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Old 02-03-2011, 12:08 AM
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Default truck won't idle

So when I rebuilt the engine last year I also rebuilt the carb. It ran good up the alcan but after a month or so it wouldn't idle without the choke partially pulled (like 1/8th total pull or so). I've played with the idle mixture screw but that doesn't have an any affect on it. I also experience a stuck/frozen throttle on those cold days -10 or below. it runs fine while at speed I can push the choke all the way in but if i pop it out of gear it almost instantly dies, which takes the brakes with it. I've been experiencing a loss of gas milage and it seems to run hotter than normal. Should I try to rebuild the carb again? It's the stock carb.

Sean
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Old 02-03-2011, 08:04 AM
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Have you checked for vacuum leaks?
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Old 02-03-2011, 08:05 AM
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If it's the stock Aisin, have you check the idle cutoff solenoid? I had a problem last year that was similar to what you describe. Also may want to verify all linkage moves freely and returns to normal stops with just the springs holding it.
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Old 02-03-2011, 08:19 AM
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Sorry to hear about the issues you are having. Are the idle issues occurring upon start up (when the engine is cold/warming) and once at operating temperature? In reading your post, it sound like this is the case. I've had some Idle issues with my Aisan carbs and have been able to get them sorted out. With all the experience on this Forum (and MUD if we need it) you'll get issue addressed.

The carb's idle circuits are designed to mix fuel and air to achieve a ratio. Your "work around" for the idle issue is to decrease the amount of air, i.e. partially choke. Based upon that, one of the first things to check is where the "excess" air is coming from. Typically it is coming from a small gap where;
  • the carb fits to the carb base
  • the carb base fits to the intake manifold
  • the intake manifold fits to the head

To check these areas, spray/spritz some break cleaner at each area and then listen for a change in idle. When the idle changes, you have found an issue that needs to be addressed. To get a feel for what the change sounds like - do a spritz at the intake to the air cleaner and wait a bit. It doesn't take a bunch of brake cleaner to change the idle and you need to be methodical and patient. If you spray everything quickly and then the idle changes - you won't know where the air leak is.

The "excess" air can also come from;
  • Holes in the vacuum hoses connected to the carb base (for the PCV valve) and intake manifold (brake booster and transfer case shifter)
  • Tear in the brake booster
  • Tear in vacuum transfer case shifter innards
  • a crack in the bottom of the intake manifold (typically occurs on newer rigs, not so much on our early rigs)

To check (most of) these items, connect a vacuum pump to the end of the vacuum hose that connects to the carb base or manifold. Pump the vacuum unit up to create a vacuum and then let it sit for some time to see if the vacuum decreases. If it decreases, it indicates an issue that needs to be addressed.

Are you running a header or the stock exhaust manifold?

For your idle issue, I would recommend starting with the above diagnostics. If that all checks out. The next component to check is the idle cutoff solenoid. If that checks out, we can then move on to the carb itself. What altitude are you at?
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Old 02-03-2011, 08:24 AM
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Quote:
Originally Posted by RicardoJM View Post
...
the intake manifold fits to the head

...
Fixed it for you Ricardo

Good post
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Old 02-03-2011, 08:33 AM
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Quote:
Originally Posted by subzali View Post
Fixed it for you Ricardo

Good post
You are correct. I made the edit.
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Old 02-03-2011, 12:57 PM
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All the problems aren't affected by the engine temp. It looks like my PCV is bad, theres oil leaking all around it and when I pulled the carb off there was a white milky substance that looked like aerated oil. the linkage moves freely when not cold, I think there might be some grease or the like that is freezing at the low temps.
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Old 02-03-2011, 01:28 PM
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Quote:
Originally Posted by Rogue Leader View Post
It looks like my PCV is bad, theres oil leaking all around it and when I pulled the carb off there was a white milky substance that looked like aerated oil.
This should not be occurring but is consistent with a vacuum leak being the cause of your idle issue. That is to say, it seems like the PCV hose is damaged and allowing oil to get into the hose - when the engine is running vacuum is pulling oil through that hose and into your intake manifold and causing the white milky substance. That same hole would create a vacuum leak that is the source for the "excess" air. All things being equal, I would get a new PCV valve, grommet and hose put in place and see if the idle issue improves.

It is still worth checking for other air leaks to confirm there are not any others.
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Old 02-08-2011, 07:32 PM
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Installed a new PCV valve, gasket and hose. It's weird that the port on my carb base and the PCV Valve are different sizes, the hose fit perfectly at the PCV but its too small for the carb base. I soaked it in hot water and got it to fit on enough to put a hose clamp on it. I found 2 vac leaks, one the hose had fallen out, the other the nail that used to plug it up was missing so I put another one in. It ran fine for several miles after warming up but then the old problems persisted ie dying when i took it out of gear. It was also running unusually hot. We played around with the idle mixture screw and it didn't change anything no matter how much we turned it.
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  #10  
Old 02-08-2011, 09:56 PM
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One thing that happened to me for a time was when I took it out of gear or idled up to a stop my engine would die. I found a bunch of floaties in the carb bowl. Once I cleaned them out the problem went away. It's easy to check.

The idle mixture will only affect the idle mixture, it doesn't have any real effect once you're into the main and/or secondary circuits.

Running hot can sometimes be an indication of a lean mixture. Again check for vacuum leaks. Also, pull your plugs and check their condition, that's an easy way to tell how your mixture is looking. If they're anything but tan colored you've got issues you need to address.

Another test for vaccum leaks is to pull off your brake booster hose (while the engine is running) and put your finger over the port on the intake manifold and see if the situation improves. Sometimes a leaking brake booster diaphragm will leak excess air into cylinders 5 and 6 and cause them to run lean. In some cases when you press on the brake pedal the excess air is enough to kill the engine (from what I've heard of other experiences).

Also it would be good to check your timing, just to make sure there is nothing going wacky there. Having it too far advanced or too far retarded can have an effect on engine temperature as well.

Just some quick easy diagnostics to check.
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