My 71 LT-1 is recently experiencing a clutch-slipping issue, and I'm trying to sort out the root cause. It was running perfectly fine last November, but cold weather set in and I didn't get a chance to drive it for a couple months (good opportunity to button-up some loose ends in a warm garage).

However, when I pulled it out a couple of weeks ago for a drive, the clutch pedal felt "weird" (didn't travel all the way back up) and I noticed some slippage under hard acceleration. The only work that I did anywhere near the linkage was the addition of the correct reverse lock-out cable (which was missing), which passes by the clutch fork. Since I've noticed the issue, I've tried adding a newer clutch-rod return spring (the clutch pedal had "play" near the top), but the problem remains. I've tried adjusting the clutch-rod in both directions (plenty of rod left) but I can't seem to get rid of the problem and am at a loss. I did notice that the clutch fork has plenty of play (seems loose) when full disengaged - not sure if that's a problem or not.

Any ideas? I have no idea how old the current clutch is, so it could simply be worn out, but it seemed perfectly fine just 2 months ago.

 

IF the clutch is slipping under acceleration...and you do not have your foot on the clutch pedal...and the clutch pedal is at the top with CONFIRMED knowledge that the pedal is NOT engaging the clutch (which I HIGHLY doubt)....then you have a worn out clutch. Simple as that.

You should have about ONE INCH of free pedal travel before you feel the pedal stop and make the throw out bearing contact the pressure plate.
***TEST***
The car on LEVEL GROUND if you plan on having someone push for you. Put your car in gear....it does not matter which gear. Make sure your foot is not on the clutch pedal pushing down at this time. Then have someone push on your car...and NOW begin to press down on the clutch pedal and WHEN the car moves...that is telling you that the clutch is releasing. If you let off the clutch pedal while it is still moving.. the car stops. SO..that is telling me all is GOOD in regards to adjustment....and KNOWING I have the free pedal travel as needed.

SO...if it is slipping while under a load...then it is worn out..and all the adjustments to the linkage is not going to change that...and several months of sittign and IF the flywheel got rust or anything on it...it can cause this when it is put back into use....especially with you not knowing anything about the clutch and how old it is or how many miles are on it.

DUB

 

Thanks DUB - so, with the TEST, are you saying that the vehicle shouldn't start rolling until the pedal is one inch from the stop?

 

I swear I wrote it and read it so it made sense. Maybe I clearly failed in that.

One more try. If this does not work...you may need to do some searching and read on it so you understand the physics of how a clutch works.

The one inch of free pedal travel at the top is so you KNOW your clutch is not releasing.

By having this one inch of free play...you know that you are NOT pressing on the throw out bearing. And by knowing that you are NOT pressing on the throw out bearing.. The pressure plate and the clutch disc area under pressure against the flywheel.

SO...if your car is in NEUTRAL...you should be able to move it by pushing it and NOT even be in the car touching the clutch pedal PREFERABLY on LEVEL ground so the car does run run away on you.

The REASON for this is how your transmission is made. The input shaft of your transmission goes into your clutch disc. So they are now one and will ALWAYS be as one. BUT the clutch disc can slide on the input shaft forwards and backwards ( so-to-speak)...when you press on the clutch pedal and take the pressure that the pressure plate is applying to the clutch disc off of it.

When you have the car in NEUTRAL....and you push the car and it moves.

You may ask: "How can it move if the clutch disc,pressure plate and flywheel that is attached to your engine is allowing the car to move. Because IF the pressure plate , clutch disc are pressing on the flywheel...it should not be able move the car because I would be trying to turn the crank shaft while pushing on the car"

BUT that is not true...becasue when you are in neutral. The input shaft is NOT engaged with the rest of the gear cluster in the transmission. SO that is how you can move your car.

NOW...when you put it in gear. And are outside the car pushing it....it will not move becasue you are NOT pressing on the clutch pedal to RELEASE the clutch disc from the pressure plate and flywheel. THAT is why the car will not move because you are actually trying to now turn the crankshaft BECAUSE the main gear cluster of your transmission is now locking the input shaft and making the input shaft be one with the main cluster gears.....which as we know is locked into the splines of your clutch disc...which we know is being held to the flywheel of the engine by the pressure plate.

Can you push the car when it is in first gear and move it...YES... but it takes much more effort. and usually requires a rocking motion back and forth to get it to move.

NOW....when it is in gear...and you have a friend pushing on your car...start to press down on your clutch pedal and DO NOT WORRY about that one inch of free pedal travel (as long as you have that free pedal travel). When you start to press down on the clutch pedal there will be a point when the car will start to roll....and at that time...that is when you have released the pressure of the pressure plate holding the clutch disc to your flywheel...and in doing so....you and NOW not trying to turn the crankshaft and all you are doing is turning the internal gears inside your transmission. If you let off the clutch pedal while they are pushing it...the car will stop.

SO...if you try this and it does like I wrote....then your clutch is wearing out and slipping and no adjustments will make any difference BECAUSE you have that ONE INCH of free pedal travel.

BOW...when your pressing on the clutch pedal...and the car starts to roll. Take notice of where it releases. Is it releasing at the top and you did not have to press the clutch pedal down that much...or is it releasing at almost the end ..near the floor. SO... Keep pressing until you go all the way down. The amount of travel you go AFTER you know it has released and the car rolls..can often time let a person know that they are running out of clutch. Also...the threaded end of your clutch push rod that goes through the firewall and where you can adjust the clutch can often times let you know that you are running out of adjustment due to you can be getting to the end of the threaded rod and not be able to achieve that one inch of free pedal travel.

