Jet Power

[shoestring7]

The PMOs on my newly rebuilt engine need attention. One of the things Russell Lewis discovered when he stripped it down was that the plugs used to block up the old injector ports slightly fouled the inlet manifold. That resulted in air leaks between the heads and manifold, and when the car was set up on the rolling road jetting was changed to try and compensate for this.

Now its sorted I need to return it to the factory settings to get through the running-in period before it can go back on the rolling road. Richard Parr suggests a '55' idle jet (its got size 60s at the moment), and he also asks "Let me know current jets, e tubes and venturies".

I think the main jets are the brass objects here:


I'm not exactly sure what e-tubes are, - any suggestions? And presumably the ventures are the central chokes - as far as I know only the main and idle jets were changed.

Any ideas where I can get six size 55 idle jets?

cheers,

Charles

[Lightweight_911]

Many of the jets, etc used in PMO's are from the Weber DCOE's.

Idle jets, as far as I remember, are from the IDF Weber.

There are lots of stockists - a quick search throws up these:

http://www.gsparkplug.com/1x-weber-repl ... oCMvnw_wcB

[Lightweight_911]

.

'e-tubes' are emulsion tubes ...

[911hillclimber]

I spent long hours trying to correct 2 very lean running throttles on my 50mm PMOs, found the gaskets between the manifold base and the thermal insulator were "just" miss matched allowing lots of air in a hairline gap about 30mm long. Spraying carb cleaner on the area with engine running revealed the problem.
Been perfect since a home tune following the PMO guide.
New gaskets from PMO fixed this.

[fetuhoe]

I an not100% conversant with the PMO but if the use a 40DCOE parts I think that you are looking at the 'emulsion tube holder.

If you unscrew the slotted brass component you will find a calibrated jet pressed into a brass tube and this is the Air Corrector.

The brass tube itself is the emulsion tube and screwed onto the other end is the main jet.

[shoestring7]

Ok, this is what I have got. If I understand things correctly, the main jet is marked 120, and the E-tube is marked F11:





I've sent the info to Richard Parr and I'll see if I can understand the response! I'm busy next weekend, but I should be able to manage some garage time the week after that.


>Charles

[911hillclimber]

When I re jetted my 50mm PMOs for my 3.2 sent the request to Richard and got everything I needed with in 3 hours.
Back then, 5 years ago now, it was oddly difficult to buy the parts!
Once bought they were in my garage in 48 hours...

I found him and PMO outstanding.
Let us know how it all turns out.

[fetuhoe]

A 120 main seems a little small for a 2.7

Do you know the size of the Venturi and the Air Corrector? The air corrector will influence the size of the main jet and the way the progression circuit behaves towards its top end.

It is difficult to compare an F11 Emulsion Tube to the more commonly used F3 of F26 used on a conventional 40IDA3C carb as their construction is quite different.

In 40DCOE terms an F11 would weaken out the bottom end of the progression circuit but this shouldn't affect idle but could create a small flat spot.

Do you know more about the position of the idle mixture screws during the set up?

It's also not clear what 'plugs have been used to blank the injector ports.

Are these MFI heads ? because the injector posrt would be relatively easy to blank.

If they are K-Jetronic heads then some of the PMO manifolds are designed to blank the injector cutout in the port but some of the smaller manifolds don’t have this feature.

The normal method to blank these cutouts is with a suitably shaped thermal insulator.

[shoestring7]

A 120 main seems a little small for a 2.7

Do you know the size of the Venturi and the Air Corrector? The air corrector will influence the size of the main jet and the way the progression circuit behaves towards its top end.

It is difficult to compare an F11 Emulsion Tube to the more commonly used F3 of F26 used on a conventional 40IDA3C carb as their construction is quite different.

In 40DCOE terms an F11 would weaken out the bottom end of the progression circuit but this shouldn't affect idle but could create a small flat spot.

Do you know more about the position of the idle mixture screws during the set up?

It's also not clear what 'plugs have been used to blank the injector ports.

Are these MFI heads ? because the injector posrt would be relatively easy to blank.

If they are K-Jetronic heads then some of the PMO manifolds are designed to blank the injector cutout in the port but some of the smaller manifolds don’t have this feature.

The normal method to blank these cutouts is with a suitably shaped thermal insulator.

They are CIS/K-Ketronic heads, John Middleton plugged the injector ports with suitable alloy bar before he opened up the ports. The ends were a little proud of the heads - which we didn't spot.

The PMOs were set up by Richard Parr to suit the car's original specification. This was changed when it was set up on a rolling road to try to fix (what we discovered later) were engine problems. Richard has come back to me with more questions; and at the moment I'm struggling to identify the venturi and aircorrector components.

Does anyone have a decent exploded diagram?

Charles

[fetuhoe]

I am surprised that anyone would 'blank' a CIS Injector Port with a physical plug.



We have used a Tufnol Spacer with suitable gaskets for years and they work well with standard intake manifolds.



If you have PMO manifolds designed to work with CIS heads then I wouldn't blank the port.

The Air Corrector is the jet that is fitted to the opposite end of the Emulsion Tube to the main jet and I would imagine is sized between 140 and 180.



The aluminium tube sticking up out of the carb is the Auxiliary Venturi and the Venturi or 'Choke' is fitted below this part.

[shoestring7]

I am surprised that anyone would 'blank' a CIS Injector Port with a physical plug.

>C

[Lightweight_911]

This page on Partsklassic's website has pictures showing what the individual PMO jets/venturi look like:

http://www.partsklassik.com/c-65-pmo-carburtors.aspx

[fetuhoe]

That's an MFI head - much more valuable - and a simple pipe bung and sealing ring would have done the job

[shoestring7]

This page on Partsklassic's website has pictures showing what the individual PMO jets/venturi look like:

http://www.partsklassik.com/c-65-pmo-carburtors.aspx

Thanks that's helpful.

C>

[shoestring7]

That's an MFI head - much more valuable - and a simple pipe bung and sealing ring would have done the job

Well it had the CIS system on the engine when we pulled it out of the car. Possibly Porsche used different heads on the late '73 cars with the 911/91 engine, in our case they had the small I30/E33 ports. John opened them up to 36mm - valve sizes were the same as the 2.7RS across the range in '73: 46/40.



C>