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TheMrFlippers

Very High Idle and Cold Start (3k RPM)

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Factory Hitachi, was just gone through and rebuilt. Truck fires instantly, and runs fantastic however at Cold Start it instantly jumps to over 3k RPM, after I tap the gas I can get it down to around 2k. From there it takes quite awhile (more than a couple minutes) to drop, and at the lowest point I can get the idle screw to, it still won’t idle under 14-1500 rpm. Found a couple minor vacuum leaks around the vapor canister, but those shouldn’t affect the idle much if at all. 

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Vacuum leaks affect the idle!

The adjustment of the choke heater clockwise and counter clockwise affects how much the choke plate closes.  There is a bent rod from the choke plate shaft down to the throttle plate shaft.  The amount of bend in the bent rod controls how much the throttle is opened, for any given choke plate opening.   There is a cam, or a stepped stop for the throttle opening.   This may be on the idle speed screw, or it could hold the throttle open with a bent tab independent of the idle speed screw.

 

Fix the vacuum leaks.  Adjust the valves, check ignition timing, do other tune up stuff.  Fully warm the engine up, set idle speed and mixture.

Figure out exactly what holds the throttle plate open when the choke is in play.  It may be the idle speed screw, or it could be sonething else

Flatten out the bend in the rod from the choke linkage down to the throttle linkage.  This give you less throttle opening for any given choke.  

Once the warm idle mixture and speed is set, it may take a few adjustments to get the cold idle speed where you want it.  Unfortunately, you have to let the engine cool down every time you make a change in the cold idle speed, and may require a few times to get the cold idle speed where you want it.

It was fairly common for 1970-1980 carburetor engine cars and trucks to have a high cold idle speed.

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The choke is open all the way once it warms up. Everything I've looked through points to the high idle cam, however I am not 100 percent sure on how to adjust it. Due to it not idling down properly, the lowest I can get it to is 1400 rpm, I was thinking that the two might be related.

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As far as adjusting the choke heater, do I want to do it with the engine hot, and just move it till it starts moving the plate then lock it down?

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"As far as adjusting the choke heater, do I want to do it with the engine hot, and just move it till it starts moving the plate then lock it down? "

 

My opinion, that is too much choke.  I have a 521, it has a manual choke.  In the summer, most of the time starting it, I do not even use the choke. 

 

Adjust the choke heater to open sooner as the engine warms up.   Also realize you need less choke if the weather is warm, more when it it cold.  

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Carb was rebuilt due to the accelerator pump spraying fuel, gummed up from never being off the vehicle and sitting most of the time, and it also had an issue on cold starts where it would start puffing black smoke and loping like it had a high lift cam. Carb rebuild has solved that issue, now I just need to get this very high cold start rpm figured out, and then figure out the high idle. 

 

It's a 1 owner truck, with 123,000 miles, father bought it brand new off the California Coast in 1983. I drove it all through high school and into college, but it hasn't seen daily driving for close to 11 years and has sat for 95 percent of that time period.

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"it also had an issue on cold starts where it would start puffing black smoke and loping like it had a high lift cam."

Classic too rich when warming up symptoms.

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Have you turned the idle speed screw down? At it's lowest point the throttle plate will close completely. If not, find out why it's being held open

 

Check the secondary is fully closed. Use a flashlight to see. Maybe hold the throttle half open to release the interlock and push up on the vacuum rod to pull the secondary open then let close and watch it. I had one have a piece of rust stick in it. Simply pushed it closed with a pencil and that fixed it.

 

Take the rubber cap off the BCDD and turn the screw inside counter clockwise to reduce it's sensitivity.

 

 

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Little update, put a 30 day temp on it and drove it down the freeway. Truck could barely make it to 65mph, and according to my co-worker following behind me it stunk and was emitting quite a bit of black smoke. At 65mph in 5th gear I could floor it, and you could hear a difference noise wise, but it would not accelerate any further. Temp got to right below midpoint, everything else went fine. Got off the freeway and the RPM's were still over 1500. Another oddity, if I let off the gas for a second to hit the brakes, and went to hit the gas again it would jump pretty well. I will check everything that's been suggested and let you know. I appreciate the help.

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When warmed up take the top off the air filter and check that the choke plate is vertical or fully open.

 

While idling check that the fuel level is showing in the round glass window on the front of the carb.

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