Welcome to our resource for MG Car Information.



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 MGB Technical - 80 MGB won't run right

Hi All,

I've got a '80 LE with 96 K miles on it. I've never really had the car running right since I bought it. It has the Z-S carb, no air pump or cat and a 25 D distributor. The problem is that the car is hard to start, and misses and has no power once started. There's backfiring throught the carb ( despite the fact that I enriched the mixture fully). All seals including the carb diaphragm are also new. I checked the point gap, replaced the condensor, cap, rotor, and wires. The coil is a Lucas 12 V ( I no longer use the 6 volt coil system with resistor). I checked the valve adjustment and it's okay. Any thoughts or ideas?

Dave Plantz

Have you checked the timing? You really shouldn't need to have the carbs set too rich. I am not familiar with the ZS carb so can't help there.

Hi John,

I static timed this car, but it didn't seem to help at all. It also doesn't seem to make much difference which setting the mixture is at. I don't know the history of this engine, but it seems like it should run right. They're so simple, yet I've managed to hit a dead end with it.

Thanks for the response.

David Plantz

Try retarding the timing a few degrees. Also go through your ignition system. Any old or weak parts can cause weak spark and possibly a buildup of unburned fuel which can cause the carb backfire. This includes your distributor! Try weakening the mixture once you fix your ignition system.
Jeff Schlemmer

Well, I tried the timing but it didn't really make a difference. There's no power at all. I also put new gas in the tank. Is there a default setting that I need to do to the carb to make it run right? BTW, it's a Zenith with the original water choke. The metering needle is also new.

Dave Plantz

Just went through the same thing.Look in old posts under Zenith carb and try to find my post from last month.There is a default setting for the needle and fine and course idle screws. see old thred.

G Groeber

Hi Gerry,

I looked but couldn't see to find the post. I suppose that the Zenith could be the problem, but I'm not sure. I'm considering putting on daul HIF 4's. Then I would want to redo my engine to pre 75 spec to take advantage of the carbs. I'd rather have my car setup like a UK model anyway. Don't they run better with SU's ? ( properly tuned of course).

Dave Plantz

Replace your points, it's cheap and I've solved otherwise mysterious problems by replacing seemingly good points a number of times.

Have you done a compression test on the motor?

Backfiring out the carb is generally timing way too advanced, car running way too lean or a problem with an intake valve. You've addressed the first two, but if you have a problem with a valve, neither fixing the timing nor mixture is going to make much of a difference.

Matt Kulka

Usually, backfiring through the carb is a timing issue, not a carb issue. Have you really checked the timing??? What I mean is, with the valve cover off, #1 piston at TDC with both valves fully closed AND looking at the timing mark. I just repaired a B that the crank pulley had that separated from the inner piece of the pulley. I have also seen the outer piece of the pulley slip on the inner, throwing the timing mark way off. You might also want to make sure that the spark plug wiring is in the correct order. With two wires in the wrong order, the car will run but will not run strong and will backfire. Good luck, John


The points are new, and the car ran the same way with a pertronix Ignitor on it. I'll try another set of points though and see what happens. Thanks for the suggestion.


Yes, I checked compression. #1 has 140, #140, #3 110, and # 120. I know they shouldn't be off by more than around 10%, but wouldn't the car still run this way? If it is an intake valve, it wouldn't necessarily show up on a compression test, right? Thanks for the input. Please let me know that you think.


I'll use your method to check the timing. I just went by having the number 1 piston at the top and the mark at TDC. That pointed the rotor at plug wire 1. Thanks for the help.

Dave Plantz

Dave, check the firing order on the plugs. 1-3-4-2

I think the symptoms sound familier to something I did as a new owner many, many years ago when I was but a wee lad.


Hi Luigi,

The firing order on this car is correct, 1-3-4-2 counterclockwise.

Dave Plantz


Start with some basics:
Do you have oil in the carb dashpot? Engine oil is a good starting point for this.
Check the diaphram for tears and for not seating /sealing properly - even though it is new.

If you have a VOM, check the resistance of the spark plug wires. Anything above 8k ohms will aggravate any other problems you have. Also check your cap and rotor, there should be no resistance here.

Tom Sotomayor

Maybe a head gasket leak since adjacent cylinders
3 and 4 have significantly lower compression than
1 & 2?

Are the valves adjusted correctly? Maybe 3&4 are
way out?

Also check for massive air leaks from the emissions
stuff. I had a leaking gulp valve not long
ago that injected enough air into the intake
to cause a severe misfire.

