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MG MGB Technical - sputter...continues

I thought plug wires cured my problem, they did not. I had put new wires on cleaned my plugs and drove for two days with no trouble. On my way in to work this morning, about half way here it started to sputter again. When arriving I instantly pulled no 3 plug, again sooty black. All other plugs looked good (greyish white). Inside the cap (which I also swapped last week) number 3 showed some pitting, all others looked good. What is going on with no 3?

David

David.
I know it sounds funny but try changing your rotor. I had an MG-TD come into the shop running on 3 cylinders. I wnet through the entire engine as I had rebuilt it a few months before. compression check-changed plugs-replaced # 3 ign wire-replaced cap. all did nothing for it. finally I replaced the rotor and all was well. Put old rotor back in and it dropped #3 . It looks absolutely ok in all points but will not fire #3.Distributer bearings are new so no wear in that area.
I have been working with English vehicles since my first Morris 10 in 1947 and have never seen or heard of that problem .
something new every day.
Sandy Sanders
Sandy Sanders

Is it cause or effect? Try and examine No.3 plug *before* it starts missing, to try and ascertain whether the sooting is causing the missing (sooted before it misses), or the missing is causing the sooting (not sooted). Also once it *is* missing, clip a timing light on to each plug lead and see if No.3 is flashing as regularly and consistently as the others. If No.3 cap contact is showing more burning than the others, then it could be that contact is misaligned and the gap in that circuit is much greater than the others. Have you changed the cap and rotor arm (by continuing the old thread rather than starting anew I would have been able to scroll up and see)? What are the plug gaps? A friend recently had a similar problem, one of the things I suggested was plug gaps, and he found them at 65 thou! Correcting this seemed to cure it - on a short run - then his clutch release bearing disintegrated. Does cleaning the plugs cure it for a while? Maybe you have a manifold vacuum leak on No.4, and are richening the rear carb to compensate, which is fouling No.3.
Paul Hunt 2

(Paul, good point i should have continued the old post) I have played with near everything. I have put on a different rotor, switch caps, switched plug wires, checked plug gaps, switch plugs around. Since the no. 3 is sooty, I can clean it and resuse it. I cleaned it and put it in number one, it is now clean and the one I put in 3 is now sooty. It takes a couple of days for the fouling to take place. Someone had mentioned sticking valves, would that cause the sooty plug? I have an extra set of points and condenser, going to swap that at lunch. After that I am puzzled.
David

I have had this problem. It has been the rotor, the wires, and a spark plug. These parts failed three separate times on different cars, all the parts were new.
The rotor was a build defect, along with the one bad wire in the new set. The new spark plug may have been dropped when I installed the new ones.
Paul can you explain to me where the vacuum leak on No 4 would be on the example you have given?

Thanks,
James

James

James, the most likely leak to cause #4 to run lean would be at the intake manifold to head joint. Pass an unlit propane torch around the joints if the idle changes you have found your leak.
John H

James. A vacuum leak at the joint between the intake manifold and cylinder head will show up first on cylinders two and three. They have a "siamesed" exhaust port which means that cylinders two and three run slightly leaner than cylinders one and four. If, however, the rear carb is set sufficiently rich that, when an air leak is present, the number three cylinder fires properly, the number four cylinder will be far to rich to burn properly. This will result in a sooty number four plug as the mixture is burning inefficiently in that cylinder. A check of the level of the jets in the two carbs might be in order. They should be very close to being identical in depth.

David. A sooty spark plug indicates inefficient burning of the mixture. This can be caused by a weak spark, an excessively lean mixture, or an excessively rich mixture. The last time I saw this problem it was because the owner had not pushed the spark plug wire fully down into the distributor cap terminal and it was not firing the plug efficiently.

You need to hook up an engine analyzer to your engine and see what it can tell you about what is happening with cylinder number three.

Stuck valves can be diagnosed with a compression check. The standard compression check is commonly understood and a tech article is available on my website, www.custompistols.com/ if you wish to refresh your memory. This will detect a valve which is stuck open, burned, or has a burned or pitted exhaust seat. A running compression check will detect an intermittent problem.

A running compression check is performed by removing a single spark plug from the engine and attaching a compression gauge to the hole. Start the engine and watch the compression gauge. It should rise to the 80-90 psi range. Release the pressure a couple of times and watch what happens. An intermittent sticking valve will demonstrate itself by lower running compression than the fully functional cylinders.

