Poor performance, Low compression, Cam timing? (kinda long)

[kfosburg]

A little long winded here... sorry...

A bit at my wits end... 71 911E with Webers. Engine was rebuild a couple thousand miles ago and I have serious questions about it. This was quite a while ago (like 20 years) in a state far away and by a shop long since gone, so no recourse or follow up available there. Not good documentation either.

Supposedly, he rebuilt it with a 2.4 crank, S pistons, and kept the E cams. I have no idea how to verify any of it.

It has not run well for several years (or ever?, since the rebuild). I dropped the engine over the winter to clean and fix oil leaks and adjust the valves. I have thoroughly cleaned the carbs a couple times and balanced/tuned. Solid 3PSI fuel pressure. I have fitted three different ignition systems. Run with factory heat exchangers as well as simple headers.

It used to run great and was smooth like a sewing machine and completely driveable. Now, it's rough, pops through intakes and exhaust, etc... It also is down on power. From 3k through redline it struggles to get there...

One thing I keep coming back to is compression. I am getting across the board readings of only around 130. This is on a cold engine, open throttles, good battery. I did a leak down test and was around 8 to 12% on 1-5, with cylinder 6 around 18% (leaking through exhaust).

Is it possible that the cam timing is off such that the intake valves are open too long during the compression stroke? I did read up on how to set/measure cam timing and it may be a week or two before I can even really dig into it.

Or, is my perception just altered after driving modern cars (E.g. - tuned Golf R daily driver...) and I should realize it's an old car and that's just how they are???

Or, do I need to come to grips that the previous rebuild was flawed and do it again?

Thanks and sorry for rambling...

Karl

[T77911S]

you said it ran great at one time.

cam timing effects either the left side, the right side or both (usually only when set wrong initially) so I would lean away from cams right now.

i would look at the carbs. who adjusted them and do they know what they are doing.

how are you doing the compression test. are you cranking for max comp or are you counting compressions and how many.
i will usually crank for 6 revs and then i will also crank for max comp. just to compare.

what about vale adjustment, make sure they are correct. one that is too tight can hold open a valve .

has it been sitting or is it driven regularly.

you could have carbon stuck in one of the exhaust valves. you might consider getting it running and just driving it hard, see if clears up.
also, any chance you over reved it and bent a valve.

[Trackrash]

Have you checked the timing and the distributor's advance with a timing light?

Sounds like you may be having carb problems as well. I had to replace all my fuel lines recently. The rubber inside degrades and clogs things up in the carbs. Pull the intake fitting off the carbs. There is a brass screen in there which should be clean.

BTW, 130 psi across the board would not concern me, unless you are using oil. Leakdowns can be deceiving. Commonly a small chunk of carbon will flake off when you remove the plug falling on the exhaust valve. When that cylinder is pressurized, use a wood mallet to tap the exhaust valve adjuster, it should improve the reading.

[kfosburg]

Originally Posted by T77911S

you said it ran great at one time.

cam timing effects either the left side, the right side or both (usually only when set wrong initially) so I would lean away from cams right now.

i would look at the carbs. who adjusted them and do they know what they are doing.

how are you doing the compression test. are you cranking for max comp or are you counting compressions and how many.
i will usually crank for 6 revs and then i will also crank for max comp. just to compare.

what about vale adjustment, make sure they are correct. one that is too tight can hold open a valve .

has it been sitting or is it driven regularly.

you could have carbon stuck in one of the exhaust valves. you might consider getting it running and just driving it hard, see if clears up.
also, any chance you over reved it and bent a valve.

Thanks for the feedback.

Not driven regularly.
I recently did valve adjustment and think they're spot on, but I should double check.
I cleaned and adjusted the carbs and plan on taking another pass at it.
Compression test was done to max compression.
I don't think there's any bent valves.

Lastly, driving it... I'm going to make a concerted effort to drive it a lot over the next couple weeks and wring it out. Then, I'll pull the valve covers off and take some measurements to ensure valve adjustment is still in spec.

[T77911S]

check the exhaust valve adjustment on the one that leaks. if you cant wiggle it then it is too tight
do that before you drive it. you can burn a valve that way

[kfosburg]

Originally Posted by Trackrash

Have you checked the timing and the distributor's advance with a timing light?

