MGB & GT Forum

Fellow MGer's,

I decided to add this thread to the B forum for more exposure. Thank you in advance!

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...

Roger,

Ain't no way I'm puttin' any other engine in this C/GT!!! LOL... Yes, you could say I'm picky. The car is/will be used for short trips (100 miles or so), shows and just rides along the coast. Purely for enjoyment. The engine will be removed eventually for an engine bay repaint/refinish so I'll pull the head at that time and do whatever internals are required. Replacement engines in Bs are OK (Dan Master's car is my dream...) but the rarity of the C will keep me from doing any such thing... They only sent about 1450 C/GTs to the USA in 1969. The car is not original but I'd like to keep it as close as possible.

Paul

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

Paul great looking car and those engines can be very sweet. The drop in revs lift the pin can work but for it to be close the carbs need to be in perfect condition and the engine tune spot on. I find it is better nowadays to tune each carb for fastest idle after first balancing the flow. You will find that there will be an area (a couple of flats) that the idle will stay the same, so stay at the rich end of that range. If the needles etc are correct for your vehicle the mixture should then be correct over the whole range. It can also be worth letting it idle with the vacuum hose to the vac advance disconnected and plugged, as sometimes worn shafts can cause a little flutter of the advance plate. Denis

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68 B roadster, Daffodil yellow, supercharger, Burgess SC head, SC cam, Mikuni HSR 48 carburetor and engine built for supercharging.

73 BGT V8 conversion starting with a bare shell. Built the engine "3.9L Rover" early in 2016 with high comp pistons and a few other nice bits, plus a T5 ford trans. Started on the body late 2016 and complete late 2017, Did all the work myself, mechanical, body. paint etc.
Finished and going well, great to drive and quick. Now has a nice 3.23 LSD.



Edited 1 time(s). Last edit at 2015-03-02 12:04 AM by Denis.

Denis,

I understand your point very well. This was also mentioned by a member on the MGC forum. My efforts were rewarded well by the rear carb. The pin lift yielded an almost perfect rise of 50 RPM and then a drop off... The front carb did not react as good. It also seemed to show more wear in the throttle shaft so I will assume it is a contributing factor. I think my next step will be to rebuild BOTH carbs and see if things improve...

Another gentleman on the C forum mentioned this sputtering seems to be somewhat common with the 6 cylinder. Only time will tell. Until then I'm not going to sweat the situation. I just want to enjoy the ride... Thank you very much for your input!

Paul

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