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MG MGB Technical - Cylinder Head Decoke Options

I've searched through the archives and found some "mechanics fix" for decoking engines (removing carbon buildup). Various methods use straight water, others use a 50/50 diesel/water mix. Question is, do these methods work and are they safe?

I'm considering this on a 12year old, low mileage engine, with very black film inside exhaust and very black (dry) plugs, with very bad dieselling problem, cos I don't wanna pull the head right now.
Joe

I've heard that the unleaded fuel we use these days always creates a thick black deposit in the exhaust - and it's certainly true in the case of my wife's fairly new Micra. If your plugs are black then that suggests the mix is too rich and maybe that's why you're dieseling ?
Miles Banister

Very black and sooty exhaust deposits? Very black plugs? Carb mixture running a bit rich, are we?
You might want to turn the needle mixture nuts (on SU carbs) up a flat or so before decarbonizing
or it'll all soon return again. It won't hurt to check the timing, and valve lash while you're there, too.

Anyway, the factory manual recommends that the cylinder head be removed and scraped clean
periodically, but this is rather inconvenient and expensive to do regularly. This procedure may also
be beyond the skills and resources of many occasional mechanics.

Years ago, Ford dealers (USA) used to carry a noxious potion called: "Upper Cylinder Head Cleaner".
It came in a plastic bottle similar to the container that brake fluid comes in (only larger), and it looked
like used, pink-colored, soapy dishwater (except it was a bit more expensive). It's purpose was to
soften the carbon deposits on the backs of the valves and inside the combustion chambers. What
you did was start the engine (warm) and hold the carb throttle open at about 1500 - 2000 rpm and
slowly pour the stuff into the carb throat at a rate where the engine feels as though it's about to choke
on the stuff and conk-out...all while blipping the throttle to prevent it's stalling. When about half of the
bottle was gone, you shut off the engine and let sit about 30 minutes or so in order to allow the stuff to
soak in and soften the carbon. Then, you repeated this using the remainder of the liquid. Then take
the car on a run along the highway in order to help spit out any loosened carbon out of the tailpipe.

Pouring in the stuff was always rather tricky because you wanted to almost flood the engine with it
in order to affect all of the carbon - but - if you poured it in too fast, you ran the risk of "hydro-locking"
the engine and cracking a piston, or shatter a rod or two ( - because fluids do not compress).

Doing this at home always annoyed the neighbors because the process filled the immediate area
with steam, carbon, and a smell of burned petrochemicals ...not to mention all the racket this causes.
Unless you dislike your neighbors, it's best you do this in an industrial part of town, on a deserted weekend. In other words ...you do not want to do this anywhere in Mayfair.

Maybe there's a similar product on the shelves at your local parts shop?
Daniel Wong

You could be right about the mixture - although it pinks a bit under load too. One flat you say, you mean one turn of 180degrees?, but which way clockwise or anticlockwise to weaken the mix on an SU?

I'll have a look for some similar product, but what happens if the engine does stall whilst you're pouring in the stuff - will it actually fire and restart whith all that in the combustion chamber? (that was especially my thinking regarding a diesel/water mix) ??
Joe

Had my engine decoked the other week and as far as i know th eguy used barbeque cleaning fluid....

If you think about it, the deposits are kind of similar!!!

Bit of scrubbing with an old soft toothbrush too. The results seemed good enough to be and was enough to read the bore oversize on the top of the pistons...

~PHIL
Phil

One flat = one face of the mixture hex nut. A hex nut has 6 faces, so it's about 60 degrees of turn.

I've sometimes marked one flat with a felt-tip permanent marker so I could keep track of the number
of flats I've turned.

To weaken the mixture, insert the spanner around the hex nut with the handle pointing towards the
front of the car - and then turn the handle towards the rear of the car.

To help determine if the mixture is correct, you can either use a gas analyzer, or a gunson's Colortune,
or perform a series of road tests and spark plug readings (this is my preference for doing this).

I always shoot for a dry, medium grey color on my plugs and exhaust outlet.

Also, SU outlines a procedure for determining the carb mixture. Here's how it goes:

Look for a metal "piston lifting pin" at the side each carb body, below the edge of each dashpot piston
chamber. With the engine at idle, lift one of the piston lifting pins by only 1/32"...

If the engine idle rises a bit then returns to normal idle spec - then the mixture is correct.
If the engine idle rises and stays there - then the mixture is too rich.
If the engine idle immediately falls - then the mixture is too weak.

Personally, I've never trusted this procedure 100% because sometimes the engine idle is unaffected
at all when the pins are lifted (...then, what?) or the idle change is too subtle to notice on a quivering tachometer needle - and so I've always followed this up with road testing and plug readings.

---------

Should the engine stall while gargling on the upper cylinder head cleaner it may take a few extra spins
of the starter in order to clear the plugs of any fouling and get the engine to restart. Some folks wait a
minute or so before proceeding again in order to let the heat in the combustion chamber evaporate
some of the liquid into gasses or steam.

As I wrote previously, you want to avoid pouring in the upper cylinder cleaner too fast as you will run
the risk of "hydro-lock" and damaging vital engine parts.

--------

If the engine is plinking under load, and the timing and mixture is correct, and the fuel octane is OK,
then try giving each cylinder a compression test. Too low compression will do this and it may be time
for a valve job. Retarding the timing a few degrees will also alleviate some plinking a bit.

My bet is that all that black sooty carbon buildup is causing the fuel to pre-ignite, however.
Daniel Wong

I didn't think carbonisation had been an issue for many years with todays fuels. Unleaded always results in a black and sooty exhaust on our era of engines, unlike modern engines that run so weak their exhausts seem to just have a rust coating internally as the only thing that comes out of them is water.

Surely *high* compression increases the risk of pinking and Dieselling, lowering the compression ratio reduces it.

One 'cure' for Dieselling is said to be to *richen* the mixture.

Some eras of MGB heads are prone to pinking and Dieselling (even worse on unleaded 95 even with an octane booster) because of the design of the combustion chamber results in thin areas of casting that glow red hot.

Other than that over-advanced timing is guaranteed to cause pinking under load, and this can be caused by weak centrifugal advance springs in the distributor as well as simply incorrect timing.
Paul Hunt

This thread was discussed between 15/08/2002 and 16/08/2002

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