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MG TD TF 1500 - engine back pressure

I am at 750mi on my TD's engine rebuild and it still has blue smoke which in most noticable at idle.( It may smoke when I am driving but the passing air dilutes it.) I have had several suggestions as to cause. Some say it takes 1000-3000 mile to break in the XPAG rings. Others suggest I check the head gasket, however their is no oil in the water and it looks like light blue smoke. It does not smoke until it is warmed up so I think the valve seals are good. I am suspicious I may have back pressure problems. The tappet cover tube seems clear. However when I remove the oil cap in the valve cover it is full of smoke. My TD had the original air cleaner replaced with pancake filters. Now, there is a 1/4 inch hose from the original cover pipe to the left carburator with a PCV valve in it. The vertical pipe from the valve cover has a loosely fitted 1/2 inch bolt in it and a 1/4 inch stem comes out the side to attach to the carb hose. My question is how can I get more carb vacuum into the valve cover. I thought I'd try replaceing the bolt with a tighter seal and then try to put a Y in the hose so I can get vacuum from both carbs. Does this make sense?
Russ Oakley

I would first of all remove the PCV valve.You do not get enough pressure differential at the pancake filter to open the valve. Try it straight through and you should see a great difference. IOn the TF application all you have is a small hose from the valve cover direct to the back of the filter.At one time I installed a PCV valve directly into the intake manifold. It did not do any better than the original set up.
Sandy Sanders

I agree with Sandy, PVC valves can "stick" and the problem you describe might be the result. Take the valve out and see how it works.

I was told that the PVC valve keeps a backfire from igniting the gases in the crankcase. Is it safe to operate without one?
Russ Oakley

Russ, for the the pvc valve to function correctly, the plumbing would normally be from the valve cover to the manifold side of the carberators. A 1/4" line is probably to restrictive in volume if you have a non vented cap on the valve cover but the down tube should take care of the pressure. Typical vent hose configurations on LBC's was to run the line into a semi-closed air filter housing and allow the crankcase gases to be pulled through the air filter and into the combustion chamber. While this will help with smog, it is also garaunteed to oil up the filter on which ever carb you run it through. if the line is tapped in between the carb and the filter then you may risk a crankcase back fire. You will also gum up the carb. If you can mount the line near the out side of the filter without the pvc valve then as the carb pulls air it will take must of the fumes to the engine. On the issue of the smoke, I would be a little concerned. I am certainly not an engine expert. I have rebuilt two chevy V-8's (305 and 396) and four British 4 bangers. I haven't had any that was burning oil to break in the rings as many would state. My father inlaw had one of my Sprite's rebuilt and it smoked from day one and never stopped until I sold it. I'll probably step on some toes here but I think the rings bedding in thing After more than about 50 miles of up and down throtle is usually a case of excessive clearences some place (valve guides or piston ring to bore). Just my two cents.


Russ - I don't know why someone would have installed a PCV valve in a TD, they didn't come with them originally. The rocker cover vented into the original air cleaner through a restrictor. The blue smoke should like the rings haven't seated properly (I have the same thing on our MGB and I am still burning oil after some 10,000 miles). All of the other engine rebuilds that I have done on both the TD and MGB had the rigs seated by the time I reached the top of the hill a mile from our house. This is the way it should be, I would disagree with those who say it takes 1000 to 3000 miles to seat the rings. If you are still blowing smoke at 1000 miles, you will have to make a decision on whether to tear the engine down, have the cylinders honed and start over with new rings or (my choice) wait until I have another reason to disassemble the engine and do it then. What kind of break in oil are you using? The best is 30 weight non detergent. I have been told that one should never use any of the multi weight oils as they are too slick and will retard ring seating. Cheers - Dave
David DuBois

I agree with Dave except on the oil. I've always put in what I was going to run until the TF rebuild when I used Valvoline racing oil for the extra Zinc. The exception for me is I would not use synthetic for any break in. I changed over to synthetic on my B several years ago and turned a non leaker into a garage oil slick. Went back to castrol 20-50 but still leaving it's mark. I think I'd get somebody to drive behind me and see when the smoke is most prevalent. If you get the big puffs between shifts it's most likely your valves. If it's steady then it's probably the rings. Might get another opinion or check the plugs and see if it's really oil or hopefully just running rich.

