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 Posted: 05-20-2018 09:09 pm
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Dick Fickey
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Looking for fuel pressure specs at the fuel pump and at the Carburetor. The car is surging and I think I may have a weak fuel pump or bad fuel pressure regulator.

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 Posted: 05-20-2018 09:32 pm
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Jim Ketcham
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2 to 3 psi. SU fuel pump AZX-1307 delivers 2.7 psi.

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 Posted: 05-20-2018 10:17 pm
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Frank Schwartz
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check to see if your filter is clogged as well..
Frank

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 Posted: 05-20-2018 11:00 pm
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Dick Fickey
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Thanks for the infromation. I have 2.0 PSI at the carburetor. Which I see is at the bottom end of the range. The filter is new. May have other problem causing the surging.

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 Posted: 05-20-2018 11:53 pm
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Frank Schwartz
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Proper float level in the carb bowls???
Frank

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 Posted: 05-21-2018 04:10 pm
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Dick Fickey
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Plan to take the carbs off today and check floats.

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 Posted: 05-21-2018 04:34 pm
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Esprit2
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Float Heights:
17.5 mm = J-H Workshop Manual (leanest I've seen for Z-S)
16-17mm = Z-S generic Z-S, & Lotus 907 Workshop manuals.

If you have the Haynes Z-S manual, note that there is a typo in the float height spec given on page 45. The metric values are correct (16-17mm) but the inch values are bad math... wrong.

16mm = 0.630", not 0.725"
17mm = 0.669", not 0.787"

*~*~*~*
Inspect the floats closely. On some, the 'float' part itself is attached to the arm with aluminum eyelets/ rivets. Alcohol in modern fuel eats the aluminum rivets, and the float falls off the arm.

Regards,
Tim Engel

Last edited on 05-23-2018 03:56 pm by Esprit2

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 Posted: 05-21-2018 05:28 pm
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Esprit2
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The Workshop Manual specifies an SU AUF 300 pump as standard. That pump's pressure output is 2.7 psi minimum (2.7-3.8 psi range). The modern counter part is AZX 1307.

The Z-S carbs like 1.5-2.5 psi... ie, less than pump pressure. But flow losses from the pump at the back of the car to the carbs at the front, and pressure loss due to the difference in elevation between the pump and carbs, reduces the pressure at the carb inlet... but it's typically still too high.

Therefore, the factory added a bleed orifice and fuel return line that acts as a very basic "poor man's" pressure regulator. The orifice diameter was experimentally chosen ('tuned') for the J-H system, in order to drop the pressure to the correct value at the carb inlets. The orifice calibration is so specific that changing anything in the system (a non-standard pump or pump location) will throw the calibration off.

I don't recall the exact orifice size at the moment, but it's small, on the order of 0.015" (0.381 mm). It's so small, that it is easily plugged by any crud that gets past a dirty fuel filter.

I'm not pointing a finger at any one thing, I'm just pointing out things to check.

Start by measuring the fuel pressure at the carb inlets, looking for 1.5-2.5 psi. Even if the pressure is correct with the engine not running, a tired pump with reduced flow capacity may not be able to keep up with flow demands at high speeds.

If you seem to have a built-in speed limiter (engine runs fine at idle and low speeds, but falls flat on it's face at some consistent highway speed), then the fuel filter may be dirty/ plugged, or the pump may be tired or suffering from a small perforation in the diaphragm.

Do the easy thing first, replace the fuel filter and make certain the bleed orifice is not plugged. In older cars, there is a greater probability of rust issues inside the tank; so I like to install the largest fuel filter available that will fit.

Good Luck,
Tim Engel

Last edited on 05-21-2018 05:50 pm by Esprit2

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 Posted: 05-21-2018 10:00 pm
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Dick Fickey
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Got the carbs off. I had had them "re-manufactured" in town at a carb shop. Only had put 35 miles on the car since I finished it. Found out the float heights were off. The plastic jet cover on one carb had 2 sides broken off and was leaking. Called Delta and they had them in stock. Found my cold start problem. They had not cleaned the holes in the cold start valve and they were plugged. Next time I will do my own work. At least I will know who screwed it up.

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 Posted: 05-23-2018 01:28 am
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Tom Bradley
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There is a typo in Tim's float heights: 17mm = 0.669", not 0.779". So the target range should be 0.63" - 0.67".

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 Posted: 05-23-2018 03:55 pm
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Esprit2
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Oh crud! I was trying to correct Haynes' mistake, then I fat-fingered it. Thanks for catching that, Tom. I've edited my earlier post in order to avoid future confusion.

Regards,
Tim Engel

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