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MG MGB Technical - Major engine miss

Hope the collective knowledge out there can help. My 1974 C/B MGB GT suddenly developed a major miss. It ran fine until I filled up with gas and as soon as I started it again it was missing and hard to keep running.

This is what I have done so far.

First thing was to check fuel filter and it is clear with no sediment.

Pulled plugs and #1 & #2 were very wet, #3 & #4 were fine. Thinking that this was ignition related I cleaned plugs, regapped, replaced the distributor cap, new points and new condensor, along with new plug wires. Engine still running on two cylinders!

Pulled front air cleaner to make sure that the carb was functioning correctly and it appeared OK. Did a compression test and the figures are (low compression engine) 140, 140, 137, 140 so valves must be OK. Checked spark to all plugs and it is strong.

With air cleaner off and the engine running, it is very rough and is backfiring out the front carb and not drivable.

Any thoughts??

Thanks
Brian
Brian Smith

Empty the float bowl of the front carb (I use a turkey baster --- wife will never know).
Dan Robinson

Just tried a couple of other things and the front carb is backfiring and blowing vapourized fuel back into the engine compartment. That would tend to indicate that there is fuel delivery through the carb. Could this be valve problems even though the compression was good?

Brian
Brian Smith

More likely to be ignition related - check the dizzy cap and rotor arm for tracking - it sounds like stray sparks are igniting the fuel mixture while the inlet valves are open.

Look carefully at the cap between the electrodes for number 1 & 2 cylinders.
Chris at Octarine Services


Hi Brian,

Just a very long shot but a similar event happened to me. I tried to start my midget and it backfired through the carbs blowing a bung out of the top of the inlet mixer tube (SU carbs)designed to protect the carbs from this happening. After that it would run, just, but kept backfiring through the carbs and hardly ran. There was also a squeeling noise as the air was sucked into the mixer. Put my finger over the hole and it ran sweet as!

Like i said long shot but you never know, we all learn the hard way!

Good luck

Mark
Mark

Brian,

Sounds to me like you may have two problems. The first being an induction problem, possibly related to the new tank of gas. Then when you changed the ignition components the backfire developed. As Chris noted the backfire is a function of timing. I suggest you reinstall the old parts and see if the backfire goes away; the new parts might be defective. With numbers 1 & 2 wet it points to a rich mixture being delivered by the front carb.

Doug
D. Cook

Check your firing order is 1,3,4,2 *anti*-clockwise. Clip a timing light onto each plug lead and see what is happening. Check the timing on Nos 1 and 4, check it is 180 degrees away from that on 2 and 3.
Paul Hunt

I had my distributor clamp loosen once and the distributor would turn as I drove throwing the timing way off.
Fred H

Doug,

<With numbers 1 & 2 wet it points to a rich mixture being delivered by the front carb.>

Not if the plugs aren't firing properly - you will get wet plugs even if the mixture is spot on.
Chris at Octarine Services

Brian,

Your problem was immediately after you filled with gas without even lifting the hood. Provided that you have still got the new leads on correctly then the firing order is OK. You replaced the dist cap etc, was it with new components ?? How did you clean the plugs ? wire brush is no good, swap them for 3/4, timing has already been mentioned, the only other point is the baseplate earth but this would not produce the symptoms you suggest.

Whilst all this may point to ignition I believe this is fuel related. Can't quite work out why the plugs are wet but a fuel/induction fault would be the obvious for putting out the front two cyls.

Check that there is actually fuel in the front bowl by removing the screws and having a look inside. Make sure that the needle valve is not stuck shut, get someone to flick the ignition switch (coil LT disconnected) and see that fuel flows when the pump starts.

There is also the possibility of a lump of muck being stuck in the jet of the tube between the bottom of the jet and the bowl. Remove the dashpot, lift out the piston & spring and again flick the ignition switch to ensure that fuel flows from the jet.

If all that fails I would look to the manifold gasket and tightness but I can't relate that to your pit stop except that all things fail at sometime.
Iain MacKintosh

Since your problem started immediately after filling the tank, I would tend to think you got a bad batch of gas. This time of year in BC it's pretty cold and it's not unusual for gasoline supplies to get water in them. If your # 1& 2 cylinders were marginal before getting gas, a little water could push them over the top and begin fouling plugs. Try a couple bottles of Dry Gas (or similar water remover) in the tank before you go any further.
Bill Boorse

Needle valve stuck open on that front carb?
P J KELLY

Thanks for all of the great ideas. I have determined that the original problem was a combination of burned points and dodgy distributor cap. The second problem was me having a moment! I have driven and worked on MG's since approx 1968 and you would think that I would have it all down pat by now.....NOPE.

Paul

Thanks for your email, I started reading it and thought, yes I know that it is 1,3,4,2, but as I read on the light bulb went on. I wired it clockwise not anticlockwise. Unfortunately I had to hitch rides today because of my mistake. As soon as I returned home, I switched the wires and it runs beautifully.

I feel really dumb at this point but I would like to thank all of you who answered and I hope that I can help someone else out at some point. I will try and give GOOD information though.

Red faced Brian
Brian Smith

Don't feel bad....I once spent all day trying to figure out the same thing. Sometimes the more you work on cars, the easier it is to miss the bleedin' obvious. Did I tell you guys the story of slightly forcing my trunk lid shut over a cardboard box???? Well it ended up in the bodyshop and I still kick myself for that one!!!!
P J KELLY

This thread was discussed between 22/01/2004 and 23/01/2004

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