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Post Posted: Sat Nov 23, 2019 4:16 pm 
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Before i start, I will just say that I have done some searching for issues other people have had and I have had a crack at reading through the workshop manual.

I will be adjusting the choke and rack one tooth to see if that helps but I have a feeling this might be something electrical to do with timing which is a bit beyond me at this stage.

So this started with me doing relays for my headlights and then after going for a drive.
After driving for about ten minutes my idle was really high, i guess around 2k (no tacho).
I don't beleive it is a vacuum leak problem and I have also previously run the car without a choke valve because that seemed to fix a bug on my first sierra so I did the same with this engine.
I have re-installed the choke valve but that hasn't solved the problem, it seems there is plenty enough air going down the hole with the choke valve for the revs to run high.

The idle problem improved when I unplugged the idle-up and fuel cut solenoid plug but it is still a bit fast, guessing around 1.5k.
I also wound out the idle-up screw on the front of the carb and there is clearance now so that is not playing a role.

It smells like something is getting more fuel than it should at idle and I watching some youtube videos they're like "don't make adjustments to anything because that is just band-aiding the problem" so i haven't touched the fuel mix screws.

I have deleted the resistor for the coil pack because I have a coily engine in my sj51t and some things happened when I put the engine in but i only just deleted that resistor but it did run fine for a while before that test drive.

I am wondering if that may have advanced the timing or something and that is the electric bit.

at this stage I feel like I can play with the mechanical side of the carby to force it to idle lower but that may just mean it runs terribly forever.

I've also heard that putting a J20 and aw-4 will solve my carby issue.

help/comment/hate. I know deleting stuff isn't cool but it was fine before and then changed.

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Post Posted: Sat Nov 23, 2019 6:10 pm 
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was running a lot lower if i disconnected the vacuum line to the distributor advance actuator and I took it for a drive like that ran fine but when I returned home it was revving it's ass off as I parked it in the driveway, back to square one.

i am going to see if I can put the resistor back in but not hopeful re: carbys being shit and all

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Post Posted: Sat Nov 23, 2019 6:27 pm 
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actually it wasn't a resistor I deleted it was a filter running from the negative on the ignition coil and from the distributor back to earth via the filter so i will put that back but I can't imagine how that would change the idle revs and not sure why it just changed after one drive.

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Vehicle: 98 5 dr SQ420, 05 5dr JB420

Post Posted: Sun Nov 24, 2019 7:08 am 
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I can't see fitting headlight relays impacting idle speed so the sequence of events has to be purely co-incidental.

There is, as I believe you're aware, an "idle-up" mechanism that raises the idle speed to compensate for alternator load when certain electrical loads are energized, but the headlight circuit is not among them, the park light circuit is and the presumption here is that the park lights come on before the headlights can be turned on, so the idle speed compensation has already happened.

Moving on - whilst ignition timing does impact idle speed, there's nothing electrical that's going to affect the ignition timing, so you can stop thinking along those lines. The resistor in the coil feed if there is/was one is there to compensate for the voltage drop that occurs due to the starter current when cranking the engine - bypassing/deleting/removing random stuff when you don't understand how it works/what it does will get you into a world of hurt, which is where it sounds like you are now.

Choke valves are cold start aids, they restrict airflow to increase vacuum which increases the draw on the carburetor jets to "richen" the mixture so that the engine starts more easily, most choke mechanisms also have a "fast idle" mechanism that brings the idle speed up, to make the engine less likely to stall after a cold start. Vacuum leaks on carbed engines generally won't cause an increase in idle speed, on a fuel injected engine, it typically does, but that is because the EFI system has a way to compensate for the weakened mixture cause by the excess air from the vacuum leak, carbed engines generally don't, and a vacuum leak means more air less fuel, so it's more likely to stall.

What causes a carbed engine to idle high is an open throttle plate - air flowing through the carb body past the throttle plate pulls fuel through the jet(s), more air, more fuel and the idle speed increases, so for some reason your throttle plate is not closing as it should, you need to find out why and fix that.

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Post Posted: Sun Nov 24, 2019 6:42 pm 
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yeah i have come to the conclusion that it id air flow. ive taken vacuum hoses off one at a time to see if anything changes. the choke actuator at the top which is like triangular is the only thing that really changes when the vacuum hose is removed which makes sense because it closes the butterfly valve.
I think the next step is to remove the carb to see if there is anything allowing bypass of the throttle valve.
I hear what you r saying about normally you would expect an engine yo stall which could be a whole bunch of problems but at least with this it can only be one or 2 things.
will also get new vacuum hoses.
i wondered if it was a an inlet manifold leak but covering the air intake stalled the engine pretty fast

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Post Posted: Mon Nov 25, 2019 11:21 am 
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I think you need to find someone who is familiar with those carbs to assist you with this - you're still babbling about vacuum hoses and choke plates after being told that they cannot cause a high idle speed.

I also really don't see you removing the carb and finding "anything allowing bypass of the throttle valve" - I seriously doubt that there are any passages that allow air flow around the throttle plate, and it's highly likely that some part of the linkage you're going to be removing to get the carb off is holding the throttle plate open, if I remember correctly that vehicle should have a double barreled carb with a vacuum operated secondary, it is possible for the secondary throttle plate to get stuck open if the linkage is dirty or binding - that will cause an inconsistent high idle speed with symptoms similar to what you describe.

Try taking a thin screwdriver, insert it into the secondary and make sure the throttle plate is closed - do this with the engine off and then start and see what the idle speed looks like, adjust if necessary - drive the car, and if you end up with the idle high, switch off and see if the secondary throttle is open - it should close by itself.

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Post Posted: Tue Nov 26, 2019 10:46 am 
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I can't see any mention of what engine and carb we are actually talking about. If it's the standard automatic carb, which is heated by water from the inlet manifold, it's probably just the hoses being blocked.

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Post Posted: Tue Nov 26, 2019 6:03 pm 
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solved. it was a piece of wire insulation stuck in the secondary throttle valve

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