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MG MGB Technical - 74 Has Issues need help!

Ok here the deal: 74 B, rebuilt head, new coil, condenser, rotor, cap, fuel pump, filters. The car runs a bit rough at idle, with sporadic misfiring but will even out when I rev. When I put load on the engine it will run great for a few yards then the engine just wants to spudder and die out. Sometimes it will keep running at load if I take my foot off the gas and slowly aply fuel....and sometimes it just dies.

I've found the fuel pump ground hanging in the breeze and reconected it. Fuel has good flow and pressure but its still doing the same thing. All of the guages are showing good, no loosing the tach or anything. This is my first B and this is driving me NUTS. Plugs show Im running a bit rich and temp warms up to about 3/4 on the guage and stays steady....is this normal?
Also have taken the carbs off about 5 times now, I am 100 percent sure that the floats and needles are working properly. Have noticed redish crud in the bottom of the float chamber on both the HIF4 carbs. There is a fuel filter before the fuel pump and Im going to put another up front before my fuel regulator (have fiddled with it as well to no avail)

Can anyone help? I have a feeling that the anti-run on valve might be malfunctioning but wouldnt that affect the engine with or without load?

-THANKS!
Murphy

Murphy. Your plug reading technique is invalid. You are showing sooty plugs at idle, showing that either the choke was used to start the car or the idle mixture is rich. However, a proper "plug reading" requires that you be able to take the car on a high speed test drive to determine whether the needle is correct for cruising operation. Idle mixture is an easy adjustment, hence we do not read plugs there. Cruise mixture is determined by needle profile and we do read the plugs to determine if the needle is correct for our application.

Normally, I would suspect ignition as a primary source of your problem. A low speed miss, followed by increasingly poor performance as the engine is accelerated is, most often, an ignition problem. There are instructions on testing the ignition system on my website, www.custompistols.com/ then click on the MG section and articles. Check out the ignition system first because, if it is not working, the carb system cannot be properly tuned.

You case, however, may be the exception to the rule. I say this because of the "reddish crud" in the float bowls. This is a fairly classic indication that, at some point, the car was stored for a long period with fuel in the tank. The fuel evaporated leaving a shellac which is disolved by fresh fuel and carried throughout the fuel system. It builds up on floats, needle valves, inside jets and on the needles. If this is the problem, the entire fuel system needs to be taken apart and cleaned, including a hot tanking of the fuel tank. No fuel filter will capture this shellac and prevent it from being carried to the carbs. Have had this problem with cars I have purchased and now do a through cleaning of the system before running if the car has been purchased out of storage.

Do not know what fuel pump you have. The factory supplied SU fuel pump does not need a regulator in the circuit and one should not be used with the SU pump. David DuBois has an article on the SU pump on my website which is worth reading. It includes the correct pressure and volume requirements in US units of measure.

This may get you started and, after the tests are completed, allow us to focus on the causes of the problems which are, now, a rather large number of possiblities. Les
Les Bengtson

If you suspect the anti-runon valve then disconnect it temporarily and see. Better still remove the overflow pipes from the carb float chambers, this will disconnect the anti-runon valve and also allow you to see if the carbs are overflowing. If it misfires at idle then hook up a timing light to the coil lead and each plug lead and watch the flashes. Any irregularity that coincides with a misfire indicates an HT problem (an LT problem would almost certainly affect the tach and you say this isn't happening), if this occurs on the plug leads but not the coil lead it is probably the cap and/or rotor (replacement noted). It could be an iffy plug, but if you use the 12v timing light with inductive pickup that shows breaking down plugs too. If the flashes are steady it must be fuel. Also check the dwell and actual timing is steady when the misses occur. As it will run for a few yards after applying load fuel supply has to be the first suspect, the pump should deliver at least a pint in 30 secs.
Paul Hunt

This thread was discussed between 17/08/2003 and 18/08/2003

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