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MG TD TF 1500 - Backfire - ran great / now won't run

Hi guys -

Here I am with another question about my 1955 TF-1500.

The car is backfiring and won't run above an idle. Where should I start looking to find the problem?

The car ran awful when I bought it about four months ago. I thought that I finally had it running well, but the gremlins have returned.

Drove about 90 miles round trip to a club meeting. About 45 mph (speedo is broken). Ran great. But on the return trip, it backfired loudly out of the rear, not through the carb, under acceleration about 3800 RPM.

When I returned home, I replaced the condenser, in case that was the cause. Checked the timing - still at 9 degrees BTDC.

After about 30 minutes of city driving, it began to run poorly around 2500 RPM, then began running rough and backfiring at lower RPM. I barely made it home.

I replaced the plug wires. Checked the points - new and still at .015" gap.

The car will start but runs rough at low idle I cannot increase the RPM - it shutters, shakes, and backfires. Sounds like it's running on only a couple of cylinders.

TF-1500 with TC/TD head #22952
Valves at 0.015"
Distributor head re-peened to correct loose wobble
No wobble. Shaft tight to bushings.
New points @ 0.015" gap
New condenser
New rotor
New plug wires
New plugs (1/2" reach for TD head)
Timing @ 9 degrees BTDC
Lucas Sport coil B.Y.S SP12
Carbs rebuilt with SU Master Rebuild kits. Rebushed at throttle shafts.
Float level at approx .016" below bridge
Jets at approx .070 below bridge, minus two flats up
90 Octane non-ethanol gas
Compression appears to be about 150 psi at each cylinder. I couldn't hold the comp test probe in the spark plug holes tightly enough to get accurate reading.

Don't know if this is related ... I set the carb float level and jets by measuring fuel level from the bridge, after about 30 minutes of driving, the engine would surge at idle and lose RPM until it stalled. I raised the jet adjusting nuts two flats and the problem diminished.

Once again, thanks for pointing me toward a solution.

LM Cook

Lonnie, is the spark advancing as you rev the engine? Bud
Bud Krueger

I suspect it is either erratic spark due to stuck distributor weights, or the fuel is arriving in 'fits and starts', which would suggest that the carb is flooding intermittently, possibly the float is sticking on the spindle or sticky Gross needles?
Ian Bowers

I suspect a bad condenser.
D. Sander

"Where should I start looking to find the problem?"
Pull the plugs. Are 4 black or maybe 2? White from lack of fuel? Are some wet?

You failed to mention the distributor cap. They're notorius for cracking. I found one with the condenser offset so the cap couldn't set down square. You can fire it up in the pitch dark to look for sparks grounding out. Carbon tracks inside cap can short sparks but that would show up on the outside- check inside for visible lines that don't belong there. I plug a timing light onto the coil wire to look to see if the flashes are steady or erratic.

A vacuum leak, possibly between manifold/head, or related to one carb, can be a pain. You might shoot some starting fluid around the manifold & carbs.

Please report back with your findings.

I'm with JRN Jim on this - vacuum leak at manifold. Try tightening the clamps that hold the intake and exhaust manifolds to the block.
Gene Gillam

Is the rotor black/brown or red? Make sure the plug/coil wires have not pulled loose from the thumbnuts/washers. George
George Butz

Along with checking the cap, also check if you might haver one of those bad rotors that floofed the market a while back,, they were red I believe.
And as mentioned above, plug color tells a lot,,,,When it's running bad, shut it down right away, and check the plugs...


I would recheck the spark plug wires routing. This sounds like you may have a couple reversed in the firing order
Don Harmer

I followed your suggestions, plus my own troubleshooting. But the problem returned.

Any addtional ideas or suggestions?

I started the engine after completing the steps noted below. It ran very rough and was difficult to increase RPM above idle. But it began to run well after a few tries to increase RPM. Afterwards it sounded good. Took a short test drive. After about five minutes the engine started running rough / miss / backfire again at about 2500 RPM and continued to get worse, like it did a couple of days ago. I dashed back home.

