MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - miss at 2500 rpm

I need help chasing a persistant miss at around 2500 rpm when holding a steady throttle. The engine is an 18G in original form except for pertronix ignition instead of points. I have adjusted the valves, checked that the vacuum advance is working, adjusted the carb mixture, made sure the needles (new) are centered, and balanced the airflow. The spark plugs were regapped and then checked after running for awhile - they were all a consistent tan color. I checked the timing at 900 rpm, 1600 rpm and 2200 rpm to make sure the distributer is advancing correctly. The car idles fine, accelerates fine and pulls smoothly through the gears. I had a slight hesitation on applying the throttle but a heavier weight oil in the dampers solved that. I may be chasing perfection but the miss is annoying when trying to cruise at 55-60 mph on a level road. Anyone have any other ideas??
don scott

Float level ??
Alyn

Hi.

Does the tacho twitch significantly when the engine misses, suggesting an intermittant ignition supply fault ?.

Could there be a little water in the fuel tank, which makes it's way through the fuel system in odd drops ?.

Don
Don

A vibration is upsetting something electrical. Some race cars have this problem. Check your dizzy. Check your belts and other parts for tightness. Sounds strange but true.

If it's a miss it's generally electrical. Gas problems usually involve backfiring which you didn't mention although I know an MGA owner who had that problem from a worn dizzy.

Mike MaGee

Don,

I recently had a problem very similiar to the one you've decribed but with my TF-1500. It would idle fine, goes up thru the gears just fine but at about 3300 RPMs it would begin to miss out but would smooth out again around 3800 RPMs. My mechanic and I decided to pull the distributor and send it out to be checked for straightness etc. It was straight but the bearings were worn. We rebuilt it ($80.00), fitted a Pretronix unit and everything is as should be now. Ya might want to check the inside of the distributor cap. One of the contact points for the plug wires was nearly chewed in half by the rotor on mine indicating an out-of-round condition resulting from the worn bearing. I'm only guessing here but I could certainly see how an out-of-round condition would cause such a problem as harmonics would have played a roll here causing the miss in that RPM range. Pull your distributor and see if there are any little metal filings building up on the inside of the cap.

Cheers - Dennis
Dennis Rainey

Thanks for the ideas - I will take a closer look at the distributer this weekend when I get home.
Don Scott

I was going to suggest similar to Dennis. This is often a distributor problem, and if so can be anything from moisture inside the cap to lack of lubrication on the points cam, to dodgy primary wire, to worn bushing... (or even Pertronix unit failing!) I personally doubt it's a fuel problem, but wouldn't presume to disregard that thought completely. You can check by pulling out the choke when it's occurring.

Oh, and replace your plug wires if it's been over a year since you did it last. They don't really last forever, but most people treat them as if they do.

"Gas problems usually involve backfiring" Yes, and it's most polite to excuse oneself to the men's lounge or the garden before this occurs.

Matt Kulka

Hi Dennis , you say ;
"One of the contact points for the plug wires was nearly chewed in half by the rotor on mine indicating an out-of-round condition "
Interesting point of view . I noticed that on a dizzy of mine and never concluded about the reason why. Might explain the problem on a very early engine as a 18G!
Renou

Checked the dizzy today and while I don't have another handy for comparison I'm guessing you shouldn't be able to grab the shaft and feel lots of play when you shake it back and forth. Looks like a rebuild is in order.
don scott

Hi Don.

I think you may have found your problem.

I would seriously consider buying a new distributor unless you are really keen.
Sorting the main shaft bearings isn't a DIY job, the centrifugal weight pivots are probably well worn, the centrifugal springs tired, etc.

Don
Don

This thread was discussed between 06/09/2005 and 11/09/2005

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.