Intermittent power loss 1975 rb roadster

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Paul Scott
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Intermittent power loss 1975 rb roadster

Post by Paul Scott » Sun Sep 02, 2018 5:19 pm

Hi Everyone.
I need a little help to narrow down a power issue.
We did the Midget run today with some mixed issues.
My wife drove our B the 60 miles to the starting point. No issues what so ever.
Then I get to drive the 60+ mile route of rhe run. Occassional power loss, tacho dies engine dies for a second or 2 then back to normal. No pops or bangs. But does this several times on the run. When it dies I release pressure from throttle. Later driving from Stowe to MK (home) still cuts occassionally. So trying to add maximum throttle and no throttle I could get it to happen more frequently. Got home, then asked the wife to drive to our cars garage. She now has it cutting out but she doesnt release the throttle on power loss. Power comes back with a backfire.

Any ideas what could give a power shut down for short periods stopping the tacho?

Can the advance mechanism earth wire cause this?

Any Ideas would give me a head start. I'm guessing a bad connection somewhere.

Thanks in Advance
Paul
1975 MGB Roadster
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Vic Butler
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Re: Intermittent power loss 1975 rb roadster

Post by Vic Butler » Sun Sep 02, 2018 8:33 pm

Points or electronic ignition? It's ignition low tension circuit related as the tacho dies. Has your car got an ignition relay mounted in the engine bay on the offside. It's a cylindrical black object on the inner wing.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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Charles Farran
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Re: Intermittent power loss 1975 rb roadster

Post by Charles Farran » Sun Sep 02, 2018 9:21 pm

I agree with Vic's thinking that it is on the L.T. side as the tacho dies.The relays are notorious for this sort of problem. You could inniiaaly check all the contacts are clean on the ignition relay. I would also check the fuse box & make sure that the bottom two in particular have clean contacts as well as the ends of the glass fuses. (You could try swapping the two relays ( ignition & starter) round as they are exactly the same if you haven't a spare & see if that sorts the L.T. issue.It may of course give you starting problems, but at least you would be identifying the problem ).
Cheers, Charles
1980 Roadster

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Paul Scott
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Re: Intermittent power loss 1975 rb roadster

Post by Paul Scott » Mon Sep 03, 2018 6:36 am

Thanks for your thoughts.
Points type distributor 45D

I will have a look under the bonnet today looking for a relay. Unsure it has one past the starter according to wiring diag but I need to find the issue.

It is like turning the key of for a second then back on again. I cant say if anything else goes off at the same time as I was busy aiming the car in the correct direction.

I have had the tacho -brake light-indicators die before but the engine did not. Changed all lucas joining blocks under the bonnet. That appears to have fixed that issue because that was in April has not happened again.

My plan was to change any lucas blocks fitted to the car. Unsure how many maybe under the dash, but thought maybe one of these could be an issue too.

I will let you know what I find in the relay area.

Regards
Paul
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Vic Butler
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Re: Intermittent power loss 1975 rb roadster

Post by Vic Butler » Mon Sep 03, 2018 7:36 am

Paul, could you give the chassis number of your car then I can check the relevant wiring diagram in the official workshop manual to see whether your car was fitted with an ignition relay.
. Vic
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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Paul Scott
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Re: Intermittent power loss 1975 rb roadster

Post by Paul Scott » Mon Sep 03, 2018 7:47 am

Will Do

Thanks
Paul
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Charles Farran
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Re: Intermittent power loss 1975 rb roadster

Post by Charles Farran » Mon Sep 03, 2018 9:09 am

I would also check that both + & - contact tabs on the coil are tight on the mounting screws & that the tabs themselves are clean as well as the female sleave connectors as a poor / intermittent contact here might create the symptoms you are experiencing.
Cheers, Charles
1980 Roadster

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Charles Farran
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Re: Intermittent power loss 1975 rb roadster

Post by Charles Farran » Mon Sep 03, 2018 9:22 am

Apologies for my earlier post refering to two relays. I don't think a separate ignition relay was added until 1978 as shown in the Bentley wiring harness diagrams for the MGB.
Cheers, Charles
1980 Roadster

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Paul Scott
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Re: Intermittent power loss 1975 rb roadster

Post by Paul Scott » Mon Sep 03, 2018 9:42 am

Thanks Charles,
A shortened reply as my detailed one didnt submit!! Don't know why!! I only look after the forum :-(.

I will be under the bonnet later, with my torch sandpaper and maybe some cutters.

As an appliance repair man I know intermittent probs are hard to find, but normally easy to remidy once located.

I will update later.. hopefully

Regards
Paul
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Vic Butler
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Re: Intermittent power loss 1975 rb roadster

Post by Vic Butler » Mon Sep 03, 2018 11:52 am

Charles, the ignition and starter relays were fitted to the 1976 onwards cars with the forward mounted radiator.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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