Air leaks can also arise in the vapor recovery
and breather hoses. Quick test is to
shut off the intake hose to the anti-run on
valve. If the engine stalls, the vapor
recovery system is intact, if not there's
a leak. This will cause the Zenith to
run extremely lean.

Hi Tom,

The carb has Moss' carb damper oil in the dashpot. I'll double check the diaphragm and I'll check for continuity at the cap, rotor, and wires. The wires are new ( Beck Arnley) as is the dist'r cap, and rotor. Thanks for the info.

Hi Ronald,

Could be a head gasket leak I guess. Would the difference in compression be enough to make the car run this way? I'm about to just go ahead and pull the head to see what's going on in there.

I adjusted the valves after I bought the car. I rechecked them and the clearances were still fine.

The airpump, and gulp valve setup were taken off before I bought the car. The hose for the brake booster, run on valve, and valve cover vent are still there. The crankcase vent isn't plugged in. Could there be a big vacuum leak in the brake booster?

Dave Plantz

What do you mean by no power after you finally get it started? Undrivable? Is the backfiring constant or at certain times? Is the spark weak looking?

Hi Greg,

By no power I mean that the car won't drive at all. I can move it forward gently on flat ground, but forget trying to go uphill. The spark looks white and strong ( I guess). Backfiring is constant ( under even the lightest acceleration).

Dave Plantz

Dave, the crankcase vent (front tappet cover)
is supposed to be connected by a hose to
a port on the side of the carb closest to the
firewall. If this hose is missing, and the
carb port is open, then the mixture will be
excessively lean which could explain your
problems also.

"The crankcase vent isn't plugged in. Could there be a big vacuum leak in the brake booster?"

Hold the phone. As Ronald says, if it's the hose between tappet chest cover and manifold, that's your problem. It's supposed to suck air against a resistance. If it's open to atmosphere, you're sucking a ton of air into the manifold.

Have you connected a vacuum gauge to the manifold? What reading do you have at idle?

To answer your other thought, you sure can have a vacuum leak through the brake booster. Find a way to plug the vacuum port at the manifold and see if that improves things. I know you can drive most cars with the brake booster disconnected if you're willing to use some oomph to push the brake pedal. I don't know about MG's. I suggest you wait for someone else to chime in on that safety issue before you try it.

Matt Kulka


I'll hook up a line from the crank vent to the carb. What size hose should I use? Looks like 3/8" but I'm not sure.


I'll try the crankcase hose first and see what happens. If that doesn't do it, I'll plug the brake booster to see if it gets better. I have a feeling that the problem may be that disconnected hose. There's not even a hose there. Maybe a DPO took it off at some point. ' Guess this system can't do its job properly if everything isn't connected.

Thanks guys for the responses.

Dave Plantz

I'd thoroughly check for a vacuum leak before pulling the head! When a Z-S carb fails it normally tends to run too rich, not too lean - the autochoke can leak or stay on, the diaphragm can be punctured, and it goes way, way, way too rich - seldom lean. If you had to enrich it all the way it's prob. not the carb - you've almost certainly got unmetered air entering the system. If you can keep the car running, you can pinpoint the vacuum leak with a can of carb cleaner. Spray a little cleaner on the various joints in your intake system (not forgetting the manifold gasket itself!) and when the spray hits the leaking joint you will notice a marked improvement in the running of the engine.

It may sound silly but i had a similar problem with a gt that i bought and after so much fidling around, the problem was the firing order was wrong.
Do a thumb compresser test by moving the fly wheel by hand on #1 cyl, make sure the piston is up them check where the the rotor is point at, thats your #1, them work from there,

A (an original anyway) ZS won't have a breather port to connect the crankcase to. The front tappet chest was connected to a port on the inlet manifold, and a restrictor on the back of the rocker cover controlled the flow. If that port on the inlet manifold is open, or any of the other ports are open, or the brake booster is flowing air instead of sucking a vacuum, then it *will* run like crap. And if a Welch plug has been blown out of either end of the inlet manifold it will be even worse. For the purposes of diagnosing the poor running it doesn't matter if the crankcase vent is open, as long as the manifold vent isn't.
Paul Hunt

And don't forget, Your air cleaner assembly must be in place for the carb to work properly on this model.

Also, the air flows from the inside to the outside of the air filter on original ZS setups. They will clog up but still look new from the outside.

I have to correct Sam on one point: if the diaphragm is punctured, the ZS WILL run lean. I agree with Sam on everything else, but if the diaphragm is punctured, the piston won't lift, the needle won't open and the engine starves out. ' Been there!


