Carburetor Issues

[SeamusNZ]

Hi all from NZ.

I now have the second refurbished carb from Clarks.

Unfortunately not great.

Symptoms are as follows:-

Starts and runs well on the chokes.

Once the chokes have opened and the choke step has cleared the throttle, the engine dies.

Will not run unless the idle screws are right in, and at that point runs roughly.

I have noticed that the auto chokes are not effected by the vacuum diaphragm.

I’m sure that they opened the chokes previously when the engine was revved when choked. It’s not doing it now. It seems that it is creating too much vacuum. Is this possible?

Cheers

SeamusNZ


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[terribleted]

I have put a vacuum gauge on both carbs, but the gauge is all over the place. I understand that this is reflective of the Otto performance cam.

I will certainly check for vacuum leaks and go back to scratch with respect to the carb set-up. Cheers SeamusNZ


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Have you done a compression test? Vacuum gauge jumping all over could be showing compression loss in a cylinder or cylinders from any number of causes like bad rings, dropped valve seats, poorly sealing valve, or if you are lucky only badly adjusted valves. You need to perform a compression test to see if there are any low cylinders before proceeding with anything else. If you find low compressions the next step is to adjust the valves to see if it makes a difference. Low cylinders might simply have their valves adjusted too tight making them never fully close.

I see you are running a Pertronics electronic ignition. You should certainly remove this and convert it back to standard points ignition for troubleshooting. Electronic ignition units can give some very odd and often inconsistent symptoms when they are not working properly.

Assuming the compression checks out. The vacuum diaphragms (choke pull offs) should suck in as soon as the engine starts and essentially stay sucked in until the engine is shut off. How is the fuel delivery? Any blockages, fuel pump delivering the correct volume of fuel? Do you have a vacuum leak somewhere?

[WinginEngineer]

Random thought. Have you checked to make sure the throttle blades are closing at idle?

[SeamusNZ]

Random thought. Have you checked to make sure the throttle blades are closing at idle?

No they are slightly open. Given that to get the engine to idle, the idle screws are nearly all the way in. Cheers SeamusNZ



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[bbodie52]

IDLE SYSTEM
The idle system is an independent system and is used to meter small amounts of fuel during slow engine speeds when engine air and fuel requirements are very low.

The idle system consists of idle tube, idle passages, idle restriction, idle air bleeds, an off-idle discharge port, idle mixture adjusting needles and the idle discharge hole.

During engine idle operation, airflow through the carburetor venturi is not large enough to meter fuel properly through the main discharge nozzle. Therefore, the idle system is used to provide the proper mixture ratios during engine idling and low speed operation.

Will not run unless the idle screws are right in, and at that point runs roughly.

I have noticed that the auto chokes are not effected by the vacuum diaphragm.

Are you talking about the idle speed or idle mixture screws? The initial setting for the idle mixture screws is supposed to be 1½ turns out from the bottom (closed) setting. If you have them closed all the way, the engine would not want to idle once the fast idle (choke) system was disengaged.

The engine idle speed screws should be initially 1½ turns clockwise after they initially make contact with the closed throttle linkage. Without this preset, the idle speed screws would not hold the throttle partially opened for engine idle once the choke system has disengaged.

When the engine is started, manifold vacuum is applied to the spring-loaded vacuum break diaphragm. This moves the diaphragm plunger into the assembly until it strikes the cover, which in-turn opens the choke valve to a point where the engine runs without loading or stalling. The length of the connecting link determines the amount of choke valve opening.

As soon as the engine has started, the choke vacuum break diaphragms should be fully exposed to maximum intake manifold vacuum, which should pull them inward continuously to slightly open the choke butterfly to permit more air to enter once the engine has started.

I would suggest that you review the description of the idle system (starting on page 4 of the attached DELCO ROCHESTER - Models H, HV Carburetor Service Guide) and the choke system (starting on the right side of page 8). This should help you in your observations of the idle system and automatic choke system.

[SeamusNZ]

Hi there, I have now discovered via a local engine rebuilder that cleaned and measured my engine before assembly that the fitted cam shaft is actually an Isky high lift hydraulic grind No477.

I’m not sure if the cam is stamped with No477 or they measured the lobes.

Does this ring any bells with respect to the lumpy idle and low speed performance??

As an aside I have now installed two newly rebuilt carbs from Clark’s and apart from the lumpy idle and low rev performance, the car is now running very well, although it doesn’t run that economically. I am generally getting 18-20 miles per gallon around town.

Any help or advice is very much appreciated.

Cheers SeamusNZ


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[joelsplace]

That's all I get with stock engines. I know some say they get up to 26 on the highway but I've never been able to do that.
The best I've ever averaged was about 21.