MGC Forum

Fellow MGer's,

Please let me provide some pertinent background information prior to asking the question...

I've had a B/GT since 1981 so am very familiar with these cars. I have been an aviation mechanic all of my adult life. I am not an ASE vehicle mechanic but do 99% of my own work on an assortment of vehicles (can't do machine work, auto body, alignments, etc., due to tooling).

I purchased the 69 C/GT back in August. She is a wonderful car, what I'd call an 85%er due to my ailment of perfectionism... I have been through the engine as far as tune up work and can see that the carb spindle bushings will need replacing. Not devastatingly loose, but again, this is my "level of perfection". The spark plugs looked rather ugly and I knew the carbs were out of tune so I started with the basics; set valve lash, confirmed timing, tuned carbs, replaced plugs.

One of the previous owners installed the Maniflow Dual Exhaust system on the engine. This allows the front three cylinders to breath through one pipe and the rear three through another. On my car the left pipe at the rear is for the front three cylinders, right pipe rear three.

I performed a compression test (engine at operating temperature) several months ago with the following results:

#1: 145
#2: 150
#3: 150
#4: 140
#5: 145
#6: 150

I have receipts which appear to show the engine was rebuilt back in 1989 but I have no idea what the mileage has been ever since...

Now for the dilemma... All the plugs now look perfect except for cylinders 2 and 3 (both 150psi compression). They are much darker (almost black) whereas all the others look excellent (like those you'd see in a rebuilt engine!). She burns practically no oil but you do get the blue puff when starting again after a long drive (which is normally worn valve guides and/or seals). I also have some sputtering back at the left tailpipe at idle which would be from cylinders 1 thru 3.

I put a vacuum gauge on the intake manifold during the tuning process. Idle vacuum was good other than a very slight, but rapid wavering of the needle. This wavering would completely stop at any RPM above idle, nice and steady. If you go to this link and select scenario 3 you will see what my needle was doing:

http://www.secondchancegarage.com/public/186.cfm

The author of this webpage says the "needle shake" may be caused by a high performance cam and the vacuum will be lower than normal. Mine vacuum is not lower than normal (20 InHg). She does have a slight lump at idle but I have have no other C to compare her with and no paperwork showing a camshaft purchase. I'd have to remove the valve cover and measure travel to confirm valve lift. Worn valve guides can cause the vacuum gauge needle to waver but this is usually a fast fluctuation between 14 and 19 InHg. Mine is slight, like the link above shows...

When setting up the forward carb (which feeds 1 thru 3) I had to set the jet quite a bit richer than the rear for the 50 rpm rise/fall-off as part of the normal SU tuning.

I am quite certain that 2 & 3 may have either worn seals or guides but why the sputtering? Could this be a combination of the worn carb spindle bushings along with guides and/or seals causing this whole scenario? I know that a carb out of tune can make the exhaust sputter when too rich (black smoke) or too lean, yet the number 1 plug is perfect in color so... Back to the confusion!

Please take some time to digest this information. Any feedback you could provide would be more than welcome! I will gladly provide any other info you may need. She runs like a champ down the highway so I am no overly concerned. I just like things done right!

Paul

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Those who confuse Burro and Burrow don't know their @ss from a hole in the ground...

Paul,
The design of even the North American intake manifold ( which placed the carbs further apart than the home market cars ) is such that 1 and 6 really don't get the charge they should, and can end up running lean when the remaining 4 cylinders display perfect plug color.
The piston lift method to determine mixture assumes the carbs are in "as new" condition, and is fraught with problems, not the least of which is that the butterfly in carb not being adjusted needs to be fully closed. Another is that the pins can wear and not lift each carb piston equally/enough. Worn throttle shafts and or bushings will affect mixture far more at idle than at WOT when vacuum is much lower. High vacuum at idle will have you setting the carbs rich if the shafts or bushings are worn, to compensate for the resultant lean condition. Most of the need to compensate goes away off idle, which makes using the lift pin to set mixture at idle, nearly useless.


Putting the carbs into " as new " condition would be a first step I think, then repeat the set up of them and see what you have. Going to a triple SU setup will solve the problem of feeding each cylinder equally, and help overall driveability, and be the perfect enhancement for your improved exhaust system.
It is tough to point to one specific area on which to concentrate. Though your problems may be " annoying " , living with them may be the only solution short of a rebuild if the carb rebuild doesn't fix it.

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Andy

NAMGBR# 20-7738

Andy,

I'd like to go "triple carb" one day in the future. This will probably be accomplished after I finish with my complete rebuild of the B/GT... I plan on removing the engine/tranny to repaint the engine compartment along with all the other normal things accomplished with the engine removed. I'll ceramic coat the manifolds and plan on lightening the flywheel (or going aluminum). Let me ask you this question if I may... Can I use the two original HS6 carbs and add another (plus linkage & intakes) to accomplish this?