I feel that the symptoms are pointing to carb float and mixture settings -- I'm really having a hard time setting them by using measurements at the bridge. The float forks are almost flat ... much less than 3/8" drop specified in the WSM. The symptoms could also be a result of an electrical problem that appears after the engine warms up.

Here's what I did today.

> Confirmed that distributor head is not loose from shaft.

> Rotated rotor about 5 degrees and could feel the pressure of the centrifugal springs. Rotor returned to starting position. Rotor from Moss is black, not red.

> Inspected inside of distrib cap. One high tension tab is scored, possibly caused by rotor when distributor head was loose, before it was peened solidly to the shaft a few months ago. I have a new Moss distrib cap, but it rocks when installed - will return it. A new Lucas distrib cap from British Parts Northwest will arrive tomorrow. I'll install and see if the rotor scores one or more of the tabs, possibly indicating that it is not turning concentrically with the cap.

> Inspected connection of plug wires to distributor cap and spark plugs. All are firmly seated. Copper washers are soldered on. Wires at spark plug caps are doubled back and soldered for extra thickness. Plug caps are attached with screw terminals. Did not replace coil wire because it has a push-in terminal for the coil. Confirmed that the wires are in the correct firing order.

> Removed the new Moss condenser and installed the one that I had removed after the "backfire" episode.

> Confirmed point gap at 0.015"

> Removed the NGK B6HS spark plugs. All are covered with black soot, like they were before I rebuilt the carbs. (I wiped them off, but did not clean when I installed the carbs.) The outer electrode on #1 plug had less soot than the other plugs. #3 and #4 may have a little more soot than #1 and #2.

> I brushed the plugs clean with a brass brush. See photo below.
+ Top photo before cleaning
+ Mid photo after cleaning
+ Bottom photo after test drive

> I reduced the plug gap from 0.025" to 0.020" and installed.

> Checked the fuel level in the float bowls. Front float seemed to be higher. Did not measure, nor measure the levels at the bridge.

> The needles are not Grosse needles. They are the rubber tipped needles and mating seats that came in the SU Master Rebuild kits.

> Confirmed that manifold and carb bolts are tight.

> Started engine. Ran rough at first but eventually ran well. Idled at 1500 RPM -- higher RPM than before.

> Lowered idle speed from 1500 to 800.

> Checked timing. Reduced slightly from 9 deg BTDC to 8 deg BTDC. Timing mark on the front pulley was not rock-steady. Difficult to set less than 8 deg. Engine idled best / fastest at about 20 deg BTDC.

> Hooked timing light to coil wire. Good steady flash. Timing advanced with engine RPM. I did not do the test again after I returned from the test drive.

> I started it again after it cooled. Ran rough on start up, but cleared somewhat. Idle was about 400 RPM. It was 800 RPM earlier.

> Ran an unlighted propane torch around the manifold and carb mounts. No change in RPM. I may have not used enough raw propane.

> I ran the engine in the dark. Couldn't see any wayward sparks near the ignition wires.

> Forgot to note that the fuel pump is a new Facet #40104, 1.5-4 psi. I checked the pressure when I installed it ... 2 psi. No bubbles in line.

Sorry for the long-winded dissertation, but I am trying to eliminate as many variables as possible.

Thanks for your help,


LM Cook


All this may be out of left field however something similar has occurred on my TF and other vehicles over the years.

Confirm that the choke mechanism return spring hasnt become disengaged.

Other things maybe to look at, do the dash pots contain oil and do a compression test.

An old mate of mine always advocated ruling out ignition problems prior to chasing fuel problems, 99% of the time this is factual. Engine temperature appears to be involved here however I would try a few ploys in the elimination process.

Make sure the distributor drive gear is still pinned to the shaft, I once bought a car and this pin was missing. Car ran fine until I stressed the engine.

Other thing to look at is to ensure the points are opening the same amount on every lobe of the distributor cam.

Disconnect the LT feed from the coil and make direct connection to your battery terminal, run the engine and note any change in performance.

If you have access to a spare coil and a set of HT leads I would also do exchanges.

Other basic test would be to check the continuity of the LT lead between the coil and the points, do this at the same time as manually operating the advance mechanism you may have a severed wire that intermittently goes open circuit when the engine is running.

