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MG MGB Technical - Leaking SU Carb Problem Fixed
|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 carburetor. 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 3.5-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
|Great thread and example of problem solving!|
It's curious that the leak was present with a pump rated at 3.5 PSI, but was somehow solved with the regulator set at 3.5 in the engine bay. My hypothesis is that the 3.5 pump is incorrectly rated, or not well designed to sit where it is in relation to the fuel tank and carburettors on an MGB.
|Easy solution - Use the OE SU fuel pump with the proper pressure rating. Cheers - Dave|
|*If* the pump was indeed putting out exactly 3.5psi, and *if* the regulator did indeed regulate to exactly 3.5psi, and one fixed the leak and the other didn't, then it would be strange I agree. But the chances of that happening are virtually zero. I suggest the pump will have had a higher actual output than the regulator, despite the numbers apparently being the same. The fact that one carb always leaked and the other one never did even though valves, floats and lids were changed and swapped around shows that there was still *some* difference in the rear carb that made it more likely to overflow.|
But at the end of the day if you don't have an SU pump and changing float valves doesn't cure a leak, then fitting a regulator should always be the next step. In fact I'd go so far as to say that unless you *know for sure* i.e. with a pressure gauge that an aftermarket pump doesn't have the same output pressure as an SU you should always fit a regulator. I suggest the Mr Gasket regulator is one to avoid, pressure is different to flow, and in regulating pressure it shouldn't affect maximum flow rate at all. Maybe it was just a faulty one.
I went down the same road several years ago with an aftermarket electronic pump [can't remember the make], after changing floats, needles etc and becoming the fastest HIF SU remover in the west, fitting a Malpassi 'Petrol King' regulator cured the problem.
The strange thing with mine was that the problem didn't start until several years after I fitted the aftermarket pump, but once it did nothing made a permanent cure till I fitted the regulator.
I also get the impression that the car runs smoother with a regulator fitted though that might be my imagination.
|Frank's mystery cure has started me thinking about a comment in a thread on this matter, some months ago when a member suggested that the reason for the flooding could be vibration. Earlier this year I was plagued on a 2000km trip by the rear carby flooding at exacttly 100kph. If I travelled at 96kph or 98kph, no trouble so I also began to think "vibration".|
Like every one before me, I changed the float needles but also fitted an in-line filter in the flex before the rear carby and the problem has not returned which, of course, I put down to the new needles.
Maybe, the fitting of the my filter and Frank and Ron's regulator in the flex line just before the rear carby has resulted in a dampening effect on a vibration period in the float bowl which would cause the fuel to froth and the float to sink.
Similar damping devices [rubber mounted weights]are fitted to high voltage transmission lines to reduce vibrations caused by the wind.
This thread was discussed between 14/11/2008 and 16/11/2008
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