sticking high idle cam

[Cowboybob]

Hi, I've only had my vair since last sept It runs and looks great but lately it won't drop off high idle most of the time when it is at running temp. The carb is a recent rebuild and I have used choke cleaner on the linkage of both carbs. I tried some graphite but that seemed to make it worse so I cleaned the linkage again. Any suggestions?

[terribleted]

You mean it stays on the fast idle step of the cam once warm after operating the accelerator? They will not necessarily come off high idle without actuating the accelerator to release the linkages allowing the choke to pull away. If it is staying on the fast idle step even after accelerator has moved then either something is wrong with the choke, the linkage is binding, or the carb was badly adjusted during rebuild.

[bbodie52]

The fast idle cam has graduated steps so that fast idle speed can be lowered gradually during the engine warm up period. The fast idle cam follows rotation of the choke valve. When the choke valve is completely open and the engine is warm the fast idle tang on the throttle lever will be off of the steps of the fast idle cam. At this point the idle stop screw controls normal idle speed.

As shown in figure 10, the fast idle cam rotates into position to hold the throttle open at a faster idle when the choke butterfly mechanism at the top of the carburetor air horn rotates to close the choke. The design of the choke lever and choke collar applies pressure to the fast idle cam to rotate clockwise the first time the throttle is manually opened by the driver. The fast idle cam works in conjunction with the choke butterfly, and the two together both enrich the fuel-air mixture and increase the idle speed while the engine is cold.

As the engine warms, the pressure that holds the choke butterfly valve closed is gradually released, which allows the choke butterfly to open and return the fuel-air mixture to normal operation. As the choke butterfly valve opens, pressure on the fast idle cam mechanism is removed, and the balance weight of the fast idle cam allows it to return gradually to a position where it is no longer in contact with the throttle linkage. At this point, the idle speed adjustment screw position determines the amount that the throttle is held open, which establishes the idle speed when the engine is at normal operating temperature.

Carburetor Choke Mechanism - Fast Idle

I would suggest comparing the fast idle cam and associated linkages with the illustration shown above. You should verify the correct positioning of the various components in the fast idle cam and the linkages that tie it to the choke butterfly. With the choke disengaged and full open, you should be able to hold the throttle open (engine not running) so that the fast idle tang does not come into contact with the fast idle cam. You should be able to manually rotate the fast idle cam back and forth to see if there is any resistance or binding in the mechanism. The large screw in the center of the fast idle cam can be temporarily removed to release the mechanism for further examination, if needed. And under normal conditions the fast idle cam's shape acts like a counterbalance, whereby its own weight returns it to a neutral position that would not be in contact with the carburetor throttle linkage. There is no spring or physical lever that forces the fast idle cam back to the neutral (disengaged) position. Gravity is used so that the weight of the cam will normally be parked in the disengaged position by its own weight. If the fast idle mechanism on the other carburetor is working properly, you should be able to compare the fast idle mechanisms on the two carburetors to determine what is causing the fast idle cam on the faulty carburetor to remain engaged with the throttle linkage.

The attached DELCO ROCHESTER - Models H, HV Carburetor Service Manual provides a description of the operation of the Rochester model HV automatic choke system on page 8. The overall manual is very good at describing the operating functions of each subsystem within the carburetor. The manual also is a good supplement to the factory shop manual to guide you with repair, troubleshooting, and adjustment of your carburetors.

[64powerglide]

Did you disconnect the choke rod at the carb & see if it's adjusted correctly. If the choke with the engine hot drops screw the choke rod down to align with the hole. If that's not it good luck with it.

[Cowboybob]

Thanks, I have tried tapping the throttle, which works once and awhile. Seems the hotter the engine is, the less it works.

[terribleted]

Be sure the chokes are adjusted properly...with engine cold and choke held fully closed (open the throttle a little throttle to allow full choke plate closure) adjust the choke rod to cleanly enter the choke arm then adjust 2 turns longer (up). If the chokes are properly adjusted and functioning, once the engine is warm the fast idle cam should never be in a position to hold the idle high unless something is binding it somehow, or the fast idle adjustment on the carb is way out to lunch (tang overbent?) . Are you sure the engine is warmed up? Without being driven it can take quite a while.

