J-Grams Posted October 31, 2015 Report Share Posted October 31, 2015 Howdy folks! New to this forum and new to working on vehicles in general but so far I'm having a great time with it. So I just bought this '84 720 about 2 months ago. Everything was running great, but the other day it started to stall out whenever it's in idle. On a cold start it will do fine but after about 10 minutes it will start stalling out in idle. So then I have to give it gas constantly to keep it from stalling. I've adjusted the idle speed screw(about 5 turns CCW) but it hasn't seemed to help at all. I checked all the vaccuum lines, didn't see anything wrong, but then again I'm new to this so I don't know if I missed anything. Oh yeah on the day it happened, I was driving up this bumpy as hell country road and when I got to my destination I noticed the positive side battery cables popped off and was dangling probably for awhile. Right after I put that back on is when the issue arose. Plus the temperature dropped in the area I'm living (Mosier, OR) if that counts for anything. Any help would be much appreciated, Jensen Quote Link to comment
datzenmike Posted October 31, 2015 Report Share Posted October 31, 2015 I would imagine the idle cut solenoid is unplugged. The solenoid in energized when the ignition is on and allows gas down to the idle circuit. These engines tend to diesel or 'run on' when turned off so the solenoid closes and shuts off the gas to quickly stall the engine. The idle cut is a hexagonal part usually with a single Red wire powering it. To check, turn ignition on off on off and listen for a faint click sound. If you can't fix it, just jumper power from any ignition switched power source so it is on and off with the engine. The reason it works when cold and doesn't when warm is simple. When cold the choke is on and this supplies a rich mixture and the missing idle gas is hidden. When the choke warms up and turns off the engine no longer has gas to idle with and stalls. Welcome to Ratsun Jensen. Quote Link to comment
J-Grams Posted November 1, 2015 Author Report Share Posted November 1, 2015 Thank you sir, I'll go try this out in the morning when I got some light to work with and let you know how it goes. Thanks again, Jensen Quote Link to comment
daniel5on Posted November 3, 2015 Report Share Posted November 3, 2015 Your problem your having is the exact opposite of mine, rough idle while cold, but once she warms up she does good. I even got mine around the same time as you! Hope you can figure out whats going on with your 720! Quote Link to comment
J-Grams Posted November 3, 2015 Author Report Share Posted November 3, 2015 Well folks I've had some luck! I checked the idle cut solenoid and it seemed to be working fine. I tinkered around with the idle speed screw some more and finally found a decent place where it doesn't stall on me when in idle. However, the idle rpms are a bit high, I don't have a tachometer so I can't give out a reading, so it's does shake a bit when I turn it off and seems a bit clunky in 1st gear. But I'm happy it's running well enough to drive again! Thanks for all the help! Quote Link to comment
Rhino13 Posted November 3, 2015 Report Share Posted November 3, 2015 Is your choke turning off? Let the truck warm up with the air cleaner off and see if they open up after you tap the gas pedal once warm. I'm not sure if you do this, but to turn the choke on you press the gas pedal 2 (or 3? I don't have a stock carb) times and as the coil in the choke warms up it expands which turns a little fast idle cam, which in turn lowers your rpms as it warms up. If your choke was stuck on, it would run great when warming up, then take a shit once it's warm because there's not enough air flow but plenty of gas. Edit: when you checked the idle cut solenoid did you unscrew up to see if the needle was in good shape or if the hole was possibly plugged with gunk? Quote Link to comment
J-Grams Posted November 11, 2015 Author Report Share Posted November 11, 2015 So I'm continuing to have issues with this truck, I don't know if it was related to the last issue, or if my altering the idle set screw did anything. Rhino13, I checked the choke and once I get it warmed up it seems to stay open. But the new issue is that I'm losing power at speeds of about 45 mph and above. The engine seems to sputter, but I can let the gas off and it stops till I put the gas on again. Also I noticed when going up hills at lower speeds(2nd gear) the trucks starts rocking pretty violently until I take the gas off. Any ideas? Thank you, Jensen Quote Link to comment
Lockleaf Posted November 11, 2015 Report Share Posted November 11, 2015 Regardless of what you have done already, change the fuel filter again. It's oddly common on these 720s for a slightly clogged fuel filter to cause exactly this kind of issue. Quote Link to comment
Rhino13 Posted November 12, 2015 Report Share Posted November 12, 2015 I had those exact symptoms when my fuel pump started going out. I'd listen to Lockleaf and get the filter first since it's cheaper, but if that doesn't solve it order yourself a facet brand pump. Quote Link to comment
