Pmo help, please

[steve meltzer]

I wasn't sure if I should post this here or on the engine forUM, but here goes...

I bought a ’72 911 with a a 2.7 L motor freshly built by Ed Mayo, with 0802-0 PMO carburetors. The car runs great except for a hiccup on acceleration exclusively at about 4000 RPMs. This happens probably 80% of the time. The carbs were recently synched by a pro mechanic, but this problem has persisted. I'm thinking that it might be worth a few bucks to change the main jets from 135 to 140. However, I'm not sure where the main jets are located on this carburetor! I hate to admit my ignorance, but in fact, I don't know where any of the jets or other fittings are on this carburetor.

I've attached two pictures and have circled the areas about which I'm uncertain. Can someone help me with specific explanations of the areas that I've circled?

Also, I’d take opinions about whether or not I'm on the right track here in replacing the main jets, and whether or not I should also consider changing the emulsion tubes as well.

thanx
steve meltzer

[snbush67]

Reference your pictures. Yellow are the jet stacks, black is the idle jets, red are the float bowl drains.

Is the hiccup a hesitation, or just rough spot. Try richening the idle mix by turning all mix screws out 1/4 turn overall mix see if it goes away. If it goes away then turn the screws in 1/16th turn incrementally until the hiccup returns, then back out, to the last best setting.

If it ends up that you get it to go away but your idle is too rich then consider jetting changes.

Start by listing all the specs on your carbs,

Idle jets
Venturis
Emulsion tubes
Mains
Airs

Also you should get some decent filters.

[HarryD]

Have you reached out to Ed Mayo? He may be able to help you.

[steve meltzer]

Thanks guys. That is the information I needed. Ed MAYO built the engine, but did not set up the carburetors your tune, the engine as best I can tell. steve

[stownsen914]

The main jets are in the jet / emulsion tube stack that screw into the top of the carbs. You've circled the top of the jet stacks in your first pic.

See pic below, grabbed from 914world. PMO uses Weber IDF jets and parts. The top of the stack is at the bottom of the below pic.

[Jeff Higgins]

Originally Posted by snbush67

Also you should get some decent filters.

I would elevate from "also" to your absolute first priority. You do not have a jetting problem per se, you have an air flow problem. In other words, you will never satisfactorily solve your jetting issues until you install something over those carb throats that will be conducive to proper air flow.

The little screens and filters you are using have two inherent problems. First, and not necessarily the most obvious, is that they severely restrict air flow. Metal mesh screens like that are rated in percentage of flow restriction. In other words, "25% restriction", "18% restriction", etc. Most metal screens similar to the ones you are using fall somewhere in that range. Add to that the restriction imposed by whatever "filter" (yellow) material underneath those screens, and you have hopelessly choked off the air flow through those carbs.

Even worse (yes it gets worse) is the interface between those screen/filter bases and the air horns, or short velocity stacks on top of the carbs. It's way, way too much to get into here, but there is no shortage of information available regarding air flow into these kinds of flared inlets. Suffice to say here, one of the most critical aspects is the open space around and above the flared lips of the air horns. Anything too close axially or vertically not only reduces flow, it massively increases turbulence, which is the last thing we want. Turbulence sends mixed signals to the jets. Not what we need. Those air horns only reasons for being are to straighten air flow and to reduce turbulence. Those screens don't even give them a chance.

So, yeah, you need to start with that. Either proper filters or open air horns. I assume this is a street car, so the former is the preferred option. Lots of options available that both look great and offer the performance you are looking for. And they will make it way easier to tune.

[steve meltzer]

@snbush67 thanks so much for beginning the conversation about airflow and these great looking but poorly performing stack filters.

@stownsen914 it is interesting to me that many so-called experts on the web believe that the PMO carburetors are based on a 40 IDA, when in fact it's really the 40 IDF that is the closest Weber to the PMO.
Thanks for the reminder

@Jeff Higgins thank you so much for the education about airflow, which I had not even considered. Though I'm not an engineer, I completely understand exactly what you're saying and appreciate your thoughtful comments. In this regard, can you give me a specific recommendation or two for the best air cleaner to give me the performance I want so I can ditch these striking, but ineffective and interfering air cleaners?

