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MG MGB Technical - Hesitation,miss,lack of power

Ive got a 79B with 85K,Rebuilt zenith,electronic conversion ignition(Crane) Timng is good,so are plugs and wires.Rebuilt the carb,tuned several times,even a new metering needle and float valve. Tach holds steady.Idles a little rough and I can live with that,but very little power,hesitation and missing to the point of stalling on acceleration.
Anything I missed?

Thanks in advance
Michael

Could be due to a lean mixture.

Did you synch the carbs to each other?

What are the color of the plugs?
Daniel Wong

Michael,

Since you have the ZS carb, you don't have a carb synch problem. You may have a mixture problem, as might be evident by plug color. However, you could also have a timing / ignition problem. Moreover, perhaps you need to adjust your valve lash. Ignition and valve clearances should be checked before tackling the fuel end of the spectrum. Lots of archives available on setting everything up correctly.

Barry
Barry Kindig

Some thoughts that may or may not be helpful --

How did you adjust the mixture? With the later
Zenith, you really need a CO meter to do this
properly, but try the lift the piston test. (Idle
should be relatively steady when lifting the
piston by about 1/16".) As Daniel points out,
you may just be running really lean.

Check for vacuum leaks everywhere, EGR valve,
gulp valve, anti-run-on valve.

What is your compression? Are the valves
adjusted to .015" cold?
is the diaphragm in your Zenith ok? Fuel filter
in good shape? Is there oil in the dashpot?

You stated that the timing is correct, but is
the timing advancing with increasing RPM?

If compression and timing/mechanical advance are
correct, then maybe go back and look for some
gunk clogging the carb jet or metering needle.
Ronald

Also, I had a similar problem due to a leaking
intake manifold gasket. Check that the gasket
looks intact and that the manifold nuts are tight.
Should be an easily solvable problem if you've
got good compression.
Ronald

Reading through your posting I have a few comments to make which whilst does not clear up your problem but I hope will go someway towards answering your queries.

I see that you state that you have electronic ignition, however later in your first posting, you said that you have changed the points. I maybe wrong because I don’t know the particular make of ignition, but I thought all electronic ignitions did away with points.
There are a few things that you have not checked; 1 is the timing of the engine; 2 is the set up of the carbs; 3 have a look at the air filters too. All of these items would also affect the engine and lay the ‘black soot’ you have described on the spark plugs.

Moving onto the rev counter swinging round I would suggest that you replace the wire with the damaged covering. It is the only way to ensure it is working properly. That may stop your rev counter playing up but in case it is not, I will ask this question:
All rev counters, which measure the ignition pulse swing when you first start the car up. How long does the needle pin itself to 7000rpm?
cecil kimber

Sorry my last posting should have been posted on an answer to another question.. But how did it get here?...weird!
cecil kimber

Thank you for all the input guys!

The valves were adjusted about 2500 miles ago. when I first started having performance probs,I changed out the fuel and air filters.Coils hardly ever go bad,but I changed it anyway. Two plugs are dark and sooty,the two in the middle are light gray.The vacuum advance hasnt worked since ive owned the car,the air pump wasnt there either and it ran pretty good for years. When I rebuilt the carb,I changed the diaphram along with the needle,float valve(top of floats measured correctly) There is oil in the piston. Ive had the piston out several tmes to insure the tool is making the adjustment.Ive tried rich,Ive tried lean and still no difference.The coarse idle nut and fine adjust screw work OK as does the main idle speed screw. The anti-run on valve seems OK.It clicks once a few seconds after shut-off. When I raise the pison while the engine is running it slowly tries to stall which means too lean, but when I richen it up it still runs bad. I think Ive tried every adjustment possible.Ill check the manifold bolts. No vacuum leaks. Is the ZS carb so sensitive that it has to be adjusted so exact to run at all?
michael

michael,

I'm not an expert by any measure, but my experience is that the ZS is not at all sensitive. After having mine rebuilt, I did the adjustment by ear, and it ran fine. I would definately recheck the valve clearances if I were you. I checked mine after about that same mileage and was surprise to see the discrepancies...

cheers,
Dan
Daniel Vukmanich

Thanks Dan Ill try that next. This morning I took the piston back out of the carb,adjusted the needle seat(brass ring) flush with the bottom of the piston,returned the coarse,fine and idle screws to the center of adjustment range.No difference.Valve adjust seems like the logical choice. .015 cold......
ME Michael

Michael,

I would check two things before proceeding further
with the carb. (a) Adjust your valves first, then
recheck compression. I think a burned exhaust
valve or really worn rings have to be ruled out
first.

(b) Although as you state the vacuum advance is
inoperative, the mechanical advance must function
to get any kind of reasonable performance. Do you
get a significant timing advance when revving
the engine, as seen by the timing light?

Good luck, and please keep us posted as to progress.

Oh, one more idea. Does your 79 still have
the catalytic converter? If this is clogged,
the engine will bog down under load. Hook up
a vacuum gauge to the inlet manifold (say gulp
valve signal port) and check for at least 16"
at idle. Rev the engine, and if the vacuum
drops you may have a clogged cat or blocked
exhaust system.
Ronald

Well,I adjusted the valves according to the Haynes book,and got SOME improvement.Ive got some ticking going on,so Ill go back later in the week and try it again.This was my first time. Its not a clogged cat-conv.(doesnt have one) and the mechanical advance seems OK. So,my next thought is,when the valves are dead-on and the compression is,well,marginal and I still cant get any power out of it,will I need an engine or head overhaul?

Thanks again for all the advice
Michael

What are the compression readings exactly? If the problem
is a burned valve then you can just try replacing the valve
and grinding it in, but you may want to take the whole head
down to the machine shop. If the head is cracked then you
will need a new one.
Ronald

Michael;
I hope your problem is different , but my 76 had three wiped lobes on the cam. It ran fairly smooth, but
was terribly anemic on power, and went flat at 3500rpm.
I had a similar problem when I first got the car, and it turned out to be a bad coil. there was a pinhole corroded thru, and the oil leaked out of the coil. When it heated up, spark was extremely weak, and consequently no power, and backfired.
We had a clubmember that had a recent gold lucas coil in his Healey, and it went bad when about 1 year old. Caused it to run terrible, backfiring and stalling.

Pete Haburt
Pete Haburt

Back on the road!!!! This time,I took my time with the valve adjustment and what a difference!!!! Running nice and strong.Havent checked compression yet,cant find that bloody tester.Thank you for all the help. Now I can fine-tune that Zenith!
Michael

This thread was discussed between 30/03/2002 and 04/04/2002

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