Dave- Your diaphram on the ZS is micro cracked. I went through the same heartache a few years back and spent hundreds on by gosh and golly "could be's". The diaphram dries out in disuse and cracks. These cracks are visible to superman and anybody else with a magnifying glass. Hook up an accutune/ color tune before replacement, then adjust your best blue after. Problem solved my son. Vem
vem myers

Another thing you could try is to loosen your distributor pinch bolt, and hook up a timing light.
Then start the car and move the distributor to advance the timing. Try it in 5 deg increments.
At some point it will seem stronger, then on the next step weaker. Go back one step.
I recently did carbs and a cam on my '79 and had a similiar problem.
It seems that the cams on later cars were made of an infrerior (softer) material and wore quickly.
If your lobes are worn down the net effect would be retarding the timing.
Mine runs best at 40 BTDC.
I am going to replace the cam this spring.
(I got this advice from Bob at Brit-Tek. You could call him too.)
Good luck.
C. A. Fuller

Tappets tappets tappets....had similar issues....check your valve clearances...these motors seem to run rich and lumpy espcially around idle if your clearances are too might sound like sewing machine at 15 thou....but if it runs better than you are on the right track. Could also be worn cam syndrome if all else checks out...measure valve depression to check.


I'll look into the cam thing if the vacuum leak doesn't help matters. I don't really know the history on this engine, so I guess anything is possible!


I set the clearances to 15 thou, with the engine warm. I ended up doing this twice because I was worried that I had set the clearances to tightly.

Thanks to both of you for the info. I'm going home this weekend to work on the B, and I'm hoping to get it running right.

Dave Plantz

Speaking of Zeniths and not working without the air cleaner hooked up, Does anyone know if an after market air cleaner, (K&N for example) will work on a Zenith 175cd carb. The sell kits for this application, but apparently they cause the carb to run too lean, True or false?
Nick Rossino

how high is the voltage on the ign.coil, look at it.
your car had a resistor in the wiring loom, the extern resistor is not for sinking the voltage on the coil.
remove the wire from the hot site of the coil, take an temporairely wire to put 12 volt on your coil.

With regard to Wim's comments. My mechanic added a resistor to the wiring on my '79 which solved a lot of problems. Seems the resistor/resistance built into the later model wiring looms tends to fail.
Frank Sperling

Note Dave's original post where he says he uses a 12v coil and *not* the original coil or ballast resistor.
Paul Hunt

Hey Guys,

I got the B running yesterday. I took off the rubber vacuum hose going to the brake booster and plugged it at the manifold. The car started after a few turns and actually had some power. It quit almost all of the backfiring through the carb and revs up okay. The line from the crankcase vent to the carb is in place. I set the timing again while I had it running. Also put in the Pertronix ignition setup that I bought for it a while back.

I drove the car this morning around the block. It's a little sluggish going uphill. I really don't think that the Zenith's auto choke is working. The car misses and runs rough when I first start it, but does better after it warms up. I can feel coolant flowing into the auto choke water jacket. The car runs fine until I really try and step into the gas pedal ( especially when going uphill). BTW, the fuel filter is also new. I also replaced the rubber diaphragm just to be sure that it wasn't cracked.

Any ideas as to what I should try next? The mixture is set to full rich on the Zenith, but the car still runs too lean.

Thanks in advance and I couldn't have got this thing running without you guys.

Dave Plantz

Have you checked the timing with a timing light and the vacuum advance disconnected? Are you certain that the vacuum advacne is functional? Has the 25D distributor that you have in the car been rebuilt, or at least disassembled and checked for wear at the bushings (shaft), advance weight pivots, and the points plate pivot? ANY problem with the distributor will cause the driveability issues you have. The Pertronix will make up for main shaft play, but the rest is critical. What is the number on the side of the distributor?
Jeff Schlemmer

Hi Jeff,

I used a light to set the timing with the vacuum advance disconnected, at 1500 rpm. I don't know if the advance works. How can I tell? Everything inside the dist'r looked okay when I installed the Pertronix, but it hasn't been rebuilt. I'll get the number off of the dist'r tomorrow.

Thanks for your help.

