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MG MGB Technical - Weird idling problem

I've got a '79 MGB tourer, basicly stock as far as I can tell. She has sat for about 11 years in the elements with occassional work and engine turn over during that time.

Honestly, I'm a little ashamed of myself. :(

About a month ago, I got serious about getting this car back on the road, so I've been working on many systems (changing hoses, doing the basics for a tune up, etc), but I have not done any major rebuilds (except just yesterday I totally rebuilt the carb and a month ago on the clutch master&slave cyclinders).

Issue: She runs great, idles nice at about 1000 RPMs when warmed, but if I put my foot in on the clutch pedal the RPMs drop slowly and sometimes even kill the engine. I can keep her going if I hit the gas. It makes absolutly no sense to me why pushing in on the clutch pedal has anything to do with the car's idle.

I hope I'm making some sense.

Thanx for your help in advance,
Reg
Reg

Pushing the clutch pedal puts drag on the engine. As I recall, the book says to set the idle speed with the pedal depressed.... I've found that on my '69 it makes little difference, but I've heard ZS cars are more sensitive to it.

HTH!
Rob Edwards

Thanx for the response. I'm fairly new to auto mechanics so I don't know. Pushing in on the clutch puts drag on the engine? I thought that it released the engine causing less drag. Could you explain how the clutch system works? I will definately need to understand clutch theory of operation.

Thanx again,

Reg
Reg

The drag when the clutch is disengaged is that of the Release bearing which is now holding the pressure plate (solid bit in the cover with all the springs) away from the friction plate (moving bit on gearbox shaft) against the spring tension in the cover. It's worse on "our" cars than most as they have a solid, ablative graphite bearing rather than a "real" bearing. This is, incidently, why the damned things wear out so quickly and experienced MGB drivers don't allow their left foot anywhere near the clutch unless it's absolutely necessary.
When the clutch is engaged and the car is not in gear, the only drag on the engine is that of rotating the gearbox shafts (i.e. not much). When the clutch is engaged and the car is in gear, the drag on the engine is that of moving the car around (i.e. lots). When the clutch is disengaged you get release bearing drag (some). I was going to say that it shouldn't get worse over time as you hold the clutch pedal down, but I've just realised that I've never done this for more than about 5 seconds so I've no idea whether it does or not(!) Changing the release bearing is such a pig of a job that I'd suggest that you stop doing this too :-)
Tim Cuthill

Actually sounds like a worn thrust washer.

I had a Triumph Spitfire with the same symptom. Have someone watch the front of the engine (crank pulley) as the engine is running and you press the clutch. I'll bet you a donut they see the crank surge forward a bit.
Bill Pilon

Fraid I agree with Bill - in fact the carbon release bearings are far longer lasting than ball race types, but it sounds to me as though the crank is moving forward and rubbing on the block.The alternative is that its a ball race type release which has seized

If there's nothing else wrong, changing the thrusts is not too big a prob - sump off, measure crank end float at centre main bearing, centre main undone and then remove old thrust washers and replace with appropriate size to take up all but about 3 thou ( from memory) of end float

Chris
chris

Hi Reg,

This site has a very good explaination of a clutch:

http://auto.howstuffworks.com/clutch.htm

Simon
Simon Jansen

Thanx for your help guys. 2 sets of questions though.

1. Chris, what do you meen? Are you saying it will cost about 3 thou(sand) dollars to fix? Also, are you saying I can fix my clutch without pulling the engine? My Clymer Shop manual said the engine had to be pulled to fix it. Please tell me I don't have to. Pleeeease... ;-)

2. How do you drive the car not using the clutch? Do you put the car in neutral as you come up to stops and have to slow down? Do you "power shift" (shifting without clutch). My brother said he would teach me this for emergency uses, but recommended not using this for normal driving. I've been driving for a while, I just want to be supper sure I'm nice to my "B".

Thanx again.

Reg
Reg

Don't worry, he meant three-thousandths of an inch, not $3000.

That's an interesting thought though; never considered the force one applies to the crank shaft while disengaging the clutch.

