American MGB Association Message Board

Technical Area => Upkeep and Performance Hints => Topic started by: september2017 on August 15, 2017, 09:42:21 PM

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Title: Rough Running
Post by: september2017 on August 15, 2017, 09:42:21 PM
Q:   I have a 1974 mint MGB but I am having trouble.  It started running roughly so I took it to a service station, two in fact,  Replaced plugs, wires, cap, rotor, condenser, points, vacuum advance and I replaced the coil.  It did run well on a 60 mile trip and then started running roughly again.  I inspected the carburetor parts on both carbs cleaned and reassembled .  I checked the flow from the fuel pump and It seemed alright, but the fuel filter doesn't seem full when running. 
I replaced the fuel pump about 30 years ago.  Any assistance would be appreciated.

Ron Mansi
North Providence, Rhode Island
Title: Re: Rough Running
Post by: Art on August 15, 2017, 10:09:22 PM
A:   Sounds like you've done the basics, but let's first talk about the fuel pump.  They do have a finite life.  Mostly it's about 15 years, even for the electronic ones.  So, after 30 years, that's a possible culprit.

I'd also strongly suggest a change from points to the Pertronix ignition kit.  Goes in without alteration other than a wire change for the primary lead and can overcome a lot of wear in the distributor to even-out electrical distribution and eliminate setting the point gap.  Maybe not necessary on your car, but it just eliminates another headache. 

Also, in checking the carbs did you check/replace the floats and their valves?  Leaking floats or stuck needle valves will tend to allow more fuel in and flood one or both carbs.  They also can keep fuel flowing, often to the extreme of having overflow from the charcoal canister, as well as leaving the level low in the fuel filter (which does not always fill completely, but should have a level higher than the outlet tube to the carbs).  I switched my '73 to the Gross Jet ball type valve, replacing the needle type years ago and have not had any issues.

On the '74, there's another thing to consider. The stock HIF4 carb has a thermal jet adjuster, called a 'temperature compensator'.  It's mounted internally in the body of the carb in the float chamber.  These work to adjust the jet height and mixture as the engine get warmer/colder.  They are bi-metal construction and do wear and fail, which makes adjusting and keeping the mixture adjusted very challenging.  They are still available from Moss or Victoria British; about $35 each.

Hope that can help you. Let me know what you find.

Safety Fast!
Art Isaacs
Title: Re: Rough Running
Post by: september2017 on August 15, 2017, 10:13:10 PM
Q:   This is a follow up on the rough running 1974 mgb .  I havenít replaced the fuel pump as yet for it still seems to be pumping.  The coil worked and it did start but it still runs rough and uneven and there is a smell which indicates to me a bad fuel mixture.  These are the original SU carbs that came with the car.  I have cleaned them and adjusted the mixture screws.  I do have another set of carbs in good condition and am tempted to replace the originals with these.  I think that the car sitting in the garage all winter with gas with ethanol in it might have caused some trouble with the float etc.  Before I change these carbs I would appreciate your advice.
Ron Mansi
Title: Re: Rough Running
Post by: Art on August 15, 2017, 10:16:20 PM
A:   From what you say and that these are the original carbs says both a lot and raises questions.  By halfway through the 1975 model year, the HIF4 carbs as in your car were history, so unless you can find rebuilt used ones, they are all over 40 years in service
Your reply doesn't indicate that you looked at some of the issues mentioned, so you may be missing something.  That the coil works, for example, has little to do with wear in the distributor affecting the idle.  The suggestion of moving to a Pertronix kit was because the kit covers a lot of the ills that come with distributor wear. Since it uses a sensor that does not rely on a mechanical gapping of the points, which could vary in a worn distributor, it evens out the spark giving you a consistent, stronger spark, better idle and less chance of missing during acceleration or at speed.  A weak spark will not burn the fuel completely, giving a gas smell from the exhaust.

In the 43 years they have been in service, have your carbs ever been completely and professionally rebuilt?  Even if routine DYI cleaning and maintenance were done, after all these years,  there's the probability of wear in the throttle shafts and bushings in the carburetor casing, that the end seals have long since deteriorated allowing air to enter around the shafts and that the carburetor jet and needle have worn, so the throttle butterfly can be out of place and the mixture cannot be properly adjusted and more. New bushings, seals and stock or oversized throttle shafts are available, but require line-bore tool, which is costly and not something used routinely, so paying a shop to do this is the best solution.

Even the oil-filled domes at the top wear and the springs weaken after all this time.  BTW, I found that going to 20W50 or even gear oil in worn dampers helps.  Not sure what you're using but replacing the SU carb oil with this has been helpful.
Even without wear, if the floats have not been checked, changed or their tabs adjusted.  Leaving the carbs full of fuel, even under daily use, eventually the tab on the top that actuates the float chamber valve will bend and need to be raised or the float and fuel level would be too high in the chamber and allow too much fuel into the carb, giving you both the rough idle and fuel smell. Gunk and wear on the float chamber valves also prevent them from moving, opening or seating properly, allowing too much fuel into the carb. And for the thermal compensators, there's no way to tell by looking at them if they are performing.

I don't know the source or actual condition of your spare carbs, but I'd do a thorough check of them before considering installing them. I would also suggest you send them out to be completely rebuilt before installing them. Maybe live with the rough idle a bit longer and let someone like Joe Curto (1-800-726-7878) or Apple Hydraulics (1-800-882-7753) rebuild them.

Also, when was the last time you adjusted the valves?  These engines use mechanical lifters, so there is normal wear and the valve gaps have t be adjusted periodically.  Too closed, there's not enough fuel getting into the cylinder or exhaust getting out and the engine runs rough.

Hope that helps.  Good luck and keep me posted.

Safety Fast!
Art Isaacs

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