Paul

P.S. I'm probably going to rebuild the two already installed in the very near future.

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Those who confuse Burro and Burrow don't know their @ss from a hole in the ground...

Paul,

Downton supplied one short neck SU to be used in the front position, along with linkages etc to fit with the factory SU's.
You only need an additional HS6 for the conversion.
With Herbold inlet manifolds you can use any HS6. A short neck front SU is not required.

The difference in angle used by Downton is no more than changing the float bowl insulating rubbers to a different set to get the inclined HS6.
All HS6's are the same just different mounting rubber inserts used in the float bowls where they fit to the SU body. [ comments that they are different SU's is all Bull S**t ]
The short neck piston chamber was the only difference as well as which side the float bowl mounts onto the SU body.
There are 2 versions of the HS6 depending on the float bowl mounting position.

I have attached 2 photos of my setup as well as 2 photos of my lightened flywheel.

Bruce.

Paul,
Yes, you can use the original carbs and add a third HS6. Your carbs will have floating needles, so a similar carb can be found, or you can use one with a fixed needle and convert your original pair to fixed configuration.
It is a matter of preference as to whether you add a second "front" or "rear" carb, but fronts seem to be favoured by most.
As for some of the other things to look for, please see my comments to Bruce, below.


Actually Bruce, not so much Bull S**t. The Volvo HS6's I sourced had different float chambers which are mounted such that the tops are considerably higher than those used on the MG engine. I'm not at home, so can't refer to the numbers, but not all HS6's are created equally. In my case, I had to source new float chambers to match those on my original carbs.

Joe Curto was a huge help with parts and advice FWIW. great guy to deal with, and likely forgotten more things about SU carbs than most of us will ever know.

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Andy

NAMGBR# 20-7738

Gentlemen,

So then the I would estimate the biggest question to be... Where does one acquire a set of Herbold style aluminum manifolds (I'd rather not have the steel ones...).

On page five of this link Michael Lippman mentions the possibility of manufacturing them again. Post #69

http://www.mgexp.com/phorum/read.php?48,2559151,page=5

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Those who confuse Burro and Burrow don't know their @ss from a hole in the ground...

Andy,

What I was referring to was that the main body of the HS6 is the same with 2 versions to take right and left hand float bowls and choke linkages etc.

You are CORRECT about the different float bowls, Downton supplied me with the wrong one for the front SU, the mounting position on the bowl for the holding bolt was 1/4" different to the factory fitted SU's.
I only discovered this a few years after installation when I wondered why the jet nut for # 1 (short neck) was so different to the factory SU's to get the mixture correct (or nearly correct) after buying the correct part # float bowl the jet nuts were all in a similar position. The float bowl can mount at a number of different angles to suit different engines it is just a matter of swapping over the neoprene mounting rubber.

This difference in float bowl mounting position is not obvious to the eye, even my Dynamic Balancing engineer did not spot it.

SU make a huge variety of different models but the idea that the inclined SU was a totally different carby to the factory horizontal one was B-S.

Bruce.

Paul,

Back in1986 I bought a set of standard as well as a set of +.010" throttle shafts expecting that I would have to ream out the bushes in the body and fit the larger shafts.
To my surprise the body bushes were hardly worn but the shafts were well worn. I fitted the new set of standard shafts and all was good again, they are still in there as I have not touched the SU set up since 1986 other than to add the intermediate insulation layer and the vacuum motor to lift #1 throttle shaft when the compressor relay cuts in.

I take the complete SU set up and heat shield set off as a one piece unit (as in the photo) and simply refit them as a unit. I don't even change the manifold gasket although this time I will have to fit a new one as the often used one is past refitting.

The modified GGL is a pretty good engine to work on in car. Take off the air cleaners then the SU set up then the headers and refit in reverse order, much easier than the factory clumsy arrangement with the manifolds connected, with the RHD ones a lot easier than LHD ones.

In your climate I suggest you make up a good heat shield, it took me decades to finally get mine right. Now the idle speed is 800 to 850 rpm in all normal conditions with or without the compressor operating.
The Downton set up as supplied with NO heat shield or black insulation blocks for the SU's was absolutely hopeless in our sub-tropical climate.

Bruce.

Aside from the obvious weight savings, and nicer looks of the Herbold manifolds, there really isn't any reason not to use the Maniflow setup, which is at least available.
Unless someone picks up where Gary left off, or you can find someone who purchased a set and now doesn't intend to use them, those manifolds won't be available to you.
You might try a WTB in the BST forum to see if anyone out there has a set they no longer want.

