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 MGB Technical - Non starter

Hello, it's been a while since I have posted anything. I have a problem with a 1974 MGB GT, I replaced the HIF4 carbs recently for some HS4 carbs. The car did not start after that, I changed the Points & Condenser as I have had to do this on a couple of occasions to get it started. I left tuning the carbs properly for another day as it was starting now,I tried starting the car the next day and it would not start. The battery is fully charged,fuel is getting to the engine because when I removed the plug there was signs of fuel on it. I have checked the plugs gap,Points gap, I asked a friend to turn the ignition while I removed the Number 1 plug lead to check if any spark was there. I could not see any signs of a spark, the engine is turning over. If I remove the distributor cap & rotor, turn the ignition on and move the contact apart with an insulated screw driver there is a spark at the points. I should also mention there was a coolant leak which dripped onto the distributor for some time,which I think could have caused the distributor to stop functioning properly. I am going to replace the plugs tomorrow as well, I suspect the distributor needs to be replaced. It is a 25d, not sure if this is the correct one for this car. The engine number is 18v700f, the haynes manual doesn't quite help with this. Any help will be much appreciated.
Glenroy
G Thompson

I suspect the distributor rotor is shorting through its insulation.
This has been a common ailment as of the last few years
A new rotor should get things working again.
Daniel Wong

Thanks Daniel, I will get a new rotor and see if that fixes the problem.
Glenroy
G Thompson

I hate swapping parts without proper disgnosis. If you have a timing light clip it onto the coil lead and each plug lead in turn and watch the light while you crank. If you get flashes on the coil lead but none of the plug leads then the rotor probably *is* at fault. But given the coolant leak onto it the cap is more likely, and in this case it may well flash on some plug leads but not all. If no flashing on the coil lead then it closer to the coil/points/condenser. If you don't have a timing light remove the coil lead from the cap and check for a spark from that the same way you did before, or just by flicking the points open with the ignition on. Again no spark points to coil/points/condenser.

If you get a spark see how far it will jump, a good system should jum at least 1/4" with a sharp crack. If the condenser is bad it will barely jump a plug gap and only fizz.

If you *do* replace the rotor make sure you get one with either no visible rivet head i.e. OE, or if it has a rivet head make sure it is outside the base circle (see http://www.mgb-stuff.org.uk/rotors.htm). The ones with the rivet inside the base circle are well known for breaking down as the rivet is too close to the steel shaft.
Paul Hunt 2010

Thanks Paul, I don't have a timing light, been meaning to get one. I did remove the coil HT lead and could not see any spark or hear any sound of sparks jumping gap when trying to start the car. Ordered points, condensor, rotor parts but the wrong ones turned up, waiting for correct ones. Don't know what else I can do to get MG started.
G Thompson

You really need a voltmeter to check the ignition low-tension circuit, but what you get will depend on whether it was originally a chrome bumper i.e. unballasted ignition, or a rubber bumper with ballasted ignition, as it changed during 1974.

On a chrome bumper car with the ignition on and the points closed you should see 12v on the single white wire which should be on the coil +ve, and almost zero on the two black/white wires on the coil -ve. With the points open you should see 12v on both.

On a rubber bumper with the ignition on and the points closed you should only see about 6v on the coil positive, which should have two white/light-green wires on it, and almost zero volts on the coil -ve as before. With the points open you should see 12v on both coil terminals as before. When cranking on the starter you should see 12v on the coil +ve as this is the coil boost voltage.

If you get that, but there is no spark from the coil lead to earth when you flick the points open and closed, then the coil is almost certainly faulty.
Paul Hunt 2010

Paul, thanks for the advise, it has been really helpful. Just wanted to update on the situation with the MG. I replaced the rotor, which did not fix the problem, I had to replace the plug leads because they were coming apart when removing from distributor cap. I also replaced the distributor cap, and condensor after which I could not get the car started. But on your advise Paul, I was getting at least 12v at the coil using a volt meter. I decided to spray some carb cleaner into the carbs, because I was getting spark to the plugs with all the changes. I got a response from the engine, which prompted me to check the carbs for fuel. I did have fuel in the tank while I was having this problem but it was low. I added more fuel, checked the front carb which had fuel, but the rear did not have any. I could hear the fuel pump ticking away when the ignition was turned on, but it was not as strong as it normally would be. After opening the carb bowls to check for fuel, I might have released air trapped in the system. Becaused the pump was back to normal and I got the car started, this has me wondering whether I had two problems from the start. I plan to replace the new parts with the old ones I removed to recreate the problem to get to the bottom of what was wrong. I am a bit surprised at the fuel problem, because that was fine before I had the ignition problem. I will update on the progress as I am trying to get it M.O.T'ed tomorrow.
G Thompson

Opening the carb bowls shouldn't have had any effect on releasing air from the system as the float lids have vents in them to do that anyway. If those vents get blocked, far from preventing the fuel pump operating, it actually stops the float rising to shut off the pump and the fuel pours straight into the inlet manifold and combustion chambers. If the rear carb had no fuel when you removed the lid it would have been because the float valve was stuck closed, or there was some other debris in the lines, possibly dislodged during the removal and refitting of the lid.
Paul Hunt 2010

Thanks Paul. Does anyone know the best way to clean the fuel system. That is the tank, pump, fuel lines etc, I suspect there is debris getting into the carbs.
G Thompson

Have you still got problems?

If you have luckily changed the carbs to HSs it is simple job to look in the float bowls for debris. None visible in the bottom means there isn't any, unless one lump got to each to go inside the jet pipe and block that! Debris wouldn't normally stop fuel getting into the carbs, it usually stops the float valves closing hence causing an overflow.

If there is debris, in the shape of brown particles, it could well be rust from the tank, especially if the fuel level has been left low for a long time, although the pump should have a filter on the inlet to stop particles blocking its one-way valves. If this is the case you could fit an in-line filter, but if you find that getting choked more frequently than once a year you may well have to plump for replacing the tank. I hear Americans talk about stripping and sealing the tank, but you have to be careful with that or you can end up sealing the tank strainer as well.

I also hear tell of gummy deposits in carbs causing problems after a layup, also fuel 'going off', but I have never had any problems not using a car for up to three months over winter and having fuel six months old by the time I start filling it up again in Spring. The gummy deposits can be cleared by spraying carb cleaner in the float chamber inlet, outlet to jet pipe, and down the jet from above after carefully removing the piston cover and piston.
Paul Hunt 2010

Thanks Pauls for all the information. The reason I asked was I do get a lot of debris in the Fuel filter and suspect that the tank could be the main culprit. The vehicle is running and it is just a matter of running maintenance now.

Glenroy
G Thompson

Bits in the filter do indicate a rusty tank, but of course the filter should trap those and prevent them getting to the carbs, even if it needs changing more frequently than normal.
Paul Hunt 2010

This thread was discussed between 15/03/2010 and 29/04/2010

MG MGB Technical index

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