Thank you again gentleman,

Steve

[steve meltzer]

Oh, and one more thing! The fittings that are to drain the float bowls have a banjo bolt configuration. Why would they do that more sophisticated fabrication, when it's not necessary? What am I missing? Thanks Steve.

[Hi_Fi_Guy]

Rain hat style K&N filters are popular. There are also basic flat top covers or ITG with foam on sides and top.

[steve meltzer]

thanx so much. s

[stownsen914]

If you get different filters, remember that the top of the PMO carbs is different from the top of the Webers. The float bowl vent hole on PMOs will be blocked if you use Weber-specific filters. Newer PMO filter have the vents drilled for both types of carbs. It's easy enough to drill the hole in older Weber base plates, you just have to know to do it. If the vent gets blocked, the float bowls will drain from suction when throttles are closed. Ask me how I know

[steve meltzer]

@stownsen914 May I assume that if I get the PMO air filter set up ( Part # 0314) I will have provided the most effective airflow and avoided any problems with the problem you described? Thanks, Steve.

[Jeff Higgins]

I see folks with more carb experience than me (I'm a hardcore MFI junkie) have chimed in with recommendations and specific knowledge regarding suitable filter setups. We use the same ones on MFI, it's just the specific fitment has its own little details.

Don't overlook the stock air filter housing. On my 3.0 MFI inducted motor, it makes more power than the separate K&N filters. We saw the same results over 20 years ago on a good buddy's 2.2 S motor, and on my son's 2.4 T as well. All three verified by running on a chassis dyno. Some don't like how it hides the top of the motor, but it really is the best from an airflow perspective. Granted, individual filters look way cooler, and on a street driven car they are so close performance wise that I would defy anyone to tell me that they can tell the difference.

The open foam ITG filters, either the squared off or the rounded, seem to match the stock housing for flow. I'm just not sure that they are a "street" filter, especially if you get caught out in the rain. Racers love 'em though.

Diving deep into the esoteria of the K&N filters, the rain hats do flow marginally less than the flat top housings. The effective inlet opening is essentially the "slot" that would be visible between the face of the filter and the "skirt" of the upper housing as viewed from the bottom. Fortunately, much like the difference between the stock housing and individual filters, any performance differences would likely be unnoticeable on a street car. Really picking nits here.

So, yeah, several popular, proven choices available. Pick one you like and go with it.

[stownsen914]

I believe the current PMO offering is usable for both Weber and of course PMO carbs, including re: the point I highlighted above. It's the older filter housings that you have to check and possibly drill an appropriate hole for the float bowl vent. The factory housing flows just fine for a stockish engine and even up to at least 250 hp per my knowledge. Note that the factory housing bases would have to be drilled for the vent hole, but it should be possible to adapt them.

[steve meltzer]

Thanks again guys. I decided to go with the PMO set up with a short version as I might want to put an air conditioning condenser over the engine bay. The parts numbering system used by PM is amazingly stupid .....e.g. you must differentiate the letter “O” in front of 312 from the numeral “0” at the end. So, it becomes PM-O312-0. I ain't that bright, but I know not to do something confusing like that. steve

[nickelplated5s]

I'd suggest just running with open stacks for a baseline. Then I'd suggest taking Jeff's advice on going stock.

FWIW Jeff other than some plating I stayed stock. The dyno run with the rebuilt pump will be interesting. Mark told me he was reshaping a space cam for a 3.5, never heard of such.

[steve meltzer]

@nickelplated5s as I was fooling with the car this afternoon and getting the mushroom filters off the stacks, I was thinking exactly the same thing be nice to know what happens with no filters. thanx s

[nickelplated5s]

Think about it the same as taking an ear plug out. Now you have me thinking about where the 69 motor went. I call mine a Karmadugeon as the 2.2S in it.

[steve meltzer]

@nickelplated5s I had a modest chance of getting the original motor for my '69E, but when I bought the car with a 73T motor it was, and continues to be, running great. He wanted too much money for a pig in a poke, and matching numbers just weren't that important to me. I suppose I should update my profile, because the car in discussion here is a 72T with a rebuilt 2.7 motor out of a '74 car. Therefore, also a bastard. s

[HarryD]

Steve. Did you see these? https://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/1186057-early-911-air-intake-complete.html