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Andy

NAMGBR# 20-7738

Bruce,

Yes, a good heat shield would be mandatory (polished stainless of course!). I will also have the exhaust manifolds ceramic coated to help.

Andrew,

I guess one advantage of the steel manifolds would be to have the outside ceramic coated (same color as exhaust manifolds) for heat reflection and good looks too.

Paul

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Those who confuse Burro and Burrow don't know their @ss from a hole in the ground...

Paul,

I just read your first post, for the first time, your compression figures look much better than mine ever did. A variation of 5 psi is normal and up to 10 is OK for a high compression engine. When my Downton head arrived I had a variation of from 3 to 5 psi, that was the only time it was ever that good. It didn't stay like that for long. I don't even bother to measure compression now, with this engine I would only be concerned if I measured it. It runs perfectly so I just accept it as it is. Sometimes it is best not to know, this definitely does not apply to aero engines which run at constant high power for hours on end.
Very difficult to pull over aircraft and check things after all, they must always be correct if one intends to land normally.

Now on to the exhaust spluttering at idle. This seems to be normal with these engines running dual exhaust systems with good headers, it is usually one pipe only and I have no idea why. My car has done this for what seems forever and it varies from time to time for no reason that I can think of. My engine runs perfectly so I just assume that this is normal for this engine, idle mixture is good everything is good but this occasional splutter happens from time to time mostly from one pipe but sometimes from both pipes. First off it seems to indicate a miss but I do not think it is ignition related. It could be momentary plug fouling when very hot after driving for a few hours.

Perfection with a BMC Morris 'C' series engine seems to be definitely in the "Too hard basket". The whole thing is at best a compromise but developed it really is a great engine for it's purpose and has a very long life.

Bruce.

Bruce,

The sputtering from my engine is only from the front three cylinders (which also have the two dark plugs in cylinders 2 & 3). I also mentioned having to enrich the carb for these three cylinders more than the rear carb. What I really think this boils down to is a combination of valve seals and/or guides for these two cylinders and a worn carb spindle shaft and/or bushings. I'm going to start off with the easiest route by rebuilding both carbs, then reevaluate...

I tend to sway towards "perfection" more than most I guess. Do it right or stay on the porch! I suppose that comes from my aeronautical background (36 years of maintenance/maintenance management and a Master of Aeronautical Science)... A few years ago I had a temporary job teaching Airframe & Powerplant students to help them obtain their FAA license. One of the things I told them about how important their job would be was comparing their work to an M.D. If the doctor screws up he might kill one, if we screw up we can kill hundreds... (wouldn't it be nice if our pay reflected that statement!!!) They got the point!

Paul

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Those who confuse Burro and Burrow don't know their @ss from a hole in the ground...

Paul,

A friend in the UK has all "Maniflow" manifolds and is very pleased with his MGC now.

Unless you can find a set of Herbold manifolds go for the Maniflow steel ones. Manifold was started by ex employees of Downton when Downton Engineering Works closed down.

I discussed ceramic coating the inlet manifolds inside and out with the people who did my ceramic coating. They said NOT to coat the inside of the inlets but coating the outsides would be very beneficial (they look much better than flaking paint too) they coated my stainless steel headers inside and out.

They do a lot of work for the general aviation industry, at the nearby airport, as there were all sorts of aircraft engine components hanging up alongside my manifolds when I went to collect them. So I would not coat the inside of steel inlet manifolds. They didn't go into details about this, when we were discussing it.

Bruce.

Paul,

Good to hear you are teaching your students how important it is to do everything correctly and carefully with aircraft maintenance.

A good friend of ours lost their son and his three passengers while flying over dense scrub on Cape York [tropical far north Queensland] when his single engine gradually lost power and returned to idle.
A recent routine maintenance procedure had replaced the throttle cable but the lock nut moved with vibration as it was not tightened up, the nut moved and the throttle gradually closed. This accident [a minimum of manslaughter in my view] changed the procedure so that if this was to happen again the engine would go to full throttle. This would give the pilot a better chance of putting the plane down.

Back to the spluttering, I seem to think that it is the left pipe (front cylinders) that do this more than the right one. Like you I wish it didn't do this as other inline 6's seem to idle OK, Jaguars in particular just sit and purr [ yes I know Jaguars don't purr, purring cats cannot roar and roaring cats cannot purr sort of thing] beautifully no sign of a miss-fire with the XK engine.

Bruce.

Bruce,

That was my plan... Just outside on the intake manifolds (to help reflect heat and match exhaust coating). A completely smooth surface on the inside of an intake manifold is not recommended. Inside & out on the exhaust... Thank you for all your inputs!!! Great information...

Paul

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Those who confuse Burro and Burrow don't know their @ss from a hole in the ground...