QuestionQUESTION: 1972 MGB, SU carbs, std ignition. Was running ok until this spring. I replaced the starter and took it for a short test drive. Ran ok for about 2miles. I pulled onto a side road to turn around and the engine quit. Restarted easily, but was continuously misfiring all the way back. I had removed the dist cap when I changed the starter, but otherwise did nothing else. Since then, I have:
1. Replaced everything in the ignition: coil, condenser (4 different ones), points, cap, rotor, LT terminal, wires, plugs. Dist was rebuilt 2 yrs ago and looks fine. Timing is set to 10 deg BTDC, but I've tried 10-20 with no difference.
2. Replaced the carbs with rebuilt. Repeatedly set mixtures. Checked floats. Fuel pump is SU electronic, 2yrs old, seems to pump fuel fine. Fuel is fresh, tank is 1/3 full. No apparent leaks in fuel lines.
3. Checked compressions: 165-170psi on all 4 cylinders. Leak down test: 20, 25, 35, 20. On the leak down test I can hear air hissing from the top of the cylinder head somewhere on cyls 2 and 3.
4.I checked all the LT wiring. I tried jumping 12v directly to the coil and jumping to the LT terminal on the dist.
The engine starts easily, idles very rough when cold, but less rough when hot. It revs up when the throttle is opened, without missing, when there is no load--ie in neutral, or when the rear wheels are off the ground. But when under load, it misses and backfires (black smoke out the exhaust). By load, I mean in any gear, starting up or accelerating forward. If the car is moving in gear, it doesn't miss if I let up on the throttle. It also doesn't miss as much in reverse. The spark plugs always are covered with black soot shortly after I clean them. I found what looked like oil in the intake when I removed the carburetors. I smell gas in the oil. I've spent hours working on this, rechecking everything and I can't think of anything else to check or do.
25D4 distributor
ANSWER: Hi Geoff. What a tale of woe. You have already tried just about everything that I can think of.
I do have one idea. Inside the distributor there is a moving plate that the condenser is attached to. There is a thin flexible wire that connects the moving plate to a small screw. You can see it at the top of this picture (attached). These wires do fail, causing intermittent misfiring or complete failure of the ignition.
Another possibility is a leaking float in one of the carburetters. You say the plugs are black. This is an indication of an excessively rich mixture. Are the front two plugs blacker than the back, or vice versa? That would point to one carb flooding.
Engine lacking power under load is often a faulty condenser. I have fitted new ones that were bad.
Finally, try connecting the LT wires to the coil the other way round. You may have gotten them reversed.
---------- FOLLOW-UP ----------
QUESTION: Well, I've tried another condenser connected between a good ground and the lt terminal, both before and after removing the condenser inside the dist. No change. I tried bypassing the tachometer by jumping from the switched 12v on the fuse block to the coil, and jumping from the coil to the lt terminal on the dist. I've tried two sets of carbs, one newly rebuilt by a "pro". But I had also checked the floats and needles on the 1st set. I've removed the dist again and can't see anything wrong with it or the points or the cap or the rotor all of which are new. I am perplexed by the fact that the problem is only when the engine is under load. It idles and runs fine unless it has to push the car, then it just loses power, and misses and backfires. Since the compression is fine, it must not be valves or rings. I notice a voltage stabilizer in the circuit diagram which is peripherally related to the ignition. Do you know if failure of this could cause a problem. All the instruments work, except the speedometer which (back when the car could be driven) stops at about 50mph. Thanks again for your help.
AnswerHi Geoff. The voltage stabilizer is only there to provide a reliable 10 volts to the fuel gauge (and temperature gauge on some USA cars). It will not affect the ignition. Speedo is purely mechanical, cannot affect the running of the car.
I would go back to first principles:
Symptom: Engine ticks over fine, but loses power under load. Possible causes:
1) Lack of oil in the carburetter dashpots.
2) Jets in carburetters not centred correctly, causing needle to stick. (on BOTH sets?)
3) LT wires to coil reversed. Check again that the plain white wire goes to the + terminal.
Keep trying. You will find it.
