Petrol overflow

[MKTR]

Hi all,

My HS6 carbs have developed an annoying ad-hoc problem and I thought I would ask for some guidance.

Every now and then the rear carb float chamber overflows with petrol from the overflow valve when - mainly when the engine is switched off but sometimes when running.  It suggests to me that the float valve is not closing correctly, but why not overflow all the time.

I have checked the float integrity, looked for chamber debris, added washers under the valve seat to activate it earlier, and changed needle valves.  What next?

Cheers Mark

Edited November 30, 2023 by MKTR
missing words

[Lebro]

May some dirt (rubber flakes etc) occasionally holding the needles valve off it's seat. ?

Bob

[Michel Higuet]

17 hours ago, MKTR said:

Hi all,

My HS6 carbs have developed an annoying ad-hoc problem and I thought I would ask for some guidance.

Every now and then the rear carb float chamber overflows with petrol from the overflow valve when - mainly when the engine is switched off but sometimes when running.  It suggests to me that the float valve is not closing correctly, but why not overflow all the time.

I have checked the float integrity, looked for chamber debris, added washers under the valve seat to activate it earlier, and changed needle valves.  What next?

Cheers Mark

On both carbs? I had a problem but only on one carb the reason was the little hole in the lid was closed with dirt, so when the float goes up but not enough because the air is compressed, can't escape and the valve don't shut.

Otherwise very high fuel pressure? 

[RobG]

I had this for a while until I changed to the newer black float type and this solved it.

Possible old type and new ethanol petrol not a good combination.

[Z320]

5 hours ago, Michel Higuet said:

On both carbs? I had a problem but only on one carb the reason was the little hole in the lid was closed with dirt, so when the float goes up but not enough because the air is compressed, can't escape and the valve don't shut.

Otherwise very high fuel pressure? 

Sorry, this explanation doesn’t match.

As RobG states I would check for a leak / sunken floater.

Ciao, Marco 

[MKTR]

Many thanks all - I will double check the float and needle valve, and take a look for a vent hole.  I also see on other forums that people do not think the brass tipped valves are always 100% at sealing off.

Mark

[Michel Higuet]

8 minutes ago, Z320 said:

Sorry, this explanation doesn’t match.

As RobG states I would check for a leak / sunken floater.

Ciao, Marco 

He can check what he want but in my case the vent of the lid of the back carb was blocked, Marco if you like to have the vents of your carbs blocked be my guess... How a tiny hole like that can be blocked? maybe rubbing the carbs with paste to make them shining, I don't know I am not Sherlock Holmes... :-)

[Z320]

Hi Michel,

Mark states petrol comes out of the „overflow valve“.

There is no such valve on the HS6 and very likely he means the ventilation hole in the lid,

connecting the float chamber with the atmosphere.

How can petrol come out the „overlow“ if air is locked in there and the (same) hole locked?

Something doesn’t match here?

Ciao, Marco 

Edited December 1, 2023 by Z320

[BlueTR3A-5EKT]

Trust me on this one as someone who did it.
  Trying to stop float chamber flooding on a car with twin H4 SU carbs

I blocked the vent pipes by joining them with a length of screen washer tubing.
  Blocking  the float chamber vent pipes caused the engine to run very badly, not revving at all.  

Clearing the blocked vent pipes by removing the tube joining the two float chamber vents solved the bad running instantly.  
 

Renewing the needle valve and seat solved the float chamber flooding.   The floats were in fine floating order.
 

So, without the vents the float chambers struggle to fill and will definitely not overflow.  

[RobH]

It may also be the type of needle fitted. The viton- tipped ones are good but will not seal properly if the viton gets worn or damaged.   I had a problem with Grose jets which didn't seal properly at all after a while. 

[Z320]

Dear Mark,

please be so kind and show us on a photo what you mean with „overflow valve“ on your HS6 carbs, where petrol flows out.

Ciao, Marco

[Michel Higuet]

15 hours ago, Z320 said:

Hi Michel,

Mark states petrol comes out of the „overflow valve“.

There is no such valve on the HS6 and very likely he means the ventilation hole in the lid,

connecting the float chamber with the atmosphere.

How can petrol come out the „overlow“ if air is locked in there and the (same) hole locked?

Something doesn’t match here?

Ciao, Marco 

OK now I understand, also for me there is was no "overflow valve" on HS6, I was puzzled. Though he wanted to talk about the float valve. If the gasoline comes out through the ventilation channel (at times) it certainly also comes out through the jet, except very high fuel pressure the cause should be in the float valve(s), it is always interesting to know if the problem is only one or both carburettors.  

[Lebro]

18 hours ago, MKTR said:

Many thanks all - I will double check the float and needle valve, and take a look for a vent hole.  I also see on other forums that people do not think the brass tipped valves are always 100% at sealing off.

Mark

I would always used the viton tipped version.

Bob

[Z320]

1 hour ago, Michel Higuet said:

OK now I understand, also for me there is was no "overflow valve" on HS6, I was puzzled. Though he wanted to talk about the float valve. If the gasoline comes out through the ventilation channel (at times) it certainly also comes out through the jet, except very high fuel pressure the cause should be in the float valve(s), it is always interesting to know if the problem is only one or both carburettors.  

100% agree

[Z320]

Hi all,

allow me to tell my view,

because I notice (not now) for some mates it it not clear what the float chamber is for.

It is a pressure reducer to get the fuel pressure down to atmosphere, and for this it has to be vented to the atmosphere.

What some guys indicate as a "overflow" is the ventilation hole or ventilation channel (on my BMW Delortos).

Also fuel vapours from hot fuel all vent all out there.

Is it blocked, no fuel / vapours and no fuel comes out there.

And with the ventilation hole it blocked the carbs are pressurised by the fuel pump and can't work.

Ciao, Marco 

 

 

[BlueTR3A-5EKT]

19 minutes ago, Z320 said:

Hi all,

allow me to tell my view,

because I notice (not now) for some mates it it not clear what the float chamber is for.

It is a pressure reducer to get the fuel pressure down to atmosphere, and for this it has to be vented to the atmosphere.

What some guys indicate as a "overflow" is the ventilation hole or ventilation channel (on my BMW Delortos).

Also fuel vapours from hot fuel all vent all out there.

Is it blocked, no fuel / vapours and no fuel comes out there.

And with the ventilation hole it blocked the carbs are pressurised by the fuel pump and can't work.

Ciao, Marco 

 

 

+1

My experience proved this.

 

 

[MKTR]

Thanks everyone,

I am surprised by the amount of debate my layman's use of the term 'overflow' has generated.  Of course you are right it is technically the 'vent hole' which is behind the baffle plate, but that is where petrol comes out when the valve does not shut off correctly - in my experience.  Please excuse my poor terminology.

I took advantage of the wet weekend to check things and to also change the needle valve for a different rubber tipped one.  So far so good but a will wait for a few runs before I cheer too much.  I am not sure how old the needle valve is and if it is ethanol resistant viton, so will purchase new ones, along with resistant floats, to be ready for next time.

Maybe it just goes to show that remedies are often simple and changing the same part a few times can work out.

Mark

Edited December 4, 2023 by MKTR
spelling

[Z320]

I knew from your first post what you was writing about....