I haven’t driven the MG much over the winter; only a couple of times, and started it occasionally.
But today is sunny and warm, so I had to take it out. I warmed it up while I was taking the top off and doing some other things, then drove it about 25 to REI. Ran like a champ.
Only when I got off the freeway I noticed it was running a little bit rough. When I left the store it seemed to be running rough. I sat at a stop light for a while and it didn’t get any better. When the light changed there was hardly any power. I could get it up to about a mile an hour. I pulled over to let people by, and revved the engine. BLATT!!! BLATT!!!, it backfired. For about half a mile, as I made my way to the freeway and got on it, I wouldn’t have been surprised to see flames coming out the back like the Batmobile. I could only get it up to 50, and I was wondering if there were any repair shops. But then it cleared up and ran fine and I cruised home at about 70 or 80. When I got off of the freeway it was still running rough, and didn’t accelerate well; but wasn’t backfiring.
The engine has about 2,000 miles on it SMOH. I kept the revs down as recommended for the first thousand.
I’m guessing it wasn’t firing reliably on all cylinders. Gas would get into the inactive ones, and eventually they’d fire – or backfire. I don’t see how it could get so far out of time in just a couple thousand miles, and the problem pretty much cleared up after a bit. The dual SU carbs were rebuilt at the same time as the engine, and new K&N filter elements were put in at the same time. The original spark plug wires were replaced with silicon ones, and the car got a Pertronix electronic ignition.
If the car has been sitting for the most part over the winter, the gas may have gotten a considerable amount of moisture in it. Since it isn’t extremely expensive, try a can of gas drier added to the gasoline and see if that helps.
I always top off the tank to reduce the amount of condensation before I park it for a while.
Yesterday was Sun-day. Today sprinkles are expected. Looks like there’s some sun next weekend, so I’ll fill it up then.
So if it’s the gas why did it run well to start with, then run like crap, and then clear up? The fuel tank was cleaned and coated before installation, so it’s highly unlikely there’s any debris in it. Water in the system should have manifested itself at the start.
Seems gas quality is not the issue - how about delivery? Fuel pump up to snuff mechanically or electrically (not sure of type you may have? Fuel filter(s) clogged or collapsing with pressure?
Electic gremlins? Sounds as if you have updated much of the system but there may be some unstable remainders courtesy of LUCAS - god of darkness. Coil may not be responding well to heat.
Sitting for a while and temperature changes can / will create corrosion. Fine when starting up but under hood (under bonnet) heat can open up connections or cause circuit failure. Perhaps related to rate of heat generation; things got better after some cooler running?
Sorry I’m not of much direct help but some other things to think about - might jog a better answer out of someone.
My guess is that the misfires are due to stale gas fouling the plugs. The rest of the ignition system is working, and nothing has happened that would affect the timing or engine compression.
Top up the tank with fresh gas and replace the plugs. I’m willing to bet that one or two plugs fouled from stale gas which caused the car to run poorly, especially under load. Once the engine warmed up, the plugs cleared and the gas burned more readily.
smithsb: Fuel pump, filter, and lines are all new. Electrical system is new. (Still Lucas, but new.) Water temperature is steady at about 160º - 165º. The camshaft is only slightly hotter than original, which results in a little ‘lope’ when it’s cold, but it’s nice and smooth when it warms up.
cornflakes: Sounds like the consensus is stale gas. I’ll top it off with Premium (it likes Premium) when there’s sun again, and check the plugs for fouling.
Good idea, but I’d replace the plugs. I’m used to motorcycle ignitions which are weaker, but a small amount of carbon on the insulator can cause a misfire.
It may not be the gas in the tank, per se, that’s the problem - if there was any sediment or varnish in the carb float bowl(s), that could have partially and intermittently blocked one of your metering jets causing rough running. Try putting some miles on the car, preferably at speed, and see if that helps, too.
Good advice. I hadn’t thought of carrying extra plugs, but I’ll get some.