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MG TD TF 1500 - Missing on #2 and #3 cylinders

TF1500, drove it yesterday, ran well, had adjusted the carby sync a few days ago. Idling nice, when shorting each plug to block get a similar RPM drop.

This morning, set of for the local car show, started missing on 2 cylinders after about 2 miles, pulled off the road, cleared it's throat and off we went again, same happened.

Came home, ran OK most of the way home, last 150 yds back on 2 cylinders.

Shorted each plug to block with a screwdriver, #1 and #4, the engine quits, #2 and #3 make no difference. Often spits out carburetor, more often rear carb than front. Plugs pretty black.

Thinking out loud, if it can clear up, it is not mechanical/serious? But rule out the mechanical side first.

Pull out the 40 year old gunson compression tester, connect it to #2, the little "release valve is sent into orbit never to be seen again.

Head to big auto parts company to buy a new one, none in stock, get the free loaner, bring it home, it does not hold pressure, but all cylinders look the same, 30-40PSI jump per compression stroke...

Return that, head to another big parts company, get 25% off a tester, bring it home and do a check with throttle full open, get 185/170/190/185, not perfect but not enough to sweat.

Tried new set of plugs, same result. Fuel is not common to #2 and #3 so not carburetors, is this electrical?

Distributor cap looks perfect, rotor button is from the last century, but very low mileage.

Do I switch out the rotor button for a red one as a first step? Anything else I should look at immediately????

Thanks in anticipation.

P G Gilvarry

Check distributor cap for "arcing". A faint arc line may be present. If you have a spare cap and rotor change it out. Also check to see if plug wires have worked their way loose. I've had that happen.


79 MGB
gary hansen

Checked wires for thightness. Examined plug, could not see any arcing, but will check again.

Contacts on cap are clean, no arcing marks, but I will go take another look.

Looks like $30 + shipping gets me a cap and rotor so might be the route to go here.

Thanks for the input.

P G Gilvarry

Last year something similar happened to me. Eventually I found the problem. It was a hardly detectable blow thru between block and gasket. After grinding off the volcanoes that had formed around the head bolts due to over tightening and replacing the old gasket, it has now worked well for a year.
Y Strom

Make sure you get the latest red rotor. The earlier repros in black were notorious for shorting out between the rivet and the spring clip on the underside.
John Quilter (TD8986)

If it's intermittent then I doubt it's a head gasket or crack. And if it's only 2&3 then it isn't fuel. I'd look for something electrical first. Since the plugs are black, maybe they're fouled? Try new plugs and check the mixture again.
Steve Simmons

Oh, could also be a distributor cap issue.
Steve Simmons

Unusual for a cap to cause a missfire on opposite cylinders on the cap, but anythings possible
As far as checking for arcing- start it up in the dark while it's missing and see if you can see anything
I've got a headgasket feeling
William Revit

Found a couple of dodgey connections on distributor cap, cleaned them up, now have less than 1 ohm for all contacts inside cap to plug connection.

Will get new plugs and give it a try.

P G Gilvarry

Ongoing saga ....

Tried new plugs and rotor yesterday, still not working on #2 and #3.

Tested the plug leads from contact inside distributor to plug connection, all are less than 1 ohm.

Have carefully examined the distributor cap, nothing obvious.

Next thought is to get a new distributor cap and leads.

On advice I have also ordered a new coil and condensor, going to make new leads also.

Anybody got any ideas?


P G Gilvarry

Wobbly distributor cam?

Timothy Burchfield


Does not seem to have much movement.

When I fitted the new rotor I marked the contacts and cranked the engine over with the crank handle, got no interference.

Thanks for the input.


P G Gilvarry

I agree with Steve on the plugs, I have a 30hp Kohler engine that will drop off a cylinder once in a while and a plug change fixes the problem and also the possibility of an issue with the distributor cap. Not saying it is, but a distributor cap can be defective with no visual indications and can short between two terminals, usually an old cap that has been subject to heat variations over the years, old 6 cylinder Chevys of the 30s and 40s would do this on occasion with the original GM cap. Be nice to know what the final cure is. PJ
PJ Jennings

Thanks PJ, placed an order with Jeff Zorn for quite a few parts. Coil and Condensor along with distributor cap too.

Making up new leads later today, tried new plugs yesterday. I have enlisted the advice of Tom Lange so I sure hope it is back up and fixed next week when the parts arrive.

Luckily my local Advance Autoparts has the Champion N5C plugs so that is a quick fix.

P G Gilvarry

Peter, I never asked but, is your plug wires solid core? PJ
PJ Jennings

To check for a bent dist. shaft
Set the contact gap to spec and then rotate the engine and check at each dist. lobe ,The point gap 'should' be equal on all lobes
If it's not it's bent
I can't see that being the problem though if it clears itself, specially with 2-3 being on oposite sides of the cap

Still got a h/gskt feeling
William Revit

The problem will not be rotor, coil, condenser, etc because any of these would cause misfire on all cylinders. It has to be cylinder specific, which narrows it down to cap, plugs, wires, or something really odd that we aren't thinking of. I'd definitely run it in a pitch black room (be careful about that radiator fan) and watch for arcing.

I've seen some strange things happen with shorts from places you might not expect. Like the time my MGA water pump leaked a bit. Coolant droplets were blown by the radiator fan back into the engine bay, where some of it landed on a plug wire, and then dripped down into the cap. It picked up electricity which traveled up the wire in the coolant, then arced from there to a braided oil hose located about an inch away. Took me a while to figure that one out.
Steve Simmons

This thread was discussed between 10/06/2018 and 16/06/2018

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