Thanks Ian - I checked throttle plate positions when I had the carbs off. All 3 appear to be in very close alignment. I sprayed carb cleaner on the back of the plates on 5 & 6 and each seemed comparably sealed.

Manifold Gaskets?
 

Wondering if this could be the problem:
http://forums.pelicanparts.com/porsche-911-technical-forum/873140-pmo-carb-issue-3-2-911-engine-2.html

Gaskets between the head, insulator and manifold. They appear good, but based on the above link - apparently can wear out with few miles...

As Ian said, it's best to diagnose this with the carb off - if you would like a visual clue, hold the carb up to a light source - if one plate is not aligned equal when the throttle is closed, you will see more light around that plate. Actually I believe this is a fairly common problem. I have wondered about how this might happen, but so far have found no way to explain how this happens.

PMO Idle Circuit
 

http://forums.pelicanparts.com/uploa...1601299474.gif

This describes a problem that is kind of rare but something that should be checked. The the image for a good visual...

FYI. my problems were solved by Dawe et. al. but unfortunately, without a real bona fide smoking gun. Just thorough disaasembly on bench, clean etc. However, They did find two blown gaskets/gaps in gaskets. Did not find out if they were at manifold-carb or carb-insulator interface or how large. either way, seems that would create a huge vaccum leak and at least account for no idle... and also an afr approaching 16-17 when made to idle at high rpm. My car was backfiring and spitting quite alot , before this out of tune incident, and i drove it on track like that for quite awhile because at upper ranges 7-8k , it did not really matter. car still pulled nicely up there. i suspect that interval led to gasket blowing along the way, unnoticed by me..... jmho.

Thanks folks - thinking I may try a propane leak test around the head-insulator-manifold area this afternoon.

Will report my findings.

how do you do that gordo?

Propane Intake Leak Test
 

Vehicle outside, safety glasses on, fire extinguisher at the ready...

Involves waving an unlit (turned on/gas flowing), handheld propane torch around areas of the intake manifold where it may have a leak - with the engine running.

If the intake manifold leaks / draws in air, then it should draw in propane as the torch head is waved over the leak - which should make engine RPMs momentarily increase.

-----------------------------------------------------

I tried a propane leak test, but the results were inconclusive.

I don't think the propane leak test method will work for the area I am investigating.

I think the head / intake manifold mating area is subject to airflow from the alternator fan - ducted from the engine shroud. As such, I think propane from the torch is caught in this turbulence.

Ordered new head-to-manifold gaskets anyway... Project for next weekend.

Thanks for the thoughts folks.

Are your manifolds from PMO & selected for your heads or are they OEM manifolds?

Thanks, was hoping you might chime in.

I have PMO manifolds sized for stock 3.2l heads/intake ports and 46mm PMO carbs. Phenolic spacers that cover the injector cutout ports, sandwiched between gaskets.

Ran great for 5+ years with occasional idle jet cleaning and adjusting. No configuration changes leading up to current issue - spark but no idle on #6, strong/normal on mains, all carb passages cleaned multiple times.

Gordo

PMO Installation Links
 

Couple of threads covering install of my PMO carbs & manifolds back in 2014 for reference:

http://forums.pelicanparts.com/911-engine-rebuilding-forum/746290-studs-b-b-exhaust-pmo-manifolds.html

http://forums.pelicanparts.com/911-engine-rebuilding-forum/801692-pmo-46mm-carburetor-installation-3-2l.html

thanks for that explanation gordo. yeah i can imagine its very tough to get that to work withn fan air going past the intake ports. but cool idea. peter dawe told me he uses a laser like heat probe to shine on each header as it exits the block. this tells him whether the cylinder is actiually firing .

So, #6 only? I thought #4 through #6 were similarly afflicted.

PMO fuel galleries have threaded ends with sealing screws, I assume that you have opened, mechanically cleaned & visually observed the galleries for #6 that feed the idle fuel jet. This series of galleries starts with a horizontal run to the bottom of the emulsion tube followed by a vertical run that is parallel to the emulsion tube which is intersected at the top of the run by the horizontal passage that the idle fuel jet screws into.

PMO uses brass for their mixture screws, the tips of these can score & shear off in Weber bodies leaving the tip in the fuel metering hole. PMOs use a brass insert for the metering orifice but check for mixture screw integrity just to be sure.

Remove port plug that covers where the transition holes are drilled & check for blockage there.

#4 and 5 seemed odd - more difficult than usual to find a mixture screw setting that produced a nice blue flame (w/colortune) within (+/-) 1/4 turn of the mixture screw.

