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MG MGB Technical - my BGT misfires above 2500rpm

I recently bought a 74 BGT which is only showing 35,000 miles from new (according to speedo and all old MOT certificates from 1977 to date.

It hadn't been used since 1999, but has just been MOT'd and I plan to use it daily.

However, when I got it it ran, but it wasn't great as it misfired and ran roughly, so I replaced points, condensor, distributor cap, plug leads, plugs, I also reset the timing (according to haynes which may or may not be correct! the midget haynes manual shows wrong data!) I have also set the tappets. Also I have replaced the fuel pump, added a fuel filter and put fresh fuel in it. I also changed the carbs for recently rebuilt units.

Now the car starts on first turn of the key, doesn't run on and ticks over great. It drives nicely and pulls smoothly. However, once 2500 rpm is reached in any gear, the engine note changes (it almost sounds restricted?) then it starts to misfire which can be felt through the car and makes driving very unpleasant. You cannot drive thrugh the misfire either to higher revs.

So basically i'm now driving around like an old granny and changing up gears earlier and not going much over 55mph.

I'm basically running out of ideas with my car now so any ideas would be greatfully received.

John
John Collins

Well you seem to have covered everything. But you say you have replaced the original carbs. Check that they have been furnished with the correct, jets, needles, dampers and springs. Also check the output (fuel flow) of the pump.
After that you're probably looking at head/valve problems. Do a compression test first. Check the exhaust for steam, oil for "mayonnaise" and the water for oil.
Good luck.
Allan

Any differences hot or cold? Any flickering of the tach with the misfire? Can it be reproduced at a standstill just revving the engine or it it only under load? Did you check the functioning of the centrifugal and vacuum advance systems?
Paul Hunt 2

a couple simple things-

you stated you added a fuel filter- can we assume you replaced the old one?
Also, have you checked for a good ground (or earth, as i believe you folks on that side call it)

my 69 gt was doing the exact same thing when I bought it. the fuel filter was clogged.

then it started again. eventually it wouldnt start. it was a bad ground strap. i cleaned it off, and now it's fine. well, except for rotted floors, bad brakes, and no clutch hydralics.... lol

let us know what happens
don
don g

Could very well be that debris in the tank has clogged the screen on the fuel pickup. Might want to disconnect at the pump and force air back into the tank (with the filler cap removed) to see whether this affects it. With a car that has sat this long, it might be a good idea to put a filter ahead of the pump, too.

It could also be related to the mechanical advance mechanism and to the new rotor. I would suggest putting the old rotor back on as a test, too.

You violated the old racer's mantra of "make one change at a time".
David "sounds like fuel to me, too" Lieb
David Lieb

sounds to me either a plug or plug lead is breaking down under load
check plugs for oiling up which will give you which lead or plugs going faulty
you dont say if you changed the rotar arm?
ste
Ste Brown

Checking the centrigigal and vacumn advance is well worth doing, a car used as little as this may well have them siezed leading to an inabilty to rev. Take the dizzy cap off and see if you can twiddle the rotor arm relative to the body, then suck through the vacum pipe and see if moves. Advice on freeing off is in the archives, I oil mine so have 0 expereince of doing this job.
Stan Best

It should be easy enough to check the centrifugal advance with a timing light while revving the engine.
Dave O'Neill 2

UK 74s didn't have a fuel filter from the factory, hence 'fitted' rather than 'replaced'. Check fuel delivery by removing a pipe from a carb and directing it into a container. Switch on the ignition and it should deliver *at least* one Imperial pint per minute, and in practice closer to two. It should deliver steadily and consistently, with negligible bubbles.

Sucking on the vacuum pipe should move the points plate clockwise, but you usually have to suck quite hard. If you can continually suck air through the diaphragm is ruptured, which will cause a weak mixture, although the loss of vacuum advance otherwise will affect mpg more than running.

With centrifugal advance, you *should* be able to twist the rotor, anti-clockwise, but against spring pressure. You should be able to move it so far against one spring pressure, then and second spring should come into play making it harder to twist further. Releasing it slowly *should* return it all the way with no 'free play' at the end, but it is inevitable that used distributors will have some free play. If you can twiddle it freely throughout its travel the springs are faulty and effectively you have no centrifugal advance, or more correctly maximum advance throughout the rev range. To check centrifugal advance with a timing light you must disconnect the vacuum pipe and plug the port at the carb, then it is easy to see the timing advance and retard as you go up and down the revs.
Paul Hunt 2

Is there oil in the carb piston dampers? They are often left dry for shipment. Ignition coil could also be breaking down at higher rpm.
John H

HI,

many thanks for all of the help and advice, I really do appreciate it.

OK, to work through the replies;

the oil was changed by me along with the filter, the old oil was fairly clean with no signs of mayo or other contamination.

It was giving the same symptoms both hot and cold, the tacho didn't fluctuate at all and it only misfired when under load.

I used a new fuel filter just before the carbs and also checked the fuel delivery as stated above (this is why I changed the old pump as that was only dribbling fuel through) I also blew the fuel lines through both ways with an airline.

I also tried a new coil on it (I was saving it for my 64 midget!) but that too made no difference.

BUT....tonight, I came home and remembered that my midget uses a 25d4 distributor as does my MGB, and I had a new recon dizzy ready to use on my midget, so I tried that....and basically the misfire has dissappeared! the car now drives beautifully, it is lively, loads of torque and runs great.

Just in time as well as I've sold my MX5 tonight and need the B for daily use.

So basically I reckon it was sticking weights in the dizzy not allowing the timing to alter with engine speed.

Hopefully this will also be useful to others who have simialr problems.

Once again, many thanks,
Hopefully I can get on with my midegt restoration now instead!
John


John Collins

This thread was discussed between 20/01/2008 and 21/01/2008

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