Sounds like you may be having carb problems as well. I had to replace all my fuel lines recently. The rubber inside degrades and clogs things up in the carbs. Pull the intake fitting off the carbs. There is a brass screen in there which should be clean.

BTW, 130 psi across the board would not concern me, unless you are using oil. Leakdowns can be deceiving. Commonly a small chunk of carbon will flake off when you remove the plug falling on the exhaust valve. When that cylinder is pressurized, use a wood mallet to tap the exhaust valve adjuster, it should improve the reading.

I have actually tried 3 different ignition systems - stock distributor with points and with permatune, stock distributor with crane optical pickup to crane electronic ignition and high output coil, and now a crank fire Electromotive. Advance is 35 degrees at 6k.

Carbs -I have cleaned them 2 times now and did a baseline balance/tune a couple nights ago. I'll do another balance/tune on them this weekend hopefully.

130 PSI... seems low but I'll keep moving forward with the things I can actually do something about.

Thanks for the feedback.

Karl

[kfosburg]

Originally Posted by T77911S

check the exhaust valve adjustment on the one that leaks. if you cant wiggle it then it is too tight
do that before you drive it. you can burn a valve that way

Good point. Thanks. I have weekend homework.

[Tyson Schmidt]

Retorque the cylinder head nuts.

I’ve seen two 911s in the last 2 months that we’re both recently rebuilt that had very loose head nuts.

[wreckah]

try to think back when it last ran great and try to remember what you did before it started acting up...
cylinder head nuts yes, valve clearance yes, carbs yes, all possible. Although i had loose cylinder head studs after a rebuild, and did not notice any change in performance...only more noise and more oil leaking.

[DanielDudley]

Carbs need more initial advance on spark timing, but not more total advance.

You need to take your car to an independent Porsche specialist who knows how to set up a 911 for carbs. There are a lot of variables, and when you start messing with things to fix the problem, you have to start at square one and go through everything from scratch.

[kfosburg]

Some follow up...

After fixing some hot start starter issues...

I determined that the pistons are not "S" for sure and are flat topped "T" or "E" pistons using my borescope.

I did a full carb rebalance and tune this weekend and it's running somewhat better. Still a few pops back through the carbs on high idle and decel and still some idle instability, but certainly better than before tuning. I'll move on to testing for leaks around gaskets and shafts next.

Also, I'll be doing another valve adjustment next weekend to nail that down. I'll check head stud torque at the same time.

FWIW - I found a pretty great app called PerfExpert for "dyno" testing to help get some objective results as opposed to just seat of the pants. It is surprisingly accurate - I verified using my Golf R which I have known values for torque/HP and it's within a couple % for sure.

Here's the graph of my last run of 3. All runs were within 1% of each other. This is wheel HP:



Thanks for all the helpful suggestions and I'll update as I make progress and put some miles on it.

Karl

[kfosburg]

Finally have updates after quite a bit of work...

It was very frustrating to get a solid idle - especially as it heated up. Hunting, stalling, etc... Finally, I dug in and looked deeper into the Webers.

First, I installed dual O2 sensors, controllers, and LED gauges. This was very helpful to really see what was going on with mixture.



It was pretty obvious that I was getting a lot of leakage around the shafts. I could easily wiggle the end of the throttle shafts easily .1mm. The mixture would move from 12 to 18 and nothing stable at all. I looked into local machine shops, but no one wanted to touch re-shafting them. I didn't really feel like shipping them off, so I reached out to Alfa1750 user on Ebay who I've bought Weber parts from in the past, and bought the custom reamer and oversized shafts.







Definitely a leap of faith reaming out an expensive set of Webers, but it went very smooth. Since I had them apart, I soda blasted them and thoroughly cleaned them.

After re-installing and doing a quick balance it was immediately better. Idle was smooth and no longer had a hunting mixture, even after heating up.

Yesterday, I finally checked all the head bolt torque and they were 100% fine. I adjusted the valves using the SnapGap system that I decided to try out (pretty cool method and it's dead-nuts on gap).

Took for a drive today out to Summit Point and it ran great despite being in the 90's and humid here.



Thanks for all the feedback and helpful hints. Pretty sure I'm in the home stretch now and can start just driving for a change!

Karl

[lespaul]

I adjusted the valves using the SnapGap system that I decided to try out (pretty cool method and it's dead-nuts on gap).

Thanks for the shout out Karl!