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MG MGB Technical - Uneven Running - Update

I got this posted in error on the MGB General section, so I am adding it here again. I am trying to trace the origin of an uneven running condition when driving on level ground at 2,500-3,000 rpm. I have done all of the usual tune-up things - set the timing, checked the timing, adjusted the valves, balanced the carbs (mixture and airflow). I pulled the dizzy and checked both the mechanical and vacuum advance - both work properly and show only minimal signs of wear. The spark plugs when pulled after driving are a tan color and consistent among all 4 plugs. The only thing I found when reinstalling the dizzy is the rotor did not snap back into position when in the car like it did on the bench. If pushed and then simply released, it only returned about half way and seemed to lose tension. Could this be the problem? If so, what could be causing it. I am about out of ideas. The engine idles perfectly and revs without hesitation. If climbing or otherwise under load at 2,500-3,000 rpm the engine does not stumble - only under level throttle conditions does it run unevenly. Please help - I have used all of my vocabulary and still have no solution.

Thanks, Don
don scott

Don, I posted some suggestions on your thread in the General section.

Clifton

Clifton Gordon

Don, fatigued old springs will cause the problem you just described. They no longer have the strength to return the advance weights back to the "closed" position.

Since your car is running well, the problem you describe sounds like a lean condition. You may have a problem with fuel supply, mixture adjustments, or jetting.

Jeff Schlemmer

Don, could be the throttle shafts. It seems that the problem is there at a "normal" cruising RPM which is where most shaft wear occurs.

Tony
Tony OIiver

Not likely to be any of the above. The advance sounds pretty much as normal, there is enough friction to make the rotor/cam stay partially advanced when the engine is not running, since the weights are laying on the cam surfaces on the drive plate, and the points add drag. When it's upright on the bench it doesn't do this. You should have lubricated the shaft, weight pivots, and the cam surfaces the weights lean on though. Throttle shafts are also out, as the % leakage with the throttles off idle is miniscule, unless they are worn near clear through. It is likely a mixture problem, and possibly a very obscure one at that. Get a cup of coffee!

C1966, I was learning all this, we had a pretty OCD customer with an immaculate B. He traveled a lot for work, putting about 30K per year on the car, was obsessive about maintenance and tuning, 3K oil changes, regular tuneups, etc. My boss Paul had maintained it once out of dealer warranty, and we kept telling him he was going to wear it out in the shop instead of on the road, given the ideal conditions of operation. It started to develop a similar ailment, getting gradually worse over time. We went over everything meticulously, time after time; it got worse. Finally it would shut itself off completely, always at "cruising speed". He'd coast off the road, poke around, start it right up, and continue. Neither Paul nor I could make it happen, though he told us exactly what conditions were required - level road, constant speed at the speed limit.

At 85K, he told us to pull/dismantle the engine and examine everthing, he was satisfied he had gotten his money's worth, but none of us could stand the mystery.
We told him it wouldn't help, but he insisted. We got to see what a perfectly maintained and operated engine looks like inside, ground the valves, replaced bearings (which were as new); all that equaled NO difference.
We then went out with the customer driving, and found that he was also obsessive about not pushing the speed limit - 50 in PA at the time, and he had the knack of keeping the speed dead constant for hours, as a good driver in a familar vehicle can do. It came out to something like 52mph on a dead level road, and even a head or tailwind would make the problem not happen.

For about the tenth time we pulled the carbs, but this time we stole some off another car - problem cured, but not solved.
Aside: the piston drop test
The vacuum chambers and pistons are matched, so that it takes 5-7 seconds for the piston to fall out of the chamber with the damper out, the transfer holes in the piston bottom plugged, and the chamber and piston clean and dry. Both carbs should have very similar times, and pistons and chambers should never be mixed up. Though it is a 50/50 thing, I find that 75% of the time the chambers have been switched on a given car - thanks Murphy! Mark them when you get them right. The clearance between chamber wall and piston is on the order of .0001, which is why dirt and especially abrasives are an absolute NO on them. Clean with solvent and soft cloth only; once I had this experience, it IS part of a tune-up.

