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MG TD TF 1500 - Looking at engine issues

Right now, as I'm starting to work on my 51 MG, the engine appears to be firing on only two or three cylinders. It's very ragged and uneven. My dad blames the carbs, but here's what I'm thinking...

With only about 26k real miles on this car (but plenty of other issues) I'm thinking that what I should do is:

a) install an oil filter - there isn't one
b) decoke the head. It was last removed in 1979 and we have no idea of the internal condition. I'm thinking if there is any gunk around the valves, it may be dropping compression in one or two cylinders, and then the fuel mix has been enriched to compensate for low compression.
c) then look at the carbs.

I'm just wondering if other people have other suggestions or thoughts on how best to begin trying to get the engine back up to scratch...

Of course, a compression test is in order before the decoking.

Thanks, Geoff
G M Baker

Hi Geoff, I would not remove the head as yet. It's probably running on only one carbie as the needle valve could be stuck on the other one. Just remove the float bowl covers and check they both have fuel in them. (new fuel of course)


Paul van Gool

Remove air filters, lift carb pistons, spray "carburettor cleaner" or "fuel injector cleaner" right at the needle and jet and let it sit for a day. Put the rest of the injector cleaner in the tank. Start it up and run it a bit. fuel evaporates from the jet and leaves deposits at the normal fuel level point, which is also the idle position.
It also doesn't hurt to remove the rocker cover and spray carb cleaner at the valve stems, right where they disappear into the guides. I've freed up a number of sticky valves this way.
Leave the head alone until you know there is good reason to remove it.

FR Millmore

And 75% of carb problems are electrical...go over the wires, cap, plugs, etc.....
Give it a compression test...then check the carbs....!!


My process is "Breathing, Fire, Fuel".

I check the compression, wide open throttle, warm if I can. Then I set the valves. If I'm positive I know the cam in the engine I use the valve cover tag, or setting plus .001 on a cold engine, if I'm not sure of the cam I use .015, plus .001 on a cold engine.

Then, as Gordon says, I change the plugs, check the wires, clean the cap, set the points and static time the engine. I make sure the firing order is 1342 counter clockwise... going 1342 clockwise changes it to 1243 and will make it do what you are saying! Pull off each spark wire and check that it is making spark. The spark should jump a good 1/4 inch, if not check the coil.

Then, I do as Fletcher suggests because, as Gordon says, 75% of carb problems are electrical in nature. Set the carbs up using a dial caliper with the pistons out, measure the depth of the fuel in the bridge, I like .160, you can use the jet as a measuring point. Then set the initial jet setting down .070 and adjust from there.

If you want to decoke the head, simply run a pint of SeaFoam through the engine with the first tank of gas.

Dave Braun

Hi Dave,

I am interested in your comments to Geoff and in your 3rd paragraph you mention setting the depth of fuel at the bridge. Are you saying .230 ( .160+ .070 ) before adjusting and that the measurements are ( in inches ) from the bridge down to the jet top?

Also Seafoam, have not heard of that before. Do you know of a UK equivalent?


David Tinker


No, I mean that the float level should be adjusted so the fuel is .120 to .200 below the bridge, which can be determined by moving the jet to fuel level, and then measuring the depth of the jet. I use .160 for the carb rebuilds I do. Then, you raise the jet all the way up to the nut, and adjust the nut until the jet is .070 inch below the bridge. Reassemble.

SeaFoam is basically naptha. Hope this helps,

Dave Braun

Hi Geoff

Since you say install an oil filter, I assume you are saying that someone removed the old canister filter unit. If you need them, I do have a set of oil lines for the older TD. Pick up a spin on filter adapter and the pipes will bolt right to them.

The others have posted what you need to review prior to pulling the head.

Where in AZ are you?
I'm in Glendale AZ.

Bruce, I'm definitely interested in your oil lines/banjos. I've sent you an email on the subject. MGrogan, I'm in Tucson.

Thanks for all the suggestions, and Dave, I'll be following your advice to the letter. I may still open the head up soon if these suggestions don't fix the problem - but I'll do these first. Quick question, Dave; you have a lot of measurements there on carb and jet settings. Do I need any tools to take these, like a dial caliper or micrometer?

I've never played with SUs before, it's sure to be interesting...

thx again

G M Baker


Yes, a dial caliper works best. The end opposite of the jaws will have a little depth measurer.

Don't forget to check your carburetor balance and to fine tune the mixture adjustments. Once you have balance, and the .070 depth measured, make all future changes EXACTLY the same on both carburetors.

Dave Braun

Before you rip the head off, give it a compression test. That will reveal whether that effort is called for or an expensive waste of time. The cost of the head gasket should get your attention, not to mention wasting a great deal of time overlooking a really simple problem.

Pull spark plug wires to identify suspect cylinders.

I recently had very rough running/missing show up out of the clear blue sky- turned out to be the intake gasket deteriorated. Vacuum gauge may disclose that, or some starting fluid sprayed around manifold/head or carb/manifold mating surfaces.

Rust accumulating in a float bowl can take one carb out of service, disabling a pair of cylinders.
jrn Northrup

This thread was discussed between 08/11/2009 and 09/11/2009

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