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MG TD TF 1500 - A bang and a dead engine

Out for a lunchtime drive on a hot summer New Zealand day in my restored 1954 1250 TF. Has electric dizzy conversion and starts and runs very smoothly. Unfortunately 20 min into run at 50mph etc, large bang/ backfire? and loss of power. Thought vapour lock, but it will not start next day when cold. I have a spark at plugs and sometimes starts at weak idle, but any attention to throttle and the car dies. Any thoughts before it is off to garage. Ian
IF Shears

I had the same problem after an electric dizzy conversion. You may have a faulty electronic ignition which is giving intermittent spark.
See my posts and responses under MGB labeled "pertronix" There's lots of good advice there from our fellow enthusiasts
Keith Yarbrough(TD 1275)

A well sorted MG TD/TF does not vapor lock at speed. You can cross that off your list.
Regards, Tom
tm peterson

If you rule out electrical gremlins then was the engine under heavy load when the bang occurred? Was the water temp and oil pressure normal when this occurred? Is there any cross contamination ?

I would also check a mechanical cause. Check that the cam timing is correct. It may have jumped due to a badly worn/stretched timing chain and or gears. How many miles are on the current engine since overhaul and were these items replaced? A quick compression check when compared to previous readings would be a possible indicator. If the numbers are significantly different then stick a degree wheel on the crank end with a dial indicator on number one pushrods to further verify all is well and to the cams timing specs.

Check the top end for sticking valves. The bang may have been a valve contacting a piston.

Check for increased blow by as an indicator off a broken piston

W A Chasser

Broken crank is possible. Any small in-out movement where the crank handle attaches when turning over on the starter?
M Magilton

Broken crank? It still turns over and runs..just at low idle. No mention of any unusual sounds upon start up.

It only takes proper fuel mixture delivered to cylinders with compression and a spark at the appropriate time. Good fuel, good compression, proper spark and it will run. Verify all three.

Regards, Tom
tm peterson

Ian, I had the same thing happen with 2 petroniix ignitions, after installing and running for about 20 miles the car started back firing then would hardly run. I phoned petronix and they said if the car ran I had it wired correctly I then re-installed the points and condensor and the car runs as it should.
Kim Herford
W K herford

Petronix / 123 = always carry a spare points dizzy!
Chris at Octarine Services

transistorised systems are very voltage sensitive, check all connections and earths. A slightly oxidised fuse holder was enough to cut my engine. It was not until I twigged the engine acted up every time I braked or indicated. My meter never showed it up till I put another load on the electrics and the ignition voltage dropped.
Ray TF 2884
Ray Lee

See http://www.ttalk.info/Failure.htm for my failure experience with a Pertronix system. Bud
Bud Krueger

Tom, a T Type will still run with the crank broken at the front web. I was able to drive my TF onto a car trailer with a broken crank. Ran fine at low idle.
M Magilton

I love TCs. I understand how they work, and if something goes wrong, I can always trace the problem. I think that is one of the main attractions of classic cars. My road car and my wife's car are both "modern plastics" with the associated modern electronic/computer stuff attached. When they won't work, I have no idea how to fix them and I call the RAA. If I could, I would fit the old systems to the moderns. I would not fit the "modern" systems to the old cars. (I guess I might get some flack for those comments?)

The nature of the "bang" is important. Assuming it was not a mechanical sound but a backfire/explosion sound, that points to loss of ignition - then filling the muffler with explosive mixture - then ignition of same. (Like the hoons used to do by turning off ignition and then turning it back on again) Alternatively it could be backfire due to ignition in inlet manifold due to ignition timing going haywire? The latter would not be so loud and would come from under the bonnet.

Bob
R L Schapel

Bob, in days of old we used to use the throttle and spark advance controls on the steering wheel of our Model A Fords to make a great backfire. Problems occurred when doing so set the wooden floorboards on fire. Bud
Bud Krueger

I can confirm Matthew's statement re a broken crank. I too, had the same thing happen; had enough power to limp home, where a compression test combined with strange noises, revealed the bad news.

I pray that's not your case, Ian. Please keep us informed.

Gord Clark
Rockburn, Qué.
Gord Clark

Gord, but that is the thing, unless I missed something in my read, there are no strange noises. I am not saying it isn’t a broken crank, but with the information presented here, I am doubtful.

I have had a crankshaft break in my Tiger Moth at cruise. It does not sound like a backfire or after fire. There was no doubt a big piece of engine failed as the prop departed.

Regards, Tom
tm peterson

Wow, Tom, that is scary!. We must be lucky to still have you with us. Incidentally, I have heard Tiger Moths described as "The TC of the Sky."
Bob
R L Schapel

Once had a friend who bought two cars (cheaply) with what turned out to be broken cranks. A Morris Minor and an XK150. Drove them both home from point of purchase, slowly and gently. Both made a continual knocking noise and had low oil pressure but did run. All the bearings and thrust bearings kept the cranks more or less together.
John Quilter (TD8986)

Thanks for all the thoughts. It turns out the round magnet part of the electronic conversion was faulty. Instead of being tight on dizzy, it started to work loose and move, thus putting timing well out and causing the backfires. Points back in while I await a replacement. Ian
IF Shears

Ahh- Ian
I've had that happen with a Powerspark conversion
I thought the original one was a tiddle loose fitting over the old lobes when I first put it on but guessed that that was how they were as I'd ordered it specifically for the car
Wrong, it gradually wore away and ended up moving far enough to get out of line and backfiring/pinging was the eventual result
When I replaced it I stuck the new one onto the cam/lobes with Sikaflex and let it set and have had no more problems
Apart from glueing the magnet set on the only thing holding it there was the rotor button
willy
William Revit

I had a similar problem with an MGB. One bang and then no power. The inlet manifold is sealed with a core plug at each end and the back one had popped out. It took ages to find the problem.
Barrie Jones

This thread was discussed between 07/01/2019 and 28/01/2019

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