1971 MGB GT - Massive curve ball thrown - see page 23

[Peter C]

15 minutes ago, Zelandeth said:

Dicky ignition switch or fuse in line (no idea if applicable) making the power supply temperamental?

🤔

[Dj_efk]

Interesting - I've used that coil before but as above, only with electronic setups - as my understanding was you can't get the full capacity of 40k Volts from it through points and condensors - but I may well be wrong (make that almost certainly wrong if Dizzy Doc recommended it).

For what it's worth, I've normally defaulted to a Lucas Gold one on A and B series applications, but the one you have there is definitely better in capacity - and I would wager in quality now that Lucas just seem to whore out their brand and produce the cheapist shite from China like most other once-great British brands (hence the cost differential between the two).

Either way, great update and glad you got it working eventually. There will be some finickity reason it didn't work from the off like the wire in the dizzy wasn't quite making good contact or something like that - anyway it works now so I would defo call that a win  - Well done!

[Peter C]

I was busy with work yesterday but I found some time this afternoon to take the MGB for a proper test drive. The weather was perfect, the roads were dry, so I took the opportunity to take the MGB up and down the A40, then, once the engine had definitely fully warmed and all was ok, I decided to take it up and down the M40 between J3 to J4 and back again, a total trip of around 15 miles. 

Check out the rear view mirror, the driver of the skip lorry behind me was no doubt oh so pleased that a little red MG (not Corvette) was crawling in front of him at 55MPH.

On board checks, all gauges reading ok.

When I got back home, I left the MG to idle on the drive for about 5 minutes.

The good news is that there was no more spluttering or misfiring and the engine now idles smoothly. So, my efforts have not gone to waste and whatever was wrong with the ignition system has now been put right. My money is on the coil.

I am still experiencing a slight hesitancy when initially depressing the throttle but that is definitely related to the rear carburettor dashpot piston, which is not lifting as quickly as the front one, thus briefly starving cylinders 3 and 4 of air and fuel. Depressing the throttle gently, thus allowing both pistons to rise more slowly, does not cause any issues. I need to investigate this issue further but it's not a deal breaker, the MG now drives ok.

So it went back into the garage.

Hitherto, I've carrying tools and spare ignition parts in a pretty pink cardboard box. Last year I was gifted a posh bottle of vodka for my 50th birthday, which came in a pretty wooden box. I have now transferred the tools and parts into the wooden box.

Which fits snuggly in the lower boot compartment.

Time to throw a curve ball.

I met with a client this morning, who has a lovely 1970 MGB Roadster, which he fully restored ten years ago and despite being in regular use, still drives and looks mint. He is moving to a property in central London and will be losing the house and garage in Windsor soon, which means that the MGB has to go and he offered it to me for what I think is a very attractive price. He hasn't advertised it for sale yet but he says that he will do so soon, for more than he is willing to sell it to me for. 

I haven't committed to any kind of deal but, between us gentlemen, I am tempted. I don't really need the GT's additional luggage capacity and the thought of driving an MGB mid summer with the top down certainly has an appeal for a man who is celebrating his 51st birthday next week and is yet to experience a midlife crises.

I  can afford to run both MGs, as they cost peanuts to insure and tax etc are all free, however I don't have garage space for both and I am not keeping either outdoors due to my extreme OCD.

So, does anybody want to buy my MGB? If I don't sell my GT, I will happily keep it and forget about the Roadster. I am not going to bother to advertise it anywhere else, so if nobody from here is willing to take it on, then I will save myself the hassle and money and stick with what I've got and enjoy it. I've come a long way in a short time with this car and I am enjoying the experience, so I am not desperate to move it on.

It owes me knocking on £4k plus hours days weeks of hard labour but I will let it go for £3k.

More soon, one way or another. 

 

[Mrs6C]

22 minutes ago, Peter C said:

that is definitely related to the rear carburettor dashpot piston, which is not lifting as quickly as the front one,

Sounds like the jet needs to be recentred, to allow the piston to rise and fall cleanly.

If you're not familiar with the process, there is some useful discussion about it here:
https://www.britishcarforum.com/community/threads/su-jet-centering.68729/

Check the same type and quantity of oil is in both pot tops too, of course.

As well as @Mr Pastry's suggestion below, if the carb. operating linkages  are worn or slack or just out of adjustment, that can make one carb. work before the other too.

 

[Mr Pastry]

17 minutes ago, Peter C said:

I am still experiencing a slight hesitancy when initially depressing the throttle but that is definitely related to the rear carburettor dashpot piston, which is not lifting as quickly as the front one,

Possibly the carbs are slightly out of balance?

[Peter C]

29 minutes ago, Mrs6C said:

Sounds like the jet needs to be recentred, to allow the piston to rise and fall cleanly.

