As usual, you guys have lots of good info to consider.

I did casually check total movement of the fuel distributor during my rebuild; I didn't detect any issues there. I'll check again.

I have not put a scope to my Pertronix yet, but that is a very good suggestion. It may be better to swap the points back in and eliminate that.

One more bit of possibly-related info - I'm also not running a stock coil (I have a Pertronix 45111 coil instead). This came up in another thread:

http://forums.pelicanparts.com/porsche-911-technical-forum/782721-pertronix-flame-thrower-coil-compatible-cdi.html

More info back when I get some testing done...

Still haven't had a lot of time to play with this. I did test my decel valve and found that it was cracking open at much lower vacuum than where I thought I had set it (it was opening ~15 in.Hg which would have had it open at open and some part-throttle conditions).

I reset the decel valve to open at 22 in.Hg (car was idling at ~20 in.Hg at previous test). Drivability is much improved, but I still need to go through the other items mentioned above.

Decel valve operating pressure.........
 

Fan,

The operation of the decel valve (opening/closing) has no effect to the vacuum supply to both decel valve and WUR. Unless the diaphragm inside the DV is ruptured or broken. The release of excess vacuum during deceleration of the engine is channeled to the lower chamber of the DV. A valve is activated to open at a certain vacuum setting (?). So the question is at what value? I searched for this value and came short. But when I tested six (6) decel valves for opening pressure, I found that around 22"+ Hg, the valve pops open. So I've been using this value for setting the decel valve.

Tony

Thanks, Tony, for testing a few decel valves. It looks like I guessed correctly.

Here's a picture of how the vac lines are routed at the decel valve:

http://forums.pelicanparts.com/uploa...1387084161.jpg

The larger-diameter vacuum line that "leaves" the picture to the right is the line going to the WUR as vacuum reference. Let me try to explain my previous statement a bit better.

I recognize that the decel valve gets opened by a separate vac line than what operates the WUR. However, when the decel valve is opened, the WUR no longer references manifold vacuum directly; the pressure reference for the WUR will increase (decreased vacuum) because of the air spilled into the line by the decel valve. Therefore, when the decel valve opens up, two things are really happening: Air is bypassed around the throttle plate and the WUR slightly enrichens the mixture.

Wrong connection.....
 

That is not the right connection for the vacuum line!!!!!! The tee-connection is hooked up on the top of the decel valve not below as shown in your picture and goes to thermo valve. Then thermo valve to WUR. Your logic is correct about the vacuum lost during operation based on your current set up. But the fact is that you have the wrong set-up.

I'm not home today and could post the vacuum diagram later.

Tony

Tony - Thanks for the info. Please post that vac diagram when you get a chance.

I've got a '75. No thermovalve, and one vac reference for the WUR (located on top - no atmospheric reference ported to the throttle). I didn't think the thermovalve came into play until ~'77. This is how I found the vac plumbing done when I disassembled the engine and is also corroborated by some research.

Here's an excerpt from the early CIS microfiche that I found online somewhere; it references that the WUR I have only gets a post-throttle-valve vac connection:

http://forums.pelicanparts.com/uploa...1387263047.jpg

Well, I took a "hiatus" from this problem to strip the car apart over the winter and make it pretty (replacing front fenders to eliminate the rust holes, fixing a bunch of body stuff, full wrap to get it all one color, etc). I finally got it back on the road briefly last weekend and again today for a longer drive. Oddly enough, the problems aren't as bad as I remembered. (Doesn't mean they're not there...) Back to troubleshooting.

One question: How much does indicated oil temperature (engine temp) affect operation of the CIS? I think my on-engine oil thermostat may be stuck open - engine is not getting above 180F. (I have verified oil temp with independent temp probe right in the oil tank.)

The engine temp should not affect the wur as the wur is heated via the electrical element on the bimetallic arm. You should be able to see pressure changes simply by running the fuel pump and applying power to the heating element.

Thanks, L.J. As soon as I read your response I smacked myself on the forehead with a "I knew that" moment. I've been doing too much bodywork. I made myself go back and re-read the whole thread and dig through my Bosch CIS book.

Resetting my punchlist:

[ ] Verify timing at advance is 32-38° BTDC (was 30° last time I checked, will probably bump to 34°)

[ ] Retest the pressure change from the WUR with vac/no-vac. Check that there isn't a malfunction in the the WUR.

[ ] Injector flow and spray pattern test

[ ] Pull the air box and properly test all seams for leaks

Nope.........
 

Larry,

My friend, I have to disagree with your statement!!!!! The control fuel pressure is directly affected by the ambient temperature (meaning surrounding temperature). This includes the heat generated by a warm engine in the engine bay. As an example is the chart of the of the control fuel pressure for -009 attached in the previous post. As you could see from the chart, the value of the control fuel pressure is linear to temperature change.

Tony

That's correct, i.e. both affect how the CP changes as the engine warms up, but the
heater element reduces the warm-up phase and allows the CP to reach it final value
which may not occur without the heater functioning properly.

OK - Finally got to play with the car again.

I verified timing is at 34° BTDC at full advance.

I retested my system pressures:

System pressure: 4.8 bar
Cold control pressure (20°C): 1.6 bar w/o vacuum
Cold control pressure (20°C): 2.1 bar w/ 400 mm Hg vacuum (2.2 bar @ ~200 mm Hg?)
Warm control pressure: 2.85 bar w/o vacuum
Warm control pressure: 3.4 bar w/ 400 mm Hg vacuum (3.6 bar @ ~200 mm Hg?)

