25 replies to this topic

[Clifford S]

I can't help but think that restricting the carb breather hole isn't going to fix the underlying problem.  The oil filler cap should restrict the air flow through the engine enough to not cause a problem at low throttle openings.  Somewhere there's a lot more air getting into the crankcase/transfer gear case or the breather pipework than there should be.

I've now obtained a new filler cap and breather hose. I shall try these at the weekend and see how it goes. Hopefully that will solve the problem. Will report back on my progress.

 

My concern about the breather hole was that I had read somewhere about a restrictor sometimes being used, I assume this was Leyland's penny-pinching alternative to a proper PCV valve. All the 'A' Series engines I've had before have had HS2 carbs and either an open breather circuit or one fed into the air filter. The only engines I've seen (before this one) that breath into the manifold (high vacuum) side of the carb have employed a PCV valve, which is incorporated specifically to eliminate the problem I'm having. Too much air entering and weakening the mixture when the throttle is closed and so pulling maximum vacuum.

[Lplus]

 

I can't help but think that restricting the carb breather hole isn't going to fix the underlying problem.  The oil filler cap should restrict the air flow through the engine enough to not cause a problem at low throttle openings.  Somewhere there's a lot more air getting into the crankcase/transfer gear case or the breather pipework than there should be.

I've now obtained a new filler cap and breather hose. I shall try these at the weekend and see how it goes. Hopefully that will solve the problem. Will report back on my progress.

 

My concern about the breather hole was that I had read somewhere about a restrictor sometimes being used, I assume this was Leyland's penny-pinching alternative to a proper PCV valve. All the 'A' Series engines I've had before have had HS2 carbs and either an open breather circuit or one fed into the air filter. The only engines I've seen (before this one) that breath into the manifold (high vacuum) side of the carb have employed a PCV valve, which is incorporated specifically to eliminate the problem I'm having. Too much air entering and weakening the mixture when the throttle is closed and so pulling maximum vacuum.

 

I can imagine a restrictor being used instead of a proper pcv valve if the breather goes into the carb downstream of the throttle plate, or into the manifold, as fitted to the likes of the cooper S and MGB, but both my minis (1275 and 1098) have no restrictor and feed into the carb upstream of the throttle plate
 

[Spider]

 

 

I can't help but think that restricting the carb breather hole isn't going to fix the underlying problem.  The oil filler cap should restrict the air flow through the engine enough to not cause a problem at low throttle openings.  Somewhere there's a lot more air getting into the crankcase/transfer gear case or the breather pipework than there should be.

I've now obtained a new filler cap and breather hose. I shall try these at the weekend and see how it goes. Hopefully that will solve the problem. Will report back on my progress.

 

My concern about the breather hole was that I had read somewhere about a restrictor sometimes being used, I assume this was Leyland's penny-pinching alternative to a proper PCV valve. All the 'A' Series engines I've had before have had HS2 carbs and either an open breather circuit or one fed into the air filter. The only engines I've seen (before this one) that breath into the manifold (high vacuum) side of the carb have employed a PCV valve, which is incorporated specifically to eliminate the problem I'm having. Too much air entering and weakening the mixture when the throttle is closed and so pulling maximum vacuum.

 

 

I can imagine a restrictor being used instead of a proper pcv valve if the breather goes into the carb downstream of the throttle plate, or into the manifold, as fitted to the likes of the cooper S and MGB, but both my minis (1275 and 1098) have no restrictor and feed into the carb upstream of the throttle plate

 

 

The CCV Crankcase Ventilation set up is one thing I think Leyland got very right, it's way better than a PCV Set up, though, they are fairly good, but the CCV Set up runs rings around it. Yes, it would have to be cheaper to manufacture the cars with a CCV but I doubt that was their motivation for doing so here.

Adding a PCV Valve to a CCV System isn't something I'd previously contemplated, if nothing else, it might work as a restrictor in the system.

It might be getting off topic but there's a bit of info in this thread on the systems, their pros and cons;-

https://www.theminif...ather-question/

[Lplus]

The only situation I can think of where a restrictor at the carb might help is if someone connected one crankcase breather to the carb and fitted one of those small filters to the other crankcase breather, thus increasing the air flow through the crankcase.

