MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

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 - Bob Munch.....

Hi Bob,

Your a/c keeps rejecting my reply to your e-mail, so I hope you don't mind my posting this in open forum.

Wow, where do I start?..............

My view is that a worn chain is unlikely to lead to rough idling - the dynamics are quite simple, as the chain wears it stretches and the cam becomes progressively more retarded in relation to the crank - which is why I set up new engines with a couple of degrees of cam advance. (Cam advance pushes the power band up and retard pulls it down the rev range, but neither results in rough running). The slack is taken out of the chain by the tensioner, so unless that is seized, the cam can't chatter back and forth even with a worn chain. Unless the chain is so worn as to have lost some of the rollers ( I have only seen this once).

The cyclic nature of your problem suggests that this is more likely to be something that varies its position - like the cam followers, worn and rotating, these can produce varying valve clearances which will affect the running. A sticking valve will do the same, but usually accompanied by backfiring as unburnt fuel gets into the exhaust or hot gas into the inlet manifold (i have seen 6inch flames coming out of the carbs!).

The other favourite is worn dizzy bearings which alter the contact breaker gap - do you have breakers or electronic ign? Sticky fuel float valves are another possible reason - starving the engine of fuel on one carb but clearing when the engine is opened up on the road.

Another cause might be bad HT leads or the leads grounding out somewhere ( I had a misfire on the V8 which finally came down to a bad HT lead - in a brand new set of leads!)

The plugs sound as they might be a touch on the lean side - I would expect plugs 2 & 3 to have a light chocolate brown deposit and 1 & 4 to be slightly lighter. What plug gap are you running - a 68 should be on 25 thou - no wider.

Is the car used daily or just occasionally?

Is there any pattern to the rough running? Changes in fuel supplier? Temperature of the weather? Rain/dry weather? ( My V8 ices up the SUs on the motorway in cold damp weather!)

Have you changed the fuel filter lately?

The only sure way to get an engine running spot on is to have it tuned on a rolling road with a gas analyser to ensure that the mixture and timing are set up throughout the rev range.

I think you just have to be methodical, trying one thing at a time and don't assume, because you just fixed something ( like the valve clearances) that they are OK - check them again!

To summarise, plugs, mixture, timing and valve clearances are the things to check whenever you get the rough running - perhaps a pattern will emerge that will lead you to the cause.

Chris
chris

I had a very similar sounding problem on a Ford Cortina GT about 30 years ago and tried everything I could think of. A colleague sugested a good look around the engine when idling in the dark, the darker the better and give your eyes time to acclimatise to the dark. I could not believe what I saw, sparks everywhere and one HT lead glowing from end to end, very revealing. I have used this method several times since but have never heard it mentioned anywhere else.
Forgive me if I am trying to "teach my grandmother to suck eggs"

Maurice.
Maurice Blakey

Bob-
Chris is right about all of the possibilities that you need to check, but....he's made a mistake about the effect of the chain tensioner. Now, follow my logic: Ever drive down a smooth highway behind a motorcycle? Did you notice that although the rear wheel wasn't moving up and down the slack side of its drive chain seemed to be undulating? This is because chains don't wear evenly. True, the cam chain tensioner on the B Series engine does take up the slack and keep it from thrashing around inside the cover, but it can't make one section of a badly worn chain the same length as another section. This variance in length has it's effect on the drive side of the chain's circuit, causing the cam to vary in its timing, resulting in all sorts of symptoms which can range from increased vibration to, in extreme cases, rough running. As Chris pointed out, as the chain wears it goes out of phase as the tensioner takes up the slack, causing the timing to become progressively more and more retarded and shoving the power ever downward toward the low-rpm part of the powerband. Many owners of MGBs who complain that their engines seem to run out of steam at high rpm usually have a worn cam chain but just don't realize it. Here's another bit of advice: when cam drive chains become old and worn, their "stretch" increases the distance between the rollers. This in turn distributes the load across fewer and fewer teeth on the sprocket and thus increases the rate of wear on both the sprocket and the chain. Hence, if a new chain is put on an old sprocket, it will wear out prematurely. Moral: Never reuse an old sprocket.
Steve S.

Maurice-
Your method of observing the engine compartment in a darken garage is indeed as old as the hills, and a good one. Here's another old-timer's trick: While observing the engine running in a darkened garage, use a spray bottle to create a fine mist in the engine compartment. If you see a flash like lightening, you've got bad spark plug leads.
Steve S.

Steve,

Sure, an unevenly worn chain would act as you say, but I have only once seen one where the wear was so severe as to break up the rollers round part of the chain - now that engine still ran and ran smoothly!

The normal amount of wear in a chain is very small - held horizontally they droop about 1/2 inch at most - equivalent to around a thou per link.

New chains droop about 1/4 inch.

I don't agree with the motorbike analogy - the chain is muh longer and the untensioned side is made to undulate by the very action of coming off the front sprocket - ever watched a fan belt do exactly the same thing on the slack side between crank pulley and alternator? Same reason - how it comes off the crank pulley. The tensioner is there to stop this chattering and to ensure that the cam rotates as evenly as possible.

Most chain sprockets are perfectly reusable, wear is often very light - rarely is there any measurable sharpening of the teeth unless the engine has been run with no oil or blocked oil feeds.

chris

Chris-
You said: "I have only once seen one where the wear was so severe as to break up the rollers round part of the chain...". Broken rollers are the result of bad heat treating, not wear. Thus, the fact that that one particular engine still ran smoothly and the fact that rollers had broken in itself proves nothing concerning wear. If the engine ran smoothly, the camshaft was still being timed adequately well, so wear was obviously minimal.

