1971 MGB GT - Engine now purring like a kitten - WHY??? - see page 20

[Peter C]

@SiC

Would you have any idea what’s going on?

[Mr Pastry]

Do both pistons move freely up and down the dashpots with the dampers removed?  If not, slacken the 3 screws at the base of the dashpot, wiggle it a bit and try again, as they do not always align perfectly if they have been apart.

[Peter C]

26 minutes ago, Mr Pastry said:

Do both pistons move freely up and down the dashpots with the dampers removed?  If not, slacken the 3 screws at the base of the dashpot, wiggle it a bit and try again, as they do not always align perfectly if they have been apart.

Both pistons do move freely with the dampers removed.

A chap on a Facebook MG group just said the same about loosening the dashpot screws. 👍

[Peter C]

I had another go at sorting the carburettors today.

I started by removing both dashpots and pistons.

I gave everything a good clean.

I checked both needles and dampers for straightness, all looked good.

With both carburettors reinstated and re-filled with engine oil (as recommended by people on the internet), the piston in the rear carburettor still needed significantly more effort to lift compared with the front carburettor.

I drained the oil from both carburettors and filled both with 3-in-1 oil (as recommended by other people on the internet). With everything reinstated again, both dampers provided almost zero resistance. 

I decided to mix some engine oil with 3-in-1 oil.

 

I poured the correct quantities of my concoction into both carburettors, checked the movement of the pistons and they seemed fine, both needing about the same amount of effort to lift.

I fired up the engine. It idled beautifully but wouldn't rev.

On the basis that I could do no more with the carburettors, I had another look at the ignition system.

When I bought a job lot of parts from Moss Europe a couple of months ago, amongst other bits I bought a spare condenser and points, which I've kept in the little toolkit in the boot, for emergencies. 

I decided to replace the condenser and points with the spares. Unfortunately, the spare parts were also made by Intermotor. 

Replacing the condenser and points is fiddly but I got them done in about 20 minutes.

I fired up the engine and not only did it idle properly, it also revved without any stuttering.

Now, either the previously fitted condenser and points were defective (as suggested by some good folk here) or I did something wrong when I fitted them last week. Considering that they can only be fitted one way and I've set the points gap to 0.38mm, the same as before, I can't see how I could have done anything wrong.

Who cares, problem solved.

More soon.

 

 

[Mally]

Put old points and condenser in bin.

Buy spares  from Distributer Doctor as recommended by @SiC

Store them somewhere dry in the car,

Tend to run forever* once it's right.

[SiC]

1 hour ago, Peter C said:

More soon

More soon... when the next new intermotor condenser fails? 🫣

[SiC]

I've only ever used 20w50 engine oil in dashpots and never had a problem. One day maybe I'll get some proper SU oil. 

I know others use 3in1 but I find it's a bit too thin and doesn't offer much resistance. Maybe that should offer snappier throttle response but then also will risk leaning out further.

If you change between the two, it is likely to need a tweak on the needles. I wouldn't fiddle with the mixture or balance them just yet. May well need it but it sounds like they're good enough to make it work for now and you don't want to add more variables into the mix by altering anymore things. 

[plasticvandan]

I think the proper stuff is sae 30,I did have a bottle for a while,different weights give different responsiveness.

I would avoid 3 in 1 though,mainly because it is or at least was organic and gums everything up,and is a bit thin for dashpots

[SiC]

10 minutes ago, plasticvandan said:

I think the proper stuff is sae 30,I did have a bottle for a while,different weights give different responsiveness.

I would avoid 3 in 1 though,mainly because it is or at least was organic and gums everything up,and is a bit thin for dashpots

Looks like the stuff Burlen sell now is SAE20

https://sucarb.co.uk/dampers-caps-oil/damper-oil/damper-oil-sae-grade-20.html

Tbh 20W-50 is probably pretty close to that if the carb doesn't get too hot (i.e. near ambient and not engine temp).

[Matty]

37 minutes ago, SiC said:

only ever used 20w50 engine oil in dashpots and never had a problem. One day maybe I'll get some proper SU oil

I'd stick with 20/50s. The SU oil is thinner. I've been pissing about for a while now trying to cure a hesitation getting back on a closed throttle. The piston is snapping open a bit quick and leaning the mixture out momentarily.

