1980 RX7 clutch fades mid-race

I have a perfect feeling clutch when the car is cold and pulling around the pits, after 3 or 4 laps it goes away. When the clutch pedal is depressed completely I can just barely shift but is not without some grinding or double clutching. After pulling in and sitting in impound for a while it is fine again. I will flush the line and reservoir before the next even for sure, but what would be causing this? I would not think that it would get hot enough to have this happen from boiling fluid. I guess there could be some water in there but I thought I would ask to see if anyone has experienced this before. The slave is fairly new, has 5 races on it. The clutch master is unknown age, but like I said it works great cold. In case anyone is not familiar with the car the fluid is completely independent of the brake fluid. I would like to feel good about flushing it and going back out to race but no way of really testing it until I am back on track.

Any thoughts?

Matt

Matt,

I think that you are on the right track. Is the master cylinder known good? Flush and fill fluid using high quality fluid.... Make sure the the release bearing is doing its thing and releasing completely. Then......and perhaps this should be first....Make sure there are no fluids finding their way to the clutch assembly.....no motor oil from a leaking rear seal or O-Ring.....and no transmission fluid from a leaking front seal of the transmission. It takes very little of either to make it slip and it doesn't have to be leaking onto the ground under the car for it to be leaking badly enough to get to the clutch. If the clutch disc is at all oily, or has ever been oily without having been really well cleaned, slippage will likely occur at some point in a session.

I just went through the very scenario that you describe. My clutch would feel and operate perfectly for a 30 minute SARRC race and then fade mostly completely away and be significantly useless for the last some-odd laps of a 45 minute Carolina Cup series race; fixed the leak(s) and problem solved (well, I put in a new clutch, too, just to be safe).

Another thought.....are you running a race clutch or a stock clutch? If stock, and assuming you have verified all else is good, then you would do well to consider a different clutch assembly.

Check the potential leakage thing out very closely....you can see the front side of the flywheel, and a little bit of the bottom of the transmission bell housing by removing the small metal cover at the top of the rear housing of the motor and looking in with a flashlight.

Hope this helps.

Cheers.

Matt,

I'm pretty sure that the Clutch M/C was changed in 05 or 06. I'll do a little digging to find out.

My money would be on the clutch slave. Can't count the number of these I've replaced.

How old is the clutch? (just sayin'-)

Thanks for the help. The clutch disc has 9 races on it, it was new this spring. It is a stock disc, but the disc itself is not slipping. It will not disengage, the clutch pedal is very soft when this happens. Clutch uptake occurs 1/4 inch off the floor instead of 4 inches off the floor. After the car cools down all is back to normal.

Be sure to remove the boot on the slave, and check if it is wet. I didn't, and we went in circles! It doesn't have to leak a lot. Also, if you have a chance to pressure bleed, do it, as there is very little stroke / fluid movement from the pedal.

Also, if you do pressure bleed, it should flow nicely just by cracking the bleeder. If it doesn't, and it requires that you push the pedal slightly, I think then that you've got a problem with the master.

Quote:

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Originally Posted by ebb

I have a perfect feeling clutch when the car is cold and pulling around the pits, after 3 or 4 laps it goes away. When the clutch pedal is depressed completely I can just barely shift but is not without some grinding or double clutching.

Any thoughts?

Matt

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Matt,

.... I had this very same problem in a car that I drove in the ECR Series. We took the car apart several times, made some changes, and it would do the same thing every time we put it back together.

.... First we tried the pilot shaft bearing..... not it.

.... Second we tried a different transmission.... not it

.... Third we checked all the hydraulics..... not it

.... Now trust me the symptoms were almost identical to what you described. Car was good for about 3 to 4 laps then the clutch would not disengage. Talk about frustrating ....ARRGH

... Went to Road Atlanta for an ECR...... Car did good for qualifying, but I noticed that it was starting to act up again right at the end of the session. Went to the pits, talked it over with the crew, and we decided to take it apart while it was hot. The clutch was not releasing fully when I came in so we knew it had to be something with the clutch. The travel on the slave cylinder had not changed so we broke it down to find a hot spot on the pressure plate. Which means that the diaphragm was not releasing the clamping force inside the pressure plate on one side of the disk. That was causing drag on the clutch disk, and making it difficult to change gears.

.... Mike and Alex changed the pressure plate and it was like driving a completely different car..... It is a hard part to diagnose unless you can test it under "REAL" conditions. Which in this case was .......while it was HOT

.... So, if your slave cylinder travel does not change, and your pilot shaft bearing is good, Then you stand a good chance of having the same problem.

.... SEDIV SCCA SARRC IT7 Champion 2000,01,02,04,&05
.... Rickey Thompson, NCR Driver of the Year 2007:cool:

Hotshoe, that is very helpful. Did your pedal feel change when it happened, like where the clutch would engage from a stop? I have no problem changing the pressure plate and clutch if that would do it. For that matter I might just redo the entire thing from the master all the way to the disc.

Quote:

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Originally Posted by ebb

Did your pedal feel change when it happened, like where the clutch would engage from a stop? I have no problem changing the pressure plate and clutch if that would do it..

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Matt,

..... The pedal feel was definitely as you described it. Good when it was cold with the release / engagement point at a normal position. But, after it would get heat soaked it would get spongy and not have that "feel" of releasing....... It could be compared to Brakes that are boiling the fluid type of feel..........Since the pressure plate was never fully disengaging, the pedal felt the same all the way through the stroke.... Instead of more pressure at first and then less after it disengages, like normal.


RT

So did you have the flywheel resurfaced, and replace the other parts? I was using a generic clutch because I had one explode on track and needed one on a saturday night. I never changed it again after that. Is there a proven clutch that will last me a long time? How about pressure plate?

Quote:

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Originally Posted by Hotshoe

Matt,

..... The pedal feel was definitely as you described it. Good when it was cold with the release / engagement point at a normal position. But, after it would get heat soaked it would get spongy and not have that "feel" of releasing....... It could be compared to Brakes that are boiling the fluid type of feel..........Since the pressure plate was never fully disengaging, the pedal felt the same all the way through the stroke.... Instead of more pressure at first and then less after it disengages, like normal.


RT

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Matt,

My old engine had a Center Force dual friction clutch and it worked great. No slippage at all and no visable wear after 6+ years of use.

My current engine has a used stock clutch and it slips on the 3-4th shift. I am ordering a Center Force DF for it too. Summit has them for $$295

Unfortunately I can't swap clutches between the two engines because one has the early flywheel and the other has the latter flywheel

thanks Scott, I need to check what year engine is in my car and order one.

A stock disk will also swell up with temperature or oil and will become too thick to disengage. It will seem OK when cool and you look at the disk and it looks new. Had a friends ITS RX7 do this at school.

I think you should just park the car and get out of racing. Or I could trade you a nice "very stock" IT7 for it.


Or you could just put a Centerforce clutch in it, replace the clutch master and slave cylinders, pressure bleed it and be done with your problem for a good season or two. Just get it done by October for Gateway!