If you're not familiar with the process, there is some useful discussion about it here:
https://www.britishcarforum.com/community/threads/su-jet-centering.68729/

Check the same type and quantity of oil is in both pot tops too, of course.

As well as @Mr Pastry's suggestion below, if the carb. operating linkages  are worn or slack or just out of adjustment, that can make one carb. work before the other too.

 

With the dashpot damper removed, the piston goes up and down just fine.

I have replaced the oil in both carbs.

Linkage is all good.

[Peter C]

22 minutes ago, Mr Pastry said:

Possibly the carbs are slightly out of balance?

Quite possibly. 

[Mrs6C]

3 minutes ago, Peter C said:

With the dashpot damper removed, the piston goes up and down just fine.

You could swap the two dampers over and see if the issue is transferred to the front carb... it may be the rear damper itself is out of true.

If no difference, then try recentring the rear jet.

Both pistons should lift cleanly with one finger pushing them up, then drop cleanly with a small 'thunk', with the dashpot dampers in situ.

[Peter C]

Just now, Mrs6C said:

You could swap the two dampers over and see if the issue is transferred to the front carb... it may be the rear damper itself is out of true.

If no difference, then try recentring the rear jet.

Both pistons should lift cleanly with one finger pushing them up, then drop cleanly with a small 'thunk', with the dashpot dampers in situ.

I've posted about this, see previous thread.

Swapping the damper transfers the problem to the other carb but the damper rod is straight, which is why I need to investigate this matter further. With the good damper rod fitted, both dashpots can be lifted easily. With the bad damper rod fitted, a lot more effort is required to lift the dampers.

[Westbay]

Is the 'c' clip centred correctly on the damper ?

Have you got/seen a copy of this ?

 

[Westbay]

[Mr Pastry]

Since the problem stays with the dashpot when moved to the other carb, and the piston/needle falls freely with the damper removed, surely it has to be something to do with the damper piston - the brass bit on the lower end of the damper rod.  Compare the two?

I think one of the worst mistakes BMC ever made was to publish tuning instructions for twin SUs in every driver's handbook.  So everyone became an instant  expert without really understanding what they were doing, and gave SUs a reputation for "going out of tune"  No they don't.  They go out of tune because people mess around with them, so they are all a bit suspect now if you don't know the history.

[juular]

May have already been mentioned but it's worth checking the needle is attached to the piston properly and in the right place. The retaining screw can work itself loose enough to allow the needle to shift and wreck the mix.

[Peter C]

@Mrs6C @Westbay @Mr Pastry @juular

CORRECTION!

I've had another look at the carburettors.

I removed the air filter housings and fingered both pistons.

After I replaced the oil in both dashpots last Sunday, both pistons now rise correctly, with equal amounts of force and, as far as I can see, both drop at the same rate. 

I removed the carburettors twice. First time, when I had to replace the exhaust manifold and second time, more recently, when I had to re-tighten the exhaust manifold nuts. On both occasions, the carburettors got squeezed up against the inner wing and there is a chance that something got disturbed.

I have noticed that the fuel chamber on the rear carburettor wobbles a little more than the chamber on the front carburettor but it definitely does not leak. Neither fuel chamber is rigidly mounted to the side of the carburettor. 

The throttle and choke linkages look all good, correctly adjusted with no slack.

So why am I getting a flutter when the throttle is depressed any more than very gently? I'm pretty sure it's going to be a fuel related issue.

 

 

[Peter C]

This might be useful.

https://www.howacarworks.com/fuel-systems/adjusting-an-su-carburettor#:~:text=If it falls and the,clockwise makes the mixture richer.

[Peter C]

This looks very relevant.

https://www.mgexp.com/forum/mgb-and-gt-forum.1/slight-hesitation-upon-acceleration.471210/

Next time I drive the MGB, I will see what effect pulling the choke out will have on the hesitancy. 

 

[Mr Pastry]

Flexible mounting of the float chambers is normal.

Have you tried revving the engine gently with air filters removed so that you can see what the pistons are doing?   They should rise and fall exactly in sync.

[Peter C]

3 minutes ago, Mr Pastry said:

Flexible mounting of the float chambers is normal.

Have you tried revving the engine gently with air filters removed so that you can see what the pistons are doing?   They should rise and fall exactly in sync.

Yes, I did that whilst I tinkering with the carburettors last Sunday. Unfortunately, at the time, the ignition system was defective and the engine wouldn't rev, so the inspection of the moving pistons was inconclusive. 

I plan to:

Do what you've said again, now that the engine will rev up.

Reinstate the air filters.

Take the MGB for a drive and see how the engine behaves when I pull the choke out.

I'm not giving up!!!

And I am now very much in two minds whether I want to sell my GT and buy the Roadster. Every time I step into the garage the GT puts a smile on my face.

