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MG TD TF 1500 - backfire through carbs

Dear all, here is my current problem. The XPAG starts at the first pull easily. Run it uo to 3000-3500 to lubricate the inside. Try to move the car, even at 3000, it loses power and backfires through the carbs. where to start? Tom
Thomas McNamara

My first guess would be a cracked distributor cap. I've had it happen on a couple of cars. Next guess would be a burned valve. Compression test would reveal a low cylinder pressure.
John Masters

Or a sticky valve
W. A. Chasser

Check your ingition points, plugs, compression & timing, but it sounds like it could be lacking fuel.

While checking your ignition, pull the plugs and look at the color and gap. Brown/tan good; white is lean; black is too rich. See if one pair is significantly different than the other (1 & 2 vs 3 & 4) as it is a dead giveaway as to one carb funky.

Make sure your pump is delivering fuel, but it again doesn't cost anything to clean the strainers/screens in the pump and carbs. Also look down the float bowls for accumulated brown rust. You might also pull the main jst to make sure it didn't get crapped up.

Then come back here to report what you find, then we can lead you down the path of checking for an air leak around carb & intake manifold!

Similar issue a few years back on my B. Got up to to 3000 + RPM and started to backfire, tach jumping, etc.

First check the coil main lead. Mine worked its way out just enough to cause this problem. refitted wire lead and issue resolved..

Is the tach steady or is it jumping. Check the points, and condenser. A failing condenser will cause the same symptom.

Check cap and rotor for arcing on the distributor cap.

Let us know how this works out


79 MGB
gary hansen

All any engine needs is compression, proper fuel/air mixture delivered to the combustion chamber and a good spark delivered at the proper time. Work your troubleshooting in a logical sequence with those factors in mind to determine the cause of your backfire.
Have you done any recent work on the engine? If so, I would start there, still working in a methodical manner to test and verify the proper mixture-timing-compression.

I am not sure I understand the " run it up to 3000-3500 to lubricate the insides"...42 years as a mechanic on various transportation modes powered by internal combustion engines and I have never encountered that in any operators manual. The greatest wear on any reciprocating engine is revving on initial start up before the oil warms..multi-viscosity oils help a great deal.

Regards, tom

tm peterson

Check your timing...if it runs well without a load then I doubt it's valves, points or wires.
Gene Gillam

Is there a ground on the dizzy. Had a similar problem and as I moved the dizzy around to check timing, it would cut out. Installed a small ground line on the holding bolt and it cleared right up.
M Grogan

I have had exactly the same problem and eventually traced it to a warped cylinder head allowing a blow through cylinders 2 and 3.

Jan T
J Targosz

Are the rocker arms/valves set properly, none too tight? PJ

You did not say if you had done any work. Made any changes? History of the engine, etc.

I had a similar issue and here is how I solved the issue. I started at the top.

1. Adjust the valves. I did them cold and added .001 to the middle of the road setting of .015.

2. Moved to the distributor. Replaced the wires and the cap. Noticed oil inside. Found I had a oil leak at the bottom of the tappet cover. Replaced that. Due to the oil I replaced the points. Replaced and reset the the points and the rotor. Did not change the condenser. I did not deal with the coil.

I checked for spark at the plugs by grounding it to the engine.

3. Next moved to timing. Used a socket to align the pointer on the timing cover with notch on the crank pulley. Then opened the cap on the distributor and noticed where the rotor was pointing. Found what should be number one was actually number four. Simply moved the wires to give me 1432 firing order.

3B maybe the distributor needs to be rebuilt?

3C did a compression check and put some oil in the cylinder. It was good and the delta between wet and dry was 30# in all cylinders.

4. Fuel. Found I was not getting full choke nor was it releasing completely. Installed new cable. Too messed up to work with the old one. Checked linkage while I was there to make sure I had full throttle.

5. Cranked the engine and let it idle. It started right up. Adjusted the idle to 700 rpm. I should note this was a totally rebuilt carb so I was confident that it was in superb shape. If it was not perhaps you need to look into the carbs and vacumn leakage. Think vacumn leak. Test with brake cleaner for a rise when squirt at the shaft between carbs.

6. I used my timing light which has a adjustable dial and set the idle to 8 BTDC and made sure I had 30 BTDC at 3000 rpm.

Hope this helps. It is not a quick fix but a logical sequence that worked for me. It replaces a shot gun approach of try this and then try this.


JWP Policastro

Forgot. Prior to the above I drained the the fuel tank. Removed and cleaned all the fuel filters and made sure the pump was working.

JWP Policastro

Joe, I guess you meant to type 1342? Cheers
Peter TD 5801
P Hehir

Hey all, worked on the problem off and on all day. Changed plugs, rotor, dizzy cap checked timing. No Joy. :( I pumped a quart soda bottle 3/4 full in about 40 seconds. Fuel to the carbs appears okay.

I can take the car to 3000 rpm and it runs smooth as glass. As soon as one tries to move the car it starts to backfire through the carbs and has very, very little power.

The one anomaly I did find is that the dizzy has about a "loose" 1/8 inch vertical play on the shaft. The top bushing seems okay for side to side play. Could this somehow be the cause of my problem? Tom
Thomas McNamara

Did your tach jump about??? That would indicate an ignition issue. When you changed out the plugs, distributor cap and rotor, did you change the condensor??

Check the secondary coil wire to the distributor also.. Over time this wire loses its conductivity and will break down once the rpm load is increased.

79 MGB
gary hansen

What is history of problem- most important to help figure out. Condenser source and age? Rotor red or black? These are most likely suspects. Remove coil wire from cap and coil end and make sure not burned off/loose. Check for major air leak? Stuck carb piston? Float/fuel level? Shots in the dark: make sure tach drive not rotating and shorting on dizzy terminal. Also try separate ground from battery to block/dizzy- load causes engine assembly to move, if cracked/loose ground strap. George
George Butz

Are the ignition wires in the right order?

Klaus Harthof

Second above. Also, pull plug wires one at a time (carefully) and see if same RPM drop for each cylinder.
George Butz

the tach on the TD is mechanical. Faults in the ignition only show up on an electr(on)ic tachometer.
Willem van der Veer

Backfiring through the carbs is almost always a symptom of too lean a mixture (not sufficient fuel). Only one person has mentioned that and I see no such advice. This is not a fuel pump issue but rather an issue of carb settings.
Check the float bowl levels and ensure your mixture is properly set. I don't need to tell you how as that information is well covered elsewhere.

Hey all, some joy today. After chasing my tail for a week I decided to go back to basics. What I finally found was an un-tightened points base plate. Somehow it worked until under load? At any rate, I set the points to .012 and the car, thought still not "right" does move under it's own power. Thanks to all and please enter this into your memory banks. Tom
Thomas McNamara

It also may be possible that the advance weights in the dizzy may be sticking and may require a little bit of lube. They could stick advanced when they should return for more power.

Just a thought.... CR
C.R. Tyrell

Did you check your plug colors? You may find one pair black and the other pair white. That would isolate one problem carb.

Try driving it with the enrichener (choke cable) pulled out... make sure it actually pulls the main jets down. Take it for a test drive.

Jim, he says he fixed the issue. Regards, tom
tm peterson

This thread was discussed between 03/07/2016 and 15/07/2016

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