I was already on the SU oil. 10/40s improved the job a bit. 20/50s better still. Next step is to pack the fucker with grease 🤣

[Matty]

43 minutes ago, SiC said:

More soon... when the next new intermotor condenser fails? 🫣

The DD condensors are head and shoulders as you say Si. And even they still fail just a lot less. I've always got a spare as they are only a few quid. Wouldn't bother with anything else for all them reasons.

[Matty]

While I'm block posting. @Peter C. Whilst it appears that your issue was ignition related, if you're still concerned about your pistons. Take the dashpots off, clean them, lightly oil the inside of the bell. Plug the transfer holes up in the pistons with blue tac. If you then hold the dashpot upside down there is a timescale for how long it should take for the piston to drop out into your hand (i can't remember but goggling su piston drop test will tell you). Its a range of between x and y seconds. It might just set your mind at rest that there's no issue. (Obvs don't leave the needles in whilst doing it).

[Peter C]

24 minutes ago, Matty said:

While I'm block posting. @Peter C. Whilst it appears that your issue was ignition related, if you're still concerned about your pistons. Take the dashpots off, clean them, lightly oil the inside of the bell. Plug the transfer holes up in the pistons with blue tac. If you then hold the dashpot upside down there is a timescale for how long it should take for the piston to drop out into your hand (i can't remember but goggling su piston drop test will tell you). Its a range of between x and y seconds. It might just set your mind at rest that there's no issue. (Obvs don't leave the needles in whilst doing it).

Good advice, thank you but I've already done research on the SU drop test and I'm now fairly sure that my carbs are functioning ok. 

[SiC]

Buy a cheap SU HS carb, even if knackered, take it apart and see how it works. Likewise the distributor. Then work out how to put them back together.

They're all mechanically visually obvious once you've got the bits in your hands and a whole lot less intimidating to work on your own car once you know how they go together. 

[SiC]

Something like this if it goes for less than like a twenty quid or so.

https://www.ebay.co.uk/itm/335775566446

Cheaper than a book and a lot more productive to learn when doing it hands on. 

[Zelandeth]

The alternative approach to condenser issues is to site a high quality polypropylene film capacitor external to the distributor.  I did this as I was sick of the utter tripe that's being turned out these days.

Lives in the little black box underneath the voltage regulator in my case.

[Mally]

1 hour ago, Matty said:

 

I was already on the SU oil. 10/40s improved the job a bit. 20/50s better still. Next step is to pack the fucker with grease 🤣

Only works on lever arm shocks.

[Peter C]

8 minutes ago, SiC said:

Something like this if it goes for less than like a twenty quid or so.

https://www.ebay.co.uk/itm/335775566446

Cheaper than a book and a lot more productive to learn when doing it hands on. 

Good idea.

I will look out for bargain MGB bits when I go to Beaulieu in May.

[Matty]

49 minutes ago, Mally said:

Only works on lever arm shocks.

Till they rip out the bulkhead 🤣

[Matty]

58 minutes ago, SiC said:

Buy a cheap SU HS carb, even if knackered, take it apart and see how it works. Likewise the distributor. Then work out how to put them back together.

They're all mechanically visually obvious once you've got the bits in your hands and a whole lot less intimidating to work on your own car once you know how they go together. 

Having refurbed a couple of sidedraught webers , I'd take a hs series SU anyday. No wonder they work better. Beautifully simple idea that's just effective!

[plasticvandan]

Some su have a lifting pin that you can push in to indicate the state of mixture,from memory of you lift the slide using the pin and it dies it's too weak,if it races it's too rich,and if it rises slightly then returns to idle it's about right. I always found they need to be richer than "book spec" I would do the usual fastest even idle then richen it up a flat or two,then adjust idle to suit.gets rid of any hesitation on acceleration.

[SiC]

If you do use that lifting pin, it only needs a light press to bring up the piston slightly. It does allow the piston to move up pretty far and pushing it all the way will almost certainly cause stumbling from the piston open too far, not because it's weak. The usual recommendation with the air filters off is turning slightly a flat headed screwdriver. 

Watch John Twist on University Motors channel on SU carb tuning. He specialises in MGs and especially the B. 

[Westbay]

14 hours ago, Zelandeth said:

The alternative approach to condenser issues is to site a high quality polypropylene film capacitor external to the distributor.  I did this as I was sick of the utter tripe that's being turned out these days.

Lives in the little black box underneath the voltage regulator in my case.

 

Very neat ! any chance you can pass on 'How to' part numbers etc ?