Meanwhile #6 simply fails to fire (at idle) for any/all idle mixture screw settings. I can choke the carb with my palm to get it to fire (possibly drawing from main circuit), but clearly not getting normal idle circuit fuel flow.

#6 is exactly as described in this thread:
http://forums.pelicanparts.com/porsche-911-technical-forum/873140-pmo-carb-issue-3-2-911-engine-2.html

I have focused on #6 and removed all galley screws to blow out with carb cleaner & compressed air. I studied & made a whiteboard drawing of the idle circuit fuel flow - to ensure I wasn't missing anything. Observed carb cleaner flow from E-tube/float well, up to Idle Air Corrector, down past/through idle jet and down to progression holes and finally to mixture screw & throttle body port.

Bottom line - I'm pretty confident I don't have a blockage (you should feel that way after you've removed the carb for the 3rd time to clean...).

Rather confusing - leaving me to think I have an intake leak (at the manifold to head mating surface) that is reducing vacuum to the point that it will no longer adequately draw fuel from the idle mixture port at low RPM's (leak = least resistance).

Thats' my current theory.

Next time I have ambition to type - I will write my theory on how manifold gaskets could prematurely fail and leak (phenolic/plastic spacer vs metal expansion / contraction & resultant stress on uncompressed gaskets...).

It's a good thing I enjoy a challenge - and the assistance & support I get from folks on the board...

Thanks again,

That is a major air leak. OK, pull manifolds & observe gaskets. The reason I asked if you had PMO manifolds is OEM manifolds on CIS heads with phenolic insulators can develop air leaks due to mismatch of the ports to the circular port on the OEM manifolds. Aluminum PMO manifolds are rather stout compared to the magnesium, OEM manifolds and are less susceptible to warpage. Flanges will warp at the points where the fasteners are and not across the throttle bore due to load applied to area of gasket supporting the load. Flanges mechanically yield resulting in less sealing capacity at the throttle bore.

Intake Manifold Gasket Leak on #6
 

Started this AM doing a compression test - relieved to find all cylinders hitting at ~175psi / no notable delta's... :cool:

Removed 4-5-6 intake manifold and immediately saw the problem:

http://forums.pelicanparts.com/uploa...1601739061.jpg

Gaskets on 4&5 ports were intact - with #6 clearly blown out.

As unusual as this may seem - I'm now aware of 2 other instances of manifold gasket failures causing a cylinder to misbehave at lower RPM.

Colortune was essential to troubleshooting - narrowed to individual cylinder & clearly showed it was an idle circuit issue vice spark.

Glad to finally figure this one out. Thanks for everyone's help.

great gordo. do gaskets go on each side of the phenolic spacer when mounting
the manifold to the block?

Intake Manifold Gaskets
 

Yes, gaskets go on both sides of the phenolic spacers.

With the intake manifold removed, I was able to get a good look at the back sides of the intake valves - pretty clean, not bad for ~ 6 years and ~30k miles:

http://forums.pelicanparts.com/uploa...1601781468.jpg

I will be re-tuning the carbs tomorrow AM - hoping to find even STE #'s as I sync.

I also want to see if I can record an instructional video using the Colortune. Won'y be easy - need to see if I can find a way to do it that doesn't require 3 hands (hold mirror, camera & adjust mix screw).

Btw
 

Missed this... Yes, I've done the same - used a cheap Harbor Freight laser thermometer to determine which cylinder is problematic (different temp than others) by pointing at each exhaust tube coming off the head to determine which isn't firing well (cooler).

Works quite well - my brother "Cabmando" intro'd me to that troubleshooting method. Quick way to figure out which idle jet is plugged - but normally when you have one idle jet acting up, it's best and easy to remove and clean all of them.

I focus on idle jets when troubleshooting - they are by far the smallest passage for fuel flow, and most likely to experience a stoppage. Fortunately they are also super easy to remove, clean and replace (without needing any adjustments/tuning) - cleaning the idle jets is always my first step in troubleshooting any new hesitation / coughing problem.

Meanwhile, Colortune provides best insight to whats going on / cause - but using it is more involved.

Thank you Gordo. great knowledge to store. Are the idle (air) jets you are referring to, the ones that vent to the top of the carb? meaning the ones which will flood the carbs if they are covered over by the mounting plate for filters? I thought those jets normally only conduct air, not fuel.

Would be very interested in a tutorial on colortune use if you can pull it off.. THanks