Issue with 84 720 carb and engine redlining in neutral

[Brownie]

Hi there, I’ve scoured the threads for this issue and while some have been useful k have yet to diagnose the problem. 

my 2.4 L 720 is stuck at a dangerously high idle speed even in neutral. Up until a road trip last week it was running smoothly. I’ve tried to lubricating everything on the carb but to no avail. I’ve inspected the lines and there doesn’t appear to be an air leak either. One thing I have noticed is that the butterfly flap stays open after giving it gas and does not close when I release my foot from the pedal. To do so I must manually play with the system and it shuts. Could this be my issue? 
another thing I’ve noticed is the fuel level in the carb is not existent and I see no gas even close to the center dot that it’s supposed to read at. Another possible issue or perhaps this stems from the first? 

apologies in advance, I am learning to be more mechanical inclined through this vehicle.

 

 

https://imgur.io/a/X3n6VpN

[NC85ST]

1 hour ago, Brownie said:

the butterfly flap stays open after giving it gas

Do you mean the one at the top of the carburetor? If so, that’s the choke and it should be open after the engine warms up.
If it’s the one that you have to look down the carburetor to see, then a likely cause is the throttle cable hanging up, probably a frayed cable inside. 

[datzenmike]

Hold the choke flap fully open and have someone step on the gas pedal once and release it. Use a flashlight and look down both barrels while holding the choke flap fully open. At the bottom of both  barrels is a round brass throttle plate. You should see that the secondary is fully closed and the primary just a hair open. Make sure there is no debris blocking either from closing. Are they closed?

[Stoffregen Motorsports]

Rusty or dirty throttle cable?

 

Floor mat hanging up on pedal?

Edited August 29, 2023 by Stoffregen Motorsports

[Brownie]

Update: thanks guys it seems that both are staying open. The secondary is a bit open and the primary is much more. Would this be an issue with the throttle cable? Going to try and pull it apart tomorrow. There appeared to be no debris keeping them open and I gave both a good clean 

[Brownie]

Another suspect issue is that I heard it hissing last week. None of the visible lines are damaged so perhaps a gasket leak?

[datzenmike]

An air leak does not make the engine rev up. The air and fuel mix is close to correct for a proper idle. Adding more air leans the mixture and the idle falters not revs up.

 

To rev up it needs air and fuel as in the throttle plate is not closed all the way or the fast idle cam is stuck. My guess is the BCDD is set to sensitive which turns it on but won't reset. The BCDD is a small carburetor within the carburetor. When activated when slowing down it adds a small amount of gas and air to relieve the weak mixture on deceleration the does not burn completely. Normally you don't notice it because long before you stop the BCDD turns off.

 

It's the round thing with the 6 Philips head screws and the black rubber plug in the center. Remove plug and turn the screw inside counter clockwise a turn or two. This will turn the sensitivity down. Keep track of the turns so you can put back in case....

[Stoffregen Motorsports]

Remove the cable from the carb and see if the throttle plates go back to their closed position.

 

It's not hard to mess up the linkage assembly on the carb. There are stepped washers and each linkage piece needs to be in the exact precise location. They can also get bent and cause them to hang up.

 

One other possible cause is a throttle plate that has come loose and is hanging up in the throttle bore. Are all the throttle plate screws present? Are they tight?