Dropping Engine Revs

Post by **SJSMinor** » Mon Apr 23, 2007 5:00 pm

Hi there, I have a 1950 low light tourer fitted with 1098cc engine and drive train. The car is generally very sound but one thing bugs me. When the clutch is depressed the engine revs drop slightly and the engine begins to sound a little unhappy. Not crank bustingly noisy, not rattley just a little unhappy. It doesn’t matter if the car is on the move or stationery but it doesn’t happen if the car is left in neutral where it idles happily.

I think it would be best to sort this out now as a have a vague memory of this being a symptom of thrust washer or crank pulley wear and can turn serious.

Any help from a minor guru would be greatly appreciated so if you can tell me what is causing this and how I fix it I would be very grateful.

Post by **picky** » Mon Apr 23, 2007 6:23 pm

most minors do this, it is the clutch fork pushing the crankshaft forward, and putting pressure on the crankshaft thrust washers, so there is a little more friction and the engine revs drop slightly. doing this for long periods of time is not a good idea as the clutch bearing will wear out quickly. you can increase the idle speed if you want to make the revs slightly higher when the clutch is depressed. Im not sure why it doesnt do the same in neutral though, maybe another forum member can explain??

Picky

Post by **MarkyB** » Mon Apr 23, 2007 9:46 pm

The problem is that the crankshaft thrust washers are very worn which allows the crankshaft to move forward when you push the clutch.
This puts the big end and small end bearings out of line with each other which causes the extra friction and slows the engine.
It is caused by riding the clutch and or keeping the car in gear while waiting for traffic lights to change etc.
I think it is probably possible to replace the thrust washers by dropping the sump and the main bearing caps.
I'm sure somebody here can say definitively whether this is possible or not.

Post by **rayofleamington** » Mon Apr 23, 2007 10:01 pm

The problem is that the crankshaft thrust washers are very worn

I'm not sure it's possible to determine that so easily - this does sound a normal trait of the 1098 engine, but if it is 'excessive' then the thrust washers are the likely cause.
Bear in mind that you can be putting 1000N end load onto the crank when pressing the clutch!

Post by **picky** » Mon Apr 23, 2007 10:10 pm

if you have someone press and release the clutch repeatedly with the engine running and the bonnet open, and look at the crank pulley, if it is badly worn you will see the pulley moving backwards and forwards.

Post by **Kevin** » Tue Apr 24, 2007 9:53 pm

this does sound a normal trait of the 1098 engine

Yes mainly down to the clutch being of the mechanical type

Post by **dunketh** » Wed Apr 25, 2007 9:16 am

Both my 1098 engines did this and now my 1275 does aswell.
I'd go with the 'mechanical' argument as theres no mechanism to stop you pushing too hard.

Post by **MarkyB** » Wed Apr 25, 2007 9:45 am

Yes mainly down to the clutch being of the mechanical type

How does that make a difference? surely the same amount of pressure is needed to operate the clutch whether it is delivered mechanically or hydraulically.

if you have someone press and release the clutch repeatedly with the engine running and the bonnet open, and look at the crank pulley, if it is badly worn you will see the pulley moving backwards and forwards.

Safer to do this without the engine running. Then you can feel how much movement there is.

From this page;
http://austin-healeys.com/spritespecs.htm

Crankshaft end float (all Capacities) .002 to .003

The big end and small end bearings are designed to run parallel.

The bad news is the crankshaft needs to come out to replace the thrust bearings.

Post by **dunketh** » Wed Apr 25, 2007 10:42 am

How does that make a difference? surely the same amount of pressure is needed to operate the clutch whether it is delivered mechanically or hydraulically.

I'm assuming a hydraulic system will only push the clutch enough to engage/disengage it whereas with the mechanical lever you can potentially push the mechanism a lot further past this point.
For example, my clutch bite point is around a third of the way up the pedal, however I depress the pedal right to the floor when changing gear.

The harder I press the pedal the slower the revs get - its as if the lever is actually pushing against the engine turning. Now if I only press the pedal down to the point of engagement the engine revs arent that affected - doing this is very difficult though!

Post by **picky** » Wed Apr 25, 2007 11:13 am

I think it is possible to replace the thrust washers with the engine still in place - remove the sump and the centre bearing cap. But going back to the original SJSMinor's question, I dont think this is needed.

Picky

Post by **millerman** » Wed Apr 25, 2007 1:20 pm

You can replace thrust bearings, big ends and mains without removing crank. It's not difficult!

Post by **picky** » Wed Apr 25, 2007 1:28 pm

millerman,

how do you replace the all the main bearings without removing the crank? I tried in vain several months back! I could only remove the centre main bearing cap, the others are cannot be removed without taking the engine front and backplates off, which means removing the engine from the car anyway. But I agree that the big ends and thrust washers can be done.

Picky