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MG MGB Technical - idling rough after fuel drain

I had to drain some fuel before having my car shipped here for my move. I simply removed the fuel line before the carbs and turned the key to start pumping fuel into a tank. The first time driving after that the car worked fine (only 2 blocks). After that the car runs real rough at idle and dies out if I dont put my foot on the gas. Under normal driving everything is fine. Its only when i have to stop do things get tricky.

What the heck happened? All I did was disconnect the fuel line, pump out gas, then reconnected it. Please help.

Ronald
Ronald

Ronald. More information would be of use to us. Year, distributor and carb type would help.

Do not assume that the last thing you did, drain some fuel out, is the root cause of your current problem. If the points happened to be closed when you turned on the ignition switch to pump the fuel out, you could have damaged the points/points replacement system. You need to disconnect the wire going from the coil to the distributor if the ignition system is to be left in the on condition for any length of time. Burned wires and/or points can be a problem if that is not done.

Ignition system troubleshooting and fuel system troubleshooting are covered in tech articles on my website, www.custompistols.com/ and might be worth looking at.

Les
Les Bengtson

Okay thanks,
The car is a 1974.5 with HIF SUs. It has the Crane Cams electronic ignition system. Would the ignition system be the problem if it runs fine during driving?

Ronald
Ronald

You may have disturbed some dirt in the fuel system disconnecting and reconneting the fuel line, also the faster flow while emptying the tank may have caused the same. A little dirt in the float needle and seat may allow the carbs(s) to flood slightly. If it is minor it will effect your idle but when at road speed you are consuming more fuel and it won't show up.
John H

John,
What you proposed is very likely. I'll try cleaning them out and post the results.

Ronald
Ronald

Ronald. Yes, it is possible that the fuel system might be at fault. But, if you have a working fuel filter, I would not have expected anything to be able to get into the fuel bowls of the carbs.

However, what altitude did you come from? What altitude are you now living at? This will have some effect on the mixture setting at idle and will have a significant effect on the needles required for proper operation.

Add in the fact that there may have been some change in average temperature and the fact that HIF carbs have a built in compensation mechanism and your problem could be fuel system related.

It can also be ignition related. When you used the ignition switch to power the fuel pump, you also powered up the ignition circuit. If the position of the chopper window/magnetic pick up was such that current was flowing through the system, you could have damaged either the wires, going all the way back past the ignition switch, or your electronic points replacement system. If an inspection and tuning of the carbs do not correct the problem, remember that most "carb faults" are ignition system related. Also remember that a low speed miss is hidden at higher engine speeds.

Les
Les Bengtson

Thanks Les,
I moved from Golden Colorado so the altitude is the same. Also this problem started in Golden so the elevation is not the culprit.

I am hoping that its not ignition related. I looked through your troubleshooting on your web page. It was helpful but I don't know where to begin. Do you have any suggestions for how to determine if the ignition system is at fault?

Thanks

Ronald
Ronald

Okay, I replaced the hose from the fuel pressure regulator to the carburator, then removed the airfilters and cleaned the needles just by lifting up the piston and spraying in a load of Gumout Carb cleaner. It took a little while to start but once started it ran great! So for now it seems that the problem is solved, hopefully it starts next time. Thanks for the advice.

Once running I called the local mg specialists to ask if they had time to inspect my ignition system. First they said that they are booked for a week. Second they said that the ignition system shouldn't have been damaged by what I have done. I appreciate the advice though, Les.
Thanks

Ronald
Ronald

Ronald, glad it helped but the needle and seat I was talking about is the float bowl inlet valve. If you look at Moss's site it is item 31 under the HIF carb section.
John H

John,
I knew what you meant. I was hoping that this simple cleaning would help. Wanted to try something very easy before performing surgery and retuning. I got lucky this time.

Ronald
Ronald

This thread was discussed between 20/09/2007 and 21/09/2007

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