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MGB the mysterious disapearing spark

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Sopwith_Camel

Sopwith_Camel

Jedi Knight
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MGB gt post 75 engine.
ok I think i found my poor performance issue with a blocked up Vacuum tube to the carbonator.
But i can't get it started to find out, it was running two weeks ago.
ok we have a new coil, new wires, new plugs, the plugs are getting wet with fuel and we can here the fuel pump. yes fresh gas. and new gas tank last year so no 30 years of crud.
I put the engine at top dead center and went through the set up procedure on the new dizzy to set the timing.
now we put a light between the spark plug lead to the spark plug and it lit up nice and happy.
but when we put the timing gun on it it dose not fire. yes we tested the gun on the midget it is working.
all the fuzes are good.
it really defies logic

ideas guys ?
i did just by a 95 jag xj but thats another story

steve
 
Last edited:
Is it firing on the compression stroke? Or is it one cycle out?
 
that is a good point. ill double check, the roter would be in the wrong place since it goes around once for the crank doing twice.
but dosent explain why the timing gun dosent fire.
 
Sopwith_Camel said:
MGB gt post 75 engine.
poor performance issue with a blocked up Vacuum tube to the carbonator.

ok we have a new coil, new wires, new plugs, the plugs are getting wet with fuel and we can here the fuel pump.

I put the engine at top dead center and went through the set up procedure on the new dizzy to set the timing.
now we put a light between the spark plug lead to the spark plug and it lit up nice and happy.
but when we put the timing gun on it it dose not fire
Click to expand...

The blocked vacuum line may cause lower fuel economy during part throttle cruising but it won't really impact performance.

You have installed a lot of new ignition components. When you change a lot of things at once it is easy to introduce problems by accident.

As mentioned above, check that the points are set properly. That means open by about .015" with the heel block resting on the high point of a distributor cam lobe. (Don't trust my 0.015" value... look it up in the specs). Be sure to lube the distributor cam lobes with a bit of high-pressure grease. If your distributor has a felt wick that rubs against the distributor cam, make sure the felt wick is saturated with oil.

As mentioned above, check that the distributor/plug wires are not 180 degrees out. Remove the valve cover. Turn the engine over by hand until the timing marks line up. Grab the rocker arms for cylinder #1 (water pump end). You should be able to wiggle the rocker arms. If they are "tight" and not free to wiggle, turn the engine over one more complete rotation. When the timing marks line up and the rockers for #1 can be wiggled, you are at/near TDC for #1 on its firing stroke. With the engine sitting at #1 TDC on its firing stroke look at where the distributor rotor is pointing. Put the spark plug wire for #1 on that distributor cap terminal. Fit the remaining plug wires in the order 1-4-3-2 going COUNTERCLOCKWISE round the installed distributor cap. That will confirm the plug wires are on in the correct order and not 180 degrees out.

You put a new coil on. Did a '75 have ballast ignition or standard ignition? Did you buy the correct coil for the ignition wiring that is in the car?

Let us know if going through this again gets the spark back.
 
As to the order of plug wires, should it not be counterclockwise 1-3-4-2? Our '74 GT runs pretty well set up that way...

Over 60 years ago when we rode our bikes to the 'foreign car shop', those wise guys told us that the firing order for an English 4-cylinder was 2-1-3-4. We liked those guys anyway.
 
1-3-4-2 is correct. 2-1-3-4 is the same thing, just with a different starting point...
 
oh and this was suppose to un-confuse me how?
im not getting any bang at all
no luck with the optical dizzy my friends spare dizzy and the old wobbly shaft dizzy.
could it be i fried the loop in the tach?
plan today test the white wire from key to tach, tach to coil.
put engine to top dead center and put in the good old dizzy.
oh and we tested the compression we are good.
 
White loop on tach? OK, I knew you had a mixed setup. The white loop is used on earlier impulse (Smiths RVI) tachs. If you have a tach with such a loop it is extremely unlikely that it has failed and/or is associated with the problem you are having.

I cannot speak for the others but the reason I focused on the firing order as a possible cause was your statement that you replaced a LOT of the ignition parts AND when you had an inline lamp on a plug wire... it lit up. Was the light inline with the "king" lead in the center of the distributor cap OR was it on one of the wires going to a spark plug?