Just because you still have a lot of threads on the clutch push rod...does not mean your clutch is still good. It can get glazed and thus not hold the torque when you accelerate.....thus....you clutch is is spinning BETWEEN the pressure plate and the flywheel.

DUB

 

Based on what you said - the clutch did not come all the way back up, I would check the linkage for something binding. The rubber boot, cross shaft / Z bar for grease, boot on the firewall, linkage under the dash... etc....

It could also be the throw-out bearing is not sliding on the transmission shaft, due to dirt or a slight rust issue. A possibility but not likely.

In a rare case it could be the fingers on the clutch/ pressure plate failed and thus the clutch will slip and not return to the full up position.

My 85 is a problem child in the spring as it sits 4 months ( in a garage attached to a house) and the linkage gets stiff. It's the throw-out bearing on the shaft (early 6 speed Richmond's are noted for it) . It takes about an hour of local driving for everything to return to normal. If I get on it - it will slip as the clutch is not fully engaged. Every year I say I need to fix it - but don't.
It self fixes and works fine after the drive.

If the linkage is working DUBs method works well for testing the clutch.

 

Thank you Dub and Blue - very helpful.

 

So, I performed the test and made some adjustments per DUB's advice and took her out for a drive this morning. Still some slippage under moderate/hard acceleration (as before), and it popped-out of 4th gear twice on me under hard acceleration. This has never happened to me before, so not sure if it's related to clutch linkage adjustment or not. Thoughts?

 

Popping out of gear is an internal transmission issue. You clutch does not have anything to do with this 'new' problem.

IF you have successfully adjusted the linkage and got a nice smooth even NEUTRAL gate. then it is not having anything to do with your linkage....ASSUMING...that your linkage rod for 3rd-4th gear is good and no slop in it...and ASSUMING that your shifter handle rod is not hitting your console plate...which I doubt if you just made it short throw.

DUB

 

One thing I would do is to check your motor mounts and transmission mounts. I would make sure everything is tight and not cracked or loose.

 

check bushings on linkage and shifter.(if they have them)

 

Thanks for all the feedback, guys. I did a little more poking around today, and determined that all mounts are solid as a rock, so that's not a concern. I also scoped inside the bell-housing to see if I could get a closer look at the throw-out bearing. Some pics (apologies for the quality - was using a cheap scope):






Seemed to look okay to me (pretty uniform), so I rolled the car forward in an attempt to get the full picture, and the opposite side looked MUCH different:









Any of you experts pick out anything here? I would think the teeth should look uniform all around the bearing, and they clearly do not.

 

I can see what you are describing...which DOES fall back to what is wrong....or can be a part of what is wrong.

Knowing that it is not my car...but if it were...I would be putting in a new SPEC clutch and even possibly a new flywheel with it also so they are matched/balanced. OR...if i was just not wanting to put in a new clutch. I would not be driving it with any 'spirit' and let it just putt down teh highway until it possibly left mew on the side of the road ...and then I would call a roll-back and take it to work and fix it.

I know it does not make sense that it was fine months ago and now. WHAM it is not...but crazier things have happened when working with or dealing with cars.

One thing is for sure...it is not going to get better.

DUB

 

Photos are fine. Your pressure plate is toast. You are right. The pressure plate springs should be evenly in contact with the throwout bearing. You need a new clutch and pressure plate.

Edit: I should add there seems to be more grease in there than there should be. Most likely the last guy to install a clutch in that car used a little more lube than he should have.

 

Thanks DUB and drwet - at least things are starting to make more sense now. I wonder if something happened suddenly, which could explain why the clutch-feel seemed to change overnight.

Looks like a new clutch kit is in my future. Will probably replace the flywheel as well per DUB's advice while the thing is cracked open. I need to search around to see what people recommend as far as replacement goes - seems like there are a bunch of options out there. I'd like to stay as close to SPEC as possible.

Thanks guys.

 

The name of the company is called SPEC.

Not that this matters...but I have been using them for a long time now. And the MAIN reason is this. Call about any other clutch manufacturer and ask them what they balance there assembly to. They will normally come back and say "Detroit specs"...which can be (as I was told by them) 50 grams out. And ...28 grams is an ounce. So go figure out how bad it can be is right out of the box.

I had an engine at the balancer and he was having a fit due to the customer was bringing him on different clutches. You name it...he had them. When I sent him a SPEC clutch and flywheel...and he installed it on the crank...he said he barely did not need to do anything due to how well balanced the assembly was.

SO...since then..I use SPEC and do not get caught up in what 'others' feel is better due to marketing. And SPEC offers clutches that are stout..as you will soon see.

Not trying to sell you anything due to I have no financial interests in SPEC...so the choice is yours. Just letting you know that just because SPEC is not well known and does not roll off the tip of every ones tongue....DOES NOT mean that are not awesome.

http://www.specclutches.com/?gclid=E...SAAEgIn0PD_BwE

DUB

 

Okay, gotcha. I'll check them out. Also need to sort out if I have a 10.4 or 11" clutch. Looks like I have some work ahead of me.

Thanks again.