You also said: "I don't agree with the motorbike analogy- the chain is much longer and is made to undulate by the very action of the front sprocket". If the sections of the chain were of identical length, the chain would move in a steady arc. It would not undulate.

Also, the proper way to measure chain wear requires two step process: 1) Lay it out againt a metal straight edge and compress the links and measure for length. 2) pull the ends of the chain and again measure for length. If the difference is greater than 3%, the chain is worn beyond its engineering tolerances and should be replaced if excessive wear of the sprockets is to be avoided.

The slack side of a fan belt is of constant lentgh, but it undulates due to the acceleration/deceleration of the crankshaft as it goes through compression and combustion strokes. The harmonic balancer minimizes this effect, but on a four cylinder engine it is still there.

The way to measure wear of the teeth of a sprocket is to take a roller and smear a light coat of machinist's bluing on it and gently place it between the teeth of the sprocket. When the roller is removed, the entire profile of the tooth should be blued.
Steve S.

Bob, in case you don't see my entry on the other thread you contributed to, I too need a new email address for you to send you some interesting pix I took at Vancouver Indy, of a highly competitive MGB.
Ken R
Ken Rich

Sorry everyone, Here is my new address. idbritcars@att.net. I will have to start posting it again. I had so much trouble with bizarre email problems with AOL (NOT viruses or worms, but email going off to unintended addressees I had never heard of!), that I had to go elsewhere, even tho my webpages remain with them.

Back to the topic: Thanks everyone for your responses! I will start rechecking what is going on with my engine to see if I can sort it out, given your ideas.

I thought I had the bugger cornered a while back when I replaced the float valves, but no, it came back and it is the fact that it comes and goes that is most troubling. It's coming and going is not really of a truly cyclical nature, but more sporadic. This first made me wonder about the fuel system and those things you have seen me post about before, like water/crud in the fuel or clogging filters, bad fuel pump, etc. Those were not the issues. And now the induction system has all new gaskets and shows a very healthy 17 inches of vacuum with a good steady needle at idle.

The dizzy I use is a Eurospec 45D (points type) recently installed new and I have exchanged it for the one on my MGA (also the same unit used on a 3-main MGB engine in that car) which uses the pertronix Ignitor. This engine ran perfectly with no roughness. However, after installing, no difference in the problem. I then installed a Crane XR-700 unit I also have on hand and again no difference. I exchanged plug wires, rotor, cap, and coil from the MGA and again, no difference.

This led me to believe that the problem was something inherent in the engine mechanicals. I had read a posting, possibly by Steve, about roughness and the timing chain, and having had a couple of escapades with chains on other cars, it did seem plausible, tho my experiences both had backfiring. The possibility of cam/cam follower wear is also plausible. If any of these problems are the cause, I think I will take the opportunity to follow Steve's notes on building the head and camshaft systems back up to snuff.

It really is frustrating as when it does decide to idle just fine, it also goes like scat with plenty of power! It is just that damned roughness that comes and goes at 900-1000 rpms that makes me wonder what is up.

Again, guys, I thank you for your input. Perhaps something I have said here will help you to focus on it too. If and when I find the culprit, I will be sure to let you know what it was.
Bob Muenchausen

Bob,
The intermittant nature of the problem seems to tell me that something is 'sticking', either in the carb(s) or distributor. Like floats or centrifugal advance weights. This is over simplified, but I think overhaul of those two areas are where I'd concentrate my efforts. You've probably already done that, but there may be something that's not obvious, that you may have missed the first time?

Joe
Joe Ullman

Hm, I was the one who missed something! Neglected to see that you had changed distributors twice...guess it's not the advance mechanism.
Joe Ullman

Hey, Joe, I thought I was the only one who did that! ;-) I too thought of the sticking "something" of the dizzy, but found all was well in both distributors.

I was out at the car this afternoon and while looking at something else, I happened to wrap my hand around one of the oil cooler hoses while the engine was running (idling) roughly. What was interesting was that I could feel a pulse INSIDE the oil line that seemed to be exactly in time with the rough lumpy vibration of the engine. I may be wrong about the pulse I felt actually being within the hose, but if it were, what would that indicate?

To me it would indicate something acting like a check valve that shouldn't or vice versa. Now what could that be? And, if it truly was the oil in the line pulsing as well as the engine vibrating, that seems just a little bit weird. I will take a look at the shop manual tonight and see if I can figure out something that could cause the pulse AND the vibration. Bizarre. Or purely coincidental. Hmmm.
Bob Muenchausen

Bob
From what you're describing, there appears to be two possibilities:

1)The oil pressure check valve spring is weak and the seating of the valve is defective, causing the check valve to flutter,or-

2) When the engine runs rough, the acceleration/deceleration between the strokes of the crankshaft is worsened, causing fluctuations of the speed of the oil pump.

Either of these could be symptomized by the fluctuations of pressure within the oil cooler hose.
Steve S.

I will check that out, Steve. I don't recall the dash oil pressure gauge pulsating, but then I don't think it is all that sensitive to pulses of this frequency. I have a couple of other oil pressure gauges I can connect just to see if either of them is more sensitive.

As you say, it could just be a collateral effect of the roughness and not really be symptomatic of the true cause. I will do some further digging too. Thanks again for your help!
Bob Muenchausen

This thread was discussed between 03/12/2001 and 05/12/2001

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.