For driving without using the clutch, you have to place the tranny in neutral between gears (this is all while you're moving along by the way), bring the engine to a speed in revolutions that matches the speed it would normally be turning while the vehicle is travelling at the speed, and in the gear you want to use. Then give the shifter a solid push into the gear you want it in. So to recap: If accelerating, ie. from first, you would accelerate through the first gear range, bump the shifter into neutral, let the engine slow down, and then shove the shifter into second when the engine speed sounds right for the speed you're travelling. For gearing down (ie. from 4th to 3rd) : When you get to the lower end of the revs, and it's time to down shift, place the shifter in neutral, rev the engine up to the speed that would match the speed you're travelling, and push the shifter into the next lower gear you want to use(in this case 3rd). Don't shift into a gear that isn't protected by a syncro though, as the syncro is what is making up for the differences in gear speeds.

Driving your vehicle in this manner will lead to premature synchro wear, and you will find yourself grinding the gears occasionally aswell, so I wouldn't recommend doing this a lot, but it does work; I know, I drive like this when I'm feeling lazy.

That was long winded, feel free to comment though folks, in case I've said something wrong. :)

Anthony
Anthony Morgan

I can't believe what I am reading, driving a MGB on the road without using the clutch?
It is possible but your local gearbox rebuilder will be able to retire to the South of France much sooner than he expected.
Do these people know the relative cost and difficulty between the choice of replacing crankshaft thrust washers and clutch release bearings at 5 or 10 year intervals compared to rebuilding your gearbox every year (or sooner)!

Mick
M F Anderson

Reg, You mentioned you rebuilt the clutch slave and master cylinder. What was the condition of the cylinder bores, especially the slave cylinder? Usually the slave cylinder on a car that has been sitting for years will be junk. I bring this up because you said the idle slowly drops, which may indicate the hydraulics system is leaking and will cause the clutch to drag. Also MG clutch hydraulics are not easy to bleed, you could have some air in the system. You need to look at the push rod to clutch fork movement while you have an assistant push the clutch down. There should be about 3/8" to 1/2" of rod movement, it should not change until the clutch pedal is released. If it does change you have a clutch hydraulics leak or air in the system.

I doubt if your thrust washers are the problem. If they are worn to the point the crank is rubbing the block you will need to replace more than the washers.
Most Triumphs use only two thrust washer halves, the MGB usees four so excessive thrust washer wear in Triumph's is more common than in MG's.

If you want to be nice to your car, please use the clutch to shift. If you are sitting at a stop light, shift the transmission to neutral and release the clutch pedal. When the light changes depress the clutch pedal shift into gear and go.

MGB throwout bearings will usually last as long as the clutch.

Good luck, Clifton
Clifton Gordon

Regarding driving without clutch, it is certainly possible as a get you home measure. When my slave cylinder let go 10 miles from home, I got home with very little bother. It's true the roads here are not too congested, but I planned my route carefully to try and avoid known bottlenecks and traffic lights as much as possible. I got the engine nice and warm in neutral so it would start easily. Switched off. Plonked in second gear and turned the key. A couple of lurches and we were off. Changed into third and then drove the whole of the rest of the journey in third and O/D third. The MGB engine is amazingly flexible and doesn't mind really low revs as long as you don't want sparkling pick up. When I approached traffic lights, I slowed right down and crept up to them and they changed before I had to stop. I actually only made that one gear change in the whole trip.

Mike
Mike Howlett

Reg,

I don't think anyone suggested driving without using the clutch. By all means use it -- that's what it's there for! :-) What you should _not_ do is to leave the car in gear with the clutch depressed whilst sitting at a stoplight or stop sign. Instead, as Clifton says, take it out of gear and release the clutch until it's time to proceed....

HTH!
Rob Edwards

I drove 60 miles across country without using the clutch as I felt that something was about to break so wanted to use it the minimum possible. Got round roundabouts and through traffic lights and junctions just using 4th and overdrive, never had to slow any more than that. Even backed it into the garage without using the clutch, then pushed the pedal twice, there was a 'bang', and the release bearing broke. Lucky or what? 'Not using' the clutch usually means don't sit at traffic lights on red in gear with the pedal down. Other than the above quite soon after I bought the car I've never had a release bearing fail, nor a clutch from new. Like a mug I fitted one of those roller release bearings with the new clutch only for it to start squeaking a few thousand miles down the road. Think about it - the next time you change the clutch are you really going to leave that old roller release bearing in there? Having said that the V8s use them as standard, but I still wouldn't put an old one back. No car I have had slows the idle when the clutch is depressed, but Americans seem to complain about it a lot. I suggested thrust washers once but it was poo-poohed.
Paul Hunt

This thread was discussed between 17/02/2003 and 21/02/2003

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