25 replies to this topic

[Clifford S]

Should there be some sort of restrictor in the HS4 engine breather port?

 

Couldn't get my 998 auto to run below about 2500 rpm, after much searching and playing around I discovered if I pull the breather pipe off and plug the port it runs OK. This port has a 1/4" hole directly into the carb body, which doesn't seem right to me as it just allows a huge amount of air in to weaken the mixture.  I did search this forum and seem to remember someone mentioning a restrictor with a 3/32" hole in it, but of course, can't find the reference now. I also can't see any indication in any of the parts diagrams available.

 

Can anyone enlighten me please?

[timmy850]

I believe if it’s all hooked up correctly then the only source of intake air to the crankcase is through the factory vented oil cap. If you have any other sources of intake air (like an open breather filter) it’ll cause a high idle

You can block the port if you have the breather system set up accordingly

[Clifford S]

I believe if it’s all hooked up correctly then the only source of intake air to the crankcase is through the factory vented oil cap. If you have any other sources of intake air (like an open breather filter) it’ll cause a high idle

You can block the port if you have the breather system set up accordingly

I do have the standard setup. Air in through the oil filler cap, out via a canister on the tappet chest cover, then piped into the carb. The problem seems to be way too much air entering the carb. Which is what led me to think perhaps the flow through this 1/4" port should be restricted somehow.

I've ordered a new filler cap and breather pipe to see if it makes any difference.

[68+86auto]

What happens if you disconnect it from the carb and leave it open? the engine should cut out or at least be close to it as it will then be too lean.

 

I would also suggest resetting all the carburettor adjustments.

 

I have a 998 auto with the same setup.

Edited by 68+86auto, 30 August 2023 - 06:01 AM.

[Steam]

Never had to reduce the diameter of the breather inlet. You may have a leak in the ventilation system or other carb or manifold leaks.

[nicklouse]

Sounds like the carb needs serving up. There has never been a restriction in the port.

[Spider]

Some engine set ups did run a restrictor in the CCV Port. It is a standard factory part.

I'll look them up shortly.

[Spider]

Some engine set ups did run a restrictor in the CCV Port. It is a standard factory part.

I'll look them up shortly.

 

12G3436 was the part. They are NLA, but there's nothing much to them and can be done in many other ways - if a restrictor is needed.

There's some info in this thread about them;-

https://www.theminif...gine-breathing/

The CCV Port will admit more air (and other gases) o the engine, so the needle does need to be selected accordingly.

[Clifford S]

What happens if you disconnect it from the carb and leave it open? the engine should cut out or at least be close to it as it will then be too lean.

 

I would also suggest resetting all the carburettor adjustments.

 

I have a 998 auto with the same setup.

If I leave it open I get the same as connected. Only run on choke and not below 2500 rpm.

 

Have checked and double checked both timing and carb. settings. Simply too much air getting in!

[Clifford S]

 

Some engine set ups did run a restrictor in the CCV Port. It is a standard factory part.

I'll look them up shortly.

 

12G3436 was the part. They are NLA, but there's nothing much to them and can be done in many other ways - if a restrictor is needed.

There's some info in this thread about them;-

https://www.theminif...gine-breathing/

The CCV Port will admit more air (and other gases) o the engine, so the needle does need to be selected accordingly.

 

Thanks so much for this Spider. You are the salvation of my sanity and possibly my mini too.

 

Looking into part no. 12G3436 I've found a very blurry picture showing it to be about 25mm long and a description of "New restrictor, nylon tube, HS4 carburetor. 12G 3436 $.50" (MiniCity, in USA)

 

Going to turn one up from Tufnol, with a 1/8" hole and see how runs with that. Alternatively, I shall try and find a standard PCV valve from another 1lt. engine. I'm thinking the restrictor was Leyland's cheapo alternative to using a proper PCV valve.

[Spider]

Going to turn one up from Tufnol, with a 1/8" hole and see how runs with that. Alternatively, I shall try and find a standard PCV valve from another 1lt. engine. I'm thinking the restrictor was Leyland's cheapo alternative to using a proper PCV valve.

 

Since you are able to turn one up, can I suggest starting with a smaller hole - you can always make it bigger ! I'd suggest starting at around 1/16" or 3/32".

When fitted up, with the engine running, remove the Oil Filler Cap and the engine revs should increase. If they don't, you might then need to increase the size of the hole. Once you settle on a size, check after some running that there's not moisture building up in the crankcase. Look for 'Mayo' under the filler cap.

They did use a PCV System before these (CCV), which IMO, is way better than a PCV set up
 

[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.

[Steam]

As said above If the small hole in the oil filler cap is not restrictive enough then nothing else will be
Also if you xo not let enough pressure out it will find its own exits via seals, gaskets and the dipstick making an awful oily mess

[Lplus]

As said above If the small hole in the oil filler cap is not restrictive enough then nothing else will be
Also if you xo not let enough pressure out it will find its own exits via seals, gaskets and the dipstick making an awful oily mess

I suppose if there is massive blow by from worn piston rings the sheer volume of combustion gases in the crankcase going into the carb might be enough to upset the mixture at low throttle, but in that case the engine will find a way to vent the gases all over the engine bay if it can't get the gas into the engine.

[Clifford S]

 

Going to turn one up from Tufnol, with a 1/8" hole and see how runs with that. Alternatively, I shall try and find a standard PCV valve from another 1lt. engine. I'm thinking the restrictor was Leyland's cheapo alternative to using a proper PCV valve.

 

Since you are able to turn one up, can I suggest starting with a smaller hole - you can always make it bigger ! I'd suggest starting at around 1/16" or 3/32".

When fitted up, with the engine running, remove the Oil Filler Cap and the engine revs should increase. If they don't, you might then need to increase the size of the hole. Once you settle on a size, check after some running that there's not moisture building up in the crankcase. Look for 'Mayo' under the filler cap.

They did use a PCV System before these (CCV), which IMO, is way better than a PCV set up
 

 

Good tip about the oil filler cap/revs increasing. I'm thinking while I'm at it I could make an assortment then check vacuum pulled and compare. Can also try a PCV valve as I have one from a 1300 xflow.

Edited by Clifford S, 01 September 2023 - 04:00 PM.