Moss Motors, Ltd

I have a 72 MGB with new SUs and a rebuilt engine.  I had the engine tuned by agarage specializing in english cars.  Fuel filter is clean(new).  I installed a new gas tank and have verified the gas is getting to the engine cleanly.  Fuel pump is also new and seems to be delivering the gas as it should.  My problem is when the engine is warmed up (choke is off) and  running in the low RPM range (2000 - 2500).  I press the accelerator slightly and the engine sputters and eventually kicks in.  If I accelerate faster, engines responds as it should.  Any ideas?

 Thanks

I am not an ignition person by any means but I really think your problem is ignition related. Have someone good check the whole enchilada on the passenger side of the engine. Better voices will chime in I am sure.

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Alfred, Maine
63B Waiting for resto
63B Donor
67B Ours
73BGT Mine
73BGT Hers

cfrench:

I am not an ignition person by any means but I really think your problem is ignition related. Have someone good check the whole enchilada on the passenger side of the engine. Better voices will chime in I am sure.

I'd agree.  I was just having the exact same problem with my Midget.  Kept messing with carbs, jets, all that.  Finally bought a $10 set of points and put it in, instant zip.

MG Noir:

cfrench:

I am not an ignition person by any means but I really think your problem is ignition related. Have someone good check the whole enchilada on the passenger side of the engine. Better voices will chime in I am sure.

I'd agree.  I was just having the exact same problem with my Midget.  Kept messing with carbs, jets, all that.  Finally bought a $10 set of points and put it in, instant zip.

 

I don't have experience with british cars, just aquired my first MGB.  I have, however, had experience with points ignition systems.

 

Check the play in your distributor, make sure that there's not too much in the shaft. I don't have a measurement for you, perhaps check it against a known good distributor.  As the above poster mentioned, be sure your points are good, the're cheap so you might just replace them, sometimes you just get "lemon" points.

An oft-overlooked problem is the points wire from the coil.  Cheap connections or dirt can cause weird problems like this.  I'm not sure if MG's are supposed to have a resistor wire, mine has a homemade one the DPO made in the garage.  It's serviceable for now so I haven't replaced it. 

 

If the distributor is original and wasn't replaced with a rebuilt one during the engine freshening, it could be that you need a new one.  I assume the MG distributors are expensive.

 

Good luck. 

I definitely agree that it's in the distributor. I got tired of mucking about with mine and went with a Mallory dual point. Cured it's ills, which included tach bounce.

It's a truism that 95% of fuel problems turn out to be ignition. However, one thing in the original post stands out. That is that the problem only seems to occur on mild accelleration and not under heavy load, if I understand correctly. Accellerative "load" as well as combustion temperature and a raft of other factors play into the resistance across the spark plug gap which must be overcome to produce the mixture igniting spark. Usually, the reason for a miss under accellerative load will be traced to the secondary ignition system. That consists of the coil high tension windings, the coil nose, the ignition wires, the distributor cap, rotor, and the plug bodies themselves. If any of thhese has a defect which provides a lower resistance path to ground than the plug gap, that is where the lightning will go. Voila, a missfire. Here's where it get's sticky. The greater the load the more likely the missfire. But, in this case the missfire only seems to occur under light load. Look for a vacuum leak. First, of course, optimize your ignition system. Nothing you'll do will work if there's a fault there. always make sure them sparks is goin' where and when they should. After that, look for a gasket face that is allowing air into the manifold or inlet port(s). Often, I find that when there is a very small vacuum leak it will affect mild accelleration but wide open throttle is not affected so much since the small leak is such a tiny percentage of the overall air/fuel picture. This type of problem is actually quite difficult to diagnose without being on the scene since descriptions of symptoms and other contributing factors cannot be observed. I hope I've at least put you on the right track. Anyone else?

Motorbill

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Motorbill
From Lola to Land Rover, If it's British and has wheels, it's likely I've bloodied me knuckles thereupon

My MIDGET (who needs it's own section in the forums) was running very rough at lower RPMs, wouldn't start, etc.  When I pulled out the points it was pretty clear it was a problem - warped metal and a bit of wear where it had rubbed on the shaft.  Replaced them and all was fine.  I've read at least one reference saying change out your points every 6000 miles.  Reasonable? 

I was pretty suprised that it was warped as it were.  It was just a "stock" one from NAPA.  Are there any that are higher quality?  Maybe something that can handle the RPMs a bit better? 

99% of SU problems are ignition. Yup.

But is there any slack in the throttle linkage?  (compare one carb to the other). What juice do you have in the dashpot? Try going a little lighter (if you have 20-50, try ATF).

 

(Hi, Car!<G>) 

 

 

 

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Old Fart
71 GT project
73 Roadster runner