Hi,

If anyone has any thoughts on this I will be extremely greatful, as I am supposed to be off to Le Mans tomorrow!

I have a 1972 MG BGT, and have recently replaced the head gasket and fitted a new (now unleaded) head.

After a few days setting everything up I thought it was all sorted yesterday, and it drives well for about the first half mile. After this it seems to randomly lose power, lurch forward and backward, and "kangaroo" violently. I can't get above about 10 mph.

The problem only appears to happen under load, and when setting it up I have established that...

- Good compression on all cylinders
- Good spark (new points and condensor)
- New fuel filter (old one was quite a mess)
- Carbs cleaned (were quite a mess also but were from another car - as mine were stolen!)
- Tappits need a bit of adjustment but are not miles off
- Fuel pump appears ok (?), clicks when ignition on
- Mixture seems ok (bit of choke required to start from cold)

As I said, any thoughts appreciated as I am perhaps getting a little desperate... Toblerones for all if I make it!

Thanks in advance,
Stu

Don’t like the sound of the ‘was a mess carbs’ are you sure they are properly cleaned and the choke system is really working. Will the car run up to normal temperature without driving it about? Just run it till it fully warmed up and check the chokes off and that the accelerator jets are working, it’s not those damn SU rubbish is it? My friend had a Midget and it was a nightmare for the carbs freezing, I hand to run some alloy ducting from around the exhaust and feed it to the carbs to allow it to run in the winter. Check also that the coil is not roasting hot, it may be shorting inside.

I have had similar problems with a heavy duty plastic bag that was lifted into my truck engine bay - it snagged in the bodywork and when the engine load increased was sucked over the inlet to the air filter and caused the engine to slow, which reduced the suction - bag free - engine picks up - bag sucked back....

I have heard of a similar problem from someone who had a lined fuel tank where the lining material was loose and sucked up to the tank outlet.

Do you get a strong smell of petrol when you stop when its giving the problem? If so its probably something stopping the air supply (or spark), if not, somethings stoping the fuel.

It seems strange that its only once you get going - Your fuel line doesnt run anywhere too near a heat source does it? This can cause vapour in the fuel line and engine stuttering.

If you leave the engine running on the drive, do you have the same effect after a while (if not, less likely to be heat related)

Hope thats of some use

Regards
Hugh

If you only need a bit of choke from cold it is probably way too rich, to check take a look at the plugs sooty = too rich, all the B's I've had need full choke to start then it can be backed off gradually. Also check your tining, if using unleaded now it will need retarding, but that shouldn't cause your symptoms.

well, where to start?

the B series is pretty unsophisticated but robust and will run under unfavourable conditions.

the set up as you describe should be fine but the randomness is intriguing.

I'm afraid you need to go though each bit in turn - haynes manualhas chart to follow.

ignition
- check timing
- check advance is working
- check coil is working - put plug on king lead and hold near engine earth, crank engine
do this when you have the fault (remove spark plugs to help cranking)
- check igntion leads, as above for each plug
- check plugs (should be covered by the above
- check consdener, renew as these are cheap - open points in dizzy to see if there is bog spark
- check points gap

fuel
- undo paetrol cap - listen for whoosh of air - air lock in tank
- undo pipe to carbs - twins HS4 right? see if fuel flows
- check fuel filter is connected right way round


will the engine idle when this happens?
does it happen more or less that same time after the engine started?
does it always go away when engien cools down?
does it happen from cold?

the randomness after warm up kinda points to electrical component failure.

it could be also be fuel starvation due to some crap in the fuel lines

Thanks.

The carbs (yep, rubbish SUs!) were definately cleaned and working properly on sunday, we spent hours fixing/checking all that so I would hope that isn't the problem.

It does run up to normal temperature sitting on the drive, and the symptoms are not seen when doing this. The fuel lines are away from heat, and I don't think there is a smell of petrol. The fuel tank lining does sound like a possiblilty. I suppose the best way to check for this is disconnect the fuel line from the carbs and run it into a bottle?

I have not checked the coil, so I will look at that. I set the timing with a light to the point BTDC suggested in Haynes, but I will try retarding it slightly to allow for unleaded (a few degrees?). I'll also check the plugs and exhaust, I think it may still be a little rich.

Thanks again, any more suggestions welcome!

quote:

---

Originally posted by Stuart Walker
Thanks.

The carbs (yep, rubbish SUs!)