 

[Mrs6C]

Other things you could check would be:

1. Making sure both needles are the same and of the correct typeb

2. Checking both springs are the same and of the correct type

3. Making sure both jet tubes are nice and clean with no dirt/foriegn objects sitting around in the bottom, ready to swirl up randomly and cause a nuisance when fuel is passed through...

4. Check both breathers are clear

[Peter C]

18 minutes ago, Mrs6C said:

Other things you could check would be:

1. Making sure both needles are the same and of the correct typeb

2. Checking both springs are the same and of the correct type

3. Making sure both jet tubes are nice and clean with no dirt/foriegn objects sitting around in the bottom, ready to swirl up randomly and cause a nuisance when fuel is passed through...

4. Check both breathers are clear

I haven’t changed any components inside the carburettors and the engine ran fine until recently.

How do I clean inside the jet tubes?

Noted about the breather pipes.

[artdjones]

Have the carbs been balanced?

[Peter C]

1 minute ago, artdjones said:

Have the carbs been balanced?

Not recently.

How / why would they get out of balance?

[Mr Pastry]

2 minutes ago, Peter C said:

How / why would they get out of balance?

(sighs....)They could get out of balance if the linkage has been disturbed, as it might do if they have been taken off.  Which is why I asked whether the pistons were rising and falling equally with revs.  If they are not (other things being equal) it would suggest that one carb is sucking harder than the other. 

[Peter C]

5 minutes ago, Mr Pastry said:

(sighs....)They could get out of balance if the linkage has been disturbed, as it might do if they have been taken off.  Which is why I asked whether the pistons were rising and falling equally with revs.  If they are not (other things being equal) it would suggest that one carb is sucking harder than the other. 

Thank you.

And please bear with me!

[Mr Pastry]

21 minutes ago, Peter C said:

Thank you.

And please bear with me!

Soz!  I think you are doing great stuff with this, and you clearly have the patient approach which it needs.  It is logical if you follow it through.

The carbs can only respond to the suction of the engine - that is all they do.  So in the first place you need all four pots sucking equally, i.e. good compressions, good valve clearances, no air leaks into manifold, and good ignition to burn the mixture and clear the way for the next suck.

With all that established (it probably is now) the throttle plates have to be adjusted to provide equal suction on both carbs, which is where the linkage comes in, but check piston action first, as said.

Then you can move on to setting the mixtures, but these are probably not too far off, if it starts easily and runs reasonably.

Pulling out choke may tell you something useful, but it is also important that the chokes close properly and return the jet to its highest position to restore the normal mixture.

And all these adjustments will mess with the idling speed, but one thing at a time.

[angle]

The times they are a changin...

A couple of years ago we went on holiday and got one of these as our hire car:

My seven-year old loved it, probably due to the panoramic roof and the 250bhp, so now when he's out he spots MGs. I opened the curtains this morning and told him to come and look at the cool MG outside:

He glanced outside, went 'it's not a proper MG' and went back to playing with his Hot Wheels...

[Jenson Velcro]

Have you actually sat in a B roadster yet? I seem to remember you saying you’re over 6’tall. The top of the windscreen rail is much lower on the roadster than the GT.  I can’t get comfortable in the roadster as I have to crouch down to actually see out, whereas in the GT I can comfortably sit up. I’ve always thought the best compromise would be a GT with a webasto roof. A lot less buffeting/ noise as well.

[Peter C]

2 minutes ago, Jenson Velcro said:

Have you actually sat in a B roadster yet? I seem to remember you saying you’re over 6’tall. The top of the windscreen rail is much lower on the roadster than the GT.  I can’t get comfortable in the roadster as I have to crouch down to actually see out, whereas in the GT I can comfortably sit up. I’ve always thought the best compromise would be a GT with a webasto roof. A lot less buffeting/ noise as well.

I sat in the Roadster that I am contemplating on buying but I haven’t driven it yet.

 

[Peter C]

Right, on the basis that I don't have the right tool to correctly set the ignition timing nor the right tool to assess how much air each carburettor is sucking in and I have no way of checking what is coming out of the exhaust, I have decided to have the MGB looked at by a specialist. I have spoken with a chap called David at AutoRevive London, who is based in Ickenham, which is less than 30 minutes from where I live and he is happy to give the MGB a proper once over. I'm giving him the MGB next Friday and I will get it back, hopefully properly fixed and ready to enjoy. David is an MG specialist with decades of experience and I am sure he will do a far better job of getting the MGB running properly than I ever could.

With this cunning plan in mind, I have decided not to purchase the Roadster and persevere with my GT.

[chadders]

On 08/02/2025 at 09:03, Peter C said:

I sat in the Roadster that I am contemplating on buying but I haven’t driven it yet.

 

I'm 6ft and a little bit and find that if I slouch a bit it's fine, with the seat right back there's enough legroom. I think that it'll depend on how long your legs are v.  your torso. Reclining the seat back also helps I find.