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MG MGB Technical - Sitting in traffic jams

My motor runs fine most of the time but after twenty minutes of sitting in a traffic jam it runs really lumpily and, when we finally get away the first burst of speed results in clouds of smoke from the exhaust.

What do I need to fix?

Chris
Chris Thompson

How is the temperature?
Steve Simmons

Probibally runing a little rich and loading up when you are idleing. The first time you get the revs up it's clearing all the carbon out, black smoke. Might want to try leaning it out some and adjusting the timing if necessary.
John A

Black Smoke?

and of course, would do this if just sitting at idle for that twenty minutes?

I'd say it's just by design -- a build up of unburnt petrol in an engine that likes to breath. Can you give the engine an occasional 3000,4000 RPM without causing a stir?

Or,perhaps, leaning the mixture a tad or a different heat range spark plug.
glg

Sound like your engine is sucking in excess
crankcase oil vapor during times of high manifold
vacuum (ie: long idle, or deceleration), and then
belching out the smoke and clearing up during
moderate vacuum (ie: acceleration and running
speeds).

My offhand guesses are focused on a fault lying
somewhere in the crankcase breathing system.

Possibly, a bad PCV valve diaphragm (early models) or stuck Gulp valve (later models)?

A clogged oil filler cap breather (early models) or
charcoal canister air intake hose (later models).

Worse...worn valve guides?
Daniel Wong

A long idle, particularly with HSs, can result in a poor idle, this is why when setting them up you have to rev them for a few seconds every minute or so to clear them. They don't have a temperature-compensated mixture control like the HIFs so can run rich as has been suggested, unburnt fuel can also pool in the throat affecting running. Smoke when pulling away again, if it is blue, is a sign that valve stems/guides/seals are worn and sucking oil into the combustion chamber from the rocker area.

No gulp valve or canister on UK models, which had a breathing filler cap all through. A faulty PCV, where fitted, would probably give problems on other than a long idle.
Paul Hunt

I had a similar problem last winter.
I tore down my carbs and saw that the PO had put in non-matching gross jets. I replaced them with standard jets and haven't had a problem since.
eric.
Eric King

Steve

The temperature is fine (car wise, not weather wise, nasty cold snap at the moment for the UK!)

Generally the temperature runs a little above the cold end, sitting in a jam the needle climbs steadily and hovers around half way, perhaps a tad towards the hot end. The longer we sit still the more the needle drifts, slowly but surely, towards the hot end.

The smoke is nasty looking black stuff.

Thanks for your help. I get the idea that it's probably just one of those things but, at the same time, that you are all warning me that it could be a sign of nastier things.

Chris
Chris Thompson

Hi Chris,

Black smoke indicates a rich mixture - it would be worth checking the mixture setting and the condition of the needle valves as well as float height.

Also check that the choke mechanism is allowing the jets to return to the correct position and they are not sticking down.

Are all the return springs in place on the carbs?

When were the carbs last serviced?

What air filters are you using? - in these cold damp conditions the SUs may suffer from icing up.
Chris at Octarine Services

Little above the cold end? Should be on Normal pretty-well all the time, just slightly below driving in sub-zero and perhaps slightly above stuck for a long time. Only getting up to N when you are stopped indicates a bad stat. What year is it? My roadster with mechanical fan only idles above N in hot weather with a following wind, later cars with electric fan are supposed to oscillate about mid-way between N and H as the fan cuts in and out. How did it cope with the summer?
Paul Hunt

There isn't an N mark on the gauge but if that is at 12 o clock then it is only running a little before 11 o clock. Its a 78 with the electric fan and the thermostat is new but it was running at the same sort of level with the previous stat. No particular problems with the temperature over the summer.

Hi Chris, I might be down to see you for a new gearbox soon, there are some very strange vibrations through the clutch pedal from time to time. I took it up to the Lake District last weekend and I made a point of taking your mobile number and the address just in case I needed to trailer it down to you!

I put new (recon)carbs on it after we put your engine in. Alan, the bloke who looks after the motor most of the time has taken them off and fiddled with them a couple of times since then so I suspect no problem there; except of course that an MGB can go from fine to definitely off colour whilst you pop in to the newsagents for a paper. I will have a tinker.
Chris Thompson

I too get lumpy while sitting in traffic and if you look closely I have smoke and/or steam coming from my ears and nostrils :)
Mike MaGee

No helpful advice to offer, just wondered if this is a daily problem on the A14/M11, or similar grind into the city? If not, where else in Cambridegeshire is the traffic thick enough for a 20 minute traffic jam to build up? Luckily for me, I'm usually going the other way at peak times.
Vic
V Todman

Mike,

You clearly need an additional fan strapped to your legs - just make sure it is shrouded!

Chris,

Strange vibrations through the clutch are usually pressure plate/release bearing problems - how many miles have you put on the clock since March 2002?
Chris at Octarine Services

Chris T - about 11 o'clock on that gauge is fine. But I would expect it to creep beyond 12 o'clock when idling then cycle about that point as the fans cut in and out, possibly half-way between 12 o'clock and the start of the red. Do the fans cut in and out? Does the temp continue to rise while they are doing so or does it go down a bit while the fans are running, then back up a bit when they stop? If not and the gauge continues to rise when the fans are running then I would say you *do* have problems and they will be much worse in the summer. A 78 should have HIFs which should be less affected by long idles, seems to have no effect on mine on the V8 except when it got hot enough to reach the red before I went over the cooling sytem, whereas the roadster with HSs does get a bit rough after a few minutes till I give the throttle a blip. But if I'm going to be stuck that long I switch off anyway.
Paul Hunt

Vic
This car doesn't stay just in Cambridgeshire, it's an adventurous car. We were on the way back from a bit of a run around the Lake District on the Christmas Cracker run and we got caught in a jam near Newark. Mind you the A14 any Friday night coming out of Cambridge, just around Bar Hill, could easily count for 20 minutes.

Chris
22,001 miles as of the last tank full of petrol this evening - as you say the engine went in on 14 March 2002.

Paul
Thanks for the re-assurance - no the fan does kick in an out and it does bring the temperature down a bit and generally seem to work as it should.

Chris
Chris Thompson

Chris,

Are the vibrations just as you put your foot on the clutch and then disappear as you press it down?

Your right about MGs having funny turns though - mine was on a fast run down the M1 a couple of years back and suddenly lost all power, loads of black smoke. I limped into a service area and had a good poke round and could find nothing wrong. The engine would not idle, it started with difficulty and would only run above 3000 rpm but would not pull the car, I had the carbs apart, the electrics apart and still could find nothing wrong. Gave up and went to have a cup of coffee, came out after 1/2 hour and it started and ran fine, has done ever since - never did find out what caused it.
Chris at Octarine Services

Stuck carb piston?
Dan Robinson

Chris

Well there have been several variations on that clutch theme but basically yes - push the clutch pedal all the way down and the noises and vibrations go away.

But wouldn't cars be tedious if you just got in them and they always started, ran faultlesly for thousands of miles, never displayed the least flicker of personality and were so quiet you could hear the radio?

Chris
Chris Thompson

My guess is that there is some misalignment or lack of lubrication with the release bearing / pressure plate.

As the release bearing contacts the pressure plate any wobble on the setup will be transmitted up the hydraulics to the pedal - once the bearing is pressed hard onto the plate the bearing or the pressure pad on the spring fingers lines up and the vibration goes away.
Chris at Octarine Services

This thread was discussed between 07/12/2003 and 14/12/2003

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