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MG MGB Technical - Very rough running.

Hello all, I’m looking for some advice with an engine problem I’m having with my MGB. The car is an early 1967 Roadster and is in standard trim. It has only about 3000 miles use since being fully restored (including an engine rebuild) several years ago. Having decided, once and for all, to fix an intermittent but annoying misfire when hot, I replaced the coil, points, condenser, plug leads and plugs, distributor cap and rotor arm. All seemed well on a short test drive, however the next time it was started it ran smoothly for about 10/20 seconds and then began to misfire very badly, sounding like it was only running on 2 or 3 cylinders and would not respond to any throttle. Presuming it to be a fuel problem I ordered and fitted an S.U. carburettor overall kit (both jet tubes and all seals and gaskets + both needles). There was a small leak from under the front float bowl but this appears to have been leaking for some time. The air filters have only about 200 miles on them and I obviously took the opportunity to clean the carbs as well. However, the very rough running almost immediately after start up persisted, so suspecting a stuck valve I removed the rocker cover and all the valves seem to be working fine. There is plenty of petrol in the tank and the fuel pump appears to be fine too as it will pump away if a fuel line is disconnected with the ignition on. Sorry for the long winded question, but as a very amateur spanner-man I’m now out of options and would really appreciate some help. Regards, Mark Kennedy.
M Kennedy

You might have a problem with one of the carbs. What sometimes happens in a carb rebuild is that one carb gets starved for fuel, and that is caused by not having gotten the small o-ring out of the base of the float bowl when installing the new jet hose to it. Sometimes it is having the old washer fold over, break apart into pieces which clog the line, or sometimes even the new washer will not seat properly and choke off the fuel flow from that float bowl.

You may have another problem, but since this seems to have happened after the carb overhaul, I would eliminate these problems before digging deeper.
Bob Muenchausen

Mark. A compression test would be a good first step. Valves may function, but not seal properly causing low compression and miss firing. A compression test can also show up a bad head gasket. Since you should warm the engine up for the compression check, you can find out what cylinders are not firing by carefully removing and reattaching plug wire leads from the dizzy cap. If the engine runs worse, the cylinder is firing. If there is no change, it is not firing. You might also want to make sure you attached the leads in the correct order, which is 1-3-4-2 in an anti-clockwise direction. Number 1 should be at about the 2 O'clock position, but that has to be checked. With the crankshaft at top dead center, on the compression stroke for number 1 cylinder, the rotor will be pointing to number one terminal on the distributor cap. You also need to make sure all of the wires in the low tension circuit are tight, the points are opening properly, etc. Then, check for good spark at the lead from the coil lead, followed by good spark from each of the spark plug leads. Yes, you can get a bad wire, cap or rotor, even when installing new parts. The condensor is also an area which has caused problems in the past. With a good ignition system and good compression test, it is time to look to the fuel system. Les
Les Bengtson

Many thanks for the great advice. I'm fairly sure its not the old "o" ring in the base of the float bowl as I was carefull to remove these. I know the firing order is right as I'm getting a great spark at the plugs and you can see i'ts right when you take the plugs out and arrange them in the 1-3-4-2 order. No1 is at the 2 O'clock position and I have'nt changed the order since she last ran well. Also it runs O.K. for a few seconds after start up. I'm beginning to wonder could the timing have slipped when I was fitting the new points etc? Regards Mark.
M Kennedy

When changing the points, you do need to reset the timing. The exchanging of points do affect the timing.
You also could have the age old problem, that the point assembly has slipped and the point gap has gotten smaller. This problem usually shows up when accelerating, but not at idle. But you said the problem shows up after a few seconds. Could those few seconds be at idle after starting the car and the problem occurs when driving away?

Ray Ammeter

Mark,
Sounds like you need to go back and try some basic tests. If it is running ok for a while then gets bad I would assume it is fuel and not ignition as ignition tends to be more consistant. Just off hand it sounds as if it is maybe a fuel starvation issue in one carb. This would allow good running for a few seconds untill the bowl ran empty.
There are several basic things you could try. If you remove the plugs one by one, you should be able to pin it down to a cylinder or two. You can use some type of insulator to stop the shock. You can also cover the carb intake and you should see a considerable drop in revs. If you suspect one carb, ie you have two dead cylinders front or rear, take the bowl tops off the carb in situ. I would guess you will find the bowl empty. If this is the case it may be a stuck/miss-adjusted float. You should also check the carb piston moves up and down freely and the throttle disk rotates.
If it is only one cylinder it may be the distributor. If this is the case I would pull the distributor and take a voltmeter and check the point continuity for every cylinder. Also check for shaft wear. You can also take the top off it and make sure the weights/springs inside are connecteed and there is no major damage inside. Hope this helps.
J Arthurs

You may be right, M Kennedy. When you do find your problem, let us know what you have found. Some problems are true anomalies, and some are just at the far reaches of the "bell curve" of likelyhood, and so, add to the store of possibilities that another MG enthusiast might need to look for. Good luck!
Bob Muenchausen

You say the fuel pump is pumping OK when disconnected. How fast? The fuel should *really* pee out of the thing (fill a milk bottle in a few seconds). I've seen it in the past where the pump is on the way out and you get a mere trickle. Car idles OK but a few seconds of heavy throttle and, lo and behold, all the symptoms you describe.
Tim Cuthill

I've personally had a case where the seat wasn't tightned down enough on my SU and it backed off causing extremly poor running and fuel leakage..I wonder...like it was running on two cylinders caughing and sputtering..poor starting etc etc..

In my case..it WILL NEVER happen again..you could say I learned my lesson to check this.
Eric Willis

Thanks again. I'll check the float bowl's tomorrow and also have a look at the fuel pump as Tim suggests. It does'nt seem that strong. This motor will only just idle, lumpy as hell & shaking around on it's mounts. Giving it gas just results in a sucking noise ! and more often than not it will stall of its own accord. One thing though, the front carb piston seems to stick in the fully down position. applying choke releases it, presumably as this (I think) lowers the jet. P.S. Does any one know of a link to a page that shows how to adjust the ignition timing? Appreciate all the help. The score is still, Engine gremlins -- 1 Mark -- 0 !!
M Kennedy

Try this!
Hope it helps.
http://www.mgcars.org.uk/electrical/index.html
Robert Dougherty

Mark. You may find my website of some use. Go to www.custompistols.com and then click on the MG section, followed by the "articles" section. There is some information on static and dynamic timing, along with some other articles which may be of use to you. The basic "rule" is get the igntion system in order, make sure you have good compression, then go to the carbs. David DuBois has some very interesting information of SU fuel pumps, volume and pressure, that he has sent me. He rebuilds the SU pump as "hobby/part time job" and has some insights I have not seen before in a rather extensive collection of MG related books. E-mail me off line and I will provide a copy of his e-mail on this subject (with his permission). I hope to be able to get him to do a tech article on this because most of the information available is rather dated. Les
Les Bengtson

Mark,
if your front carb sticks until you move the choke, perhaps you have a problem with the jet centering? As you may know, if it is not centered in relation to the needle, the two can bind. But when you pull the choke cable, you are changing the point at which they bind by the act of lowering the jet in relation to the needle.

If your jet's guide/packing nut was not tightened properly at adjustment, it could have vibrated loose and allowed your front jet to decenter itself, causing the binding you have noted. Worth a looksee. Good luck.
Bob Muenchausen

This thread was discussed between 04/02/2003 and 06/02/2003

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