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MG MGB Technical - Fuel leaking from Carb over flow pipes

I’m having trouble with my carburetors that were rebuilt 2 years ago. I just got the car back on the road and noticed there is fuel coming from the overflow pipes when I park it. I’ve taken the tops off the chamber bowls and carefully cleaned everything and replaced the needle & sets. I noticed that Moss sells shims but I do not know whether they were installed on mine originally. Comparing my floats with the manual they do not appear to be adjustable. They are all plastic, just hanging hinged in the chamber. There is no evidence the floats are leaking. My only idea now is to replace the floats and have the shims as a backup if that fails. I did replace my fuel pump with the appropriate moss part but have no way to check if the pressure is correct or not. I realize it is not a good idea to drive the car with this kind of problem but are they leaking while I’m driving the car?
Danny Jacob

If it only leaks when you switch off, that indicates that the normal heat-soak and expansion of the fuel in the float chambers is causing the leak, which implies the float level is too high to begin with. There will be no more pressure with the engine switched off than with the ignition on, and after about one minute there won't be any pressure left in the system anyway. If it were a pump pressure or float valve problem it would overflow with the engine running, or at the very least with the ignition on but the engine stopped. In this latter case the pump shouldn't click more often than once every 30 secs, if it does then either there is a leak from a pipe or carbs which is hazardous, or the non-return input valve in the pump is leaking back, which is less of an issue as long as it isn't bad enough to cause fuel starvation at speed. If *both* carbs are doing it then either incorrect parts have been used or they have been rebuilt incorrectly. If only one then there is something wrong with that carb, if HSs remove the float lids and floats and compare the fuel levels in the chambers. More of a problem to ascertain with HIFs. In either case inspect the floats *very* carefully for fuel inside, it can be difficult to see.
Paul Hunt

The below discusses a problem similar, but different from yours, and a solution. You may get some useful info from my tale of woe.

I thought that I would share with the club the solution that I found for fuel leaking from the rear SU carburetor's overflow vent.

I had an intermittent, but substantial leak, from only the rear SU HS4 carburetor on my 1964 MGB engine. I lost approx. 1-pint of gasoline every 10 miles that, in turn, resulted in a fuel consumption rate of about 15 MPG (US). Additionally, the engine flooded and sputtered when the leak was at its maximum. I was able to monitor the fuel leak by capturing the lost fuel by using a Mayonnaise jar with a hole in the center of the lid through which I inserted a tight fitting 1/4 inch plastic tube the other end of which was connected to the rear carb overflow vent. I also vented the jar with a small 1/16 inch hole near the center hole.

As a point of information, the fuel pump I was using was an aftermarket Airtex with an output pressure of 4.5 PSI. The pump is located under the car and mounted on the frame close to the rear axle.

I tried the following in an effort to solve the problem.

1. Checked to see if the float adjustment level was correct, the needle valves seated and sealed and that the floats had no leaks.
2. Replaced the standard float needle valves with Grose jet assemblies. …No help!
3. Switched the Grose jets from the front to the rear carb. …No change, the rear carb still leaked!
4. Replaced the rear float bowl cover, Grose jet and float with known good units. …Rear carb still leaked!

After trying the above, I decided that the problem must be too much fuel pressure from the Airtex fuel pump because the SU pump puts out only 2.5-PSI. I searched the Internet for the lowest output pump that I could find. This turned out to be a Mr. Gasket 3.5-PSI pump. I thought, for sure, that there would be at least a 1-PSI tolerance in the Grose jet assemblies. I purchased a new pump on E-bay for $25.00 and installed it, in line, with the Airtex pump.Using an "on-off-on" switch I am able to use either pump, with the Airtex being the backup. To my surprise, the leak was still there.

My next attempt was to install a fuel pressure regulator. I found a Mr. Gasket fuel pressure regulator with an adjustment range of 1-PSI to 6-PSI. on Amazon.com for about $30.00. I installed it on the underside of the car between the Mr. Gasket fuel pump and the carbs. To my surprise the pressure regulator reduced the pressure too much even at the 6-PSI setting, and starved the carbs of sufficient fuel to enable the engine to run. Now I was really perplexed. As a last resort, I removed the regulator from the under side of the car and reinstalled it in the engine compartment. Low and behold at a setting of 3.0-PSI the fuel leak is gone and the engine runs great. Problem solved!!!

A suggestion… If you are experiencing high fuel consumption, a smell of gasoline while driving or see an occasional puddle of gasoline on the pavement below the carbs, you may be experiencing the same problem that I had. Try the Mayonnaise jar idea described above to see if you do have a carb leak.

E-mail me or call If you have any questions. 843-838-0822
Frank Grimaldi

I now use a pressure regulator and teflon tipped needle valves from Burlen and that nailed it. The problem only started after the politicians decided to "improve" petrol.
Stan Best

What fuel do you use, Stan? Can't help thinking there was still something wrong with your setup, I've not had any problems with either HIFs on supermarket 95 or HSs on Shell 98 and 99.
Paul Hunt

We will have to agree to differ here Paul. I had no problems from 1969 until 1999-ish. We were en route to an event in the Morelands Brewery in Abingdon and got no further than the A40 at Stokenchurch where the car started missing. After a lot of aggro a new pump
needles and a pressure reugulator seemed to fix it for a couple of year it recurred on the way home from the Uxbridge Car Show last year. I put the teflon tipped needles back in and so far so good. The problems only happened after unleaded was introduced. As the fuel is under control thats how it's staying. I got the car home from Uxbridge by nailing it with the ignition on and then coasting with it off to avoid pumping the tank dry, I'm not going there again (he faffing about, not Uxbridge which is great as you can see from the attached, you are very welcome to join us on our stand there this year)


Stan Best

"nailing it with the ignition on and then coasting with it off"

I've got my fuel pump and OD fused in the engine compartment where the main harness joins the rear and gearbox harnesses. When I did have a problem with th V8 because of a holed float I was able to cross-connect the two fuses and used the OD switch to turn the pump on and off as required :o)
Paul Hunt

This thread was discussed between 20/03/2009 and 30/03/2009

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