i just bought an 1984 sentra and i'm running into a problem where the engine revs super high during idle and driving(idk how high exactly, tach doesn't work), high enough to power through the breaks(i used the e-brake to stop). this is my first carb'd car so i'm not familiar with them at all. i've googled for ways to fix it but cant really find any clear answers, or even a properly labeled picture of the carb. i have found threads about an idle screw but i haven't find where that would be or how to differentiate it from all the other screws in or around the carb. does anyone have a picture of the idle screw on the carb? or know how to solve the issue?
carb trouble
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If you have no experience on carburetors, you picked a heck of a one to start out with! The Hitachi carb, itself, isn't so bad, but, you likely have the VCM system (vacuum control module) on it as opposed to the more standard choke system, used on 1983 and earlier models. If you do, you will have a big, black, throttle actuator on the driver's side of the carb with an adjustment screw...which should never be adjusted unless you know what you are doing. The base idle screw is accessed from the driver's side and firewall side of the carb by turning the throttle drum. There is a good chance the base idle adjustment may not be your problem, rather, a problem with the VCM system (*Note: many refer to these as the "hopeless chokeless" system and end up replacing the factory carb with a Weber rather than try and figure out how it works and correct it). You should really get a Nissan factory service manual for your vehicle to help you figure out what is wrong and help you determine what you need to do to correct it. You could try Nico Club's site for a downloadable manual, but I'm not sure if they go back that far. There are currently about four or five for sale on Ebay for under $20 which is a deal considering all of the information it provides about your vehicle. That would be where I would start. Also, if the VCM filter is clogged, it will cause the engine to rev. This is a small, black plastic canister that is about the size of a pill bottle near the driver's side of the carb. There is a small, what appears to be a drain hole on the end of it; make sure the hole is located on the bottom side.
2nd everything SMJ says, and recommend against adjusting either of the idle screws.
The ISCA is partially controlled by the VCM, which in turn depends upon both clean air supply and good engine vacuum, and partially controlled by the carb itself. You should see multiple hoses coming off the ISCA and 1 or 2 on the black plastic air filter assy (with the hole at the bottom).
Can you attach pics of the carb, with the air cleaner housing removed?
The ISCA is partially controlled by the VCM, which in turn depends upon both clean air supply and good engine vacuum, and partially controlled by the carb itself. You should see multiple hoses coming off the ISCA and 1 or 2 on the black plastic air filter assy (with the hole at the bottom).
Can you attach pics of the carb, with the air cleaner housing removed?
I should have also mentioned that the system is also controlled by the ECM via input from the coolant temperature sensor, so it's important to make sure the thermostat is operating properly as well as the coolant temp sensor.
Ohm the CTS, disconnected, when car is cold, and then while aiming a heat gun at it (when car is cold), and then when car is hot. What do you get for each of these?
Check coolant hose leading from thermostat after cold start - keep your hand on the hose - and out of the way of the radiator and condenser fan blades/motors. Does the hose begin to warm up after several minutes of idling or right away.
Does the car have a PTC relay on the pass's side just inside the fender top, under a rectangular black plastic cover? If so, remove the plastic cover over the relays, place your fingers on and all around the PTC one, have someone start the car, see if the relay clicks (on) when the car starts ... and keep your fingers on the relay for several minutes and see if the relay clicks off.
Look at the throttle plate w car cold and when hot. What is seen? Throttle closed/partially open/wide open?
Post pics at the site SMJ recommends and then just type in the link here. Post pics of the engine bay on the pass's side (so we can see the VCM motor, fuse block there), and the air cleaner config, and under the air cleaner - the carb, and along the firewall (so we can see the BPS and vacuum valves).
How many electrical connectors do you see going to the carb?
Check coolant hose leading from thermostat after cold start - keep your hand on the hose - and out of the way of the radiator and condenser fan blades/motors. Does the hose begin to warm up after several minutes of idling or right away.
Does the car have a PTC relay on the pass's side just inside the fender top, under a rectangular black plastic cover? If so, remove the plastic cover over the relays, place your fingers on and all around the PTC one, have someone start the car, see if the relay clicks (on) when the car starts ... and keep your fingers on the relay for several minutes and see if the relay clicks off.
Look at the throttle plate w car cold and when hot. What is seen? Throttle closed/partially open/wide open?
Post pics at the site SMJ recommends and then just type in the link here. Post pics of the engine bay on the pass's side (so we can see the VCM motor, fuse block there), and the air cleaner config, and under the air cleaner - the carb, and along the firewall (so we can see the BPS and vacuum valves).
How many electrical connectors do you see going to the carb?
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