If you install the test light in a wire going to a spark plug AND it lights when the engine is cranked... your coil is working and you are generating a spark. However, if the plug wires are installed in the wrong sequence, the timing light will flash when the timing marks are not where you expect them to be and the engine may not fire.

If you installed the test light inline with the king lead between the coil and distributor... AND the light flashes... but the plugs themselves are not firing... that says you are loosing the high-tension spark AT/IN the distributor. Did you fit the rotor (easily forgotten). Did you fit a new rotor recently (there are numerous reports of rotors developing internal shorts). Does the cap have the carbon brush in its center? Is this an older cap with craze cracks or carbon tracks?
 
this has the loop inside the can.
yes inline light was from the wire to the plug and it lite up. the gun is not flashing at all (it works on the spridget)
the only thing i can think of is the white wire from the key to the tach.
tuesday will be a nice day i will bang my head on it then. just when i thought i had this hobby figured out.
I pick up the "new" jag xj friday
 
I am not trying to discourage you but it will not be the white wire from the ignition switch to the coil or tach. If you are finding that an inline tester on a spark plug wire is flashing, there is power to the coil. However, the spark may be "weak"... too weak to trigger the timing light.

Before we spend more time guessing about what is going on, please give us the following background information.

  • Is the tachometer face marked RVI or RVC?
  • Have you tried bypassing or disconnecting the tachometer?
  • What color wires are on which coil terminals? (List them carefully please taking careful note of whether the wire on coil (+) is white or faded pink).
  • What new coil have you installed? (Brand and model and/or writing on it).
  • What ignition do you have? (You mentioned optical).
and finally

  • Please list each item you have replaced and what you have checked so far.
 
Makes no difference to having the wires wrong and not having the timing light fire. (well, not much..theoretically wrong mix when the plug fires can cause issues with spark propagation).
If you have spark out of the high tension wire from coil to distributor cap, then you have a bad cap, rotor or....horribly fouled plugs. If they are so fouled (wet?) that the spark just goes to ground, it does not allow the voltage to build up to gap the plugs.
"the plugs are getting wet with fuel and we can here the fuel pump. yes fresh gas"

If you have rubber tips on the plug end of the wire, pull them back until you feel the metal contact separate from the end of the plug, not quite 1/4", and try the light again (once you have ensured there is spark out of the coil). We used to do this on all plugs to get fuel fouled plugs to fire...then let it run to warm up and clean, then push the wires back one at a time as it ran and if it kept running, let it fully warm up and USUALLY that took care of the issue.

If you have no spark at all out of the coil high tension lead, report back.
 
well new data. I did the static timing following john twists video, i have it set fairly advance 15ish but i did get a heart stopping bang out of the exhaust. so advance? retreat... mean retard or 180 out?

now that I have the cancer on the run Im back to planing for the traditional nordic MG death of upside down on fire off a cliff into the fjord.
 
You get a bang, best make absolutely certain your distributor isn't 180 out.
#1 out, finger in the hole, crank, pow, that's compression. Make sure the rotor points at #1 plug wire, then try again.
 
well i can't crank it and be under hood at the same time. i tried the plugs 180 around no luck.
i keep pushing the dissy a bit advance. the last time it was running it had a lot of advance .
its on the battery charger now.
three more hours of daylight. sun set some time around 10:45
steve
 
Sopwith_Camel said:
well i can't crank it and be under hood at the same time. i tried the plugs 180 around no luck.
i keep pushing the dissy a bit advance. the last time it was running it had a lot of advance .
its on the battery charger now.
three more hours of daylight. sun set some time around 10:45
steve
Click to expand...

That is why God in His infinite wisdom and mercy invented remote starter buttons. Or, the button on the bottom of the solenoid for some users.
 
You can improvise the remote starter button. Locate your starter solenoid and the white/red wire on it. Connect a jumper wire to that terminal. Strip the other end of the jumper wire bare. When you want to bump the engine over, hold the stripped end of the wire to the solenoid terminal with all the brown wires. The starter will operate as long as the jumper wire is held against the terminal with the brown wires.
 
yes i have always wanted one of those starter solenoid, but the solenoid on this engine is underneath. I got a spare set of hands now, so we should make progress when it stops raining.
 
ok we were 180 out with the 45d dizzy and we did static timing at 30ish degrees. and its running but not happy and there is a bit of exhaust leak, sigh
steve
 
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