Les
Les Bengtson

DAvid,
I had a similar problem, sputtering.. tried everything as well. I was going crazy until one day the thing just stopped running. Changed the rotor presto..running smoothly and strong
Peter Murray

Still same results, it is not looking good. I have at this point changed the points & condensor, changed plugs, wires, cap, rotor, put everything in a different distributor and it still wants to foul number three. I changed points and cleaned the plug at lunch yesterday then drove the car home. Sputtering was minimal but when pulling the plug to examine it, it was already much darker than it should be. All other plugs greyish white. I am leaning toward the sticking valve now. (number 3 is still turning "sooty black")

I removed the valve cover and observed the valves moving, they are opening and closing, when fully closed the valve does have rocker to valve clearance (I did not use a feeler guage to see how much clearance though). So if it is a valve, it seems to be very slight. If the valve is not closing completely, how open would it have to be to cause this problem? (I assume anything but 100% closed is enough to make a difference.
David

*in regards to mention of a rear carb being rich. I am running a single weber 32/36 with pierce intake. I have the performance gasket between the intake and head, no leaks seem present there.
David

The siamesing Les refers to applies to the intake ports of all cylinders, of course, I was forgetting. As such any vacuum leak would affect both cylinders. The carb could be maladjusted to compensate for it at idle, but would be over-rich when running. However this would affect both cylinders on that carb.

A sticking valve will cause a fouled or sooty plug as the poor combustion probably won't allow it to get at hot as it should, but this should be very obvious from a chuffing in intake or exhaust and a rough idle - unless it only occurs at idle.

You could eliminate cap and rotor, and possibly leads if they reach the new plugs, by turning the distributor 45 degrees and moving the leads along one plug in the appropriate direction.

I'd still be very interestd to know how the plugs compare after running a while but *before* the missing starts.
Paul Hunt 2

Paul,
The plugs before missing are all clean. After driving for about 2 days, number three will be sooty black and the missing will occur. Clean the plug and you are good for another two days then black again. (I have swapped plugs around and tried different brands) Currently running Champions as called for. Since I have replaced the cap, rotor and leads and plugs, I feel all of that has been eliminated without needing to rotate the dizzy back. It does idle rough, but I do have a mild street cam in it also.
What do you mean by "chuffing"?
David

FWIW I had a valve cause a plug to foul and was able to spot it with a compression check. It was caused by a small but solid piece of ash cooked onto seating face of the valve. I suspect the cause was oil running down the valve stem (stem seals shot) after switch off.
Steve Postins

Speaking of, have you indeed performed a compression check David?

Im not saying to go ahead and take the head off but if you check John Twists University Motors YouTube vidoes there is one on how to check for valve leaks by taking the head off, turning it upside down and filling the chamber with gas/kerosene and checking back to see if it has kept its level.

James

James
I have not performed the compression check yet, I was weighing out all other possibilities. I am borrowing a compression guage this afternoon though, so I should know if it is a valve soon.
David

Paul. Actually, the simamese port I was speaking of was the center, exhaust port for cylinders two and three. My experience verifies the old contention that the exhaust gases are not expelled as efficiently from cylinders two and three, due to the simamesed port, and that the incoming charge of fuel/air is diluted slightly by remaining burnt gases. Thus, cylinders two and three tend to run somewhat leaner than cylinders one and four.

With the Weber DGV, I would have to know what is hooked up to the intake manifold. Unlike the SU system, where all of the PCV and pollution control system hoses are connected to the balance bar between the carbs, the manifolds for the Weber have these connections in the "arms" going to the cylinder head. Check any vacuum lines you may have going to the intake manifold by starting the engine and pinching shut the various lines while you listen to the note of the engine. If the idle speed or sound of the engine changes, you have a vacuum leak on that line. The intake manifold also has some fittings/plugs in it depending on who made the manifold and how it is required to be connected up. If there are any plugs, spray some carb cleaner around them as you listen to the sound of the idling engine and see if you can detect a vacuum leak.

Also, make sure the carb is tight on the intake manifold and there are no leaks around the base of the carb. Not uncommon on a DGV.

The DGV was made by Weber. Weber licensed the basic design to Holley which produced a slightly modified version of the DGV known as the Holley 5200 model. Used on a number of cars, by various producers, here in the US. Any competent, older mechanic, who started his working life back when cars still had carbs, will have no problems working on the DGV.

We have reached a point where the internet is not capable of effectively defining the problem. The engine needs to be diagnosed using an engine analyzer, needs to have a running compression check performed, and needs to be looked at by someone who can evaluate the symptoms and know what follow up tests should be performed.

