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Eccentric Carb


baymtnman

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I own a stock 85 720 with a Hitachi carb which is expressing a confusing array of symptoms.  Thanks to others for their deep insights, but the forum does not seem to have addressed the fully array of problems I am experiencing. 

 

The truck will sometimes idle smoothly at 1000 rpm, which is the lowest I have achieved to maintain stable idle.  Sometimes it will idle at 1750-2000 rpm, and other times it will simply drop out.  All this can happen on the same five mile trip. Most cold starts idle speed is about 1000rpm, sometimes it will be very low, about 400rpm and then I have to feather the accelerator for a short period and then the idle will either be as outlined above: high, 1000rpm or dropout.  

 

Choke seems fine-snaps shut in the morning, and almost fully opens when warmed up.  The anti-dieseling solenoid is energizing (yet sometimes the truck will diesel if it has experienced high idle). Float bowl looks good.  Timing is set at 3deg BTDC.  No vacuum leaks.  The idle set and the fuel mixture have extremely limited response.  I can increase idle speed, but not decrease.  Fuel mixture has no real effect. 

 

I really do not wish to re-build this unit.  I have another I can replace from my former truck which was rebuilt.  The issue here is this carb has been sitting in the garage for three years-not sure about the seals. 

 

Any suggestions would be helpful.  

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With engine running check the power to the electric choke is 12 volts. I suspect it's without power.

 

Choke should fully open. Jumper the idle cut to the choke heater wire. This in effect is a secondary source of power for the choke. I think the choke being part on is messing with the fast idle and mixture?

 

The '85 is not adjustable for idle mixture as it is set and sealed at the factory. This will not pass a visual smog check.

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Have to recheck voltage reading, as it was too weird, like 2.4Vdc connecting voltmeter positive to choke terminal and voltmeter negative to battery negative.  That said, the choke is only 5-10 degrees from fully open, and when I manually open to final full open position, it does not effect anything.

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When running you need 12 volts on the choke heater to keep it hot... and full off. Any amount on can hang the fast idle on also. Try holding the choke off (open) and rev the engine. Does the idle speed drop to normal?

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Tried manually opening the choke and at that point it would drop out completely.  And I had jumpered the choke directly to the battery source.  I have not looked at the fast idle cam-but since it was dropping out as if the idle cut/anti-dieseling solenoid was not working,I am fighting two problem sets.  

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Getting closer.  I can achieve 1000 rpm after revving to 2000.   It returns to 1000 rpm, but anytime I go below 1000 rpm idle, the idle roughens and I adjust the air/fuel mixture to even it out.   Then I am again at 1000rpm regardless of the combination of idle speed and idle mixture.  I am going to borrow a vacuum gauge and see if I can dial it in closer.  The idle speed adjustment seems to have nominal impact-which is a surprise to me. 

 

DRATTSS:  Let the truck cool and now I am back to a no-idle condition and I guess I have to feather till the choke opens.  This carb is finicky  

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You're in CA with an '85, so I'm guessing you have a feedback carb?  (Six wires, exhaust manifold has O2 sensor, etc.)

 

Consistently high idle is generally indicative of vacuum leaks in a feedback setup.  That's the first thing I'd hunt for on that problem.

 

Second, 2.4V at choke is not right.  Should be 12.  And 2.4 is probably not enough to actuate the idle cut solenoid, which would _also_ explain why it wants to die once the idle drops — with the idle screw cranked in, the primary is open and feeding fuel.  With the idle screw backed out the right amount, you should still be getting fuel from the idle circuit.  The idle circuit fuel source will be cut off if the idle cut solenoid isn't powered.

 

Easy check for now: run a jumper wire from the battery hot terminal to the idle cut solenoid.  You should be able to hear it click when you power it.  When you do that, try to avoid backfeeding the 12V into the harness from that point — could damage the ECU, depending on what other problems exist in the truck, and you don't want to be hunting for another one.

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