---

Cheers 02GF74 - plenty to be going at there!

I checked the needle type and set the lengths the same. I'm pretty sure they are in right. I will check the mixture, but as you say I did not think this would cause the problem I'm seeing.

We found particles of rust in the bottom of the carbs, under the automatic cold start device thing.

So, if my tank lining is coming out what do I do? Drain the fuel tank and flush it through somehow, or is a new tank the only solution?

Try a new rotor arm, there was a dodgy batch made that cause ignition problems only when warm...

Rotor arm purchased, added to the list of things to try... Thanks.

I had the same symptoms on my BGT and tried everything to fix it including head gasket, carbs, points etc but eventually found it to be the electric fuel pump.Athough it worked and ticked when first turned on it was stopping intermittently when driving the car, giving low fuel pressure.A new pump solved the problem.
Paul.

lined tanks - this may be wrong but is how I remember it.

there was a phase when BL made lined tanks and I seem to recall Hilliman Imp having these. It was supposed to stop tanks rusting but what happened is the lining broke off into piece that would block the fuel pipes.

If you have one of these and I have no idea if they were fitted to MG as oem ar replacement parts, then I doubt there is no other solution but to replace the tank.

you may want to shine a torch into the tank to see the condition. and look for signs of a liner, whatever that may look like.

I don't recall there being a drain tap on the tank and you will find the bolts holding the tank will ineveitably shear the lugs on the tank itself; the tanks always go on the top as there is a water trap between that and the boot floor - no need to ask me how I found that out.

^^^^ the fuel pumps can be overhauled! ^^^

[Edited on 9/7/08 by 02GF74]

Stuart
What you are describing sounds very much like fuel starvation, I would suggest a blast of compressed air down the line to the tank (take the cap off first!!!) should sort it, make sure you put a new filter in the line.
The fact that it doesn't do it on the drive when fuel usage is minimal even at high revs, but only under load leads me to think it has to be fuel related - hence it will only manage 10mph cos that's all the fuel you have available.HTH

Paul, that sounds vaguely hopeful as I think I have a spare pump!

I will have a look in the tank and run the fuel pump for a while and see if any rubbish comes through, if not hopefully a new pump might sort it... Certainly worth a try.

Thanks again

Assuming the SU pistons are rising and falling freely and the linkage bar between the two carbs is not slipping and distributer centrafugal advance is working it can only be one of two things either:

(1) a lack of fuel feed through to the float bowls. --- Remove the float bowls and check the pump is delivering enough fuel (clean and water free) past the float needle valve.

or

(2) the distributer base plate is no earthing --- remove the base plate and check the little earth wire on the underside is intact also check the condensor is tight on the bae plate and correctly fitted to the points.

What oil have you got in the dashpots? Unscrew the plastic tops and fill them up with engine oil. There's nothing wrong with SU carbs, simplicity is always the best solution.

[Edited on 9/7/08 by Peteff]

B GTs often have a problem with fuel vapourising in the lines in the engine compartment due to heat.

If you have the standard run of fuel pipes this may be your problem. Doesn't unleaded run hotter than leaded ? Was it during the recent hot spell ?

Bonnet louvres will look sexy and may let the heat out.

The dashpots are full to the correct level with 3 in 1 and fall fine (and together) when lifted.

It wasn't during the hot spell (was during the rainy spell!), but I will have a look at the temperatures under the bonnet. The fuel lines are not the standard ones, and they are routed around the front of the air filters so shouldn't get too hot there.

Not sure I have time for bonnet louvres today - a hole may have to do..

Well I tried lots of things, including a new fuel pump, but sadly it still doesn't want to play :-( so I have resigned myself to Le Mans in a Megane.

Thanks again for all the suggestions, will have another go in a week or so.....

If it runs ok for first half mile it sounds as if it is heat related.

If the stuttering is violent, it sounds like it's electrical / ignition: fuel problems are generally not violent.

Basically it's more shotgun replacement - coil, condenser, check all ignition-related wiring for solid connection.

You need thicker oil in the dashpots or the mixture will not richen up enough when you put your foot down, they lift too quickly with 3 in 1.

Check the continuity of the low tension lead inside the distributor. Once had a Mini with similar problems , replacing the said lead cured it. Also have had faulty points give similar. Broke down on the M4 once but managed to get going again with a squirt of wd40 on the pivot post of the points.