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MG MGB Technical - Temporary Fuel Starvation Problem

I have just about given up on this problem. Any help from someone, smarter that me, will be greatly appreciated.

The symptom is that the rear (only) carb on my 3-main MGB engine occasionally causes a temporary fuel starvation problem. If I tap the top of the fuel bowl with a screwdriver or some other object, the problem disappears and the engine runs great, but the problem reoccurs a week or more later. The problem usually occurs just after start up and after the car has been sitting for a week or so. Once, however, it occurred during an engine start after the car was well warmed up.

One would think that the obvious cause is a sticking float valve. I have replaced the original Grose jet valve that came with the engine with a new one. I have also replaced the original float with the newer style all-plastic non-adjustable version. I have also installed two in-line fuel filters, one downstream of the fuel pump and one upstream. I have also added a small bullet type fuel filter within a inch or two of the fuel inlet to each carb fuel bowl. I have removed the fuel bowl cover immediately after the problem occurred to see if the jet or the float was stuck and found that they moved freely. I have also checked to see if the needle in the piston/chamber assembly was properly centered by raising and releasing the piston. I get a nice loud click when released. After all of this, the problem still exists.

Should I replace the Grose jets with the original type float valve? Could the problem be somewhere else?
Frank Grimaldi

Frank - If the Gross jets were purchased in the last 10 years or so, they are probably the trouble. Gross jets used to be really good, but they were taken over by a new company and are crap now, very prone to stick closed. While you are at it, get rid of the filter between the tank and the fuel pump - if it clogs and power is left on during troubleshooting the fuel pump will sustain damage that is unseen, but it will cause rapid burning of the points in the pump. See the article, SU Fuel Pumps Facts and Myths in the SU Fuel Pump Articles section of my web site at: http://homepages.donobi.net/sufuelpumps/ Cheers - Dave
David DuBois

Frank
I would do as Dave suggests. Also if I understand your post, I wonder if the small fuel filter before the float bowl on the rear carb may have something to do with this?? I have not seen one in that position before.
Mike
MK Mike

Thanks Dave and MK,

I took your advice and changed the float valves from the Grose jet to the original design type. Things seem to be working okay, but it is to early to tell if the problem is solved.

As a point if information, I added the bullet type fuel filters as extra insurance agains debris clogging the float valve jets. I was having a problem that, I think, was caused by rubber flaking on the inside of the fuel lines. I did change the fuel lines, but as I said, added the bullet filters as extra insurance. That problem seems to have gone away.
Frank Grimaldi

As I stated in a previous April 9th return comment:

"I took your advice and changed the float valves from the Grose jet to the original design type. Things seem to be working okay, but it is to early to tell if the problem is solved."

Well the problem did return. This time, however, it occurred while driving at about 55 MPH rather than just after startup, as in the past. I pulled over, tapped on the rear carburator bowl and the fuel starvation problem disappered. Any ideas???

Frank Grimaldi
Frank Grimaldi

Frank - Take a critical look at the pivot pin and pivot opening on the float. It is possible that the float is getting cocked at an odd angle and not allowing the needle valve to open properly. Cheers - Dave
David DuBois

I have had a float valve stick in the valve body. I had to run a fine file down along each of the ribs to give it a bit more clearance.. since then no further problem have popped up.
Sandy
conrad sanders

This thread was discussed between 30/03/2009 and 10/04/2009

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