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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? "

 

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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Update, adjusted the high idle cam, adjusted the choke. Truck runs quite a bit better, still idles a little over 1k, if I try to drop it down to 750 it doesn't run very well. Black smoke is gone, and truck goes over 65 without an issue now. Other issue with the random bucking when I come back on the gas is still there. Not all the time, however when driving down the freeway today I noticed random hesitation when applying the gas. If I applied it slowly you could feel the truck back off for a slight second, and then come back to normal. If I mashed the pedal, it would sometimes be completely fine, others it would buck and then go. 

 

I am also still dealing with that very strange random misfire. It is prevalent at low rpms (in my case slightly over 1k.) It is noticeable from the exhaust, you can hear it every time it happens, can also see the engine shake, and can certainly feel it in the cabin. However, it isn't an all the time issue. On the trip back today it would do it at random and not consistent with coming off the gas, or long periods of idling. 

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When warmed up the fast idle cam should lift up out of the way of the throttle closing. If not bend the rod some more. Once the fast idle cam is out of the way, the idle speed should drop down or can be adjusted for 700-800 RPMs.

 

 

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Welp. new problem every day at the moment. Yesterday I went to fire it and pull it into the shop (been storing it at the shop for the last couple months.) Pumped the gas once to set the choke, went to crank it, it spun over twice then acted like it seized, and moved the entire truck side to side. Tried cranking again, and it did the same thing. Went back into the shop, came back out and tried again and it fired perfectly. This morning, after she sat in the shop all night, I pumped the gas once to set the choke, and it did the exact same thing. I took a video of the strange misfire going on as well. This week I'm going to put a timing light on it, and do a valve adjustment, to see if that helps. 

 

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As you probably know the Z24 is famous for blowing head gaskets for seemingly no reason. Usually at 100k intervals. It's a combination of age and heat cycles and the gasket compressing. Only 'cure' is replacement once blown BUT you can do something to avoid it by re-torquing the head bolts. The procedure is, on a cold engine, to loosen one bolt several turns and immediately tighten to 60 ft. lbs. Then on to the next bolt. Any order you like as long as only one bolt is loose at one time. This keeps the head securely clamped down on the head gasket. Check them once a year. I'm not saying this is a problem right now but this is a preventative measure that should be used as soon as possible.

 

 

First

Reluctant starting could be timing too advanced. Try checking and setting to 3o BTDC. You can run +- 2o on this if it helps.

 

Second

Set check the valve clearances.

 

Third

The idle mix screw is set and sealed at the factory but a 1/8" drill and a wood screw will allow you to pull the aluminum plug out to get at the adjustment screw. A lean-ish mixture can misfire so try turning the screw out in 1/8th turns and see if it helps. Keep track so you can put it back.

 

LjGaCgo.jpg

 

I'd say it idles pretty good!

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So, back to this. Truck has been sitting under a canopy out back of the shop since my last post. Been a busy few months.

 

I haven't touched really anything, but I have fired the truck up every couple of weeks and driven it around the parking lot. It still has that odd misfire you can hear in the video, you also cannot rev it from idle, whether cold or warm as it will literally fall on its face, you'll have to let off the gas to make sure it doesn't die. It also has the same issue when it comes to restarting. If I accidentally pump the gas after shutting it off, it will crank over once or twice, and then literally move the entire truck side to side, kind of like it's locking up. If i wait 5-10 minutes I can then restart it fine.

 

90 percent of these problems happened after the Carb rebuild (the random misfire, the off idle stumble, the restarting issue), before the Carb rebuild I could let it warm up, drive it for awhile, and have a truck that runs 85-90 percent decently. It feels like the accelerator pump isn't squirting when you hit the gas at idle, as it quite literally goes from idle to nothing, back to idle. 

 

I have to move the truck out of the shop and back to the house over the weekend, so at the minimum I need to try and figure out why I cannot rev it, and hope that it relates to other issues. 

 

I will be double checking the timing as well just to make sure, but I can be fairly certain that most, if not every issue, is carb related.

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maybe you have already resolved this by now but I would be checking vacuum hoses for either broken or misrouted (is that even a word?) vacuum lines.  

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Fixed most of the problems, somehow the timing was WAY off. Maxed the distributor out and it's still about 5-7 degrees advanced, going to have to pull the oil pump out and set it. It also has a bad dead spot right off idle that needs to be addressed, if I drop a gear and try to stab the throttle it will quite literally fall on its face if I don't let off. If I ease into the throttle she runs fine, little rich.

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50 is the correct timing so leave the oil pump alone.

 

With engine off look down the carburetor and pump the gas. What you should see is a strong squirt of fuel into the primary barrel. Yes??

 

If NO then the accelerator pump is not working. The extra squirt of fuel is to help transition from the idle circuit the the primary circuit. Without it the engine will stumblle.

 

 

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