I'm stumped so here is my first post
 

First I would like to say hello to everyone. I have used these forums for some time now and they have been extremely helpful in working on my 911sc for a couple years. Since this is my first post I am including a photo of the car so I won’t get commit that social mistake.
Here is my problem. Recently I was racing my 78 911sc at a Daytona SCCA race and when I returned for tech. inspection after the race it was idling rough. It felt like it was missing on a couple cylinders. On acceleration and wot it runs great. Track times were right on the money with previous runs at Daytona. To my knowledge there were no over revs and it was running up to it's normal 6400 rpms in fifth gear on the track.
I brought the car home and started the normal checks. Spark plugs, spark plug wires, Dist. cap and rotor. Vacuum leaks, air box cracks, and all tested fine. Now it was time to break out the compression tester. These numbers were performed cold (for the sake of the neighbors).
Left side #1-145,# 2-135,# 3-140, right side # 4-45, #5-40,# 6-55. There is no smoke being produced by the engine when running.
When I found the first low #’s on #4 I thought to myself burnt or bent valve, but all three at the same time with similar compression #’s.
I was wondering if it is possible my cam timing on the right side may have jumped a tooth, or is it possible for the cam timing itself to move?
Any help would be appreciated.
http://forums.pelicanparts.com/uploa...1313685760.jpg

Must be the tracks we were on! I'm having a very simuliar problem. Have not done leak down test yet (this weekend) but have found out that my oil restrictor in oil tube is gone and car runs better with me covering up ends of hose and filler tube opening. I hit a "bump" pretty hard...did you? Weird. Good luck and I'll be watching for your results.

Check cam timing for sure. I would remove the cam chain covers and take a look inside.

Sorry to hear you are having issues too. Daytona in August makes for a slippery track. I was using alot of the piano keys and some of those will knock the fillings out of you teeth. The transition from the front straight into turn one can give you a pretty good bump. When I get this resolved I'll be sure to post the results.

Tensioner failure causing jumped timing on 1 bank? You can check that in car by removing the chain covers and checking the timing.

Did you double check the compression numbers? I would expect the car to barely run on those if at all. Leak down test? Also supposed to be done hot, but I have done them cold to use as a diag. on issues. If you really have that bad of compression then you should have a good hole somewhere to listen for.

If it jumped a tooth you should still be able to get the valves to close when doing a compression test it just wont be exactly at TDC. I don't think there is any way for the cams to come out of adjustment (other than the chain skipping a tooth), once the pins are in isn't it set?

If your chain is tight and your ramps are all intact then it is probably bent, burnt or valves are out of adjustment.

Maybe your valves were all set wrong on that side, too tight so they aren't closing when cold.

I ran the compression test once this morning as I was searching for the source of the problem.
I'll pull the chain cover and see what I can find in the timing area. I think it only runs because the left side of the engine is doing all the heavy lifting for the right side.

BTW You guys sure respond quickly. Thanks

Did you find out where the air is getting by? Intake or exhaust? Both?

When checking compression, even if the chain skipped a tooth I think you will be off the lobe of the cam at TDC, so the rocker shouldn't be holding the valves open.

Maybe you could back the adjustment screws, on one cylinder, all the way out and do the compression check again.

Do a leakdown test before further dis assembly

911 sc
 

I sent you a PM, I live in WH area if you need some help
Mike Bruns

Leak down showed leakage through exhaust side.
I think my plan of attack will be;
* check valve adjustment and inspect valve train
* If numbers still bad, check cam timing and tensioners on right side.
* If no problems found in timing, pull engine and check heads/valves.
Thank you to everyone for the suggestions. Special thanks to MBruns who I found out lives five minutes from my home and has offered his technical assistance if needed.
These boards and Porsche people are great.
I'll report back when I find out what the problem is/was.

Update of 911Sc
 

After going through all the alternatives it came down to pulling the engine and checking the engine out. Apparently I was a little aggressive with my downshifts at Daytona and dinged the exhaust valves. You can see from the picture I was guilty of a carbon cleaning procedure I wouldn't recommend for most owners. I assume the leak is so slight the car runs ok at WOT , but rough at idle. Just wanted to update everyone for closure. Thanks again!http://forums.pelicanparts.com/uploa...1314657630.jpg

Appreciate your taking the time to close the loop w/what you found. Sorry about the exhaust valve conga drums . . .;)

Valve conga drums.... First laugh I've had all day.
Flying to the Abacos tomorrow because my fathers 31' Contender (boat) sank during Hurricane Irene in Treasure Cay Bahamas. Hate to think what that is going to cost. My engine repairs will seem like a bargain compared to a salvage and tow to Marsh Harbor. This stuff comes in threes and I'm not sure if I'm up for number three. Keeping my fingers crossed.

Just a thought or six; how many of us are real 'racers" who are even nationally (or locally for that matter) recognized, except maybe on a forum or two? How many 'weekend mechanics' do things right? Why do pretty much 'stock' 911s break, even if upgraded to whatever extent?? Maybe they are not really race cars, which themselves break as well? I am not sure I get the connection between having a great, fun car like most 911s and beating them half (or fully) to death on a track/course. Just a few thoughts, but I think logic prevails; beat it and it just might croak. It's part of the 'game', no? Takes me back to the quesstion about 250K mile 911s; not if they are whooped half to death on the street OR track, or both. Mechanical things CAN be killed, and obviously rather often are. Flame away if you like; I'm just driving around town, still enjoyin' the Hell out of mine..... :)
No ill intention here, just wondering why people are so surprised when their beaten 911 goes bad............. Peace. They were built for the road, no? Appreciate your thoughts; again, no ill will, just personal curiosity/ramblings. Legit IMHO. :cool:

I may not be Jim Clark, but I have, for moments experienced what he must have felt.

I really could not put a price on that.

May he rest in peace. I remember the day he died. His experiences were a far cry from all this. For that matter, many of my favorites from way back when as a child are gone. It all drove me to give up reading things like Road & Track, CD, and AutoWeek. Sad. I still have a hand-made, soldered brass chassis and all, slot car modeled exactly after Bruce McLaren's no. 4....I never was beaten on the 'track'. :)

Not 100% sure what you are saying, but I'll give you my take. I ran open wheel for years, before I turned the car over to my son when he was old enough to race. I love open wheel racing and a 911 can't compete with it at any level. I decided to run a 911 because they are competive cars (in their class) and look good on the track. Racing isn't cheap. A set of tires for a Formula Mazda is now around $1000 dollars and they last one weekend. I don't mind digging into an engine at all. As a matter of fact I love working on my car. When I don't have anything to do on a race car I get bored.
Taking 30 year old technology on the track has it's draw backs, but if my car could talk it would brag how it once raced at Sebring,Daytona,Homestead,Road Atlanta, before it died, versus others cars talking about how they ran and got a gallon of milk at the store for 50 years. Quality versus quanity I guess. JMHO

p.s. My brother has a '89 911 in showroom quailty so I better stop ranting.

Call Donnie Adderley on Green Turtle...he can get the boat up. Brendal or Lincoln Jones can also help.

Thanks Ben. The boat was pulled this afternoon by Steve Albury from Marsh Harbor. I am going tomorrow to check out the damage. Not sure about the hull, but the t-top is thrashed and the engines have been under water since Friday. The boat was on a lift behind the condo, but the storm surge was too much. You sound like you know your way around the Abacos. Do you have a place there? I heard GT was pretty torn up.