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Running issues. I'm out of ideas


MGTS

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@datzenmike I saw your post from 2012 stating that the non-california carb main jet should be 107. Do you have any idea what the CA carb would be? My jet reads 94. The secondary is 170 which is on spec

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The '84 Z24 California carburetor has a #94 primary jet. This makes sense as they want them to run as lean as possible for emissions.

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Starts great. Idles great. Once warm, any sort of throttle from idle and it falls on its face. Doesn't usually completely die. Stumbles a lot at higher RPM. Manually choked it and that improves things greatly. So it's either getting too much air, or not enough fuel. I'm checking the boost controlled deceleration device. It might be out of adjustment and letting too much air in all the time

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The BCDD is actually a carburetor inside the carburetor. When activated it allows air in where it draws it's own fuel from the float chamber. This speeds up the engine slightly but not noticed because it's decelerating at the time. The extra gas and air prevents it from going lean. Long before you get to idle the BCDD should have shut off. 

 

If the BCDD has an adjustment screw in the center of it under a black rubber button or plug. Pry off and turn the adjustment screw counter clockwise and this will lower the sensitivity. The BCDD is less likely to come on or stay on.

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PLDT21B.jpg

 

Jet looks good as long as not plugged. This is the secondary jet, don't mix them up. Make sure primary jet farthest from the valve cover is clean and #94

 

 

ljgoMY3.jpg

 

Under the black rubber cap is a screw. Turn counter clockwise a couple of turns. This will make the turning on of the BCDD less sensitive.

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IT RUNS! So forgetting to plug something back in actually helped me this time. I had unplugged the emissions computer to run come continuity tests on the wires and didn't bother to plug the wires back in. So when I reinstalled the carb today, it wouldn't idle because the fuel cut solenoid is connected to the emissions computer. Forgetting that I had left the computer unplugged, I jumped the leads for the fuel cut. It ran perfectly. Then I remembered that I have left things unplugged. Then I realized that I could unplug individual wires from the carb harness, so I unplugged the mixture solenoid and the throttle switch (which is redundant because the throttle switch controls the mixture solenoid). But I think it might be running rich. When I turn in the idle mixture screw, nothing happens, which means it's getting too much fuel from somewhere else. Still have to run some more tests, but major progress today

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Ok so it runs and idles great, but it's getting extra fuel from somewhere. When I turn the idle mixture screw in all the way, it doesn't hesitate or die. My assessment from reading the manual was that my mixture solenoid was stuck on all the time and that was causing my running issues

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The idle circuit only concerns the engine idle. (roughly up to 900-1,000 RPMs) For it to work, the idle cut solenoid must be active to let fuel down the the adjustment screw. If working properly the idle mixture screw can shut off or lower the fuel air ratio to where it won't run.

 

The primary barrel is only active above idle when there is enough air flowing through it to activate the venturi vacuum that sucks fuel into the barrel. If idle is at proper 700-800 RPMs then the primary isn't working.

 

 

 

Do you have a tach? what is it idling at??

Is the idle speed screw against the stop? Can you even lower the idle speed? or is it backed off all the way?

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When I was originally getting idle set, it was behaving normally. I did standard lean drop, set idle to 700 via throttle plate screw, turn mixture screw in until it stumbled and back it out a half turn. Right now it's set at 700. I wanted to re set the idle after my finding and discovered this problem. I know for a fact the solenoid is working properly

 

>Is the idle speed screw against the stop?

What do you mean? It will go lower. I do know about having to adjust the throttle cable to get it to go lower

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There is an idle speed screw that rests against a stop. Turning it in pulls the throttle plate open.

 

It's in this picture but hidden behind something. It's a Phillips head screw just to the left of the one that shows. Beneath the coiled spring

 

pGfoTdx.jpg

 

 

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I know where the idle speed screw is and how it works. By "against the stop" you mean do I have it turned in far enough that it won't turn any more? No. Things have not been adjusted since my initial idle speed set and the idle circuit was working perfectly then

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If idling, and turning the mixture screw in does not cause the engine to stall then disconnect the idle cut solenoid. This shuts off fuel flow to the mixture screw.

 

 

If it stalls then the idle mix screw is damaged or the casting around it is cracked.

 

If it does not stall then definitely getting fuel from another source. (BCDD, choke on, flooding.... ?)

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The ignition rotor can make it do that. Worked on one that was acting the way you describe. It ended being burnt thru on the center, couldnt see the melted part until I took it off and looked down inside the bottom of it. 

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