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Drop in idle when operating clutch

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Rick60

Rick60

Senior Member
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Hi,

I've been noticing a drop of about 200rpm (850 down to 650) in the engine when operating the clutch. This is when the engine has been warmed up. The drop in rpm when cold is more like 100-150 rpm. The car has done this all the time I have had it (4 years now).

An excellent mechanic (though not a Healey expert) told me that the cause is most likely axial play in the crankshaft. He warned me to have it repaired asap as the engine block may suffer serious damage.

What do you experts say?

Best regards
 
All three of my cars do this as well. I also have a hard time starting the car with the clutch pushed in versus clutch out, in nueteral. The reason for this is that the pressure asserted on the flywheel by the clutch pushes the crankshaft forward against the thrust washers. This causes a little more friction and slows the motor down. The thrust washers are fixed in the block and I do not see how this problem could hurt the block.

In my semi-educated opinion, I would not worry about it unless you get a much larger drop in RPM.
 
Hi Eric,
It's common to have some rpm drop when the pedal is depressed. The carbon faced clutch throwout bearing has some friction. The thrust bearing load also contributes a bit more friction.

Crankshaft end float should be between 0.003" & 0.006". It can be measured by setting a dial indicator on the front pulley. Push the pulley/crank back in the block with a lever. Zero the indicator & depress the clutch pedal. This will force the crank forward & the indicator will show the axial movement. It should be in the 0.003" to 0.006" range. If the movement/clearance exceeds 0.006" the crank thrust washers should probably be replaced. Don't wait too long. I'm surprised that the "excellent mechanic" didn't offer to check crank end play for you.

If you do replace the crank thrust washers it now becomes a judgement call on checking main & rod bearings for excessive wear. If the bearing inserts are worn to the point where copper is showing, time to replace them also. Who knows where it goes from there. Depends on overall engine condition.
D
PS - It isn't good to have the clutch depressed for long periods of time. The carbon faced throwout bearing is one of the weaker parts of the system. Much better to use neutral than sit with the clutch pedal depressed.
D
 
From what I have seen (including my own BJ8), this is a very common trait of these cars. The warmer my car is running, the more pronounced the drop. I thought it might be related to the throw out bearing. But after just replacing it with new, it is still the same. Some have said that if the engine is off tuned a bit, this will happen more. I just moved my idle speed up from 600 to 800 and the problem doesn't cause a stall any longer.

You just get used to left foot on the clutch, right toes on the brake, and right heel on the gas to keep it from stalling at a stop light on a hill! It keeps us young...

Mark
 
Well I disagree with the lot of you. I can remember when I was learning to tune my BJ8, if I pushed the clutch in and the idle speed dropped I knew I had not done the tuning correctly and it was back and try again. Further, this is not a characteristic of the BJ8 engine when properly tuned and the idle speed is correct.--Fwiw---Keoke
 
I think you may be right .Perhaps the cam causes the engine torque to be a bit low at these low idle speeds I see that are being used and you just have to make up for it.---Keoke-?
 
Has anyone tried a throw out bearing which has a roller/ball bearing instead of a static carbon face, like the one from DWR? Does it help with the idle drop? I have been eyeing one of those for the next time I pull the trans...

Mark
 
Hi Mark

I have a ball bearing throw out bearing in one of my cars it is not an after market part. Aside from being smoother there is no difference between it and the standard carbon faced bearing in my other car.---Fwiw--Keoke
 
Before I got involved with Brit cars, I'd never seen a non-ball bearing throwout.

From reading some of the other boards, I get the impression that ball & or roller bearing type throwouts are a major topic of concern for some marques. There seem to be many variations of these ball bearing designs, each with different problems.

A major advance in technology happened when folks discovered that the real problem was having the ball bearing jerked from zero rpm to engine speed every time the clutch was depressed. Newer cars have the bearing constantly engaged & turning. Just the opposite of what the more wear prone non-rotating carbon face bearing requires.

I personally think the carbon face ones may wear faster, especially if the clutch pedal is held down a lot, but folks soon learn to use neutral more at stops & the clutch less. In the end, the carbon bearings seem to last as long as the ball bearing types.

As has been said, the idle drop is negligible if the engine is set to idle correctly & fast enough.
D
 
Well...

Most of my early motor-head days were spent in air-cooled VWs. The early cars had carbon T/O bearings. They definitely had a lot of drag and most were changed to the later ball bearing style (which lasted a lot longer).

That being said, when I replaced the carbon T/O recently in my BJ8 with 30k miles on it (the bearing, not the car), it had barely any wear. So either the carbon lasts longer, or the previous owner didn't use the clutch very much. Based on the wear of the synchros, I have been leaning more towards the later!

bout a month ago, I re-tuned the car and pushed the idle from 600 up to 750-800. There seems to be a "knee" around that 700 RPM area (at least on mine). Anything below that and the idle has a hard time when hot and clutched. Anything above that and the car is always fine (Thanks for that advice Keoke).

My 1 1/2 cents,
Mark
 
Mark, I had the same thoughts about carbon throw outs, I know when I drove Sprites in the 70s the carbon bearings seemed like they wore out in months.

I drove my 100 for 8 years on the same carbon bearing and who knows how long before I got the car) and when I put the rebuilt engine in earlier this summer I put a new one in, but the old had so little wear I almost put it back in, from memory the new one had a working thickness of about .21" and the old one about .19".

Even more amazing than the minimal wear, it (thde old one) had no retaning springs or clips on it--I am surprised it didn't drop into the bell housing.

I don't get much drag from the bearing either

Greg 54 BN1
 
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