We had naturally done the drop test several times, however you normally hold the chamber in one hand, hold the piston up with the other, then release the piston until it hits your thumb, which you have put on the bottom of the chamber to keep the piston from falling clear out (and killing itself in the process). You are looking at the clock while you do this, to get the time. On this occasion, Paul did the test while I watched the piston, and LO! The piston fell nicely about 60% of the way, gave a little quick drop of about 3/16, then dropped at the normal rate. Carb #2 did the same. This drop corresponded to the position the piston was in at 52mph level. We refitted the correct carbs, but with the other pistons and chambers - no problem. Put the original parts on, problem returned. We found this sufficiently bizarre to remove the bonnet and the carb dampers, and make some indicator wires out of welding rod like those in some SU tuning kits. At just the right speed, we could see the pistons drop, and the car died after a few seconds. A very quick removal of plugs showed them to be soaking wet and black.
Careful (very!) measurement showed that the chambers had worn a couple of tenths only at the 52-level point, and the pistons would suddenly drop, enrich the mixture, and flood it enough to wet the plugs. As he coasted to the side of the road, the plugs would dry off from engine heat, and the car would start right up. We found that we could figure out other speed and load combos that would do it, but in normal driving it never showed up for us. 50 rpm either way in any given situation would stop it.
While it is unlikely that your car has this exact cause for its problem, and also unlikely that your trouble is so specific, other evil things like someone having "cleaned" the chambers with sandpaper can cause the same trouble, only spread out a bit. Think about the natural way to hold abrasive paper while turning the chamber, and about the fact that the chamber will get most dirty in just those positions where it operates most - idle and highway cruise. Do the drop test while carefully observing the piston motion. I have gotten so I set up both chamber/piston assemblies so I can drop them simultaneously with one eye on each; anomalies show up immediately.
FRM
FR Millmore

FR
That was a good one! Do you think we are really going to believe that story?

It sounds so good I almost bought it, until it dawned on me that at the loose spot the vaccum would drop and the piston would fall and make the mixture leaner as the tapered needle entered the jet further. He would then need to open the throttle further which would create more vacuum unless he was lugging the car.

Besides, If the story was in flat Kansas, I could see the flat speed on the flat, although, even there, a windy day would change the engine load up or down at 52 mph, but the event happened in PA, which typically isn't very flat.

Were the needles centered? Did that engine have the crankcase vent into the air cleaner and gunk built up in the carbs? Were the jets worn or incorrectly set or centered?

Was 52 mph about 2700 rpm? I note that if the exhaust mounts are poor on my car that there is a vibration resonance at that speed. It could take a couple pints of beer to resolve this one.

Barry
Barry Parkinson

If the lobe of the cam goes past the points heel when manually twisted clockwise then I can understand that just possibly it might not return fully when released. However if it does the same with the points heel between two lobes then it is weak springs, which will cause timing jitter, which may be visible at a standstill. Could also be a binding upper shaft on the lower which will not cause jitter but which will cause stiffness when you manually return the rotor all the way. Note that worn timing gear and chain will also cause jitter. I can't see that the distributor in or out of the car, vertical or not, will make that much difference.

Weak mixture will also cause hesitation, but more when accelerating lightly than at a steady speed.

You could try pulling the choke slightly when the condition occurs and let us know the result, but I have known that 'cure' ignition problems as well as carburation.
Paul Hunt 2

Barry- Your understanding of the effect of the piston dropping is wrong at the most basic level. Yes the needle drops into the jet, tending to lean it out, BUT, the air velocity through the now smaller venturi increases enough to cause more fuel to be pulled through the jet, which more than compensates for the needle effect. The increased velocity and resulting increased vacuum at the venturi section is normally tranferred through the holes in the piston, raising it and giving the "constant depression" that is the basis for the operation of SU and other CD carbs. With the worn chamber, this vacuum was leaking around the piston at that point. You can easily check the truth of this by pushing the piston down with the engine running - it will go way rich; this is in fact what happens on a ZS when the diaphragm has a hole in it, which is a precisely analogous condition. The ZS, however, has a leak throughout the range, but the worn SU only has it at one airflow point.

