PMO Carbs - Idle Circuit Issue

[Gordo2]

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

[juanbenae]

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.

[jpnovak]

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.

[911 SLANT]

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

[RWebb]

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

[Gordo2]

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...




Gordo

[john walker's workshop]

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.

[Gordo2]

Originally Posted by john walker's workshop

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.

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

[Geneman]

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

[LJ851]

Originally Posted by Gordo2

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.


Gordo

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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[LJ851]

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

[Gordo2]

Originally Posted by LJ851

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

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

[l_turn9]

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

[l_turn9]

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

[l_turn9]

Taking another shot at uploading a couple of images..

[Gordo2]

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

[juanbenae]

Originally Posted by 911 SLANT

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

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..

[911 SLANT]

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.

[Gordo2]

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.

[icarp]

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