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MG MGB Technical - Backfiring, pinging under load and wobbly timing m

OK, here's what's happened...

Car (1972 GT) has been running fince since the head gasket was replaced until this started. Drove to work one day last week and noticed that there was hesitation when cold and the idle was stumbling, this went away once the engine was warmed up and I drove the rest of the journey to work without any issues. Leaving work, got about a quarter of a mile from the office and the car starts backfiring like crazy, pull over to the side of the road and retarded the timing, still backfiriing like mad so fiddled with the carbs (HS4s) to richen the mixture. Still backfiring at revs until the engine warmed up some more and ran well enough for me to limp home.

Couple of days later after an early finish at work I managed to get the engine warm (backfiring still present when revved) so I could check the timing after I'd messed with it to get me home, used the stobe to set the timing at idle to 14 degrees, a good a starting point as any, and took the car round the block, misfiring and hestiant all the way, fiddled with the mixture again, backfiring now gone (although engine was pretty warm by now) and now had pinking under load, backed off the timing and still pinging, retarded the timing more, still pinging (by this time I'm at about 6 degrees BTDC). Sped the idle up a bit and started pulling HT leads one by one, engine wounded no worse when pulling lead 2 and little difference when pulling plug 3 so changed the HT leads for a new set. This didn't appear to make much difference so I put new plugs in and tried again, still not running right... It's now dark so gave up for the night (I don't have a garage)

Daylight again, started the car up and waved a butane torch around the carbs and manifolds didn't find any vacuum leaks so got the carb cleaner out and sprayed from just about every angle I could and still no change in the engine speed so no vacuum leaks detected (although still not ruled out). A compression test showed all cylinders between 140-145 psi so am assuming (maybe incorrectly as I'd expect over 150 but at least they're all close to each other) that the new head gasket is still in one piece.

Back under the car with the timing gun to reset the timing and noticed that when the revs were up at 3500-4000 the timing mark was jumping all over the place, maybe a good 10-15 degrees either side of the mark when the dial back was set at 34 degrees with maybe 50% of the flashes showing the white paint on the pully and the TDC mark lining up... Back down to idle and carefully look at the mark, and even at idle seems to be moving a degree or two either side of the timing pointer...

OK, I say to myself, maybe the distributor is past it's best so, last night after work, I fit a brand new distributor and exactly the same behaviour, timing mark jumping all over the place and pinging under pretty much any load.

What are my next steps? I was thining along the following lines

Check Valve clearances
Recheck timing
Rebalance carbs as I'm sure they're out now with all the mucking about I did or would just putting the mixture back to where it was before the problems started suffice? (If I kept track correctly, I'm 2 flats richer than when the problem started)

Could my coil be on the way out? What's the best way of checking that?

After that I'm down to thinking that it is worth checking the timing chain and sprockets for correct tension and wear but I'm open to suggestions as to what else should be checked...

Chances are if it's not a quick fix she'll be going to the local MG expert as the new family member is due to arrive in the next couple of weeks and I really should be spending time inside the house getting the nursery ready rather than outside the house getting oil and grease all over me...
S P Morriss

(apart from the dissy) have you fitted any new parts recently?

was the brand new dissy complete from cap down?

any twitching on the rev counter at misfires?

have you checked the fuel supply is getting to both carbs, the float chambers, needles and seats?

fuel cap vent?

set tappets, CB points, plugs, timing, mixture in that order - if you adjust any item in that chain then you'll need to check/adjust the items that follow

that's me out of ideas
Nigel Atkins

I don't remember the rev counter flickering on the misfire, but that doesn't mean it wasn't, it's not always 100% steady anyway...

New distributor is entirely new, both distributors are electronic so no points to check. Old one is unknown make, new one is accuspark.

Guess it's on to tappets when I get a chance...
S P Morriss

Does your car have the coil feed going through the tach (white in, white out) or a white/black single feed.

If it's the former try just plugging the 2 white wires together. Sometimes the tachs can cause issues.