The fact that we were in PA, Pittsburgh no less, made it very difficult to replicate the problem. But the customer, in putting all those miles on, covered a much larger area, including a lot of nice flat places like Ohio & Indiana, and drove at his "native" speeds. Once we figured it out, we could cause it at say, 48mph against a headwind, turn around and do it at 56 with the tailwind. Or various combinations of grade and speed, as long as there was enough constant road to find "the" speed and get everything stable. Our conclusion was that it was a perfect example of large scale averaging of conditions around a baseline.

Carbs were meticulously checked, cleaned, rebuilt, rechecked, as stated, and everything except the chambers was dead-on perfect. Nothing made any difference until we switched chambers, and when all was said and done, we replaced pistons & chambers with new parts = end of story.
52mph = 2905rpm by the book.

Paul- You are mostly right, but it all goes on the definition of "weak springs". Some distributors came that way, and many got that way. I first noticed the incomplete return when the cars were new, though not to the degree of worn ones. It is true that it may cause jitters at low speed, but at the speeds we are dealing with it usually doesn't do so, unless there is some specific resonance between the various parts. I've seen many cars with worn-out distributors which behave evilly at low speeds, but once you get to where the stiff secondary spring is working, no problem - and that should apply above 2500. When the distributor is vertical, the weights are resting on the little bearing ribs on the drive plate, giving much less friction than when they are laying on the cam surfaces, as they do when the dist is half on its side in situ. Try it, I did!
FRM
FR Millmore

I had the same problem and after years of searching, yes years, I discovered my floats wer set too low.
Ken Knize

"the air velocity through the now smaller venturi increases enough to cause more fuel to be pulled through the jet"

Which is precisely how the damper causes the mixture to be richened when the throttle is opened, except that is increased air velocity through the same sized venturi.
Paul Hunt 2

Thanks for all of your comments/responses. Some additional info - the throttle shafts have been rebushed/replaced, the needles carefully centered and the mixture checked with the traditional "lift the piston" method, colortune and the color of the plugs after driving. The float level is an interesting possibility as I have had trouble with sticking float valves, unfortunately sticking open not closed. I have new floats in both carbs, unfortunately the new plastic ones with no adjustment other than washers under the valve seats. Does anyone know what level the fuel in the bowls should be? I found somewhere in the archives a number of mm below the lip of the bowl and they looked close. I will try pulling the choke a little bit next time I drive it and report back.

Thanks again, Don
don scott

Did you drop-check the pistons?
Float levels with the wretched plastic floats are the same as with the old style - the side opposite the valve will be 1/8-3/16 from the lid rim raised edge which will make the "top" of the float parallel with the lid. That's with the weight of the float only on the needle valve. While float levels are the first thing you should check in any carb, and I am therefore astonished at Ken's "years of searching...", they are not all that critical - a mm or two either way won't cause any severe problems once the mixture is adjusted with the level at some figure.
Try the choke method which will make it richer in the problem range. Then you can try setting the carbs 2 or 3 flats lean and try it in the problem range - it will run poorly otherwise, but if it helps/cures the trouble you'll know it's too rich there. No extensive driving whilst lean!
FRM
FR Millmore

As FRM says float height isn't critical. When I had a problem with a float valve stuck open on the V8 I cross-connected the OD switch with the fuel pump wire (I have in-line fuses in both close to each other) and used the OD switch to power the pump to fill the float chambers then switched it off again until it started to stumble. I couldn't detect any difference in running between the two points, and the stumble only started shortly before the float chambers emptied and the engine cut out altogether.
Paul Hunt 2

This thread was discussed between 24/08/2006 and 28/08/2006

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