2F smooth then rough, smooth then rough, smooth now rough...

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May 6, 2016
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Lummi Island, WA
A few weeks ago I finally got my 1975 FJ40 running for the first time since having my 2F rebuilt a while back. After the initial startup (30 minutes between 1500 and 2500 RPMs) it briefly ran beautifully, then suddenly wouldn't idle below about 1100 RPMs.

After a bit of troubleshooting (stock carb) I guessed my idle solenoid was bad, so I ordered a new one. After installing the new idle solenoid, it ran great again for a bit - then suddenly wouldn't idle again. This time wasn't as bad as the first time, now it would stay running if I set the idle at 800-900 RPMs.

After a bit more troubleshooting I came back to the idle solenoid, wondered if maybe there was something in its little chamber that was preventing it from functioning quite right. I pulled the solenoid and removed what looked like a clear flake (of ? - not sure what it was) on it, also tried to clean out the chamber a bit (carb cleaner + toothpick - couldn't think of anything better at the time!) before screwing the solenoid back in. Voila! It ran great again and I was feeling very proud of myself, until...

Once again, it suddenly stopped holding normal idle, now requires 800-900 RPMs to stay running. And it's not a smooth 800-900 RPM idle, sounds like it's missing and occasionally dies even with the faster idle (but it runs/drives well above idle). And this time, pulling the idle solenoid and trying to clean it and the chamber didn't improve anything.

Thoughts on what my problem is? I would still guess it's related to the idle solenoid, but maybe not.
 
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What else have you checked? Is it getting the correct fuel flow? Have you checked for vacuum leaks? Timing?Pull a plug and check if it is running rich or lean? Does it have points and have you checked them?
 
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Lummi Island, WA
What else have you checked? Is it getting the correct fuel flow? Have you checked for vacuum leaks? Timing?Pull a plug and check if it is running rich or lean? Does it have points and have you checked them?
I haven't checked fuel flow or pulled a plug, will do those this weekend. Haven't found any vacuum leaks, just got a vacuum gauge and connected it for the first time this evening - was getting about 15 with variance (up and down) 1-2. Timing has been at/near the BB the whole time, and I adjusted air/fuel mixture per FSM (that was during "round 2" of it running smoothly). I installed new points while I was at the engine rebuild and set them at the correct gap - but it would be worth checking that it hasn't changed.
 
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Could it be bad wiring/connection causing intermittent loss of power to the solenoid?
@pithicus: I've had this thought, but it seems like the solenoid is getting steady power. It still clicks when I turn the key on, even though I'm back in "bad idle" mode. The solenoid getting power intermittently would definitely explain it, though.

Always re-check your grounds..esp after new install. I’m surprised at how often this is the culprit after exhausting the obvious causes
@kudzugypsy: good idea, thanks.

My intermittent "good idle" and "bad idle" periods have been long/consistent enough stretches that it seems like something has suddenly changed/failed each time. The first "good idle" stretch wasn't that long, probably a total of 15 minutes running. The second "good idle" stretch was longer, 30+ min run time at normal operating temp. And the third "good idle" stretch was long enough to fool me into thinking the problem had been solved, probably 90 minutes of driving. Then when it's gone to "bad idle mode," it has stayed that way - not gone back and forth. A suspect wire connection/ground that gets worse with heat or jostling could explain it, but somehow it seems like I'd have more back-and-forth (between good idle/bad idle) if this was the issue.
 
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1911

chupacabra
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Could it be bad wiring/connection causing intermittent loss of power to the solenoid?
This, especially if you have the emissions "computer" under your dash (not sure if '75 had that or not). It is known for the cold-soldered connections on the circuit board to crack and not provide the needed ground to the fuel-cut solenoid. You can either heat and re-flow the bad connections, or just bypass the computer by permanently grounding the solenoid. You can test this theory by temporarily jumping a good ground to the fuel cut solenoid and seeing if that solves your problem.
 
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This, especially if you have the emissions "computer" under your dash (not sure if '75 had that or not). It is known for the cold-soldered connections on the circuit board to crack and not provide the needed ground to the fuel-cut solenoid.
So does the solenoid get power through the ignition switch and is grounded through the “computer”? I saw a couple threads suggesting the computer as the culprit for the solenoid and took a closer look at my computer for the first time last night. My vehicle is desmogged, so I was (am) curious if my computer serves any purpose. Just for fun I unplugged the computer and turned the key, heard the solenoid click (I think, but maybe I imagined this?) and tried starting the vehicle; it started and idled poorly, just like it had with the computer connected. I was guessing this meant my solenoid had been rewired outside of the computer, but maybe not? But wait, the solenoid wouldn’t click without a half-decent ground - right? Thanks!
 
