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MG MGB Technical - total misfire and stalling

My 1976 mgb roadster is acting very strange, after a cold start no problems at all, very good performance on the road for a ride 16 km (10 miles). then parking the car for aprox. 1/2 hr. After that starting without a problem but then within a minute severe misfiring resulting in total stalling.
Very hard to start the engine again, managed to get home with a lot of misfiring and shooting from the exaustpipe.
The next day the same story again.

Any ideas where to look at, carbs or ignition.

looking forward to see some advice on this matter.

Jacob Halma

Sounds like a classic case of the condensor breaking down in the dizzy.
Try installing a new one.

Colin
C J Bryan

What kind of carbs?
...

I'll second Colin's statement. The backfire makes me think its definitely an intermittent ignition problem - lack of spark pushes fuel into the exhaust, then it ignites later.
I recommend removing the points plate from your distributor, completely cleaning it, and installing new points and condenser. The condenser needs to be grounded to the plate, and the plate needs to be grounded to the housing.
Jeff Schlemmer

Jacob,

sounds as if a wrong distributer has been insalled or carbs changed and the old dizzy still on the engine.
Your car should run on manyfold vacuum with the Stromberg, if changed to other carbs and connected to the vac port of a hs/hif 4, your car will have this problems as I learned from my 1975 roadster, imported from Alabama.
I changed from Stromberg to SU and changed the distributer for one of a MINI 1275, that runs very fine with the 18V pre 1974 LC engine.
If you want to do this too, be carefull with distributers from the later Minis, as Rover had changed the shafts, so go for a Leyland one that drops just into the 18V!

For further deails you can call me on 0049 2162 42648.

Ralph
Ralph

Here some more info on this one from my side, two HIF 4 carbs, new condensor and points installed last winter and has been running fine on that for 500 miles.

Jacob
Jacob Halma

Jacob,

In particular, check that the soft flexible wire that runs from your breaker plate to ground on the casing is intact. Mine broke loose from the breaker plate many years ago with precisely the symptoms you reported.

FWIW,
Allen
Allen Bachelder

Also! Check for lateral play in the distributor spindle. Grab the rotor and try to move it sideways. Any play could be problemmatic, but note whether it is enough to open/close the points. This will also cause extremely rough running. Switching to electronic ignition will "solve" the problem, but the wear in your distributor should be attended to.

FWIW again,
Allen
Allen Bachelder

Maybe some new relevant info on this thread,...
After having the same problems again I noticed the other day that my fuel filter where emty, so I checked the pumpcapacity of my (new) feulpump and this was OK, as I would expect from a new pump.
So now I think of having the vapourlock problem in the carbs? Because this is meeting the circumstances of when this failure occurs. hot weather 30 deg C. the heat of the engine and exaustpipes getting in the carbs when there is cooling like when driving the car????

Is this an option and how to check? and what to do about it.

Jacob
Jacob Halma

Jacob,

what year is the car, for which market it was built and what is the reading of the engine no. 18V...?

Ralph
Ralph

Ralph,

The manufact.year is Aug.1976, this a original european market car (Holland nr.GHN5 405815 G.)
The engine number is not on the engine but I figured out wich number it should be ???, this is based on what is installed and what is not on the engine and also compression H/L indication. 18V/846 F/ H L ..

Jacob
Jacob Halma

Colin,Jeff, and Allan checked the condensor items without any result. Also checked the distributor for lateral play. No sideway play to feel.

Jacob
Jacob Halma

Jacob: I think you may have one carburetor float sticking in the closed position. HIF 4 sometimes have that problem. Too bad, the HIF has to be removed to check the float valves. It's happened to me in the past. Check the float for binding and be sure the needle valves are good.

Clifton
Clifton Gordon

Jacob,

If there is still the original (but rebuilt) engine in the car, it aught to be a 18V 846 or 847 H ...(European markets only).
They were fitted with a 45D4 41610 distributer.
Dwell is 51 +/- 5 deg.
Static setting 7 deg.
Strobe 10 at 1000
Advance (centrifugal) 0.5 at 600
3 at 1600
8 at 2600
12 at 3400
17 at 4400
22 at 5400
24 at 6400 (!)
Ported vac starts at 3 in.Hg with a max. of 11 in Hg at 24 deg.
>The vac position was changed to manifold vac in >Septembre 1976 on these cars, so watch out for the >right pickup position!

Some information is availabel here: www.autochart.com/MGB_Tune.htm and
www.mg-ammerland.de/ in the'Technik'-colume.

Ralph
Ralph

Jacob, I would still look at the condensor in the dizzy. I have had new condensors fail "right out of the box" and after reinstalling the old unit the car ran fine. New does not always mean the item is good especially when it comes to electrical.

Let us know what the end result is

cheers

Gary :>{D
79 mgb
gnhansen

Jacob,
You really, really need to go through a step by step troubleshooting process the next time it happens because we'll send you off in a thousand wild directions without more info. Start by checking if you have spark and so on. See Paul Hunt's site http://www.mgb-stuff.org.uk/. Good luck!
Steve Postins

Firstly vapour lock is virtually unknown in practice, especially with HIF carbs where the bottom of the jet is in the middle of the float chamber. HS carbs can suffer from it in the jet pipes if the heat-shield is missing or damaged, but other than that no chance. An empty fuel filter meands nothing, mine varies between completely full and completely empty and everything in between with no effect on running. If a feed pipe disconnected from a carb and fed into a container delivers *at least* 1 Imperial pinter per minute, with no or negligible bubbling, then fuel delivery is fine.

If the problem only started *after* you changed the points and condenser, then change them again. Hunt's Fifth Law: "Many break-downs occur soon after a car has been worked on; 'new' parts can be faulty when you receive them; 'new' parts will sometimes fail soon after fitting; 'new' parts almost certainly won't last as long as the originals."

When it *is* happening then clipping a timing light onto the coil lead and each plug lead can speak volumes. If the flash from the light is regular and consistent on all leads then the problem is probably fuel. If the coil lead is consistent but the plug leads aren't then the cap or rotor is probably breaking down. If the coil lead is also erratic then it is probably the ignition LT or just possibly the coil. A voltmeter connected between the coil -ve and gound should indicate a consistent voltage, if it varies with the misfire then it is probably points (or electronic trigger) or distributor ground wire.
Paul Hunt 2

Just some update in how things are going.
Just did a correction in static timing moment from 10 deg BTDC to 7 deg. BTDC resulting in a better overall performance but not solving my maine problem.
Then removed both carbs. for a carb float check, found that there where critical low float level in both carbs.
After adjusting this according the Haines manuel my problem looks solved.
How this setting on both carbs came there must be the result of doing something stupid (by myself)last winter working on the carbs.
But what puzzles me is why only having this problem when the engine is hot? any ideas?

Still planning to do a dynamic timing check next week.

Jacob
Jacob Halma

This thread was discussed between 13/06/2006 and 19/06/2006

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