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MGB No spark and engine cranking slow

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Carlbanan56

Carlbanan56

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Hi,
I have some problems with my 1975 mgb. A few weeks back I had spark and the engine cranked fine. After working on my carburetors and letting the engine sit for a few weeks the engine no longer produces a spark. The engine cranks were slow as well, once it didn't even tour over it just start to click. I have googled around and as I understand it this is comen symptom of bad ground or bad connections??? If this is the case how do I find the fallout cabel or connotations? This might be a dumb question but I am young and inexperienced.
 

YakkoWarner

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I'd start at the source (literally) - check the battery. Put it on a charger and make sure it's topped off - if the car sat for a couple weeks there could be a small parasitic draw pulling down the battery over time, or the battery itself could have some internal loss effects (I have experienced both in various vehicles) . Or if you have another car and some jumper cables handy (and are comfortable hooking them up) see if a basic jump start will get it spinning. On vehicles I know I won't be running for a week or more I usually disconnect the battery for both safety and to prevent slow discharge.

If you have good charge on the battery but still slow/no spinning; pull both cables and clean them at the battery terminals. Remove/clean up/replace the other end of the ground (negative on the 1975) where it attaches to the body of the car. Those are usually the most common places for bad connections and occasional removal/cleaning/reconnection is pretty much routine maintainence on any vehicle.
 

JPSmit

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The ground wire (which is separate from the wire harness) should be a braided wire attached to the bottom rear of the engine sometimes sharing a bolt with the oil pan. If your battery is known to be good you can also use a jumper cable to ground the starter to body of the car. (suspension or some such non painted thing)
 

YakkoWarner

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The ground wire (which is separate from the wire harness) should be a braided wire attached to the bottom rear of the engine sometimes sharing a bolt with the oil pan. If your battery is known to be good you can also use a jumper cable to ground the starter to body of the car. (suspension or some such non painted thing)

Good call - forgot about the ground strap between the body and engine!
 

mezy

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Normally if you have a bad engine ground, depending on what type of choke and throtle cables you have
can get quite hot to the point of glowing if the earth is bad enough
 
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Carlbanan56

Carlbanan56

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I have now the strap between the body and the engine, the previous one was bad. The engine is now cranking at a good speed, unfortunately, I still have no spark. I have switched out the rotor, the cap, all leads, and all spark plugs. I am not sure if the problem is in the low-tension circuit or the high. I have removed the lead between the coil and distributor and tested it for spark against earth while cranking the engine, this gets me no spark. Because of this, I tried to see if the coil got enough juice.

With the ignition on and with the low circuit cable between the distributor and coil disconnected I get a reading of 12 volts between both CB(+) and earth and between SW(-) and earth. Between CB and SW I get a reading of 0 volts. If I connect the low tension circuit I get a reading of 7,5 volts between CB and earth and between CB and SW. Between SW and earth, I get a reading of 0,2 volts.

While getting these results I got small blue sparks between the connections that connect the coil and the distributor (with the ignition on). Losing 5 volts through the low-tension circuit sounds a lot to me but I don't know. Could there be some sort of short circuit in the distributor?

I have attached some pictures that show my reading if there is any help.

Thanks for all your help, it is very much appreciated.
 

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mezy

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images are not showing for me
But if you have power to the coil then there has to be an issue inside the dizzy,
do you still have ouint or electronic ignition,
if you have points, turn the engine by hand with ignition off, make sure the point fully close and open,
with the points closed, turn the ignition on, flick the points open and closed and see if the HT lead from the coil produces a spark,
You should get very little or no spark at the points doing this. but the HT lead should spark.
if no spark then disconect the wire going to the points and flash it to earth, if still no spark from the coil, try a new coil.
If you now have a spark, fit net points and condenser.
 

NutmegCT

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Carl - all your attached photos are "heic" format. Many computers, browsers, and some smartphones don't know how to open those. If they're important, you could "save as" jpg, then attach them to a new post here. Windows users can use Windows Photo Viewer to see heic files.

By the way "young and inexperienced". Every guy here giving advice was once young and inexperienced. And soon you won't be either of those!

Tom M.
 
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Carlbanan56

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Here are the pictures in jpg format :smile: I will check on the distributor tomorrow.

Thank you all for the advice
 

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JPSmit

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Not really an expert on electrical but, am a master on changing too many things at once. :ROFLMAO: Have you removed the distributer? Counld it be backwards (not sure if this is possible for a B - but it is for certain Midgets) Are the plug wires in the correct place (one of my favoutrites). Mostly, is it possible to return everything to the way it was when it was running? Changing one thing at a time is so much better - and there is no guarantee that a new part will work as many of us have learned to our chagrin. Keep at it - you are close because it was working not too long ago.
 
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Carlbanan56

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Hi, sorry for not responding sooner, I have had a lot of schoolwork this past week. Today I finally solved my problem and the car has spark! It turns out I had installed the pins wrong, so the low-tension cable was shorted to ground. Thank you all for the help and advice!
 
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