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If the solenoid is a single wire it grounds through the body of the carb, if its clicking then its working. There is an o-ring on the end of it, if ethanol fuel is used it can deteriorate the rubber bits in the carb. Has it been rebuilt? vacuum should be closer 18-20 depending on altitude.

also if the needle on the vacuum gauge is wandering maybe you need to check out the valves.
 
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If the solenoid is a single wire it grounds through the body of the carb, if its clicking then its working. There is an o-ring on the end of it, if ethanol fuel is used it can deteriorate the rubber bits in the carb. Has it been rebuilt? vacuum should be closer 18-20 depending on altitude.

also if the needle on the vacuum gauge is wandering maybe you need to check out the valves.
Thanks, that makes sense on the solenoid wiring/grounding (mine just has one wire). The o-ring on the old solenoid didn’t appear deteriorated (and the o-ring on the new solenoid looks good), and I did rebuild the carb while the engine was out. Speaking of, it was my first carb rebuild and I’m no carb expert - anything I might have done incorrectly that might be causing the idle issue? As far as I can tell it's functioning well otherwise.

And I'll add checking valves to my growing list of to-dos!
 

1911

chupacabra
Joined
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So does the solenoid get power through the ignition switch and is grounded through the “computer”?
Yes, exactly.


I saw a couple threads suggesting the computer as the culprit for the solenoid and took a closer look at my computer for the first time last night. My vehicle is desmogged, so I was (am) curious if my computer serves any purpose. Just for fun I unplugged the computer and turned the key, heard the solenoid click (I think, but maybe I imagined this?) and tried starting the vehicle; it started and idled poorly, just like it had with the computer connected. I was guessing this meant my solenoid had been rewired outside of the computer, but maybe not? But wait, the solenoid wouldn’t click without a half-decent ground - right? Thanks!
You can't just disconnect the computer, you need to ground the wire that comes from the solenoid to the computer connector.

I have bad hearing and can never hear the solenoid click, so I don't know what it sounds like. Try grounding the wire and see if that helps your idle.
Mine is de-smogged also, and when my computer went bad I just bypassed it by grounding that wire, and it has run and driven fine for 8-9 years since.
 
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Mine is de-smogged also, and when my computer went bad I just bypassed it by grounding that wire, and it has run and driven fine for 8-9 years since.
Does your solenoid have two wires or one? This makes perfect sense for two, but mine just has one and I'm thinking what Curt said must be the case - that my solenoid would just ground through the carb.
 
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the idle circuit is sensitive to debris and can cause rough idle even after a rebuild, i would take the carb off and give it another go. its free. also the little boot above the accelerator pump can split and allow stuff in.
 
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the idle circuit is sensitive to debris and can cause rough idle even after a rebuild, i would take the carb off and give it another go. its free. also the little boot above the accelerator pump can split and allow stuff in.
Okay, I like this theory. The little boot above the accelerator pump did split into multiple pieces (I replaced it during the rebuild, but that was a couple years ago now and I guess it got brittle!), so debris and/or pieces of the boot itself could definitely have gotten in there.

Just hit a roadblock, went to adjust valve clearances and started with adjusting rocker support bolts to specified torque (per FSM instructions) - and one broke! Guess I'll be taking a little break from the idle issue, will post an update when I'm back in action.
 
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1911

chupacabra
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Parker County, Texas
 
 
Does your solenoid have two wires or one? This makes perfect sense for two, but mine just has one and I'm thinking what Curt said must be the case - that my solenoid would just ground through the carb.
I think so; to tell the truth I don't remember. That's why I said I wasn't sure if or what kind of emissions computer your '75 has (mine is a 12/79 1980). If you only have a one-wire solenoid, then I'm guessing it's not controlled by the computer, but you'd have to check the right year schematic to know for sure.
 
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Bad fuel can cause these problems also. My 1980 first gen truck had a tank of bad gas. The debris was small enough to go through the filter and it clogged the carb. I had to drain, flush the gas tank and then it started to run good again.
 
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Bad fuel can cause these problems also. My 1980 first gen truck had a tank of bad gas. The debris was small enough to go through the filter and it clogged the carb. I had to drain, flush the gas tank and then it started to run good again.
It's possible this is my problem, or has been part of it. I drained some old gas and put in new gas before my initial startup, but all the gas I've used so far has been from five-gallon cans, and the other day I noticed debris in the bottom of one of them.
 
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