PMO Carbs - Idle Circuit Issue
 

Rebuilt 3.2L with PMO Carbs, Electromotive Xdi, Clewett wires and other stuff...
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Recently began to experience lower RPM (~2-3k) hesitation and popping. Previously ran great.

Went for the go-to, quick fix and cleaned up the idle jets - but still had the same symptoms.

Pulled the plugs - looked reasonably good, slightly rich. Replaced with a new set just for good measure and checked for stray spark (observed engine running on very dark night). New spark plugs had no impact.

Broke out the Colortune to see what's going on.

Cylinders 1-2-3 tuned in just fine - nice blue flames, just after lean drop.

Cylinders 4-5-6 not so good. Starting from closed mixture screw, each cylinder goes from lean (yellow), to light blue/yellow to yellow - essentially indicating leaning out as each mixture screw opened more. I've never seen this before.

It seems something is restricting / blocking idle circuit fuel flow on the right set of carbs.

I'm going to remove, disassemble and clean the carb today. Appreciate any thoughts on where I should be looking for blockage.

If anyone wants any particular photos as I have it apart - let me know.

Gordo

do you have inline individual filters prior to each carb bank? if so try by passing the one on the lean side for just a test if you don't have new filters on hand.

Idle jets and/or circuit are still plugged. Could also be debris in the metering mixture jet location.

Pull jets and mixture screws and clean it all out.

What cams? Doesn't happen often but if you have a lot of lean pops (bad tuning) there could be carbon build up on the inside of the carb body where the metering fuel holes exit under the throttle valve.

Personally, I would pull it off and clean out the entire carb body.

I have individual inline fuel filters prior to each carburetor on my 46mm PMOs. No lean issues.

you can shoot air thru some of the circuits

you may need to clean the float bowls

now... what fuel hoses are you using? are they new and rated for EtOH mixes? they may be shedding tiny black particles into the carbs

Idle Circuits
 

Haven't pulled the carbs in quite a while - no-kidding, removal took less time than it did to type up my original post.

In addition to cleaning the carbs, I will also need to order new base gaskets - the old ones tore while removing.

I run a single Racor 110A filter with 2 micron filter - in engine bay / just prior to the carbs. I believe evaporative properties of ethanol fuel - and associated deposits - are the primary source of blockages. Looking forward to non-ethanol pumps in my area...


http://forums.pelicanparts.com/uploa...1599857977.jpg

Gordo

There was something I remember about the idle air corrector jets being blocked by the wrong or backwards or upside down top gasket or even the air cleaner base. I ran across it once way back.

Idle Air Correctors
 

Thanks Mr. Walker, no changes made to any part of the carbs or filter housing before it started running rough.

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New question - are the base gaskets sized to carb throttle bore?

I have PMO 46's and mated manifolds - how do I ensure I'm buying the right sized gaskets?

Thanks again,


Gordo

Gordo. I am having the exact same problem on my PMO 50s on a race engine. no idle, lots of popping, incredible hesitation when you put foot into it. no low range power. Peter Dawe's shop has it now and i hope they can figure out. One thing he found was that 2 of the manifold gaskets to the motor had partial blow out..talk about big vac leak! i also only run race gas. 100+ ( so no etoh i guess) but the comment on fuel tubing dissolving is prescient . will report back. those guys can fix anything

Anything yellow in the Gunson is rich, not lean. Lean is going from blue to white. From a closed mixture screw it should go white/light blue (lean) to blue (correct) and then to yellow (rich)





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Did you check the synchronization since a whole bank was acting different?

Colortune and Sync
 

Colortune going yellow = rich... Thanks, you're right, not sure what I was thinking when I wrote that.

Progressed from mixture screw closed (black / no flame), ~1.5 to 2 turns open (white / light blue), then directly to bright yellow as I opened beyond 2 turns.

I never found the sweet spot where I could get a nice solid blue flame - which normally seems to be ~1.75 to 2 turns out on my carbs. Very unusual & noticeably different from the left bank that I had just finished tuning.

I did check the sync before I started tuning/adjusting. I found the right bank (4-5-6), was somewhat out of sync at 3k RPM.

Thought that was unusual as well, since sync seems to be a set once config - but thought I may not have checked 3k sufficiently last time I tuned. Either way, I sync'd and proceeded to colortune. Looking back - it seems the sync finding indicated something was wrong with the right bank....

Gordo

PMO Idle circuits
 

Check your gaskets - while I doubt this would cause previously well running PMOs to change if they were not R&R'd - but this is an important thing to know. Check out the attached.

As well as the gasket, the set up of Linkages is critical - even minor adjustments can have large impacts.

Hope this is helpful...

LT

I tried to upload a couple of images -- if they failed to make it pls let me know & I'll try again...

PMO Carbs
 

Taking another shot at uploading a couple of images..
http://forums.pelicanparts.com/uploa...1599957535.gif
http://forums.pelicanparts.com/uploa...1599957535.jpg

Thanks
 

Thanks I_turn9 - I looked at my install thread and realized I haven't really messed with (removed) my carbs since 2015.

http://forums.pelicanparts.com/911-engine-rebuilding-forum/821976-gordos-pmo-carb-tuning.html

what i meant was the lean bank may have a clogged filter and a quick by pass will rule that in or out. i did not suggest an inline filter in and of itself would cause a lean condition which you seem to indicate was my point..

Thanks for your clarification. I think your suggestion of disconnecting the filter would rule that out. Wouldn't hurt to try. Might have a clog.

Update
 

Cleaned right bank of carbs up as good as possible - removed carb, top plate, all screws & jets and sprayed all passages w/carb cleaner & compressed air.

Reinstalled and noted carb / cylinder #6 doesn't flow as well as the others. This isn't new - I've had to open the air bleed screw on #6 to try to sync. At idle other carbs pull ~ 4.5 on the STE while #6 pulls ~ 3.8 at best. More pronounced at 3k RPM. Curious what could cause this?

Also found I can't get #6 to idle - digging into that now. W/colortune I have spark but no fire at all - opening the idle mixture screw all the way out. I've re-cleaned - heading back out to give it another go.

I've noticed I can put my palm over #6 velocity stack to manually choke it - and it will fire, but will die as the fuel burns off.

Gordo,
One of the secrets of carb balance is to check the relationship of all the throttle plates on one carb.
This is best done with the carb off, turn it upside down and tap each throttle plate towards the closed position. You will hear a thud or a tick, thud = fully closed plate , tick = a bounce to the closed position . To get all the plates closed at the same time and equally you must tweak the throttle shaft . This is done by picking the plate or plates that need more closing . If you place a small round object ( small handle of a screw driver ) or dowel , on the opening of the good plate
at the closing side , then bounce or push the offending plate to the more closed position. This process will effectively put a torsional twist in the throttle shaft and bring it in to alignment with the other good plate. You will you have got it right when all the plates have the same thud .
This is a little known process but I promise it works. Take your time , a little twist goes a long way.
Ian

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.

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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