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MG MGB Technical - Cylinder will not fire
Looking for some advice. My 1972 (supercharged / electronic ignition) #3 cylinder is not firing. Checked distributor cap and rotor - looks OK. Maybe a little more erosion of the #3 contact. Switched plugs and plug wires around but same thing. Checked compression: cyl 1=105, cyl 2=105, cyl 3=105, cyl 4=110. Number 1 and 2 spark plugs are black, numbers 3 and 4 are grayish-white.
Does anyone have any ideas? Thanks.
|Mmm, strange. I was going to say change plugs, but you had? Still, try changing for new plugs. The lead might have a break, but youchanged this also and still number 3 not firing? Go back to basics, see if there is a spark at all leads if not trace back. If however you do have a spark and yet that cylinder does not fire, what, does it mean a charge problem? no fuel? But then the other cylinder of the pair would do the same? what do we all think? Mike|
|Thank you Mike. After I posted I was thinking it night be a fuel issue - too lean. I adjusted the mixture control on the SC carb (enriched it 1.5 turns) and it now appears to be firing at idle.|
|1.5 turns? I don't know about the SC carb but on standard carbs that is a huge amount. Let alone a single carb firing 1, 2 and 4 but not 3. If there was no spark at a plug laying on the block, you swapped plugs and leads round and that cylinder was still not firing, I'd say that contact on the cap was breaking down.|
I might suspect a vacuum leak on 3 and maybe 4. Since you have only one carb and 1 & 2 plugs are black while 3 & 4 are light, it seems it must be leaning out after the carb.
|C R Huff|
Charley - how would you suggest I check for vacuum leak on 3/4?
|Best way is propane gas, second choice carb cleaner, squirted round the joints. If the idle changes there is a leak.|
|Also, make certain that it's not a failing head gasket that is allowing small amounts of coolant to enter the cylinder and kill the spark. This happened to my '67 a few years back after some spirited driving while under boost. I replaced the head gasket, with a Payen, and installed ARP head studs at the same time. No problems since. Another common problem that crops up is the gasket between the intake manifold and the supercharger. It is very thin and tends to blow out. I made a new one, from some heavy duty gasket material, when I discovered that mine had blown out of the space between the blower and manifold. RAY|
I may have sent you on a wild goose chase. Given that the engine is supercharged, it may have no vacuum to leak (unless at low RPM). If it is under supercharger pressure I suppose a leak could cause loss of some of the fuel/air mixture. This might cause a miss, or possibly a fire.
|C R Huff|
|I have a Payen head gasket and ARP studs. So I do not think that is the problem. I adjusted mixture and engine seemed to be running fine. I was in the process of removing HT wires to determine rpm change and when I removed lead to number two cylinder the engine just quit and I cannot get it started again. I replaced distributor cap, rotor, and put in a new coil. Still cannot start engine. Any ideas?|
|Can you easily put it back to points and condensor? This may not fire the mixture at high boost, but will give you an idea at idle. last year the MGB was cutting out at random intervals, there was no inrush of air when you opened the tank, but a 1mm hole through the filler cap stopped the problem.|
|I am getting spark from the coil HT lead. But, I cannot detect any spark at the spark plugs. Could my pertronix electronic ignition be bad?|
|Maybe a bad rotor or cap?|
|C R Huff|
|Another change I made last year was to change to the red rotor arm. I have since read that the reason aftermarket rotor arms are breaking down is that the gap to the fixed contact inside the dissi cap is too big. The aurhor had cut away an old dissi cap, measured an OE and a Chinese one then soldered a bit more brass on to reduce the standoff voltage with good results.|
|If it won't start at all you need to go back to first principle and look for fuel and spark in the right quantities at the right times in the right places. Don't assume it is something to do with removing the lead, or the problem you were looking for in the first place.|
|Spoke to technical advisor at Pertronics and he gave me instructions on how to test electronic ignition. It had in fact failed. Replaced with new electronic ignition (LU-143) and it started right away but engine not happy. A lot of back firing and rpm fluctuation bewteen 2000 and 500. I think it is probably too lean.|
|"rpm fluctuation bewteen 2000 and 500."|
Unless the engine is actually varying between those rpms, i.e. the tach is always indicating the correct engine speed, there is still a problem in the LT ignition circuit i.e. 12v supply, coil, Pertronics, distributor earth strap.
I would check your cylinder compression figures as I fear they are all on the low side. I believe the Manual suggests that 130 psi is a minimum value.
My car has 155 psi on 1,3 and 4 with No 2 low at 100 psi. That said my car runs ok, but I will remove head and investigate when I take car Off-road in Oct!!
I have also bought a set of "see-through" spart plug adaters and these are brilliant to confirm a spark is present at each cylinder.
"My car has 155 psi on 1,3 and 4 with No 2 low at 100 psi. That said my car runs ok, but I will remove head and investigate when I take car Off-road in Oct!!"
If you have a (badly) leaking valve, as it seems, then this is a perfect way to scrap the head.
Have you checked/set clearances?
This thread was discussed between 22/04/2012 and 18/05/2012
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