Intermittent faults are the hardest things to resolve.


G Evans

Sounds electrical but have you checked the fuel output from the pump. A buddy's Triumph did something similar and it had crud in the tank.
Rich (TD 3983) Taylor

As Lew Palmer will say, 90 percent of all fuel problems are electrical in nature.
Tom Maine

Do a compression check so you can rule out a blown head gasket. Also check your ground strap for good continuity. Just a couple of things I would check while your at it.
MG LaVerne

Your problem reminds me of my limey motorcycle days when I'd start out running fine and then it would start acting up and keep getting worse. The Lucas electric weren't charging and the performance dropped as the voltage drooped. Throw a voltmeter on temporarily to see what's going on.

Exhaust backfiring- spark plug(s) missing then air/fuel in exhaust reignites.


First, you must be commended for such a detailed explanation !!! Most excelent!!!

>>>>I have a new Moss distrib cap, but it rocks when installed - will return it. A new Lucas distrib cap from British Parts Northwest will arrive tomorrow<<<
I'm ancious to see what this new dizzy cap will do..If the "M" one is rocking, it possibly could casuse missfiring.

As Graeme states ,, check to see that the choke returns all the way up into the carbies,,It could be disconnected springs, or that they don't return all the way up when the choke knob is pushed back in, Mine are tight, and I must sometimes press them up manually.. those plugs (before cleaning) defanatly look like a rich over choked mixture. But the photo after a test drive look almost too lean?? that is the confusing part,,

Did you do the "dash pot piston drop test"??? This insures thet the piston and neddel drop unrestricted when the engine is off


If you are using the black rotor in your picture, throw it out and install a red one before you do anything else.
George Butz

Now I am confused,,, I thought that it was a bad batch of RED rotors ??????


Have you checked the air filter elements? If someone has fitted any foam inside one or both that can suck in to the carburettor intake and cause problems.
Rod Brayshaw

I don't think I saw this mentioned yet so check that the tach drive gearbox has not rotated around and is touching the distributor side terminal. I've seen this happen and with the engine vibration the two were bouncing together causing intermittant loss of spark.
David Ahrendt


I had a similar problem after rebuilding my carbs. I also fitted the rubber tipped needle valves in the float chamber lid supplied by Burlen SU rebuild kit . I traced the problem to lack of fuel and on inspection found that the rubber tipped float needles were sticking and not letting fuel into the float chambers I replaced them with the old brass needles and hey presto the car now runs like a dream. maybe that could be your problem.

Worth a try at any rate cheers chris
C A Pick

George Butz

After you have ditched the black rotor and if the car still runs badly, shut it off, carefully remove the float bowl covers and see if the floats are bobbing well off of the bowl bottom. This crude but effective check will rule out problems with the float needles, fuel pump, tank strainer/filter, etc.
George Butz

Change the plugs.

Mark Strang

Go for electric fault first before adjusting carbs.
(As others have said.)
Personally, I would be suspect of Lucas Sport coil B.Y.S SP12, dependent on age. I had 3 bad ones that gave low spark after they warmed up. (Years ago)
Is the coil hot to touch when this happens?
(I run pertronix flame thrower now...12+ years)

Another problem I had years ago was with the advance springs in dizzy. (One was broke, other was rusted.)
Pull the points plate off and inspect those.

Another little trick I learned form someone on this board. (Think it was Jim N.)?
Run the car at full power, then shut down "under load" and coast to a stop. Pull plugs and inspect. You get a much better idea of how the fuel mix is working this way, rather than pulling them after it has been idling.

Pretty sure the "bad lot" of rotors were black.
The orange/red ones are the good ones.
Around the same time there were some dizzy caps that were crap out there.

Let us know ...we are pulling for you!
David Sheward

I second Davids suggestion:
check that the tach drive gearbox is (not) touching the distributor side terminal

Please report back when you find the cause: I'm struggling with the same fault on my Y-type, did everything you did....
Willem van der Veer

About black/red/good/bad rotors:
Willem van der Veer

I know this will sound weird, but I had the same problem.... kinda. Car would run great then would all of a sudden would run terrible. No power etc, etc.

