Carb. woes

[amarshall]

Are you sitting comfortably? Then I'll begin.

Despite finding a few extra horses on the rolling road, my car has been badly behaved for months. It been fouling plugs (3 out of 4) regularly. Not the worst problem in the world, since popping new plugs in isn't a massive job - but it's annoying.

Part of the problem is almost certainly that the carbs can't be balanced properly because some past gorilla managed to jam one of the air bypass screws and mangle the head so it can't be turned. Any attempt to turn it results in the screwdriver riding up on the damage and making it worse. Hammering a screwdriver in doesn't achieve anything, attempting to hammer the screw round doesn't achieve anything. Penetrating oil hasn't helped. Nothing. It's like it's welded in place.


"Use an easy-out" cried the crowd. So I did. I bought a set. I've drilled into the screw (removing the head in the process - this is going well, isn't it?) and driven the easy-out into the hole. It's jammed now. I can get it out, but I can't get it to turn any further.

So - easy-outs aren't easy.

Anyone have any thoughts short of "buy a new carb"?

Lucky I have another car for commuting duties - but it isn't the same.

[Alan_M]

It might be worth giving Eurcarbs a call. I’m sure they will have come across this and be able to sort it.

[Hawaiis0]

could something like this help

https://www.ebay.co.uk/itm/4x-Remover-D ... ctupt=true

[amarshall]

could something like this help

https://www.ebay.co.uk/itm/4x-Remover-D ... ctupt=true

Bit late for them now - there's no head on it any more. Not sure how well they would have worked on a flat head. IIRC, those work best on Philips which have gone round.

[Pete Boole]

Take the carb off and get some heat on it while using the screw extractor.

Pete

[CHRISYD]

try a hand operated wind handle drill and use gradually bigger bits until the screw either starts to collapse inwards or starts to feel looser.

[Grant H]

If 1 and 2 are not requiring the air screw could you swap the carbs? I have a weaker cylinder 4 which I assume is caused by the vacuum to the break servo.

[Lotus-e-Clan]

I'm a far too late with this. FWIW I would have left the damaged screw alone and referenced all of the other bypass screws to that one. The chances are that it wasn't moved by the Gorilla either, and it's hardly open.

The function of the idle air bypass screws are secondary to the physical throttle plate balance. On a new carb, the throttle plates are pristine and unworn so when they are closed as much as possible little or no air passes into the inlet manifold as long as all plates are physically synchronised.

By cracking open the idle air bypass screws, air is allowed to enter the venturis on the manifold side of the closed throttle plates. Only then should you use (need to use) a four port vacuum balancing tool.

The purpose of closing the throttle plates is to shut off the progression holes as much as possible such that only the idle fuel mixture screws plus idle air bypass ports control the fueling at idle.

With old worn carbs you get more air leakages past the plates into the inlet manifold in an unpredictable distribution, which means air-mixed fuel from the progression holes can breach the plates into the inlet manifold too. This can introduce errors in / affect the sensitivity of / idle air bypass adjustment and you tend to end up closing down the bypass screws more than when new, and the number of turns of bypass screw will be randomly distributed too.

A 4 port manometer will help acheive balance and smooth idle assuming physical balance is set first (and you have equal compression on all cylinders). But if up-stream progression holes are leaking into inlet past worn plates at idle then you will find it hard to dial out richness without screwing in the idle mixture screws. If the idle screws tips are randomly worn then it will make it more difficult to dial out richness without opening the throttle plates or idle bypass screws and you can get into a pickle and wreck off throttle progression.

If you haven't perforated the damaged idle bypass screw I'd still be tempted to leave it in and use it as a baseline reference - but set the physical plate balance first and keep in mind the relationship between throttle stop position, idle air bypass function, and mixture screw function when making adjustments. And check for equal compression before doing any adjustments.

After all, you might get the old one out only to find it wasn't worth the effort on an otherwise worn carb which will make adjustments difficult anyway. As would unequal compression pressures. Go for best compromise 'as is' first. You can easily fit new idle screws (with new o rings) if necessary to increase the chances of achieving a smoothish idle and still have good off-idle progression.

[amarshall]

I'm a far too late with this. FWIW I would have left the damaged screw alone and referenced all of the other bypass screws to that one. The chances are that it wasn't moved by the Gorilla either, and it's hardly open.

That's been tried, but doesn't work. The barrel with the stuck screw is the only one that doesn't foul plugs. Since adjustments elsewhere have had no effect at all, over many many months, it's got to the point where this screw is going to come out and be replaced whether it likes it or not.

Once it's out, I have access to the kit to do the balancing correctly, but the issue at the moment is getting the damned thing out.

[Lotus-e-Clan]

I'm a far too late with this. FWIW I would have left the damaged screw alone and referenced all of the other bypass screws to that one. The chances are that it wasn't moved by the Gorilla either, and it's hardly open.

That's been tried, but doesn't work. The barrel with the stuck screw is the only one that doesn't foul plugs. Since adjustments elsewhere have had no effect at all, over many many months, it's got to the point where this screw is going to come out and be replaced whether it likes it or not.

Once it's out, I have access to the kit to do the balancing correctly, but the issue at the moment is getting the damned thing out.

OK I see. So from what you say I guess that if you unscrew the other three idle air bypass screws those barrels will weaken to match the defective barrel and the idle speed will rise and become unresponsive to closing the throttle stop screw. If that's the case then the defective screw must be open as you say.

As long as you are sure that there is no other air leak in that barrel and it's not a co-incidence.

For example, if the idle mixture screw does not enrich the mixture on that barrel it can be entirely due to a defective o ring at the end of the spring within the idle screw assembly. But I guess you have checked all idle mixture screws assemblies already if you have been having problems for some time.

ON the the mullered bypass screw. Have you tried the sharp tap trick? Warm the surrounding alloy with a hot air gun (empty the float chamber first!) then hit the screw with a wide punch (a sharp tap downwards). If you are lucky, the oxidation within the threads fracture sufficiently to allow a significant ingress of penetrating oil. Cut a new screwdriver slot using a Dremil with a small end cutter and have another go at undoing it. Or is the end of the screw far too mullered now to cut a new slot?

[Jason.goad]

Might be worth a shot with a left handed drill bit. I think you're going to have to take the carb off Angus. There's going to be a lot of swarf dropping into the venturi. Still might be worth giving Eurocarbs a call. I'm sure they've seen damaged screws before.

[amarshall]

a**e. Charged up the reversible drill, got the easy-out well clamped in it, ran it down into the hole, same result. This thing is welded in place.

<sigh> carb's going to have to come off. And I have no time to do that until about June

[Pete Boole]

What about running some slightly different plugs 'til then - help them foul a bit less?

Pete

[amarshall]

What about running some slightly different plugs 'til then - help them foul a bit less?

I could - but I'd rather get this thing sorted. It's been taunting me every time I open the bonnet.

Last ditch attempt - it's bathed in penetrating oil again, and I've extracted as much swarf as possible using a magnetic so the area should be pretty clear. I may leave it to soak for a day or two and try again.

[bash]

get a piece of metal thats the same diameter as the outer screw hole and then warm said metal up with a blow torch or gas ring away from the car until it gets red then place it in the hole so that the heat transfers into the aluminium tower then try the screwdriver again but tighten first then try to unscrew it.
This method also works on seized brake nipples using an old socket to get red hot.

Bash
PS Keep an extiguisher handy, you shoudnt need it doing it this way but you never know.