Specs: 1975 MGB, standard 1.8L 4 cylinder engine, aftermarket Weber side draft carbs. Aftermarket electronic ignition. Apologies in advance about the length but I find it better to have too much information than not enough when I'm trying to troubleshoot something like this. I took the MGB out for a drive yesterday. Heading up I-65 doing about 60 MPH indicated the engine suddenly stalled. No different that if I had turned of the ignition key. Under deceleration there was never a single backfire or hint that the engine was trying to kick. I slipped it out of gear and coasted as far as I could, hoping to make it to an overpass so I'd be in the shade. Ended up about 40 feet away from one on enough of an uphill grade to ensure there would be no pushing the car under it. Trying the starter the engine turned over just fine and sounded like it was getting compression, but it didn't offer to start. I turned the ignition back off for a few minutes, then tried again. The engine started. It ran rough for a second or two and then began running well enough that I was able to pull forward and stop under the bridge. I let it run for a couple more minutes there, checking all the gauges. Water temp was well in the normal range, oil doing well at 30psi. Voltage holding steady at just over 14v. Cautiously I found a good break in traffic and tried for the next exit (which was thankfully pretty close). About the time I started up the exit ramp (at which point I was doing around 60 again) the engine stalled and I managed to coast to the top of the ramp but had to pull off to the side. This time the engine didn't offer to start after I waited a couple of minutes so I popped the hood and looked inside to see if there was anything obvious. There wasn't. Thinking maybe I was experiencing an ignition problem I pulled the coil wire and reseated it at both the coil and distributor. I then unplugged and reseated each wire attached to the coil. The engine started when I tried it again. It ran rough for a few seconds like before, but smoothed out after a bit. I drove to a road that parallels I-65 (Gallatin Road for those familiar with Nashville - I was at the Madison exit) and took it back into town, figuring that at the very least the engine seemed capable of continuing to run at lower speeds. I made it all the way home that way, sticking to a route that involved speed limits in the 30-40 range. I don't know if it was my imagination or not but the engine seemed to be idling a little funny when I was at stop lights. But again this could have simply been paranoia on my part. The engine on that car is not the smoothest idle I've ever seen by any stretch. My inclination is that it's ignition related and it's failing to produce adequate spark at higher RPMs, fouling out the spark plugs and resulting in a dead engine until the unburned gasoline dissipates from the plugs. Problem with that hypothesis is I just put a new distributor with a brand new electronic ignition in about 8 months ago. And we don't drive the car a whole lot. Seems strange to me that an ignition system would have failed that quickly. The previous one failed because the heater core switch is located directly above the distributer (British ingenuity) and they fail every few years, dripping coolant on the distributor until something unhappy happens. Replacing the distributor fixed that problem and the engine was running just fine afterwards. Of course it could be a bad coil I suppose. A new coil (and wires) came with the kit I purchased when I replaced the distributor, but I did not install the new coil or wires. Maybe I should have, I don't know. Car ran find once I replaced the distributor so I put the rest of that stuff on the shelf. It could also be fuel related of course. I suppose if the fuel pump was no longer delivering adequate supply at higher loads it could be the equivalent of running out of gas. I'm a little more skeptical about this possibility, but in light of the fact that there is a new distributor / electronic ignition and there is not a new fuel pump I suppose it's worth considering. I'd welcome any thoughts from you folks who are many orders of magnitude better at this sort of thing than I am. Thanks in advance.
When it craps out, and coasts to a stop, pop the hood and air cleaner bonnet and run the throttle. Accelerator pump shooting gas into the venturi? If yes, ignition related. If no, fuel related. The stumbling at (eventual) restart has me thinking fuel, as the carb is dry and only partially fueling until the fuel pump fills the bowl.
Does it have an electronic ignition? My MG Midget did the same thing. I could not easily diagnose the issue because it would usually restart and had spark when I checked. The ignition was known to heat up and fail and then have spark again when it cooled down. I replaced the old aftermarket ignition with a Pertronix and never had another issue. Under $100 repair.
I'll take a shot. Had several British Leyland cars back in the day. If it has the original type SU fuel pump still, it has a set of points.
What’s the rpm at 60? What’s the engine temp? I was thinking the ignition/ coil was over heating but it could be fuel delivery issues. sorry not really much help, I forget most of my MG issues/ fixes.
Is it possible that the fuel pump if still stock cannot keep up with the volume required to keep those Weber's filled? How long have the Weber's been there and have you had them for awhile without incident till now? Another thought is rust in the fuel tank. Check and back flush your filter. Many ditched the SU pump back in the day for a simple Facet pump
Ignition. Carburated engines don't suddenly shut down if fuel supply is an issue, the float empties and starves the engine. Note that if you cranked the engine for a bit with no sparks, it should have a rough time starting once you have sparks. Intermittent problems are a bitch. (just needed to say that!) Maybe Coil or electrical feed doesn't like level of vibration at 60 mph (or heat for the former?) Question: I never installed these kits but generally they need a teethed wheel at the crank and some feedback from the cam and a sensor (hall?) in both positions. A bad sensor can stop a car too...
Fuel tank venting properly? In the event it stalls out again, I'd open the fuel filler cap and listen for vacuum.
Thanks for all the replies. Yesterday turned into a really busy day for me so I'm just now getting back to the beeb. I'll try to answer all the questions that were raised. @pickled egg I haven't checked the accelerator pump thing, but since it will eventually start and run fairly normally after cooling off a bit I'm skeptical I wouldn't see a little gas squirt into the venturi. If it's fuel related I think it's going to be slow delivery, rather than no fuel at all. @kenessex The aftermarket electronic ignition / distributor kit I put in there less than a year ago is an AccuSpark kit, not a Pertronix. I seem to remember doing some research and checking reviews before making that choice. Perhaps I made the wrong one. @Dave K Seems like it's around 4500 RPM at 60 mph. This MGB doesn't have the overdrive transmission so you really feel like you need a 5th gear once you're getting over 60. But I've driven it at 70 on the Interstate for 15-20 minutes when necessary without problems before. Engine temp is nominal, right in the middle of the meter. @Scott S. I'm having similar thoughts about fuel delivery and may go ahead and preemptively change the fuel filter. You also asked about DCOE Weber side draft, which I don't know. I'm pretty sure there aren't any heat shields below the carbs but there does seem to be a decent amount of clearance between them and the engine itself if that's what you're getting at. @tzrider The MGB distributor drive has a rectangular base that can only be slotted in only in two positions. It's definitely not off by a gear tooth or anything like that. A failing hall sensor is certainly a possibility. EDIT: See image below, this is how the drive base slots in. @bergs Hadn't really thought about vapor lock but I did experience it on an FJ1200 I used to own. I suppose it's a possibility. We've had the car now for 2-3 years though and this is the first time this happened and we've definitely driven it in some hot weather before. Thanks again everyone for putting some thought into this. I probably won't be out there fooling with it again until this weekend.
Sounds exactly like heat is affecting something in the ignition, coil, electro-digital box, distributor, etc and shutting down. then when it cools all good. The reason you could drive slower was less heat.
If the electronic ignition kit came with a new coil, why not have installed it when installing the kit? I myself would be hesitant to mix and match old/new electrical components, especially if going from a points type to electronic. Aren't points coils different impedance? May not be your issue, but something to consider.