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 - Running problem...

Hi,

I've got a '78 MGB GT and just recently i've been having problems......

It only happens rarely, but at about 70mph sometimes the car will 'bog down' and it drops to about 50mph.

The best way i can describe what it sounds like is when you have the choke pulled out too far on a cold morning and the car struggles and you get a kind of popping sound out of the exhuast.

This has only happened 3 times in the last month at roughly 70mph each time and it sort its self out if i stick to 50mph for a mile or so and then everthing is back to normal?

Any ideas?
Gavin J

Sounds like fuel starvation, at 70 MPH the fuel system can't supply enough volume. Check for plugged fuel filter in fuel pump and any external filter that may have been added, weak fuel pump pressure, sticking carb floats, junk in the gas tank blocking the intake tube, air leak in the line between tank and pump.
John H

I agree, it's a fuel blockage somewhere. BTDT :(
Mike MaGee

Almost certainly is a fuel blockage which is caused by a lump of crap floating around and blocking the rear of an orifice such as a float valve when the flow is high. The other faint possibility is that the gas can't get out due to a loose core in a silencer which is being forced against the outlet at speed.
Iain MacKintosh

Gavin - Send me an e-mail and I'll return a fuel delivery troubleshooting guide tha may be of assistance in isolating your problem. In the mean time, my favorite thing to try (because it happened to me) is to drive with the gas cap loose or off for a time and see if the problem goes away. If the fuel tank vent, which in your case is through the charcoal filter, gets gloged or partially cloged, it will cause the problem you are describing when a vacuum develops in the fuel tank. If the problem occures more often with a ful tank of fuel, then it is a good indication that this is what is happening. Good luck - Dave
David DuBois

The pump should deliver at least 1 pt per minute into a container at the carb end of the pipe. UK cars never had the charcoal canister or any of the associated plumbing, just a sealed tank with a vented fuel filler cap. This can get blocked but is much less likely than a blockage in the the North American plumbing and canister, but the indication in both is a big 'whoosh' of air into the tank when removing the cap when the problem has occured. Bear in mind that UK cars vented fuel filler cap has a spring-loaded valve which must be overcome before air is drawn through it into the tank, and so even on a properly working system there will be a small 'gasp' when the fuel filler cap is removed having driven a few miles. Gauging a 'whoosh' against a 'gasp' is higly subjective, but if the car runs fine after removing and replacing the cap, then bogs down again after a few more miles, a blocked cap could well be the cause. But if driving at 50mph for a mile or two clears it then I doubt a blocked vent, more likely an intermittent fuel delivery problem. I had similar on a V8 for a while that had an electronic 'pointless' fuel pump, changing back to a points-type pump cured the problem.
Paul Hunt 2

Thanks for all the replies,

I took it out for a drive today again and it was doing it more often and at lower speeds.

I fitted a brand new tank around 3 months ago so there should be no bits being being dragged through the tank and teh fuel fuel cap is vented. Also checked the fuel filter and this is not blocked and looks clear.

I checked the spark plugs and they also look fine, however when i took the dizzy cap off to look at the points the HT lead fell off! so this must have have been very loose. So i have put this back on ensured it fitte nice and tight and i'm hoping that was the cause of my problems......

I will also change the points in the morning and take it a drive tomorrow and fingers crossed....


Would a loose HT Lead cause similar problems?


Gavin Jewkes

Ah! A new tank! Are you sure it had no bits loose inside before fitting? (Only partly joking).

The one thing you can be sure of is that on an MGB anything is possible where problems are concerned. If vibrations changed at around the point the problem started then yes a loose HT lead could have been the cause, although normelly I'd ecpect that to occur at particular engine revs, not road speed, although if road vibrations interact with engine vibrations in a certain way ...

Whatever, a loose HT lead is not a Good Thing. Remember that after changing the points, or the gap on existing points, you must recheck the timing.
Paul Hunt 2

Several days ago I made some photos and compared vented and non vented fuel tank caps for the MGB. The vented cap is marked inside "vented Anti-surge". I checked the valve for air flow. When sucked on the valve there was no resistance, free airflow. When I tried blowing through it I didn't have enough wind to open the valve. The valve I checked will not allow fuel vapor or fuel to escape until the tank has built up enough pressure to overcome the spring in the valve, probably 3-4 psi. I did this exercise because some people have drilled holes through a non-vented cap to vent the tank. This works for venting but if you fill the tank and go uphill or let the car sit bin the hot sun all day you will probably have fuel run out the vent. The anti-surge valve prevents that unless there is sufficient pressure in the tank to open the valve. The cap I tested was from a 68 MGB NA specification car. Photos are in my Yahoo photos at;
http://pg.photos.yahoo.com/ph/flash1929/album?.dir=a70c&.src=ph&store=&prodid=&.done=http%3a//photos.yahoo.com/ph//my_photos
My albums are open to the public so I hope the link works.

Clifton
Clifton Gordon

I was under the impression that the spring-loaded valve in vented caps opened when a certain amount of *vacuum* existed in the tank and was not free-flowing. This prevents a continual stream of explosive vapour from escaping, and leaks in a roll-over. Certainly my roadster makes a small 'gasp' when removing the filler cap if I fill up during a run. I didn't think it opened at all under pressure, or though it may be a double-acting valve otherwise considerable pressure could build up and overwhelm the float valves in the carbs.

Link worked fine for me.
Paul Hunt 2

I removed and disassembled the fuel cap valve and added some more photos. There is no spring for inlet venting, inlet venting uses a plastic button which opens when a vacuum is present. The spring is on the pressure outlet and only opens when the pressure overpowers the spring. The button type vent valve is seated on a seal when there is pressure. I used a postal scale and a dial caliper to determine how much pressure is required to open the spring. The free length of the spring is approximately 3/4" long. It is compressed to about .3" when installed in the valve. On the scale it takes approximately 10 ounces of pressure to compress the spring to that length. The spring coil binds at about .210" and 12-13 ounces. After reassembly of the valve I tried to measure the pressure required and it appears to require about 13 ounces pressure to open and allow air to escape. The example I have is free flowing for inlet suction and requires a little pressure to expel pressure build up.
Clifton Gordon

This thread was discussed between 23/11/2005 and 29/11/2005

MG MGB Technical index

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