Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Carbon build up???

I took my 73B roadster on a couple 4 to 6 hour trips a few weeks ago. Since then my milage has gone up and the car is running better then it has in the 2+ years that I've had it. It ran very well prior to that, but now seems to just roar right along. Running the engine revs up high is just so much smoother now.

Prior to those 2 trips I only drove it on short (hour or less) trips close to home....I just wasn't comfortable with getting that far away from home base until I had become a lot more familiar with the mecahincs of the car.

I've heard lots of folks say a car should be driven more rather then less. I think they mention "carbon buildup" as one of the reasons.

Would someone mind taking a few minutes to talk about this issue a bit? I'd like to understand more about why those long day trips made such a dramatic improvement in my engine's performance.


BH Davis


When I started driving My MG more I notice she ran much better. This cars are made to run not sit for long periods of time. You have found Your B to "be" reliable so You can trust her more on further trips.

I drive my midget year round weekly not everyday. This is good enough driving time for a MG but I would recommend more to keep Your spirit level up! As for carbon build up SU's over time build up with carbon. So what I do is remove the dash pot cover and piston and spray some carburetor cleaner in the carburetor body and wipe with a clean lent free rag. Also change the oil in the Dashpots howl pistons. I do this maybe twice a year.

So once again Drive Drive Drive.... That is the best thing You can do for Your B. And run a 180 thermostat year round. And a 190 in the winter. LOL.


Jump in!

James D

The carbon buildup theory sounds plausible. Old carbureted vehicles ran rich, and the MGB in particular idles really badly - cylinders 1 and 4 won't have the same mixture 2 and 3 due to the airflow in the siamesed ports. Let's take cylinders 3 and 4: 3 pulls in its charge, then 4 takes in its charge right after, while the air is still moving - then, the other 2 cylinders fire, and by the time we get back to 3 the air has stopped. (at idle it has stopped; at high revs it likely hasn't had time to.) So, 4 is getting a better charge than 3 is. Cylinders 2 and 1 work the same way - 1 gets a better charge than 2 does, because 2 has to get the air moving again. So, you Can't get your idle mixture right, ever, by design, and that's just how it is. When you're idling or running at low revs, you're building carbon. Lots in 2 cylinders, less in the other 2. This imbalance exacerbates your roughness of running.

What you experienced when you burned off the carbon, then, is not just a reduction of carbon, but a better balance between the cylinders, which makes it run more smoothly. Not too much power to be gained here, but you probably found the smoothness improvement to be pretty awesome. Smoother running may improve other things such as carburetion. Goodness begets goodness.

One more thing that driving does: it gets rid of all the damn water. Condensation. Everywhere. Stuff rusts while you're not driving. I doubt that affects your performance too much, but it certainly is the cause of the early deaths of cars that are driven too little.

I use my MG to commute to work. 25 miles each way, daily, about 1200-1500 miles a month. She runs better than ever, and needs far fewer repairs than she did when driven periodically. No backup vehicle is required. This is my transportation.

The bottom line: take her on the open road regularly!


BH Davis

BH. To add to what Sam mentions, there are two forms of carbon build up. The first is on the spark plugs and, as you and Sam note, this build up will commonly be burned off with a good, long, high speed run. Only if the plugs are severely carboned up will this method not work. The net result is the same as installing a clean set of spark plugs.

The second form of carbon build up is on the tops of the pistons and within the combustion chambers of the cylinder head. Our British friends used to call this "coke" and removing the cylinder head and "decoking" the combustion chambers and the tops of the pistons used to be a regular feature of car ownership. Something which was done every couple of years. This form of build up is more of a problem. It effectively raises the compression ratio, in an unplanned for manner, as the carbon build up on the top of the pistons and within the chambers means there is less volume available when the piston is at the top of its stroke. The net result of this build up was pinging when accelerating and run-on (dieseling) when the engine was switched off. Today's fuels are designed to burn more cleanly and, while this problem still exists, is does not build up as rapidly as it did back in the old days.

A fast drive, for a couple of hours, can have a minor effect on the deposits on the pistons and within the chambers, but not as much as it will have on the spark plugs.

Les Bengtson

To add a few thoughts to those above...
Carbon build up is less common and less stubborn with more recent oils than those used in earlier times like the 50's and 60's.
This is principally due to detergents and other additives in modern oils.

In addition to adding some extra material to the piston the material added can be heated to an extent where it can glow with sufficient heat to ignite the fuel / air mix before the plug can.
It can also continue to ignite the mix after the ignition has been turned off.

Neither of these are desirable as you can guess.

Personally I have always thought the worst thing you can do with any car apart from not maintaining it is not to drive it.

Peter Thomas

This thread was discussed between 09/09/2007 and 17/09/2007

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.