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Odd problem, can't come up with anything obvious wrong.....


BarryA

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Background: 79 5 spd L20B 620, stock w/ all CA emissions including original Hitachi carb. Ran ok, passed CA smog w/ very good numbers a few weeks ago.

 

Bought a used Weber 32/34 DFTA CA emissions legal carb, looked ok from a general inspection, installed it, engine starts but won't run below about 1500 rpm. Discovered that the electric idle shut off solenoid wasn't opening, figured that was probably part or all of the problem. Can't get a replacement locally, so removed the Weber until I can get a solenoid & reinstalled the Hitachi.

 

Hooked up the carb and all of its hoses, etc back to exactly the way it was - engine does exactly the same thing it was doing with the Weber - starts but won't run below about 1500 rpm, warm or cold.

 

Can't find anything out of place or disconnected, no cracked or open vacuum lines, does't matter whether the air cleaner is installed or removed with the vacuum lines plugged.

 

Never touched the ignition or any wiring other than that which goes to the carb - anybody got any thoughts on this?

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On the Weber and it's non-functional solenoid, I tried wedging the plunger in the open position with a piece of thin brass shim stock to see if that would resolve the problem, but there was no change, so cutting off the end of the plunger isn't too likely to resolve the issue......

 

Since the Hitachi is back on it, tonight's project is to check continuity in all the wires leading to and from the carb and emissions system's electrical components (manual says that an electrical fault in the factory idle solenoid control circuit will prevent the engine from idling by keeping the solenoid from opening) and clean all the contacts - since this IS a 30 year old truck, much of the wiring is somewhat brittle and dirty, so there may be a wire broken inside its insulation or a bad contact in a connector somewhere that was disturbed in the process of swapping carbs.

 

The next step is to buy several feet of new vacuum hose, and replace all of the hoses in the emissions system - it's amazing how often replacing all the old hoses eliminates problems.

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I can say nothing about the Hitachi and I do not understand if you mean that the motor will die if the engine goes below 1500 rpm or that you are unable to get the idle speed adjusted to below that speed.

I have been resurecting a non-running '77 620 that came with a 32/34 DFT.9A and I can tell the tuning process I have used with that if you wish.

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Check for 12 volts on the Red wire to the idle cut solenoid. If not there you will NOT have fuel to the idle circuit. Check the connector then check the fuse it's marked C and is on the ignition side of the fuse box. 10 amp.

 

Changing carbs this wire probably got pulled on when disconnecting or touched a grounded surface while running and blew the fuse.

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Found the issue with the Hitachi installation - an "open" wire in the circuit to the deceleration cut unit (as suspected, the open circuit was in an old, brittle wire - probably separated internally). Fixed it, and we're back in operation with a normal idle with the Hitachi carb and related emissions junk installed.

 

Next up, figuring out why the Weber 32/34 DFT won't idle - at this point, best guess is a blocked idle circuit.

 

Still going to spend a couple of hours over the weekend replacing all the small vacuum lines and as many of the big hoses as workable replacements can be found for - much easier to troubleshoot when you're not worried about old cracked hoses and loose connections due to aged, brittle tubing.

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My Hitachi idles really high too. It idled really low after my oil change, to the point where it would stall if I didnt give it gas, but now it's back up to 1500+RPM idle. I've got one wire that isnt connected to anything because the carb is off of a later model car that had emissions stuff and my '71 L16 didnt have any. Could that be the problem? It's not the idle cut switch, or the electric choke(which also isnt hooked up)

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My Hitachi idles really high too. It idled really low after my oil change, to the point where it would stall if I didnt give it gas, but now it's back up to 1500+RPM idle. I've got one wire that isnt connected to anything because the carb is off of a later model car that had emissions stuff and my '71 L16 didnt have any. Could that be the problem? It's not the idle cut switch, or the electric choke(which also isnt hooked up)

 

The engine could be started by pumping the accelerator pump to get gas into the manifold, then held at about 1500 rpm with the carb linkage or the gas pedal, but if it was allowed to drop back to idle, it died because the deceleration cut-out circuit was closing the electric idle valve. Probably no relation to your high idle issue......

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well this is kinda off subject but not at the same time how do you fix a screw in a hitachi i think one of the screws broke because of it the base loosens from the rest giving me a huge leak any sugestions anyone

 

If it IS broken off, the first order of business is to get the broken screw out, which can be tricky - drilling out a small screw can be difficult, especially when it's broken off in much softer material like a carb body. Odds are that no matter how careful you are, there's probably going to be some damage to the threads in the hole it's broken off in.

 

If the hole does get damaged in the process of removing the screw (or if you get lucky and the only problem is stripped threads in the hole) the best bet will probably be to repair the threads with a heli-coil (if you're not familiar with heli-coils, they're a type of threaded insert - the hole is tapped oversize, and an insertion tool is used to thread the heli-coil into the oversize threads to resize the hole back down to its proper size and thread pitch).

 

You should be able to buy a heli-coil kit at a good parts store, but if you only have one hole to fix, it may be cheaper to have a machine shop do it.

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