Checked everything, and I mean everything.

Wasn't until I talked to the guy who owns the shop that I get my paint work done at and he made the suggestion. I was running NGK plugs, and he suggested that I change the plugs to the appropriate Champion plugs. I did ( found them in the Marine listing) and everything has been fine since.

I was told NGK plugs are very finicky, once they foul, they tend not to be right after that, even though you clean them up. The Champions on the other hand are older technology and will have the tendency to burn off deposits better. (Why so many are used in 2 cycle marine engines)

For some reason all this has worked, and now I have Champions in the car and have never had a plug foul yet. No more lack of power, and the plugs are an even tan colour.

I can't remember the # on the plug, but they are for all 9HP and less Evinrude outboard engines with 2 cylinders.

Long winded but worth a shot.
CR Tyrell

I don't think anyone else has suggested it, but I once had a Ford Capri "B" sedan race car. One weekend it began running terribly at anything off idle. Tried everything until I tried replacing the new points in the distributor. This solved the problem. The spring in the points was bad and they were bouncing at any rpm above about a thousand. I didn't believe this could be the problem, so I put them back in and the problem returned.

Easy thing to check.

Joe Hine
53 TD
J.D. Hine


My TF is running again and no backfire ... yet. I drove it about five miles tonight and all is well. I'll update if the problem returns.

A new distributor cap and rotor fixed the problem. I bought a Lucas cap and Lucas rotor from British Parts Northwest Inc. The cap is dark olive drab color and has "Remove to Oil" embossed on top. The rotor does not have the dreaded rivet. I had bought a distributor cap from Moss, but I did not like the fit and finish, so I didn't use it.

British Parts Lucas Distributor cap
(shows out-of-stock. I must have bought the last one)

British Parts Lucas rotor


The tab inside my old distributor cap for cylinder #1 was scored from being scraped by the rotor. The rotor was scuffed on the trailing edge. When I bought the car in December, the distributor head was loose from the mounting shaft and wobbled when I pushed on it. A member of our MG club peened the head to the mounting shaft and stopped the wobble. I believe (hope) that the old distributor cap was scored while the head was loose. I'll check the new cap in a few days.

If the peen-fix did not position the rotor to turn concentric with the distributor cap, the problem may surface again. Then it's time for a new distributor.

Perhaps #1 cylinder did not receive a full or constant ignition spike because of the gap in the distributor cap which may have resulted in unburned fuel and caused the backfire. Still don't know why it started after about 60 miles of road miles or why it got so much worse afterwards.

The flash from my timing light connected to cyl #1 was not consistant. It was difficult to set the timing with the light. The flash is much more consistant and easier to use with the new distributor cap. Interesting -- the timing indicated by my variable timing light before the change was about 8 deg BTDC. The indicated timing was 4 deg BTDC after changing to the new distributor cap. I changed the timing to 8 deg BTDC and could probably advance even more.

I was not comfortable with my execution of David Braun's method of setting the float level relative to the bridge. So I reverted to the WSM method of setting the floats 3/8" below the edge of the lid before I installed the new distributor cap and rotor. Using the "bridge" method, I had set the floats about 1/4" or less below the lid. I did so just in case the problem was caused by a high float level. The new float level did not fix the problem. I should try the "bridge method" again.

Thanks to all of you for your suggestions. I used every one of them. Feels like I checked or replaced everything except the air in the tires! This is really a great forum.

LM Cook

Congratulations Lonnie!

Looking good, Lonnie, you must be pleased.
G Evans

Sweet...go get some bugs in your teeth!
David Sheward

Congratulations Lonnie.

I share your happiness because I traced the fault on my car today to a low system voltage. Cleaning the contact points of the regulator restored the 12V needed for a healthy spark, but ideally the voltage should be somewhat higher (13V?) at 2000 RPM, so some extra care for the regulator is in order.
Willem van der Veer

My distributor was loose and Jeff at Advance Distributors fixed me up with a later model distributor from which he removes the vacuum advance and re-curves the centrifugal advance, all for a very reasonable sum.
JE Carroll

This thread was discussed between 26/03/2014 and 29/03/2014

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