I'm thinking coil, though, as you've changed everything else and this sounds more like an ignition than a fuel issue.
Roadwarrior

It's a single black and white wire to the coil, then wires from the coil to the accuspark module from the positive and negative terminals on the coil...
S P Morriss

wouldn't have a load of water in the fuel by any chance
might pay to check the fuel filter for any moisture there
willy

or sticky carb dashpot pistons as the carbs have been off during headgasket replacement
William Revit

When you say 'backfiring' is this out of the exhaust or the intake?

Exhaust normally indicates an ignition problem, i.e. unburnt fuel from a non-firing cylinder ignition in the exhaust from the next normal combustion. This would be associated with the tach jumping around if it was an LT problem, but not if it is a condenser problem (not fitted in your case) or HT (unlikely with two different distributors). That leaves a coil HT problem.

Sudden pinking with timing retarded that much does indicate fuel problems. How many miles done since you last refuelled? Although with the timing jumping about that much if it's going 10 to 15 degrees more advanced some times then it will almost certainly pink.

As for one or two cylinders not making much of a difference to the idle speed as others when disabled, that's not unusual.

As far as the timing jumping about goes, if that was happening when there was a missfire or backfire then it's possible it's just that, but 10 or 15 degrees either side is a huge amount. It could be something to do with the distributor drive i.e. cam and drive gear, or even timing chain and gears.

I'd agree with checking valve clearances, do a static timing check (if you can with your electronic ignition) as well as dynamic. I can't see carbs causing that much of a problem but to eliminate them you should really do an air-flow and mixture balance check and adjust.

But I think the timing jumping about that much is the thing to focus on.
Paul Hunt

Plain white wire to pos terminal on coil, white with black trace out from neg terminal. The white feed goes through the Tacho from the ignition switch. , check the connections first, especially if your tacho has been erratic for a time. If the timing is that erratic it's not likely it's a valve/tappet issue and pretty rare for it to be timing chain, although possible. More likely ignition. Breaking down plug lead, rotor arm or coil are prime suspects.
Allan Reeling

I had similar problems with my 1980 MGB. Backfiring and persistent poor running with hot start problems.In my case, a "new" distributor was as faulty as the one it replaced...or at least the rotor and baseplate earth wire were very poor quality. I replaced both items with parts from the Distributor Doctor. Cured!
Make sure you go to him direct or to a reputable dealer like Moss. Rotor should be red and stamped DD. Earth wire is rugged and slightly longer than usual.
Get your timing back to where it should be and reset the carbs. Worth a try.
Have you checked the resistances in your coil? I recall they are fairly important with Accuspark. Measure across the terminals and subtract the probe wire resistance for an accurate reading.
ph barton

Long shot but do you have a battery isolator switch? Had a similar back firing missing problem that made the car impossible to drive. The timing was jumping about badly also. Replaced everything in sight but still faulty. Then I reached behind the seat for something and bumper the red key on the kill switch and all came good. It soon started again so bumped the switch again and all came good again. Replaced the switch and fault fixed. The switch had not completely failed or I would have had no battery but must have been high resistance and effected the ignition which was an electronic points replacement. Initially I also replaced this with another unit. Am wondering now what would have happened if I had gone back to points? Good luck Ken
K Stuckey

The backfiring usually indicates ignition problems, as does pinging, but the sudden onset of pinging can also result from head gasket failure bewtween either 1&2 or between 3&4. (1 or 3 fires and some burning gas squirts through the blown gasket into the adjacent cylinder, which just happens to be on compression)
However, if that was the case the two affected cylinders would be reading much lower than the other two, so this can be discounted.

Has the scatter in the ignition timing just appeared - did you ever check it recently at 3500-ish recently before this happenned? It's not unusual - B's develop it when there is wear in the gear on the camshaft that drives the distributor thanks to the load from the oil pump drive - so it's possible that it's a red herring. But then again it might not be.

The only times I've encountered these symptoms have been on cars when the distributor was loose, but that does not appear to be the case here.

So I'm wondering about something timing related but a bit more unusual, along the lines Paul suggested. Perhaps a slack timing chain with the tensioner fallen apart? That would allow the cam to wander quite a bit if the chain was badly worn, but whether it's enough to cause this I couldn't say offhand.

Paul Walbran

Did you solve this problem please and if so what was the cause?
G
Graham Moore

This thread was discussed between 03/06/2014 and 27/12/2014

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