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MG MGB Technical - extreme rough running car

I tried to tune up my 74 chrome bumper b last weekend. It had been running rough. After balancing the carbs and adjusting the valves I tried the car, but the rough running problem is now worse.The car would idle at aprox 600-800 rpm but when it is put under load it barely can get moving. I discovered that when you pull the wire off the number 2 spark plug there is no difference in the idle. There is a spark at the plug. A compression test shows 100 psi (when just turning over the engine) and aprox 80 when the engine is running. The compression is the same on all cylinders. I have tried swapping the plugs around and swapping the coil wires but nothing changes. Essentially the car is running on only 3 cylinders

I would appreciate any comments on what the problem is and how to correct it.

Thanks
rick baranyi

Rick, the compression numbers are low. How do you get a reading with the engine running? Normally you do it while cranking on the starter.
Art Pearse

You need to check the timing, static to start off with. You may not notice a difference in pulling a plug lead at idle if the ignition is advanced, after all, a lot of B engines run on for ages with no ignition.
c cummins

Rick,

Did you get the fuel lines off? If you did and have the SU HIF4's, you may have made the same stupid mistake as I did by swapping the overflowpipe and fuelline with dissasterous overflowing of the carburettor as a result.

Willem
Willem vd Veer

The correct order of things is, adjust the valves, plugs, points gap/dwell, timing, and only then move onto the carbs. Any problem anywhere else in the engine will have an effect on setting the carbs.

The lower figure with the engine running may well be because the throttle is closed. Compression testing should be done with a hot engine, all plugs out, throttle wedged wide open, and cranked.

No change in engine note with one plug lead removed is definitely a sign of a non-firing cylinder, even if an engine Diesels on switch-off it won't run normally with any one plug lead removed. If only No.2 is like this, then that proves the point.

A plug may not fire even if it has a spark presented to it for any number of reasons, the first step is to remove the plug and examine it. Is the outer contact bent? Gap correct? Fouled or oiled? A fouled/oiled plug could be cause or effect, clean it and try again, and if it still doesn't fire then it is the plug, if it runs initially but then stops oiled again then there may be some other problem with the cylinder. A timing light is a good indication of whether you are getting a decent spark or not, clip it onto each lead in turn and watch for any missing flashes. If the coil lead doesn't have the same missing flashes then the rotor or cap is to blame. You could try turning the distributor 90 degrees and moving all the plug leads round by one and see if anything changes, or swap two relatively equal length plug leads over *at both ends*.

Are you sure you don't have *two* plug leads reversed? The correct order is 1 3 4 2 *anti* clockwise.

PaulH Solihull



The 100 compression was just turning over with the starter. The 80 reading was with the engine running.

I replaced the cap, rotor, points, condensor and coil few weeks ago because the car wouldn't start.(The points appeared they were stuck) I swapped the cap back when the current problem arose. The car was running rough before that time, but not as rough as now.

All plugs are new (Bosch)

Thanks
rick baranyi

100 is very low if you are doing it as I say, should be 155-160 for a low compression engine according to the book. Doesn't seem very likely that all four cylinders would be down that much, even less likely that they should all be the same. You have to get all cylinders firing properly, i.e. the same drop in idle when each plug lead is removed, before you can setup the carbs correctly.
PaulH Solihull

Rick,

Do you know the history of the engine? If someone timed the cam wrong, you can get a large drop in cranking compression. I checked a Volvo for someone a few weeks ago that had low compression (140 psi). The cam was one tooth off, and when I corrected it, the compression went up to 170 psi.

Charley
C R Huff

How many miles does the engine have on it (since last overhaul if applicable) to the best of your knowledge? "Static" compression testing requires removing all plugs to create enough RPMs with the starter to get a useable reading. "Dynamic" compression (never tried it myself) is about half of what static compression is. Thus, if your dynamic test came up 80 psi and you assume it's about half of what static should read, you might actually be in the correct range, 150-160 psi. If your static test was done with only one plug removed and revealed 100 psi, that would be consistent with actual psi of 150 or thereabouts since leaving the other plugs in will result in a reading 30% or more below actual. If, indeed, after attempting to get an accurate static compression test and finding that you really do have only 80-100 psi, all the tuning in the world won't make the engine any stronger, it's time for a complete overhaul. From the information you've provided, I tend to think your engine is strong enough, but your static readings are incorrect. Pull your valve cover and check that your intake valve on number 2 (4th valve from front of engine) is actually opening and hasn't been adjusted incorrectly. Specifically, adjust number 4 when number 5 is completely open.
Rick Penland

What do you mean by "dynamic test"?
Art Pearse

This thread was discussed between 10/08/2010 and 12/08/2010

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