BMC 4/25 carb issue

A place for general BMC questions and chat
CNR
Posts: 20
Joined: Fri Oct 05, 2018 11:34 pm
United States of America

BMC 4/25 carb issue

Post by CNR » Sat Nov 10, 2018 10:55 pm

Image

I have a carb issue. The engine is a gas engine. It runs great for about 20 seconds then fuel comes pouring out of the brass tube at the top of the carb opening. It floats the engine out as soon as the fuel comes pouring out of it. Is fuel suppose to come out of this tube or is it a vacuum tube? Do I have a passageway in the carburetor clogged ?
Thank you in advance,
CNR



Nowtnew
Posts: 116
Joined: Sun Apr 01, 2018 8:49 am
Great Britain

Re: BMC 4/25 carb issue

Post by Nowtnew » Sat Nov 10, 2018 11:32 pm

I suspect your carburettor float valve is not working correctly. This could be caused by a fault with the valve itself or maybe a punctured float. If the latter you will be able to hear the fuel in the float if you shake it.

Regards

CNR
Posts: 20
Joined: Fri Oct 05, 2018 11:34 pm
United States of America

Re: BMC 4/25 carb issue

Post by CNR » Sat Nov 10, 2018 11:56 pm

The float should be good. I put it in a bowl of water for about an hour and when I shook it I didnt hear any liquid. I will check the valve next. Is fuel supposed to come out of that top tube?

Nowtnew
Posts: 116
Joined: Sun Apr 01, 2018 8:49 am
Great Britain

Re: BMC 4/25 carb issue

Post by Nowtnew » Sun Nov 11, 2018 9:10 am

As far as I am aware, the brass tube is connected to the top of the float chamber and is there to balance the pressure between the two zones. Under normal circumstances no fuel should come out of it.

Another possible reason for carb. flooding could be the lift pump has been replaced with one for a diesel engine. These differ in that they have a higher outlet pressure which defeats the carb. Needle valve.
Though not uncommon in the USA, the petrol(gas) engined tractor is very rare elsewhere.

If Ley154 sees this thread he may give you some more guidance. His knowledge of the lightweight range is unmatched.

Regards

Ley154
Posts: 70
Joined: Sun Apr 01, 2018 8:22 am
Great Britain

Re: BMC 4/25 carb issue

Post by Ley154 » Sun Nov 11, 2018 5:46 pm

Hi notnew is right in all aspects. The tube is a vent pipe and should not have any fuel coming out of it. It looks like the float valve is staying open. Unscrew the valve and look for any particles holding the needle open. Also make sure the screen in the roof of the lift pump and the glass bowl are clear of debris. The actual float level is set by the thickness of the aluminium washer between the brass part of the float valve and the roof of the carburettor casting. If someone has fitted the diesel lift pump you can get a static pressure of 9 lbs .sqr inch. The proper petrol one delivers around 4. I have used the diesel one in the past and fitted an inline pressure regulator set to 3.5 psi which works well. Regards John Poulter ( Ley154)

CNR
Posts: 20
Joined: Fri Oct 05, 2018 11:34 pm
United States of America

Re: BMC 4/25 carb issue

Post by CNR » Sun Nov 11, 2018 6:40 pm

I can blow through the fuel inlet fitting and air flows good. I can press the needle up (the one the float pushes up) and the air flow will stop. So, I know the needle and seat are good.
I don't know the difference between a gas and diesel lift pump. There is a new one on there.
Image

CNR
Posts: 20
Joined: Fri Oct 05, 2018 11:34 pm
United States of America

Re: BMC 4/25 carb issue

Post by CNR » Sun Nov 11, 2018 7:31 pm

Ok here Is what I did. I have a small plastic cutoff valve just before the carb. I shut it half way off and the engine ran great for about 10 mins at idle. I revved it up some and it died after about 5 seconds. Not enough fuel. I crank it back up it ran great again at idle. I opened the valve a little more so I could rev it up and it floods at idle. There isn't a happy medium. I will try a inline fuel pressure regulator like ley154 said. Will a 3 to 5 psi one work ?
Thank you,
CNR
Image

CNR
Posts: 20
Joined: Fri Oct 05, 2018 11:34 pm
United States of America

Re: BMC 4/25 carb issue

Post by CNR » Sun Nov 11, 2018 8:07 pm

Just spoke with the prior owner. He said he bought the new lift pump. It said for gas or diesel. I think that is the issue.

Ley154
Posts: 70
Joined: Sun Apr 01, 2018 8:22 am
Great Britain

Re: BMC 4/25 carb issue

Post by Ley154 » Mon Nov 12, 2018 6:11 am

Hi, I have included a photo of my 4/25 gasoline prototype which shows the fuel pressure regulator I added set to 3.5 psi. It's the small chrome unit sat on the top of the rocker cover. Regards John.
Image

CNR
Posts: 20
Joined: Fri Oct 05, 2018 11:34 pm
United States of America

Re: BMC 4/25 carb issue

Post by CNR » Wed Nov 14, 2018 7:59 am

I bought a Mr. Gasket fuel pressure regulator for a local parts store. I installed it and played with the settings. 2.5 works good at idle. No flooding and revved up without running out of fuel. That was the fuel issue.
Thank you for the insight Ley154.
Thank you,
CNR

Post Reply