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MG MGB Technical - Rough running

Hi,

Two questions in this query.

a) My gt was running a bit rough recently, I then started to check the ignition. I put a strobe on each of the plug leads. The strobe showed that there was a spark at each lead; however, plug lead 2 and 3 showed a double spark. Is this an indication of the cap, condenser or points failing - or all three?

b) Although there was a good level of fuel in both carb fuel pots, spark plug 1 & 2 were pretty dry. Car has been stood for much of winter. Any tips/guidance about how to clean the carbs (perhaps regarding the jets in particular) as I have never done this procedure.

Thanks in advance.

IA Jones

I'll leave the HT side to others.

For cleaning carbs, start with the easy and least intrusive - first, go for a good long drive, when the car is fully warmed up and running OK use the gears to get the revs up, give it a blow out - Italian tune-up.

Second - fill the tank twice with a good quality petrol that includes a cleaning additive package like Texco Momentum99 or Shell V-Power, BP Ulitmate, Texaco Supreme and use as quick as possible.

Then see how it is.

If you keep up with the regular servicing and regular use on reasonable length journeys that could be enough usually.

Nigel Atkins

If the car has stood for a long time , modern fuels tend to wax up. If you bave a fuel filter just behind / to the side of the carbs this is worth changing as clogs up. Also remove the carb piston covers & lift off the carb pistons & clean & replace taking care to replace on the same carb that you took them off. I have been amazed how the smooth running of the engine is improved ,both at idle & on acceleration following this exercise.
Cheers, Charles
Charles9

Hi,

Thanks Charles,

Would you use carb cleaner to free up/dissolve the gum?
I have been told just to use white spirit and a rag?

Would I need to get them into an ultrasonic bath for a perfect clean or will carb cleaner on its own do?

I am particularly interested in sorting out the jets, so any tips, regarding getting any crud out this, from anyone would be great.
IA Jones

You might be doing more than you need but if you're adding this to your full service of the full car then you'd be renewing the fuel filter anyway so to add to that.

air filters off, renew if paper (part of service anyway).

If you're taking the carb dashpots off and previously the car ran very well so you know the carbs are in correct order and the various parts of the carbs matched to the correct part then -
mark the dashposts for which way they are screwed to the carb body (facing engine or wing) note front and rear carb or do one at a time so as not to mix.

Unscrew dashpot damper, put all parts in a clean container (old washing up bowl perhaps) with bottom lined with kitchen roll or rag - it will drip oil.

Undo small screws that hold dashpot (don't drop them).

Carefully lift off dashpot chamber, having marked it.

Take out long spring, place in container.

Carefully lift out piston, remember the needles are attached below this you do not want to bend or damage the needle. Empty out the existing oil.

Spray carb cleaner all over and inside carb and bowl and linkages and wires.

Clean inside of dashpot chamber, piston and needle with either carb cleaner or petrol on a clean lint free cloth.

Reassemble but put a little light oil (3-in-1) on the rod part of the piston only (this is the outside part of where the damper oil goes in the inside of the rod part) - the rest of piston and dashpot chamber needs to be dry. Before putting the damper back in put fresh engine oil to about 1/2" below the top of the rod(or as per Driver's Handbook instructions).

As part of the servicing you will lightly oil the linkages and wires, excess will go to exhaust and stink later.

No you should not need an ultrasonic bath.

Oh after this (and if you're doing it a full service of the whole car) give it the Italian tune-up and run through a couple of tank fulls of good fuel.
Nigel Atkins

Nigel's on to it
The best thing you can do for your car after a layup is take it for a nice long drive and give it a bit of exersize-------
The ign. points may have corroded up a little while the car was in storage so maybe a clean and reset (or replacement)there

On the double strobing--
Points or rotor won't cause that, the cap possibly could but would be very unusual as 2-3 are on opposite sides of the cap
I'm thinking it's more like , if your timing light has an inductive pickup (clip on), they are fairly sensitive and if 2-3 plug leads are touching or close to each other the strobe is picking up the signal from both leads-I've had this happen when plugged onto one lead but the pickup is resting against another----Just check and make sure your leads are separated from each other --also maybe the insulation of the plug leads is starting to get near their use by date or dampness has crept in causing a crossfire, you could check that with the engine running in the dark and push the leads around with something insulated (wooden stick)and see if you can get a spark to jump from one to another proving a fault

But- fresh fuel and a good drive is more fun than playing sparks in the dark

willy
William Revit

Agree with Willy. FWIW modern cars often use 'wasted spark' where they get a second spark elsewhere in the cycle. As long as they are not adjacent in the firing sequence it wouldn't cause a problem. MG V8s can suffer from that as 5 and 7 are adjacent in the firing sequence and adjacent in the engine, with quite a long HT lead run. Those two leads have to be separated by the lead to No.3 cylinder or 7 can fire part way up its compression stroke.

With the air cleaners off check both carb pistons are operating together. If the front one is staying down that will account for 1 & 2 being dry as well as the rough running.

I'd also do an air-flow and mixture balance check before digging in to the carbs.