Dave Plantz

Suck on the vacuum pipe leading to the distributor while you are pointing a timing light at the marks, you should see the timing advance as you suck and return as you release the vacuum. If you suck and suck and simply draw air through the vacuum capsule is perished, which as well as affecting timing will cause a weak mixture. Likewise simply revving the engine while pointing a light at the marks should show it advancing gradually. If you have an adjustable light you can plot the revs/advance curve and compare it with original specs for your distributor. However the first thing to do is remove the dist cap and twist the rotor anti-clockwise. You should feel spring pressure, and it should return all the way when gradually released.
Paul Hunt

Hi Paul,

Well, I tried sucking the end of the vacuum line that goes to the distributor and I didn't see the timing move at all. I don't think that I was actually able to draw air through the vacuum advance, so maybe something inside the dist'r is sticking.It's like there was a lot of resistance on the other end of the vacuum line. Any ideas?

Thank you for your suggestions.

Dave Plantz


"Speaking of Zeniths and not working without the air cleaner hooked up, Does anyone know if an after market air cleaner, (K&N for example) will work on a Zenith 175cd carb. The sell kits for this application, but apparently they cause the carb to run too lean, True or false?"

Autozone offer a K&N option instead of the regular filter, you would probably have to adjust your mixture to compensate for better airflow but that shouldn't be a problem.
Dave, my wife and I picked up a 1979 B last year (it was actually her old one that we found and bought back), it ran like crap when we first got it, the guy who had owned it for the last nine years really didn't know what he was doing.

I just went through everything methodically, everything I did improved the car slightly:

Set the valve gaps.

Lubed the distributor.

Changed plugs, points, cap, wires, rotor and condensor.

Set the timing, my crank pulley has delaminated and the timing mark is way off, I just played with the distributor until it felt right, I've been driving these cars for years so I'm pretty good at setting the timing by ear now.

Checked every vacuum hose and plug to make sure they were all tight, replaced a couple and added new hose clips where necessary.

Completely rebuilt the carb, with all new seals, gaskets, needles, pistons, etc., including the autochoke, making sure everything was as good as new and working exactly as it should.

As I said, each step improved the car some and once everything was done she was running pretty well.

I don't want to sound critical but you seem to be taking a "shotgun" approach to repairing your car. Take each section one at a time, as I did above, and test the car after each step. Start with the valve gaps, then the ignition, if you cannot get the vacuum advance to work you will probably have to remove your distributor, strip it down and clean and lubricate everything. Once the distributor is working as it should and the timing is set move onto the next step.

The diagram in the Haynes of the Layout of the Fuel Evapoative Loss System is a huge help when tracing vacuum hoses and hunting for leaks, I blew it up onto a 28" poster board so I can see it across the garage! My car has most of the emissions system missing but that diagram still showed me where all hoses should go so I could check everything was actually connected up in the right order.

Hope this is of some help.


The Wiz

Hi Mike,

I'll pull the distributor tomorrow and clean it up. It could need a new vacuum advance. ' Wish I knew where to find the original type emission lines for these cars. I'll go through the check all of the hoses, but leave the brake booster disconnected for now since it's leaking. I like the Pertronix systems, so I'll leave that. My Zenith carb has been rebuilt, but I'm not sure that the choke is working. I'll probably need to pull the carb and reset/check all adjustments.

Thanks for your advice.

Dave Plantz

Hi Guys,

I pulled the B's distributor this afternoon and cleaned it up. I oiled the weights on it. The weights say 16 degrees on them and the distributor body has 42025 D stamped on the side. Is this right for my car? I think that the vacuum advance is okay because there's resistance when I pull or push the spring that comes out of it. I'm going to put it all back on the car this evening and check all of the vacuum and emissions lines again. After that, I guess I'll pull the Zenith and reset all of the adjustment screws according to the Bentley manual.

Dave Plantz

That distributor is close in curve to the stock original. It's definately not the best for performance. As far as the vac advance goes, does the spring move in when you apply vacuum to the can? If it doesn't move or you can hear a hiss, there is a leak in the diaphragm.

Was your carb professionally rebuilt, or was it cleaned with a kit installed? There is a HUGE difference and could make all the difference in the world!
Jeff Schlemmer

Doesn't look like any MG was built using a 42000-series distributor. The Lucas index doesn't show an application for a 42035D. Could this mismatach cause this much trouble?

Chuck Hassler


I cleaned the Zenith and put in a gasket kit. I talked to John Twist and he says that he rebuilds them. That might be a good idea to make sure that everything is right. I'll have to start saving $$ to pay the $300 bill on the carb rebuild.

Yes, the spring on the dist'r advance moves when vacuum is applied to it.

Thanks for your help.

Dave Plantz

This thread was discussed between 26/04/2005 and 12/05/2005

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.