The hobbyist can do a significant amount of maintenance on an MGB. But, there are times when an hour of a professional mechanic's time can save one days of frustration. I had, exactly, the same problem on an engine equipped with a Weber DGV. Spent three days trying to solve it. A professional mechanic friend found the problem, a bad gulp valve, in about 20 minutes.

Les
Les Bengtson

Final conclusion after compression check:
This didnt make sense, but after talking to a mechanic friend he explained it. As you know, my number three plug is fouling. Clyinders 2,3,&4 all checked out good. Cylinder number 1 only has 45 psi.
How does this foul number 3 was my question to him. His next question was what is the firing order, I told him 1342. He said since number 1 is not at 100% it is not taking in all of the fuel it should be. Then number three opens and takes in all of its fuel, plus the excess number one didnt "suck in". Therefore causing number three to be rich, and the plug foul.... Looks like the head is coming off.
David

Great. Please keep us informed on everything.
James

Will do. Thanks!
David

Les - I know, but it reminded me that the intake ports are shared as well 1 with 2 and 3 with 4, hence you cannot get a vacuum leak on 4 and not on 3, which was what James was asking.

'Chuffing' is some obvious beat in the exhaust or intake just on one cylinder which indicates a bad valve. If the fouling only appears once the missing has started then that indicates it is the missing that is causing the fouling, i.e. it is an ignition problem.

Interesting theory that low compression on 1 is causing fouling on 3, which goes against the above, i.e. if this is really what is happening the plug would be fouling before the missfire started. But I can't see this happening anyway, low suck will only pull a small amount of fuel/air mixture into the cylinder, but it will be a mixture of approximately the correct ratio. It won't result in a quantity of fuel being left in the intake to be pulled in through by the next cylinder. Common point injection i.e. into a throttle body maybe, but not carbs. Carbs only deliver the amount of fuel that is called for by the airflow. If that is low, the fuel delivered will be low. I also have to say that such a low compression on one cylinder should really have been obvious from the engine and exhaust beat.
Paul Hunt 2

Let me know if this makes sense. So its a single weber downdraft that has an inlet port on the left going to cylinders 1 and 2 and one on the right going to 3 and 4.

When the engine is running there is a fuel air mixture being created at one central location and being split between the two inlets.

Since the balance of vacuum between both inlet ports has been tilted, it is running richer on the right hand side due to the firing order of 1342...???
James Bekman

Paul,
Yes it does have a weber with pierce intake and the intake does take the fuel to all 4 cylinders from one common point. When the valve opens, it sucks the mixture left or right, depending on which cylinder is firing. Think about how quick the vacuum changes from 2 being strong then 1 barely sucking, it would have to leave some fuel in the intake, which would go straight to 3 being next in line to fire. The fuel air mixture could not readjust itself on one cylinder 100% accurate that fast.

James,
The best it was explained to me. With number 1 not taking all of the fuel from the intake, when number 3 opens it takes in all it would normally take, plus the excess left behind that number 1 did not take in. It is leaving just enough to foul number 3 after about 2 days of driving.

So Paul is correct, the mixture is causing the fouling, and the fouling is causing the misfire.


It is being looked at now by a mechanic, but I am open to any other thoughts. I do only have 45 psi in cylinder number 1. Cylinders 2,3,&4 all check out good, but 3 is the one that fouls. Ignition faults have been weighed out, plus either way, I must get number 1 compression where it needs to be. Now they are checking to see if I have a bad valve or piston ring.

David

Hi David
Interesting thread you have going here. Can,t quite get my head around the fact that your no.1 compression is proven to be low and yet the engine runs ok for a couple of days. Before you go ahead and remove the cyl. head I would suggest that you thoroughly diagnose as to what is causing the lack of compression pressure. Get your mechanic to do a cylinder leak down test to confirm or not that it is actually a valve or ring problem. I have had an MGB with a broken outer valve spring giving similar symptoms. Also measure how far each valve goes down at full lift it could be possible that you have a dead lobe on the camshaft the result of which is an uneven cupped out surface on the cam follower and as the follower turns in it,s bore tappet clearance is lost or changed as the follower sits in different positions. IF you are getting varied rattling sounds from the tappets or constantly having to adjust valve clearances this area could be worth a look. Hope this might be of some help
Please let us know what you find Gary
Gary

This thread was discussed between 15/04/2008 and 20/04/2008

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