[bbodie52]

Tapping the throttle while you sit in the driver seat doesn't really tell you what is going on at each carburetor. With the air cleaner removed and the engine running you should be able to observe the position of each choke butterfly valve at the top of each carburetor. With the engine fully warmed to normal operating temperature both choke butterfly valves should be fully opened. You can also look at each fast idle cam on the side of each carburetor to check the position of the fast idle cam. Both should be fully released and not touching the throttle linkage. Comparing the operation of the choke mechanism on the left and right carburetor should tell you if they are both operating properly and equally. If one choke mechanism is sticking and not fully releasing at one carburetor, while the other is operating normally you can then begin some fault isolation efforts to determine what is causing the faulty carburetor choke system to not function normally.

If the choke butterfly valve on one carburetor is remaining closed or partially closed, while the other has fully released you will need to examine the actuating lever and mechanism to see if it is binding or bent in some way. On the other hand, if the choke butterfly valve on both carburetors is wide open, but the fast idle cam on one of the carburetors is still partially engaged, you will need to carefully examine the faulty carburetor to see what part of the mechanism is binding or is in some way different than the one on the carburetor that is acting properly.

[64powerglide]

A picture is worth a thousand words.

[Wagon Master]

Tapping the throttle while you sit in the driver seat doesn't really tell you what is going on at each carburetor.

True that! the choke coils (thermostats) close the choke plates and bring the fast idle cams into play. After that, as the engine warms up, the only thing disengaging the fast idle cam, link and kicker is gravity. That's why this whole assy. must be clean, lightly lubed and free of any binding or misalignment.

[Cowboybob]

What type of lube seems to fill that situation?

[bbodie52]

...the only thing disengaging the fast idle cam, link and kicker is gravity. That's why this whole assy. must be clean, lightly lubed and free of any binding or misalignment...

What type of lube seems to fill that situation?

The picture above, the fast idle cam is held in place by the large screw in the center of the cam. The portion of the throttle lever that appears in the picture under the thumb of the mechanic must come into contact with the stepped portion of the fast idle cam that is to the right of the large screw in the photograph. When the engine is cold, the automatic choke coil expands and applies spring pressure to the choke mechanism. However, the choke cannot be set because the throttle lever tang is in the way and blocks the fast idle cam from rotating clockwise. The vehicle operator must set the choke for initial startup by pushing the throttle pedal to the floor and then releasing it one time. Doing so momentarily moves the throttle lever out of the way so that the fast idle cam can rotate about 45° clockwise. When pressure is removed from the gas pedal the throttle lever returns to the closed position, but is now held open to a greater degree because it comes into contact with the highest step of the fast idle cam, which now sits below the mounting screw. At this point, the largest portion of the fast idle cam is in the up position to the left of the mounting screw. As the engine warms, spring pressure against the choke mechanism is reduced, which allows the choke butterfly valve to open. Stepping on the throttle momentarily moves it away from contact with the fast idle cam steps. The heavier portion of the fast idle cam to the left of the mounting screw drops downward (counterclockwise) by its own weight.

I would not recommend using any form of grease or lubricant on the pivot point under the large screw, which is a special shoulder screw, similar to the one in the picture below. (Shoulder screws, have a long, unthreaded, cylindrical shoulder that allows for the free rotation of attached parts like the fast idle cam).



The fast idle cam only has to move one time about 45° when the choke is set. When the engine has warmed to operating temperature the fast idle cam returns in a counterclockwise motion with a movement of approximately 45°. The small amount of motion does not justify a need for any kind of lubricant. The special screw that holds the fast idle cam in place is shaped in such a way that it cannot be tightened against the fast idle cam. Tightening the screw brings it into contact with the carburetor body, which leaves the fast idle cam held loosely in a fixed gap between the screw head and the carburetor body. There is no significant friction to prevent the fast idle cam from moving. However, the use of a lubricant or grease may attract dirt that could restrict the motion of the fast idle cam. I would recommend the fast idle can be installed dry and the screw tightened against the carburetor body. This should leave the choke mechanism and fast idle cam free to rotate easily back and forth when the choke is set. The heavier portion of the fast idle cam to the left of the mounting screw is enough to return the fast idle cam to its neutral position with the choke has disengaged.