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PhilW

Carb connections

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Can anyone tell me what these two pipes are for.  The green arrow goes to the pressure regulator but the two short rubber pipes are just pushed on and not fantastically tight.  No clips at all.

 

Fuel pipe.JPG

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Arent those connected to the turbo plenum i assume to equalise pressure in the carbs?

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Yes, they measure (well allow the regulator to measure)  the  air pressure in the carbs to allow the fuel regulator to increase fuel pressure by the same amount - e.g. if your static fuel pressure is 4psi and you are running say 6 psi boost, at a point in time, then the overall fuel pressure supplied would be 10psi at that point in time.

NB - They are not fitted with clips as standard

Edited by 910Esprit

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Thanks Steve and Chris. Need to learn a bit more about the carbs

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39 minutes ago, 910Esprit said:

Yes, they measure (well allow the regulator to measure)  the  air pressure in the carbs to allow the fuel regulator to increase fuel pressure by the same amount - e.g. if your static fuel pressure is 4psi and you are running say 6 psi boost, at a point in time, then the overall fuel pressure supplied would be 10psi at that point in time.

NB - They are not fitted with clips as standard

I'm not sure this is accurate. Those pipes connect the carbs to the diffuser for cold starts. The pressure feed to the FPR is taken from the barbed adaptor which you can just about see to the right of your green arrow. :) 

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Yes its accurate...   Nothing to do with cold starts.   Cold starts ae via fuel enrichment, entirely within the carb

As you point out, the air feed to the FPR is common to these connections.... 

 

Edited by 910Esprit

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Those 2 short pipes are only for cold start air.

The vertical connections on the carbs which they connect to are the air inlets which are only open when the choke is activated. On non-turbo carbs there are small gauze vents there, but on the turbo, when the choke is open, boosted air needs to be supplied to them.

These are closed off if the choke is closed.

Equalization of the float chamber pressures is done by holes which run through the plenum backplate adjacent to the main air intakes.

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Can somebody explain to me why would 'cold start air' be routed into the top of the float chamber, what could that achieve?  Why would it be warm compressed air from the turbo?    

These pipes are compressed air from the turbo to manage fuel levels and air pressure as the turbo spools up.  (i.e the air pressure in the carbs is equalised with the turbo)  This is then also sent to the FPR so it can increase fuel pressure by the corresponding amount.       

If in doubt, disconnect these pipes and the car wont run under boost and the fuel pressure wil remain static

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The FPR is controlled by the tube on the right, certainly. But the two carb connections which those pipes in question go to are definitely only cold start air. If you look inside that area of the carbs you will find these dont go to the float chambers. They are air input for the choke valves, just as on the standard carbs. They are closed off when the choke is closed. On the turbo these cant be fed from clear air, they must be fed from boosted air pressure. Hence in place of the open gauze cover on the NA, there is a connection to boosted air.

I have driven 50 miles or so with one of those banjos completely missing, as it unscrewed itself! The only slightly noticeable difference was a slight lack of boost owing to the resulting air leak from the plenum. 

Hoses are shown in the parts book as follows:

 

 

Untitled.png

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Seems like I'm in the minority of one (again) and I can't argue with the description Lotus have assigned to those pipes, but I'm genuinely perplexed that its not recognised that supplied fuel pressure needs to overcome the dynamic pressure within the carburettor (ambient + boost), yet maintain a constant 4-5 psi at the needle valve.   This is the primary purpose of the FPR.   Maybe those pipes have more than one function, but without those exact pipes being connected to the FPR you do not have sufficient fuel pressure to maintain boost fuel requirements.

Can you clarify how you believe the FPR works and why it needs to be fed 'dynamic' air pressure from the plenum? 

We have a different view on why your car would not boost without these connected.... 

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Your description of the FPR is correct. That is fed with reference boost pressure from that source. The only part incorrect is the purpose of the connections to the carbs via the banjos.

So yes the crossover pipe takeoff does have dual function, they provide cold start air and the FPR reference. In my case driving with one banjo missing its possible the FPR was not providing enough fuel pressure under boost but thats likely not a big issue unless driving really hard as the reserve of the float chamber would come into play. Unless on the track its unlikely max boost would be sustained for long enough to empty the float chambers. There was no hesitation or performance problem, just the boost came in slightly later, owing to the air leak.

The carb connections are not important as they only have any purpose when boost is high and choke open. Thats not a situation that should realistically happen.

On the HC engine in addition to the cold start air pipe (a solid pipe on this version) there is another solid pipe which does enter the float chambers. It simply connects both float chambers together as a balance pipe. I am not sure why this was added.

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Andy and Steve, I still think that it’s not only the FPR that’s pressurised by the turbo but the carb fuel bowls as well. I agree that the choke has a large effect and allows for extra air in, but there is pressure from the turbo without choke too. Why would there be an extra valve that closes off the acceleration pump compartment below (item 34) when the turbo starts operating? This is unique to the turbo carb and not found on an NA carb. If not for that valve, the accelerator jets would start to spray by themself under turbo pressure. Perhaps I’m wrong here but that my conclusion.

63A07D11-E9CF-4132-ADF1-DACDA10E2FBD.png

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The float chambers are only pressurized because they are vented to the plenum. This is true on the turbo or NA. Both have vent holes. 

If in doubt, its easy to take the cover off and look inside, or even easier if you try blowing into one of those pipes, you will find, using either method, its completely closed off when choke closed.

Note the vent hole:

 

dellorto.jpg

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