If both float chambers have the same amount of fuel and the fuel pump chattered away when you first turned on the ignition, then the fuel supply that far is OK. Could be gumming in the jet, if you blow in the overflow port with the ignition on i.e. float chambers full and float valves closed fuel should bubble up from the jets, observed with the air-cleaners removed. If the front carb doesn't do that then its jet is blocked.
paulh4

As is repeatedly witnessed I would always recommend a full service on the whole car but I also think there should always be a good driving to working on the car ratio.

There can be a tendency to get these sexy tools out too early, before, instead of after, the basics like servicing and driving, so getting spurious or misunderstood results. The trend is towards boys-toys, power instead of hand tools, over complicating things before necessary.

Willy knows in detail about ignitions but is willing (no pun intended there but I like it) to suggest a very basic check which removes machine/user false readings (I'd add a spray bottle for more fun).

I know at some point more than regular full servicing and maintenance is required but that point can be greatly recessed by regular full servicing and maintenance and regular driving reasonable length journeys.

Every year we spend a lot of money to go to get to drive on the fabulous roads of Wales, I'd sooner not be delayed by a wheezing B on those narrow roads, c'mon IA get your car (in a lower) gear and enjoying those fabulous roads whilst doing your car good too. :)
Nigel Atkins

You don't actually say what carbs are fitted. Assume HS4.
As for double strobe flash;
If your timing light is an inductive one, i.e. pick-up clips around the leads, you can get stray firing of the gun from nearby leads. Put the pick-up near the plug & get the other leads well away from it.
If the plugs have been firing they would be dry!
However a flash on the gun doesn't mean the spark is a heathy one merely that there is one, which may not be sufficient to jump the plg gap..
Running rough? Define. Coughing, back-firing, hunting, stuttering, ,, etc.
Allan Reeling

Hi,

Have cleaned up the carbs. A dash-pot, needle chamber and needle clean up was quite straightforward and didn't need much of a clean. However, there was quite a bit of gunge in the jet 'tube' and needle housing; particularly in the front. So hopefully that is sorted as fuel delivery to the carbs was always ok,

When I tried to start it, still no joy; went through the various HT checks once again. No spark. Then at a number of periods after fiddling with dizzy, changing condenser, changing coil etc; realised that the problem was intermittent - sparking would come back then would disappear again - without me having changed a thing.

When I started looking at the LT circuit for the nth time found that a) there was sparking inside the clear (i.e. see through) insulation of the connector of the -ve CB wire. If the insulator had been a solid colour I would not have noticed. Replaced the connector as the wire had detached from the connector metal.

After fixing this I thought great, but ..... still no go. So now when opening the points with the ignition on I would get readings of 12v on both the -Ve and +Ve terminals of the coil .... then when repeating the tests - just to make sure - there would be no reading at all.

I suspected earthing, though I could not see how this occured? I then changed the distributor for a spare one I had, the engine fired up. Still a bit rough as I need to redo the timing after all this fiddling.

(Anyone know of a way of testing internal earthing of a dizzy; as I would like to replace faulty part/cure it - to have a spare working dizzy)

The upshot of all this is that differing problems were contributing and masking each other.






IA Jones

Have you checked the state of the earth wire within the distributor that attaches to the points? This was originally a brownish cloth covered wire which can get trapped between the points & mounting and / or the insulation can get damaged that can short in & out to earth and hence disrupt what the LT cicuit through the coil is trying to do.....
Cheers, Charles
Charles9

I second checking the CB earth (well throwing them out permanently really, nasty things).

For the jet gum I have a petrol washing method for that, same as the between services (or replacement) plug cleaning method.
Nigel Atkins

Can't get my head around your dist. test
Your test with the contacts open is correct with 12v on both terminals of the coil--
With the contacts shut you should have 12v feed to the coil still but 0 on the other terminal-
If in fact you have 0 on both terminals of the coil you have an ignition power circuit problem and replacing the distributor should not have made any difference
Where are you connecting the earth leed of your meter
What year model is your GT
William Revit

First thing to bear in mind is that apparently the engine was running, albeit briefly. If it won't even start now then you need to look back carefully at what you did, considering everything, no "It can't be that".

We don't know what year this is, I'm guessing it's a CB from guessing at HS carbs previously.

With the ignition on and the points open you should get 12v on both the coil -ve and the coil +ve. If you don't get 12v on either, as you indicate, then there is a failure of the 12v supply from the ignition switch. No point looking at anything else until you have resolved that.

The year of your car is important to taking this further as the feed to the coil can come via one of three routes. I would also double-check the meter connections and function when you do not get 12v on the coil, i.e. by testing the brown and white at the fusebox.

If you regularly get 12v on the brown and the white then the failure is in the feed to the coil, for which the year of the car is needed.

If you get 12v on the brown but not the white then there is a failure back towards the ignition switch.

As a temporary measure you could jumper the brown, or the white if that shows 12v, to the coil +ve and try starting then.

If you don't get 12v on either then suspect the meter or its connections.
paulh4

This thread was discussed between 06/05/2019 and 08/05/2019

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