I had an MGB that used to burn 1 Qt. every 300 miles....smoked like a furnace. Used Alemite CD2 at the next oil change and consumption decreased to every 1000 miles. added another at the next oil change and never burned a drop between oil changes from then on. Possibly the rings are stuck and this product works marvels. Also Mystery oil added to oil does the same thing.

Russ, my own car took about 1500+ miles for the idle smoke to completely stop. At first, if you let it idle for a few minutes and reved it, it would smoke up the neighborhood. Another TF in town here did about the same. Both eventually totally smoke-free. In theory, that shouldn't happen, but did to me-evidently a function of the type of ring and/or machine shop honing pattern. What kind of valve stem seals did you use? Modern umbrella type or the o-rings/deflectors? In agreement about losing the PCV valve- could be causing crankcase pressue to build? When this motor was designed in the late 30s, no one had even thought about a closed crankcase system- so one is not needed. I recall a time honored way to seat rings is to basically drive the heck out of it for a bit- give that a try? If no better after a couple thousand miles, something is wrong. I would still start with the valve guides/stem seals before pulling the bottom end. George
George Butz

Thanks everyone. I took out the hose with the PVC and blew through it. There was a major difference in resistance without it. The pancake filter has a connecting tube soldered in with its opening inside the brass mesh filter and about an inch from the carb intake. I'll take it for a spin in a few weeks and let you know if the smoke stops.
Russ Oakey

Russ - One of the things recommended for getting the rings to seat is to accelerate hard in top gear to increase the pressure in the cylinder, pushing the rings out against the cylinder wall. I think that is where I have has so much success (excep the last time withthe MGB), we live at the bottom of a half mile long hill and I can immediately do the hard acceleration in high gear three or four times going up the hill.

Lavern - You may be right. I have always had good results withthe 30W, non detergent oil except for this last time, so perhaps the oil is not all that important as far as ring seating is concerned. Cheers - Dave
David DuBois

My theory, take it with a grain of salt.

I think the way the cylinders are honed has a large bearing on ring seating. The honing creates a peak and valley pattern in the metal much like a file. Those peaks and valleys are useable for a short time to bed the cylinder rings, as Dave D says, the more pressure in the cylinder, the more the rings are forced against the walls of the cylinder, and initially, the honing pattern wears the rings in. The window for this process is pretty small, the first half hour of running or so.

Right now, my XPAG engine has one 10 second burst on it to assure that it would start and run. My next start will be when I will check my throttle range (no bogging down so I can take it on the road), and the third when I will drive the car under load to bed the rings. Then I'll fine tune the carbs and the ignition.

In the old days, the rule of thumb was no high speed operation, and no steady speeds for the first 500 miles. I agree to not winding up the engine without a good load, but I'm not concerned about high engine speeds during break in. I do agree with the varible engine speeds to avoid creating a ridge in the cylinder where the piston stops in its travel, as microscopic as a change as it is.

On the MGB I sometimes care for, the engine was using oil at a rate I thought we would have to live with until a reason came along to rebuild the engine, about a quart every 500 miles or so. After some good high speed long trips, especially the Gatlinburg run, consumption dropped to about a quart every 2500 miles. I'm convinced that the cylinders and ring lands had a glaze (carbon) on them from poor carburetion and poor valve setting. Getting those things right and then hard running freed everything up and solved the problem. At least I think that's what happened! In retrospect, I probably should have tried a gas treatment such as Sea Foam to clean out the carbon.

Dave Braun

Hello Russ
This was sent to me on an email, this might be the cause of your continued smoke. It is from an article by Keith Ansell on new engine oils killing our engines. I could post the whole article if any one is interested. I think our MG club is going to print it in our next issue.

Keith M. Ansell
Foreign Parts Positively, Inc.

“Redline oil and others are suggesting a 3,000-mile break-in for new engines! Proper seating of rings with today’s lubricants is taking that long to properly seal. Shifting to synthetics before that time will just burn a lot of oil and not run as well as hoped.”
John Hambleton

I agree with Dave D. The rule of thumb was always 30 wt ND oil for the break-in period (500-1000 miles) to help the rings seat. Then drain and put in oil of choice for the type of use the car will get.

Aircraft piston engines went so far as to use mineral oil for the first 25 hours.
Chris Couper

This thread was discussed between 12/02/2007 and 15/02/2007

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