MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

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 TD TF 1500 - strange sequence rough running

I have been working with engines for 50 plus years and an XPAG had me thinking funny. A total engine rebuild 1000 miles ago came back in with rough running and fuel stains on front carb bowl. I pulled the bowl top off and everything was okay. Pulled the top off the rear float bowl and the float was half submerged. Aha! I have solved the problem. No way. It was running on three with #2 no spark. checked cap and found nothing wrong but I replaced the cap just in case. Still no spark on #2. Installed new plug wire and it was no better. The points were set properly. I ended up changing the rotor and all was well.
I have never had a rotor that failed on one cylinder.
Sandy
Sandy Sanders

Seems the rotors we get these days aren't worth the powder to blow them to h**l. I had sort of the same problem, when I pulled up to a stop light the engine would not rev back up, ran on 2 cyl's. After limping for about 50-100 feet the engine would start to run well again. Finally at one of the "Triathalons" it quit completly. I replaced the whole ignition system and it still did not run, finally changed the rotor and it ran like a top. I have saved that rotor to see if anyone could see the arc path. So far no one has, it checks OK with an ohmmeter but won't run in the car. Believe me it is well marked in my box of spare parts so it doesn't get into a car without knowing that it is bad.
Sincerely,

Bob
R. K. (Bob) Jeffers

This is when my pack-rat addiction becomes a good thing. I now go to the drawer with all those old rotors, we used to replace as a matter of habit in a 'points rotor and plugs' request at the parts counter. A few minutes with a cloth and an emery board and we're golden. Makes a good counter to the spouse about the 'you haven't touched them in years' comments.

Paul
Paul Gaynor

I completely agree. These parts have caused me more trouble than any other item. I'd really like to pin down a reliable source of caps and rotors. It's amazing how a part like this can cause so much trouble!

Tom
Tom Balutis

Sandy, Thats amazing. I had exactly the same thing happen on my B about 8 months ago and much like you I was a little baffled after swaping plugs, checking continuity, changing the plug wire anyway and eventually changing out to the old rotor
( always keep the old for a spare) and then all was well. This rotor had been in for about 10 years with very low miles.

LaVerne
LaVerne

a rotor failure on 1 plug only..i'm going to file that bit of info away. i am curious, how does the post in the distributor cap look? any excessive radial movement in the distributor shaft? no corrosion down in the cap where the plug wire attaches to the distributor cap? thank you. regards, tom
tm peterson

Nothing unusal for me Tim. Everything looked fine but no matter what I did it would not fire on the #2 cylinder until I changed out the rotor. No wobble in the dizzy shaft and I'm running a Crane Ignition.

LaVerne
LaVerne

There was no wear on any part.It also had Petronix. Cap is as new.I had completely rebuilt the engine 1000 miles ago with new crankshaft.reman rods new pistons and completely rebuilt cylinder head.Bushing in dist was new. The rotor shows absolutely nothing being wrong. It just wi;ll not fire # 2. There is no spark at all on # 2.
Sandy
Sandy Sanders

Sandy; Bizarre solution to a simple miss. Wondering if the spark found an alternate path to ground as the rotor arrived at the #2 firing position-point stud, condenser bracket, etc? Would about have to be an insulation failure allowing the voltage to flow somewhere other than to the cap terminal. Since Laverne also reported a similar situation, it would seem likely that a physical characteristic in the distributor exists for an alternate pathway to ground when you happen on one of these bogus rotors. Might look for evidence of arcing on other components? I'm with you-been a professional tech for 35 years-never seen a single cylinder miss caused by a rotor. Always something new-even without on-board computers and Class II data busses. Dan
D.R. Craig

D.R.I
I would agree with you except that I changed the cap and the problem still existed. I then changed the lead to no avail. I even pulled a complete compression check and all were from 150-152 Lbs.I also changed the plugs.. No spark would come directly from the lead. I changed the rotor and all was well.I then re-installed the old rotor and it was still dead. There is zero evidence of tracking or burning anywhere in the old cap or rotor.
These part changes were done one at a time,not all together.
The strange thing was it was #2 cylinder that was dead and it was the rear carburettor that had the bad float.
It's a first for me.
Sandy
Sandy Sanders

Have had similar rotor problems, first in a TR4, then my Healey, both stopping dead. Save the old rotors and distributor caps. Also fitted new cap to my Bugeye and absolutly no continuity to wirss, don't believe the screws go deep enough into the cap. TD works fine with 25 year old cap and rotor. Bob
Robert Finucane

Seeing the comments about distributors/rotors etc., prompts me to add the following. Not a solution to the one missing plug but possibly an experience to ponder.

While tuning up a car of another make ( dare I say a Jaguar 120?) I had replaced the standard ignition with a new pertronix device. After a half a mile or so, the engine spluttered and died. I replaced the Pertronix with yet another new unit. Still no spark. Checked the plugs, leads and no spark at all. Upon inspection, it appeared, at a cursory glance, that the carbon brush in the centre of the distributor cap was stuck in a fully retracted position. Replaced the cap with a new one which was on the shelf. Car started up and ran for half a mile and spluttered out again. Carbon brush stuck in fully retracted position. Towed the car back, and tried to remove the brush. Found the brush AND the part of the cap into which the centre brush was housed to be worn down and the brush's spring was unable to make it contact the rotor. Same thing on the first new cap.

Got another new cap, but this time made sure the Pertronix rotor was pushed fully down. It was. Ground off about 1/16" off the bottom of the rotor so that it would seat that much lower and refitted the whole thing. Car has now done over 1000 miles with no repeat of the problem. Possibly the distributor cap wasn't moulded correctly and when it was fitted onto the distributor's base, it pressed down too closely and wore out the centre part complete with brush.

Hmmm... velly illitating.
Geoff Love

Sandy; What I am suggesting as a possible explanation for your #2 misfire is the spark exiting the rotor thru the bottom of the arm to something in the distributor. That's why I suggested looking for signs of arcing on components such as the point stud or condenser bracket. Don't have a distributor handy at the moment to look for myself- if you are so inclined, you might roll the engine over till the rotor points to #2 and see if anything is close to the underside of the rotor. Seems to me I read about riveted rotor blades having insulation failures-don't remember the particulars of the article. May be barking up the proverbial wrong tree- just thinking out loud.
Geoff - Aftermarket component manufacturers seem to delight in making us dink with their product to make it work correctly-sound like you got a handle on it without resorting to physical violence.
Hang in there Dan
D.R. Craig

This thread was discussed between 15/02/2008 and 17/02/2008

MG TD TF 1500 index

This thread is from the archive. The Live MG TD TF 1500 BBS is active now.