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 - Possible over- rich running problem

I have 74 BGT with almost 130,000 miles on it with the following symptoms:
Starts easily and idles well. But with any throttle it sounds like it runs on three cylinders (over all operating temperatures). When driving it is low on power. On hard acceleration lots of black smoke comes out of the exhaust.
All four plugs have a spark.
The plug for cylinder #3 is very wet/fouled and smells of petrol/gas. Plug #4 is black/sooty.
Carbs are well balanced (unsing air flow meter).
Idle increases noticeably with air cleaners off.
Compression test results are:
#1 135 psi, #2 125 psi, #3 130 psi, #4 140 psi.
There are no strange valve train noises.
The throttle butterflies open and close Ok on both carbs (HIF4).
Pistons and needles move freely and damping seems OK.
I *think* the problem is that it is running very rich off-idle - particularly on the back carb closer to cylinders 3/4. Could this be worn/blocked jets/needle valve/bad temperature compensator/bad started vale seal... and I need to clean/rebuild the carbs, or could it be something else?
- Iain
Iain Walker

Iain
If you do not know the history of the carbs I would suggest that it's time you stripped them and checked them out. It's almost certain that you are going to need new jets and needles and maybe the throttle shafts rebushed if they are found to be sloppy and leaking air past them.
If they are worn you will have a devil of a job setting them up and never succeed.
Best start off with checking valve clearances, ignition settings points and timing only when these are ok start on the carbs.
If the throttle shafts and bushes are not worn you may get away with new jets and needles, also replace the float needles whilst you have the carbs stripped.
Much advice on the web on these carbs good place is Paul Hunts site
http://www.mgb-stuff.org.uk/

Once you know the condition of the carbs or put them right with new parts you will be surprised how easy they are to set up.
Hope that helps

Colin
C J Bryan

Iain, how much oil does your engine consume? Does the black smoke occur every time you step on the gas, or just after the car has been parked or idling for some time? You probably have a combination of worn needles AND fatigued valve guides and seals, as well as possibly worn/stuck piston rings that all contribute to the problem. Have you done a wet compression test to rule out bad rings? Your compression numbers currently look a little low, as if there is a fair amount of wear in the engine.

Have you tried to replace the spark plugs and see how the engine runs? Do you still have points ignition? If so, are they in good shape and properly adjusted? Your drivelability issue is more likely a tune-up problem than from the worn parts. If you truly believe that you are rich off idle, you can try a heavier damper oil to test your theory. The heavier oil should make the pistons rise slower in relation to the volume of air entering the carbs, thus giving you a leaner mixture off idle.
Jeff
Jeff Schlemmer

Colin - I only know about he last five years of the carbs life with me. Before that they lived in LA and in the desert outside LA. Checking the ignition timing is next on my list. I agree that trying to properly set-up old worn carbs will never work out.

Jeff - the engine consumes < 1 quart between changes (every 3000 miles or so) unless I do lots of high speed driving then consumption increases. There has always been a little bit of black smoke over the five years I have had the car - but only visible at night in the headlights of cars behind me (i.e. you can't see it during daylight hours in the rearview mirror). I agree that the compression numbers are low and the engine is probably quite worn out. I have converted to breakerless ignition so I have no points ot adjust. I did check the distributor cap and look inside and it all looks OK. I'm afraid I didn't explain things very well regarding "off-idle". I really meant anything but idle - i.e., steady operation as well as transient operation. The quantity of smoke scales with throttle openening. There is no visible smoke at idle. Tailpipe is sooty inside. What I am trying to decide is if it is time for a complete engine rebuild, or just a carb rebuild (much cheaper!).
- Iain
Iain Walker

Iain:

Do you have the high compression or low compression engine? The high comp is 8.8:1 (or 9.0:1) and the low comp is 8.0:1. Comp pressure for the high comp should be in the 160 range and for the low comp around 130. Since #4 shows 140 psi I'd guess that you have the high comp. However, a wet cylinder could increase the pressure and confuse things.

Jack
J.C. Weidner

74 should be a low comp engine.
Do comp check with throttle wide open, in neutral, batt fully charged and all batt terminals and connections to starter clean - may give somewhat higher readings.
A quart in 3000 or even 1500 is very good, it's not worn out.
Black smoke = rich mixture; it is either set too rich, the vacuum chambers or air filters are dirty, the choke is partially on, the needles are wrong, or the float levels are high. The fact that the idle improves with the filters off indicates that the filters ARE dirty, and it is rich at least at idle. Fine desert dust can block them up without appearing dirty. Be sure the gaskets between filter and carb are installed so that the vent holes in the carb face are not obstructed.
The fact that the rear carb is evidently richer than the front may indicate that the rear float is leaking/sinking = a known HIF problem.
Jeff - your understanding of the damper function is exactly backwards - heavier oil will slow the rate of rise of the carb pistons and this will cause RICHER mixture on acceleration. That is the purpose of the dampers. The dampers/oil have NO EFFECT on steady state operation.
FRM
FR Millmore

FRM, as I was typing the dashpot oil bit, I was second guessing myself. Thanks for clearing that up! I hate misleading people due to my silly mistakes.

Iain, why don't you have your carbs rebuilt and possibly a valve job, and then if that doesn't cure your problems, do the engine later? It sounds pretty clear with all the new info that you ARE having carb problems, although there may be other issues as well. You don't have the pollution controls on still, do you?

I'd also replace the cap and rotor. Carbon tracking is difficult, sometimes impossible to see and can cause a bad misfire. Its cheap insurance to replace them.
Jeff Schlemmer

Thanks to everyone for their advice.
FRM: Sorry I goofed with the year - really its a '73. Does this matter with respect to high/low compression engines? I have replaced the air filters (only once in five yeras :-( ) a couple of years ago and they really helped with making the idle smooth and consistent.

Jeff - I have desmogged the motor - so there is no air pump/air injection/gulp valve assembly any more - but I still have the charcoal canister for the fuel vents. I have kept all the parts in a box just in case the California Air Resources Board (or CARB - coincidence - I think not!!!! 8-0) decides to crack down on old cars - currently my car is smog test exempt.

It looks like a carb rebuild is in order (plus new air filters). I have a spare cap/rotor so I'll try replacing them too.

Thanks agian to you all. This BBS is brilliant!
- Iain
Iain Walker

Iain,
73 is also low comp. It only takes one dust storm to plug up the filters! Depending on where you are, there could be a lot of dust floating around, even if it's not obvious. But if the filters have a noticeable effect, they are pretty bad. If you keep having a problem, convert to K&N filters - they are much more tolerant of dirt - they were originally developed for things like helicopters in deserts.
FRM
FR Millmore

Iain, as a last resort, check how your car runs without the air filters. If it really is rich, it should run a little better without them. The way my car is set up, it runs terrible with them. A change to K&N filters cured that.

Jeff Schlemmer

Running w/such a rich condition will lead to engine failure as the gas washes the oil from the piston walls and dilutes the oil.
Mike MaGee

This thread was discussed between 28/08/2005 and 31/08/2005

MG MGB Technical index

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