Aisan Carb Expert Needed - 1980 FJ40

Joined
Oct 3, 2008
Messages
12
I recently rebuilt my carburetor on my 1980 FJ40 and now it surges or stumbles (lean condition?) between 2000 to 2500 rpms with no load on the motor. It runs perfect at all other rpms. The power valve and associated jet were replaced because the bottom half broke off in the carb during diassembly. I drilled it out without damaging the threads. The power valve jet for this application is a 0.90 mm, but this part has been discontinued by Toyota so I installed a 1.0 mm jet. In addition, the restrictor for the idle jet was missing so I replaced it with an after market 0.50mm jet that came in the kit. The idle jet that came out of my carb was a 0.60 mm. I have checked all of the basics as follows: 1. the fuel level is consistently in the middle of the window, 2. the fuel cut-off solenoid is working, 3. the vacuum advance is working, 4. there are no vacuum leaks, 5. my timing is set at 7.5 degrees BTDC, and there's no junk in the fuel (tank cleaned and coated). Any thoughts or suggestions as what is causing this problem would be appreciated. Could it be caused by the smaller idle jet? I don't think the larger power valve would cause this condition.
 

Pin_Head

 
 
 
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Feb 6, 2002
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Surging and stumbling sound like two different things to me. Which is it? Is the secondary opening?
 
Joined
Oct 3, 2008
Messages
12
I would describe it as surging (rpm's up and down). I looked down the primary at the nozzle and the fuel stream cuts off and on at around 2000 rpms. The secondary is fully closed.
 

FJ40Jim

The Cruiser Whisperer
 
 
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Sep 26, 2003
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I recently rebuilt my carburetor on my 1980 FJ40 and now it surges or stumbles (lean condition?) between 2000 to 2500 rpms with no load on the motor. It runs perfect at all other rpms.
Verify that the coasting decel fuel cut system is working properly.
backprobe the ground side of the ICS connector, w/ voltmeter connected to good ground. It should always show zero volts. If the meter is showing voltage while the surge is happening, then the decel fuel cut system is the culprit.

Could it be caused by the smaller idle jet?
Not likely. A small idle jet would cause the A/F to go lean in the 1000-1500 range.

I don't think the larger power valve would cause this condition.
Right, the PV is out of the picture at low load.

The symptom could also be caused by having the boosters reversed.
 
Joined
Oct 3, 2008
Messages
12
The coasting decel fuel cut-off system was the culprit. The ground side of the ICS connector showed current during the surge condition. I connected the solenoid ground wire to one of the air horn screws and the other wire to a source of switched voltage, problem solved. The PO (father in-law) removed the vacuum switch for the decel system 20-years ago as part of the de-smog. Now I understand the reason for the "green wire mod", but would prefer to get the decel system working again. Thanks Jim. I would have never figured this out without your help.
 
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