MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - coarse engine

I've just bought a 72 GT. It's low mieage on a (i think) gold seal engine. It goes well,has plenty of torque but it's "rough/course"; i.e., it vibrates and sends them through the body. I've serviced it, timed it and balanced the carbs. It has good and even cylinder pressures, I've re-hung the SS exhaust, fixed everything loose, re-built the dizzie and checked the engine and G/box mounts. Now I've re-built several engines, all without balancing and never managed to put one together this rough. Apart from taking it out, stripping it and balancing the reciprocating masses, has anyone any other ideas?
Allan Reeling

Have you checked the engine mountings, if these are worn they will give similiar symptoms to those you describe.
Stuart
si robathan

Hi Allan,

I had similar problems a while ago.
It turned out to be a wrong jet (0.1" instead of 0.09") in one of the carbys.

Herb

Herb Adler

Herb,
Thanks for that, I was always thinking uneven combustion, hence the compression tests, dizzie work. I bought a SU re-build kit the other day, so I will keep you informed. The percieved wisdom is that Gold Seals were always more carefully assembled than the factory originals.
Al
Allan Reeling

"The percieved wisdom is that Gold Seals were always more carefully assembled than the factory originals. "

True. New engines were simply assembled from new components without checking them, Gold Seal were maintenance exchange engines where every component was checked it was in tolerance, however replaced components again were probably just used out of the box. But anything could have happened to a Gold Seal between assembly and today.

If you have been able to setup the carbs with only minimal variation from the starting point of 12 flats down from flush with the bridge and got the correct response to the lifting pin, then it shouldn't be the carbs, other than perhaps two different needles.
PaulH Solihull

I was always going to re-build the carbs as the spindles were a bit worn. Have just done it, tghe only thing I found was a completely gummed jet which wasn't moving via the linkage, but that would only affect starting. According to the MOT's the gold seal engine has done less than 4000 miles since installation in 1988, so it's done a lot of standing around. My next check is combustion chamber volumes. BUT what is the possibility that the valve timing is out?
Allan Reeling

Could this be a problem with the flywheel being loose? At what speeds does this occur? Is it only when under load or all of the time. RAY
rjm RAY

"The percieved wisdom is that Gold Seals were always more carefully assembled than the factory originals. "

I very much doubt it! Gold Seal blocks were sleeved back to standard and cranks reground - then probably put back together with parts out of the bins. I can't see them bothering to spend time cleaning and straightening parts for reuse other than the major items.

The engines were made in the BMC Longbridge plant and I suspect (but don't know for certain) that the gold seals were as well.

Certainly the conrods were graded into sets - there is rarely more than a few grams difference in the weight of an engine set, but much more between sets. Pistons and bores were graded and matched as well.

However, as Paul says, you have no idea what has happened to an engine over the years - I have had one recently where one conrod had been replaced with a very late type which was almost 100grams lighter than the other three - no wonder the engine was rough!
Chris at Octarine Services

The comment the Gold Seal were more carefully assembled than new came from Roger Parker, which must carry some weight.

I saw engines being built at Longbridge in the late 60s, from crankshaft being formed from a red-hot billet in three bangs of a huge hammer, ditto camshafts, which subsequently then had the distributor and oil pump gear induction heated from cold to red hot in about 3 seconds. Assembled engines were bolted to a moving conveyor, fed oil, and spun for about 15 or 20 minutes while travelling along.
PaulH Solihull

Just an update, had the head off last weekend, no major variation in combustion chamber volumes, no at least perceivable cracks,only a bit of "pick up" on the valves. Clean up, grind in valves grind some roughness off the ports. Back on.............no difference!! It must be the bottom end, it will have to stay like it untill the 2 litre conversion is done, does anyone in the midlands know someone trustworthy to do the machining?
Allan Reeling

Ray, sorry I forgot to reply. It's rough all the time mate, but I would have thought a loose flywheel would have given an audible clonk on take up.
Allan Reeling

Allan,

You have timed the engine and rebuilt the carbs. I wonder, could a timing chain skipping a tooth (or the crank/cam teeth not being properly aligned on a re-install or tensioner not sufficiently tight) produce the symptoms you have? Its fiddly I know, but might be worth a look.