Edited by Lplus, 02 September 2023 - 08:48 AM.

[Clifford S]

Just been doing some experimenting with this problem.

 

New hose and filler cap fitted, possibly small improvement but still wont tick-over. Hunts between 1500 and 2000  and needs a lot of choke. At this speed it's pulling 15-17"Hg on the breather!

 

Having found a plastic bung that will fit over the CCV port and inside the breather pipe, I tried running it with several different size holes as a restrictor (pipe connected of course)

 

 

1mm hole -  Engine runs lumpy at 900rpm pulling 10"Hg.

                    With filler cap removed, no change in vacuum or speed. Hand covering rocker opening, no change but pressure felt.

 

3/16" Hole - Engine runs better at 900rpm pulling 11"Hg.

                    With cap removed no change but less pressure felt when covered by hand.

 

1/8" Hole -   Engine runs fairly smoothly at 900rpm pulling 12"Hg.

                    With cap removed, speed increases to 1000rpm. When covered by hand, speed goes up to 1100rpm and vacuum rises to 13"Hg.

 

Haven't yet tried a CPV valve but I think I might be getting somewhere.

[Clifford S]

 

I can imagine a restrictor being used instead of a proper pcv valve if the breather goes into the carb downstream of the throttle plate, or into the manifold, as fitted to the likes of the cooper S and MGB, but both my minis (1275 and 1098) have no restrictor and feed into the carb upstream of the throttle plate

 

 

That's precisely where my breather does go into the carb! Downstream of the throttle, between throttle butterfly and manifold.

 

So too does the vacuum advance take-off, which is another thing I don't like about this HS4 setup.

[Lplus]

 

 

I can imagine a restrictor being used instead of a proper pcv valve if the breather goes into the carb downstream of the throttle plate, or into the manifold, as fitted to the likes of the cooper S and MGB, but both my minis (1275 and 1098) have no restrictor and feed into the carb upstream of the throttle plate

 

 

That's precisely where my breather does go into the carb! Downstream of the throttle, between throttle butterfly and manifold.

 

So too does the vacuum advance take-off, which is another thing I don't like about this HS4 setup.

 

 

 

Ok, that rather explains why you're having so much trouble, though I'm surprised there's an HS4 carb with that set up. I've not seen that before, the HS4s I have all have the breather pipe just after the bridge but before the throttle butterfly.  Is this a mod someone has done?

 

As to the vacuum advance take off, some engines have the take off from the manifold itself (MGB) or just downstream of the butterfly so that the vacuum reduces as the butterfly opens, but that's not likely to cause your problem.

 

Are you sure the port you are using is actually the breather port?  Something rings a bell in my head about a part throttle mixture weakening system which might use a port after the butterfly.  Others may know more about that.

 

A photo of your set up might be helpful.

 

 

Edit, ignore the above.  I Just looked at the photo of the restrictor you posted above in post #20, That port with the orange restrictor on it is the breather port and is most definitely upstream of the throttle butterfly but down stream of the carb bridge which holds the jet.

Edited by Lplus, 02 September 2023 - 07:52 PM.

[Spider]

Apologies here - I've just returned from a 4 week road trip and haven't got my head screwed on the right way !
 

 Hunts between 1500 and 2000  and needs a lot of choke.

 

As the CCV Port is upstream of the butterfly, it has nothing to do with the high idle issue you are having.

Given that you need a lot of choke, I'd say there's a leak somewhere between the Butterfly and the Inlet Valves. It maybe too that the butterfly itself isn't closing as it should, but I'm more inclined to thing there's a leak at a joint somewhere.

 

 

 

[68+86auto]

You don't have the vacuum advance blocked off?

[Clifford S]

Oh b*gger! I really don't know what to say apart from how embarrassed I am to have wasted everybody's time.

Profuse apologies to all. I've got it so unbelievably wrong haven't I?

Now kicking myself for being such a complete and utter idiot.

[Clifford S]

You don't have the vacuum advance blocked off?

Did have, that's where I attached the vacuum gauge. Anyway, seems I've been barking up the wrong tree all the time.

As soon as I recover from beating myself up I shall start over and look for leaks.