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MG MGB Technical - Sparkplug #4 contaminated

Hello all there, hope you can give me some advice on this thread for my 1976 RB mgb roadster. Its an original UK for European market model sold in Holland.

After a long period having problems with the engine cutting out when hot and replacing lots of ignition items, this problem is solved! Just replacing the coil with a new standard coil where only a short term solution. After installing the Lucas sportscoil my problem seems to be realy solved. Car runs great, even under heavy conditions and very long runs.

But!!! now my old problem is still there; a heavy poluted Sparkplug #4, the #3, #2 are also poluted but less and #1 is OK.

What have I done until now;
Did a complete head exchance, leadfree, new valves and guides. this gave no improvement in relation to the sparkplugs.
As far as I can judge the carbs and ignition are in good condition, new sparkplug installed (NGK BPES)
I know the car is consuming a fair bit of oil aprox. 1,5 liter / 1000 KM.

The compressioncheck shows a good performance (high compression engine)

My concern is that although a have a good compression there is something wrong with the scraperrings in the pistons. I have some questions on this;
-How can I check this before taking the engine apart? -Do I have to take the complete engine out? -or are there other reasons for such a result on the sparkplugs.

The image is showing the (new) sparkplug after a 350 KM ride.

Hope you can give me some advice on this.

Jacob



J Halma

Jacob.. If the head has good valve stem seals then your problem is definitely a ring problem. They can be changed with the engine in place.
Sandy
conrad sanders

Before going into the engine, I would replace the distributor cap and check all the spark plug wires, especially #4, for good conections.

I always like to start with the simple, easy and cheap problem solving method.
G L Gimbut

The head chooses to leak (if it does) right above the distributor and adjacent plugs.

Dan Robinson

Thanks for the reply on this thread,

GL, This is already been done, with no result.
Dan, There is no oil leak from the head.
Conrad, good to hear that the engine can stay in place.

Is there a ''roadmap'' how to exchance piston rings with the engine in place?
Jacob
J Halma

Dan is talking about a coolant leak I suspect. When this is from the heater valve it can drip onto the distributor cap and cause running on 3 or less cylinders, but that should be obvious from misfiring. The plugs are above this and the head gasket of course and so out of the way of any dribble-type leaks. If it is generally running OK, but a plug fouls after a while, it is more like internal oil fouling from valves/guides or rings/bores.

However with 4 worst, 3 and 2 a bit, and 1 OK I'm wondering if it is the crankcase ventilation system. For a much earlier model with PCV a ruptured diaphragm can cause this, and consequential high oil consumption, the oil being dumped into the inlet manifold hence affecting each plug differently. With the carb ventilation that a 76 should have both carbs and hence all plugs should be affected equally, but it's possible that the oil has bunged up the front carb.

It is relatively easy to disconnect the hose between the carbs and the front tappet chest cover, sealing the carb ports, clean the plugs, and go for a drive. Hopefully you should be able to tell if this has made a difference before going 350kM!
Paul Hunt

Thanks Paul for your comments,

I just disconnected the hose at the Y-joint (from front tappet chest cover towards front and rear carb).
I noticed a lot of black oil in the hoses, mainly towards the rearcarb.
What is the reason for this oil getting there? Is it extreme vacuum at the rearcarb? or extreme amount of oil in the tappet box or overpressure there?
Or can this be caused by an high oil level in the engine?
I have not been able to hit he road, maybe coming weekend, to see the result of the hose disconnected and sealed of.

BTW, HIF4 carbs are in the car.

Jacob
J Halma

Jacob - looks like you are on the right track. Carb suction is at a relatively low and constant level, with no components to fail, which is why it is so much better then the earlier PCV valve. However the ports can get blocked in one carb or another, so if there is excessive oil coming from the crankcase *and* a blocked carb, it will only go to the other carb and cause more fouling of its plugs than the others. On the other hand it's possible the rear carb has a problem that means it is generating more suction, which will tend to pull oil out of the crankcase and put more into that carb, but if the other carb is working normally it also should get some oil but not as much. Remove the breather pipes from the carb ports and see if both are sucking air at idle. If you have a vacuum gauge connect that to each in turn to see if they give the same readings, or alternatively with a cigarette or other smoke source you may also be able to get some idea.