It does seem that the difference in no vac/vac pressures are within spec (though at the low end).

Comparing against my previous values in the original post, it seems that cold control pressure has gone up with the ambient temp increase, but warm control pressure has gone down. Also, I'm noticing that the control pressures with vacuum applied are spiking ~200 mm Hg vacuum applied to the WUR rather than at 400 mm Hg. This doesn't seem right. Do I have a failing WUR? Is it time for rebuild?

I still need to do a spray test.

Coming back to the engine operating question: I took the car for a long drive (90 minutes) after fiddling this morning. I blocked off air access to my fender cooler and the oil temp got just over 180F while driving in ~80F weather. I verified operation of the sender and gauge this past winter (hot water bath, ohmmeter, variable resistor, independent thermometer, etc) and so know that the indicated temp is fairly accurate. Could this low operating temp be contributing to the bucking behavior? My assumption is that the WUR is hitting "max temp adjust" at ~100F / 40C.

As someone with the same problem of mid range bucking on a 74, I'd be fascinated if it turns out that the low operating temp contributes to the bucking. Like you, I've chased this problem for a while (ignition, pressures, air leaks etc) with no luck yet but I too have low operating temp perhaps due to a non-standard front oil cooler that a previous owner installed.
Did you crack this one yet?
Best,
Thomas

Coming back to this one:

The bucking problem essentially went away with high temperatures here in the PDX area. I pulled my CIS off last weekend and while it's out I'm going to change both the engine and external thermostats to be sure I'm at proper operating temperature.

Unfortunately, my whole CIS unit fell off the workbench and destroyed the airbox. I'm currently sourcing a new one. The next step is to replace that one and then start retuning.

Update (and "close-out" report):

I located a replacement airbox and discovered that it also had a few minor leaks. In searching for what to use to seal this, I considered these ideas:

- Using epoxy to seal the box as several here have done
- Welding the airbox
- Sealing from the inside with something like gas tank sealant
- Designing/building my own

I investigated the weldability of the airbox and determined that the plastic this thing is made from is not a great thermoplastic. Best I can determine, the airbox is made from carbon-filled polyphenylene sulfide (PPS). I experimented with a few different epoxies and determined that flexible epoxies containing methyl methacrylate bonded very well to the airbox material. (I eventually used Loctite "Plastic Welder" which is readily available.) I used a Dremel tool to route out a partial groove around the periphery of the seam and filled it with this epoxy.

I haven't given up on the "sealing from the inside" method yet and am considering this my "backup". I see some challenges in cleaning the inside of the airbox. My thought is to use something like POR15 gas-tank sealant or an aviation gas tank sealant product.

Having a broken airbox to play with was enlightening. I found the plastic to be very fragile and brittle. It's only a matter of time until I have the same issue with this newer box. I'm definitely looking at designing a sheet-metal version. If anybody has interesting ideas of specific features to include in design for such a box, please pass them along. (I'm already thinking integrated pop-off valve and making it separable while on the engine to facilitate installation.)

All that said, I verified no further air leaks and then reassembled and installed the CIS again.

During the airbox fiasco, I had contacted Tony (boyt911sc) and sent him my WUR for evaluation. He found it to be out-of-spec. I commissioned Tony to do a rebuild of the WUR and received it back last week. The WUR came out of its box completely spotless and looked brand new. I tested control pressures once installed and everything was perfectly in spec. The car fired right up! I did have to adjust the idle mixture to compensate for the non-leaking airbox.

All my hesitation/stutter problems are now gone. Drivability is vastly improved.

While the CIS was out I did change out the engine thermostat; I still have to change out the external thermostat. I have confirmed that the external stat appears stuck open and the car is running a bit cold. However, since the performance of the CIS is so much improved, I'm concluding that the main culprits were a combination of airbox leaks and out-of-spec WUR.

Thanks again to Tony for helping me troubleshoot this and providing a great WUR rebuild service!

The stainless Webb design air box comes up for sale once in a while.
I have one for a large intake runner car ('78/'79) you could borrow to copy if you wanted to. (not using it with my Bitz kit) :)
PM me with contact info if interested. :)

http://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/423504-webbs-machine-design-cis-stainless-steel-air-box.html

Info on the fitting of it:
http://forums.pelicanparts.com/porsche-911-technical-forum/660859-webb-airbox-fit.html

Good photos of it starting on post 45
http://forums.pelicanparts.com/porsche-911-technical-forum/621680-finally-back-game-3.html

Maybe this one is still available?
http://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/734358-webb-stainless-air-box-911sc.html

Could you expand on this, or do you have a photo? I've got a spare airbox with a split seam and would like to give it a go at repairing it before just giving up.

Dennis - YES - I would very much like to borrow that box (mostly to learn from the issues I've read about with those).

Richard - I used a small v-bit and just traced the seam best I could (1/2 of the vee on each side). On the "flat" sides, I cut a groove ~0.060 deep so there was a channel for the epoxy to lay in and seal. On the side directly under the air cleaner I used a ball bit to route the groove on the lower side of the box only. Let me know if this doesn't make sense and I'll add a sketch or something.

I did find that the Loctite Plastic Welder didn't flow like other epoxies I had used (it's fairly thick) and so I had to squish it into the groove to fill.

Ok, shoot me a PM.

Thanks, I get the general idea. Basically, give the epoxy a purchase on flat surfaces. Once I open up the box I think it'll be clear.