If that was ruled out its more likely then the bottom end.

Regards
Roger
Roger T

You could check the TDC mark is correct using a cylinder stop made up of an old spark plug and bolt.
c cummins

I'm going to make a protractor up and have a rough check of the valve timing, it should show up a discrepency.
Allan Reeling

You can print this one off

http://www.thegsresources.com/files/degree_wheel.pdf
c cummins

Hi Allan

Some of the engine mounts are too firm giving a very rough idle. Exhaust catching can also cause vibration as can bent carb heat shield catching body. Check for low rpm air leaks.

Out of balance can give a buzz unlikely to feel rough.We have seen gearboxes and clutches breaking up cause a vibration too.

Gold seal engines sleeved to standard, not all gold seal engines are standard. In early 70s the factory had the blocks cast at Qualcast Leeds. On the assembly line some of these were found to have porosities in the bores. These 'faulty' blocks were sidelined and then turned into Gold seal engines by sleeving the blocks...thus.....as my 'Old China Plate' Rog says...the Gold Seal engines were better built than the original standard engines using new parts.

Peter
P Burgess

Thanks Peter, I've eliminated any chance of anything catching, but am interested in your comment on engine mounts. They do look new.....ish, and the reason I changed the exhaust hangers was for the sam reason , i.e., they had little flexibility. You can feel the vibrations through your left foot when depressing the clutch. As I was always going to up-grade the propulsion unit, I'll probably do it sooner rather than later. Need to get a block and have some competent machine work done to give me something like a 2 litre, BALANCED unit, but i want to build it!
Allan Reeling

Thanks for "the wheel".
Al
Allan Reeling

I will add a comment here to my running problem expressed in the other forum.
I set the carbs as mentioned here 12 flats (2 turns) down from the bridge as a starting point thinking they would be close to the ideal mixture. I took the car for a 100 mile run and noticed the heavy fuel consumtion and found the plugs to be black with soot so I assume far to rich (the colourtune confirmed that with an excess of yellow). Thats when I reset the mixture. I weakened each carb until the engine just started to hunt and then richened by one flat and checked again with the Colourtune, all showing a bunsen blue colour at constant throttle.
Now this is the intersting bit that has got me thinking on comments here. Have I got a mismatch in needles/jets as the carbs were rebuilt by the previous owner. This might be a clue, what do you think. From the 12 flats out setting to weaken the mixture I screwed one in by as much as 9 flats to get the bunsen blue colour but the other by much less but one turn I think. Something sounds wrong.
mj dodridge

Allan -
The procedure for checking valve timing is in the WSM, no degree wheel needed.

mjd -
Something wrong there. I set rebuilt carbs at 9 flats down, or 10 in cold weather (.055-.060 from the bridge), and rarely move them more than one flat. Something is making it too rich and you are trying to compensate. Most errors in assembly will make it very rich at idle/low end, but will have less effect as the engine speed/load goes up. This means that your compensating lean setting causes a lean condition at wider throttle openings, not good for performance or engine life.

FRM
FR Millmore

12 flats down *is* just a starting point, enough to get the engine running. You then need to coarse tune by turning the nut for the highest idle speed, then fine tune fractions of a flat using the lifting pin. That will tell you if something is wrong with one carb or the other, because you will either have to adjust one a lot more from the starting point than the other, or adjusting one of them will appear to make no difference. Maybe American air or fuel is different to UK, I have never had to screw it back up three flats winter or summer to get from the starting point to correct running.
PaulH Solihull

Maybe I still have not got it quite right. As stated I started with 12 flats but it ran very rich, black plugs and yellow colourtune. I've now set to 9 flats and that has increased the idle, cream plugs and blue colourtune. Before this I had screwed in too far and I think the very weak mixture was causing my running rough problem and maybe too much timing advance. Yet to road test however.
mj dodridge

Jets & needles wear! The biased ones worse than the fixed ones. It doesn't take much wear to move the correct mixture at idle up the needle.
Chris at Octarine Services

This thread was discussed between 21/10/2010 and 06/11/2010

MG MGB Technical index

This thread is from the archives. Join the live MG MGB Technical BBS now