the bridge. Then back it back off 12 flats, (two full turns), this is the basic setting, and all else being normal will be near to correct. Before putting the piston damper back, check that the needle shoulder is flush with the bottom of the piston. Also switch on the ignition and see what the fuel level is in the jet. If petrol is flowing, even slightly into the carburettor the float level is too high or the needle is not seating. Then reassemble the piston damper assembly after oiling the piston guide. Lift the piston up and check that it falls smoothly and stops at the bottom with a click.
Run the car and check again.
The above covers, I think, most of the common faults with an S.U.
Alec

A clever person once gave us the following advice:

To check if it is running rich - get it warm first
Lift the throttle slide (using the lifing pin under the carb)
If the revs rise it is rich
If the revs stay the same it is ok
If the revs fall, it is lean.

Hello Ray,
I have, in more years than I care to remember, never had any success with that method.
My favoured method for accurate adjustment is to use Colourtune plug(s) made by Gunsons, a really simple tool. Having said that any oil burning in the engine gives false readings.

Alec

The 'lifting the piston' check is very simple and, in my experience,
is a good indication of the mixture setting on an SU carb, but I
agree with Alec, To use a Gunsens Colourtune( which is a spark
plug made of glass) so that you can actually view the colour of
the explosion in each cylinder has got to be the best way of
checking the mixture. It also shows if the mixture is correctly
enriching during acceleration and, with a bit of experience, shows
if the needle profile is correct over the whole range.

Gunsens Colourtune

Looks like a nice tool to me. Is it available in the car-shops or do I have to go to a specialist? How much will it cost?

I bought mine from ......Halfords! I have no idea what the kit
costs nowadays.

Found a fan of the Gunsens Colourtune:
http://www.euronet.nl/~wilaben/bagtip1.html

Found it on a german website:
http://www.scheuerlein.de/
set with 1 plug: € 45,-; with 4 plugs: € 163,-

Will try Halford first.

If you look in practical classics, there is always an ad for someone selling these, cant remember how much for though....

I paid £1 for one at a car boot sale. The vendor didn't know what it was. Otherwise try:
http://www.ajmshopping.co.uk/listing.as ... alTestTune

The 'lifting the piston' check is very simple

I agree its not perfect but recently I have used it to get 2 Minors running a lot better than they were before and it only takes 5 minutes.

Cheers for the advice I'll have a play over the weekend. I don't know if I'll buy the colourtune (being on a students budget until the summer) but I'll look in halfords anyway.

Chris

i think they are £20 - £25 at halfords.

If you are confused by all of these high-tech methods of adjusting the mixture then here is a quick low-tech method which usually gives good results.

Scratch a mark on a flat on the mixture adjusting nut so that you can (if necessary) get back to where you started.

Warm up the engine.

Turn the adjusting nut to make the mixture richer until the engine starts 'hunting'. When this happens the engine speed will go up and down and up and down continuously.

Adjust the nut to make the mixture weaker. You should find that the engine speed increases to a peak and then decreases. Finally adjust the mixture so that the engine is running at the greatest speed.

NB

If the mixture is weak, fuel consumption will improve but:
1. The engine will lack power (especially going up hills)
2. Because a weak engine tends to run hotter you will risk burning out your exhaust valves.
3. When you switch the ignition off after a long hot run the engine may 'run on' i.e. keep on going for a while.

If the mixture is too rich you will not necessarily get more power but fuel consumption will go up.

You can also tell whether the mixture is rich or weak by looking at the inside of the rear end of the exhaust pipe but this can be confusing as it will be black if your engine is burning oil. I have a friend who did a chemistry degree and can tell the difference between soot from oil and soot from petrol by tasting it but I havn't tried this myself.

Newage, the exhaust pipe colour check used to work OK with leaded fuel, but with todays lead free fuels, all exhausts appear black inside & appear to run rich by this check.
The good sign of a well tuned car on leaded used to be a nice grey exhaust, especially after a long, fast drive . A near white one indicated a very lean engine.