Intermittent power loss 1975 rb roadster

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Paul Scott
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Re: Intermittent power loss 1975 rb roadster

Post by Paul Scott » Fri Sep 07, 2018 9:44 am

Hi Vic try this link

https://www.google.co.uk/url?sa=t&sourc ... 78Or0yO6ua

Or google advance auto wire mgb

This may assist.

So should all 3 fuses in the fuse box be 35A?

Top 2 are for lighting circuits red cables & the 3rd one down is switched white live. Green on the fused side.

There is also one other fuse in the loom in a inline brown holder but have not opened that yet.

I saw on the 80 GT there were 2 inline fuses & 4 in the Fuse box.

Regards
Paul
1975 MGB Roadster
Webguru for MGB Register

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Charles Farran
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Re: Intermittent power loss 1975 rb roadster

Post by Charles Farran » Fri Sep 07, 2018 11:27 am

Ths wiring diagrams are from the source Paul has indicated. You can download the complete set as a pdf & then look at/ prinnt off the one relevant to your car.
There should be 4 fuses in the fuse box rated as previously advised. The additional line fuse you have which is n both sides is on the Hazard circuit.Agiain rated the same.When i renovated under the bonnet & reconnected the harnesses etc. As a precaution i used an 10 amp inline fuse between the battery earth & chassis to save frying anything. I forgot when testing the Hazrard lights what a load that was & the 10 amp fuse blew. (I should have used a headlamp bulb in between as well which will prove that all the various circuits are gopd albeit with lamps etc. glowing dimly). No good for testing the cranking due to extreme loading. I have used the headlamp precautionary route in the last month when completing rewiring a 1966 mini moke - i appreciate there is a lot less wiring but it proves the principle).
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 » Fri Sep 07, 2018 11:32 am

Don't forget to install the fuse box the correct way round. The bridge should be at the top on the supply side as the one feed supports two fuses covering both sides of the side lights of the car.
Cheers, Charles
1980 Roadster

Vic Butler
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Re: Intermittent power loss 1975 rb roadster

Post by Vic Butler » Fri Sep 07, 2018 12:26 pm

Thanks Paul but I prefer to use the diagrams in my official workshop manual. All 4 fuses are connected, can't think why you have only 3. All 4 are rated as in Charles' earlier posting.
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 » Fri Sep 07, 2018 1:37 pm

Hi Vic & Charles,
Definitley only 3 fuses used in the Fuse Box with a seperate Black bakelite style inline fuse just below. Looks factory fitted.

The Autowire diagram is all I have Vic & for my car it does appear to have the right colours in the places I am working.

UPDATE

Well it runs, have not had a drive yet but its running.

Still popping and banging unless the choke is out a little.

Could a fuel change be the reason for a misfire. I was just running on unleaded with additive. I had 2 suggestions of addirive with a booster and also running on Tesco Momentum.

So I did both :-( have I upset the car by adding more Octane?

On Sunday it ran sweet for 60 miles. Stopped ready for the Midget run. Then she started misbehaving - cutting out issue first then had the misfire appear later.

Should I whack my new dizzy cap - leads and plugs in now to see how she runs.

What would be the recommended plug gap as I believe that is different to out of the box.

Regards
Paul
1975 MGB Roadster
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Charles Farran
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Re: Intermittent power loss 1975 rb roadster

Post by Charles Farran » Fri Sep 07, 2018 3:26 pm

Hi Paul,
I am curious to hear that your fuse box only has 3 fuses in. Are there cups for 4? I am only aware of 2 types of boxes being fitted,either the earlier 2 fuse or later4 fuse.
The top 2 (1-2 & 3 -4 ) are for the side lights supplied by one RG & on the other side R & R with a ringed trace for the side lights.
3rd down (5 -6) are for items wired through the ignition switch 3 WN on the supply side & (on my car 3 G on the other side )
4th down (7 -8) are for fused items permanently connected 2N on the supply side & 2 P on the other side which power the horns ,courtesy lights,clock, cigarette lighter & internal boot light. (also useful to supply the memory part of a modern radio. (I am aware that where there are cigarette lighters fitted some are wired through an ignition switch dependent route).
If your 4th fuse is missing how are the purple circuits supplied/protected? What are the colours each side of your fuses including the separate one?

The plug gap should be 35thou.
Ever since i have run my original standard headed engine on Shell Optimax (now V power) or Tesco Momentum plus Valvemaster plus (some just use valvemaster), the car has run smoothly. It used to back fire occasionally prior to using the two petrol brands mentioned. Once or twice i have used bp's version but am convinced the engine does not run as smoothly. I have had no problems with it passing the emission tests of the MOT.
Have you tried removing & cleaning the car valve pistons as they do tend to effect smooth running if gummed up, particularly if the car has been little used?
it is often worth replacing on suspect area of poor running & then testing again,before changing something else,then you can pin point what is actually causing the problem , that way you may save expense,or at least know what you can hold on to as a reliable spare!
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 » Fri Sep 07, 2018 3:38 pm

Sorry - error on previous post there are only 2G on the other side of Fuse 3 (terminal 6) within the fuse box. As mentioned before i have 3 in line fuses below the fuse box on my 1980 Roadster.
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 » Fri Sep 07, 2018 3:59 pm

Hi Charles,
Top Red/Green fused to red x2
2nd bridged from supply 1 to redx2
3rd switched White to 3 x green
4th no cables - I need to check the inline fuse below as this could be the missing link.


Confused
"Have you tried removing & cleaning the car valve pistons as they do tend to effect smooth running if gummed up, particularly if the car has been little used?"

I'm suspecting Carb Dampers are the items you are refering to here. If so. Not had them out yet :-( I am aware they cannot be mixed, is there a seal under the top housing before I attempt removal.

I agree with changing one thing at a time to find the definite issue & I have ensured I can refit the original ballast wire.

I will see if I can take it for a test drive over the weekend & tinker carefully.

Thanks for the assistance.

Regards
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 » Fri Sep 07, 2018 4:48 pm

To remove the dashpots from the carburettors remove the dampers first and then undo the 3 screws holding the dashpot and lift up with the piston. Clean the interior of the dashpot and the piston with petrol. The dashpot will fit one way as the * holes are not equally spaced. Do one at a time then you won't get them mixed up. Don't forget the spring either.
. Vic
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 » Fri Sep 07, 2018 6:46 pm

Sorry Paul, i missed a "b" off my email about cleaning the carb piston valves, but i see Vic has covered it. Just be careful when lifting off the covers that you don't spill the damper oil that is held in the neck, ( i have done so in the past & also forgotten to put one spring back)!
The number of greens on the "other side" is dependent on the model year of the car. Two of my line fuses have green on one side, one of which is for the electric fan which wasn't on cars before the rad was moved forward.
it seems a PO has altered your wiring in some way.
Cheers,
Charles
1980 Roadster

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