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MG MGB Technical - SU HIF4 problem

My 1974 B sat the winter in my garage with a little less than a half tank of fuel while the engine was being rebuilt. After I got the engine in recently, it started and ran quite well. Today, I went out and started it and it ran fine the first time, but would not start when I tried a little later. I started checking for spark, and at first it did not appear that I had any. While I was troubleshooting the ignition, my astute wife noticed a large pool of gasoline on the garage floor. It turned out I did have spark, and it appears that the pool of gas came from the rear carb. I don't know much about SU carbs, but assume they have some sort of float mechanism. Is it likely that I goit some dirt in there? The situation now is that the car will start after it sets for a while, runs good on the road, but if I shut it off it will not start back up without waiting for about 15 miniutes or so. Any ideas?

Thanks, Doug
Douglas Keene

Doug. Trash in the float valve could cause the problem. You can try disabling the fuel pump
and run the engine until it runs out of fuel. Enable the fuel pump and the extra volume of fuel
filling the carb fuel bowls may flush it out. One way to disable the fuel pump is to unplug it's
power source found in the bundle of wires and connectors in the rear of the right fender well.
There is a bullit connector with 3 white wires plugged into it, I think it has three, my 74 does.
You'll have to pull one wire at a time until the fuel pump stops. Once you get the pump stopped
start the engine and run the carbs dry. Hook the pump up and see if it still leaks. If it still leaks
the carbs should be removed and cleaned. The fuel bowl is on the bottom on SU HiF carbs so
it's almost impossible to open the float chamber covers without removing them.

Hope you have a Haynes or Bentley repair manual. Basic steps to remove carbs are.
1. Remove the air cleaners.
2. Unhook the choke cable from the plate near the rear carb and pull the cable out from the
bottom of the choke linkage.
3. Loosen the gas cable pinch bolt and pull the gas cable out from the top.
4. Remove the springs connecting the gas and choke linkage to the heat shield .
5. Remove four nuts and washers attaching the rear of the carbs to the manifold studs and pull
the carbs off. Make notes on the linkage positions as they will need to come off to work on
the carbs.
6. To open the float bowls there are four screws in located at the bottom ogf the carbs. Remove
the covers and you'll see the floats. The photos and drawings should help you figure out how
to remove and clean the floats and needle valves. Hope this helps, Clifton

.
Clifton Gordon

Doug. I left out a step in card removal. Remove the fuel line and any other hoses connected to the
carbs. Good luck, Clifton.
Clifton Gordon

Clifton,

Thank you for your input. I probably won't be able to do much until the weekend. I did disable the fuel pump and was able to run all of the gas out of the float bowls, so am ready for the next step.
Thanks again, Doug
Douglas Keene

Douglas-
Clifton has covered the procedure for clearing the overflow valves quite well. Your problem with the car refusing to start after sitting for a while is probably the result of heat from the exhaust manifold causing the thermosensitive adjuster of the HIF4s to adjust the mixture too lean. Check the insulation on your heat shield.
Steve S.

Steve,
Thanks for your input. Actually the comment was that the car would start after setting for a while. I did install a brand new heat shield when I put the engine back in. Thanks anyway.
Doug
Doug Keene

This thread was discussed on 07/05/2002

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