Excessive oil coming from the tappet chest port whilst not being common *is* reported from time to time. There should be a baffle and some gauze in the cover which acts as an oil separator and flame trap to prevent this happening, but I've heard of at least one case where these were in place but it still sucked up oil until the cover was replaced with another that looked just the same. An alternative is a catch-bottle, with a port connected to the tappet cover, and another at the top connected to the carbs, the principle being that any oil that is sucked up into the bottom of the bottle will remain there until the engine stops then drain back down, rather than getting up as far as the top port and from there into the carbs. Of course that depends on how far you drive in one go, and how much oil is being sucked up!

High oil level could cause it, there were a number of dip-sticks, but I have never seen any measurements for the distance betwen the flange and the marks for each to make sure you have the right one and not one with them higher than they should be.

Excessive crankcase pressure from blow-by could also cause it, this can be checked by removing the oil filler cap (with the breather hoses reconnected). The first thing you should notice is a slight change in idle note. The oil filler cap restricts the amount of air that can flow through the crankcase into the carbs which results in a slight negative pressure in the crankcase, removing it allows more air in which weakens the mixture. The next thing is that at idle you should be able to place a sheet of paper over the filler hole and it should be lightly sucked down (incidentally restoring the idle). If it is blown off you do have blow-by.
Paul Hunt

Paul,
I done the tests you sugested it gave me some information.
First I drove the car with the crankcase hoses disconnected and carbs sealed off and open hose from the tappet chest cover. Good performance on 20 Km run (highway and city). Then measured the vacuum on carbs at crankcase hose connection at idle. could not see any vacuum on vacuum gauge. On trottle movement did see some week response on the vacuum gauge. both carbs same reaction. So I Say there is not an exesive vacuum at carbs, this a normal behaviour I think.
During the idle measerments when engine heats up more I see a kind of slightly oil mist coming from the open hose (tappet chest cover, normal?
Then had a look at the sparkplugs, noticed that #1 & #2 where dry and #3 & #4 where wet but not a oily kind of wet. unburnt fuel?
Re-connected the crankcase hoses and did the test with the paper on oil filler hole. a lightly sucking down of the paper as a result, meaning no blow-by.
#3 & #4 wet, did a check at the mixture at both carbs (liftingpin check)found that rearcarb had a rich mixture.
Hit the road next week again to see some improvements.
Jacob
J Halma

I would have expected to see some vacuum at the carb ports, but as I have never measured it myself I don't know what to expect. Seeing both give the same reading as the throttle opened is good, as the vacuum *will* increase then while the piston is being held down by the damper. Seeing light suction at the oil filler is also good. Visible oil mist at the open hose on the front tappet chest is not so good, there is likely to be even more with the hose connected to the carbs. Could it be oil smoke you are seeing and not mist from droplets? I've not noticed this on my roadster but there is smoke from the V8 when hot, switched off, and the oil filler cap removed. Initially I was bothered about blow-by and burning oil, but oil consumption is probably only about 1 pint in 2k or so, so slight smoke doesn't seem to be an issue, and is what the crankcase breather is designed to get rid of. Rear carb being rich is interesting, and is a good reason for checking the ignition and carb setup when you have a problem before delving into anything more complicated.

Paul Hunt

Paul,

You are right, its more like oil smoke. No mist from droplets.
Jacob
J Halma

After adjusting the mixture on the rear carb (towards lean), I see sparkplug #1,#2, and #3 having a nice color. Sparkplug #4 is still a little bit Black but not as black and oily as before.
So maybe need to adjust the mixture a litle bit more.
Also I keep the oil level low in the engine, this is also giving some improvement, I think.
I will have to find out if I have got the right oildipstick in this engine, with the right marks on it.
Any information on this is welcome.
Keep you informed on my progress.
Jacob
J Halma

Just one different plug points to incomplete combustion or oil in that cylinder, rather than carb adjustment. Try removing each plug lead in turn when the engine is idling (needs care to avoid shocks) and see if the idle speed drops by the same amount for each or not. Also do dry and wet compression tests.
Paul Hunt

This thread was discussed between 13/09/2008 and 03/10/2008

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