J-Grams Posted November 13, 2015 Author Report Share Posted November 13, 2015 Well folks I just got the new fuel filter in. It was an epic journey and had to bike 7 miles into town and back, but mission successful! I took it out for a spin this morning and all those symptoms seem to have disappeared. Ran pretty smooth I must say. Haven't taken it out on the freeway yet, just these country roads but I got it up to 60 with no troubles, so I'm sure it will be fine. Thank you everybody for your help. Quote Link to comment
Draker Posted November 13, 2015 Report Share Posted November 13, 2015 Excellent!! Not going to lie... I had to google map Mosier, Oregon. And I've lived here for 23 years. Welcome to Ratsun. 1 Quote Link to comment
Rhino13 Posted November 13, 2015 Report Share Posted November 13, 2015 Excellent!! Not going to lie... I had to google map Mosier, Oregon. And I've lived here for 23 years. Welcome to Ratsun. I mapped it too haha, I had no idea it was in the gorge. I've driven through it plenty of times though! Glad to hear you found and fixed the issue, now you can start upgrading things! Quote Link to comment
J-Grams Posted November 14, 2015 Author Report Share Posted November 14, 2015 It's a lovely place, this Mosier, OR. Small town of about 500 but it's got a great little bar called The Rack and Cloth. I highly recommend it. So truck was performing beautifully on the freeway but funny thing is I'm back to my original problem of it stalling while in idle. Haha! On my second test run it showed up. Had to adjust the idle speed set screw again and it's chugging along. I just bought a new spark plugs, wires, distributor cap and rotor. Going to install them on Monday. While I'm into that I'm going to give the idle cut solenoid another look over. Hopefully I get this little issue worked out and then it's time to upgrade some parts. Quote Link to comment
datzenmike Posted November 14, 2015 Report Share Posted November 14, 2015 1/ The idle cut solenoid is an electric valve that shuts off fuel to the idle circuit when in the off position. Shutting off the fuel helps stall the engine and prevent dieseling or 'run on'. What you can do is unplug and unscrew the idle cut solenoid. Have a care, as there is a spring, a washer and a needle valve, all of which will jump out and fall so be ready for them. Once removed, you can use a can of carb cleaner to spray out the inner passage between the fuel bowl and the idle circuit. (get the can with the red straw) Wear safety glasses, that shit stings. This may not fix your problem but it will remove any obstruction to fuel flow. 2/ The idle mixture is accurately set at the factory (using an 02 sensor) and the screw covered with a plug so owners can't mess with the emissions. It is easily removed by drilling a 1'8" hole and inserting a suitable wood screw. By bottoming the screw, the plug backs out or you can just wiggle it loose. Now you can reach inside and get at the mixture screw. Mixture screw is in base of carb. Idle speed screw is over to the left with the spring on it. If you have emissions testing there may be problems if it's seen that you have messed with this so... hide your tracks afterwards. Idle mixture. Set on a thoroughly warmed up engine NO choke!, in good tune, valve lash set and ignition timing correct, air filter on. Lower the idle if practical with the speed screw. Turn mixture screw in (or out) until idle drops off or worsens. Turn the other way slowly until idle improves, becomes stronger and faster. Keep going until it drops off. This good range can be 1/2 to one full turn of the mixture screw. Turn the screw back until you are approximately in the middle of this range. The idle has likely gone up so turn it down with the idle speed screw. Repeat the mixture adjustment again to get the smoothest strong idle speed and turn the idle speed down again. Repeat this as many times as necessary until you can no longer improve the idle quality and the idle is 700 RPMs. Quote Link to comment
xsdg Posted November 26, 2015 Report Share Posted November 26, 2015 Hey, DM What's the right way to keep the truck in open loop in order to set the idle mixture on a truck with the feedback carb? I'm pretty sure the feedback system is covering up a poorly-tuned base idle mixture on my truck, so that when the truck drops into open loop while idling for a long time, the idle speed drops pretty significantly. Can I just unplug the O2 sensor? Quote Link to comment
datzenmike Posted November 26, 2015 Report Share Posted November 26, 2015 The plug at the back of the720 feedback carb is problematic. The wires are too short and have to stretch to plug in, often breaking them internally. Try wiggling the connector while running and see if the idle changes. I'll check when I get home but the idle mix is set and sealed at the factory. (you can unplug and adjust but not usually required or advised) Once warmed up it should always stay in closed loop operation.... maybe something else. For example if the temp sender (a different one than the gauge) wasn't working the ECU would think not warmed up and refuse to go into closed loop. Quote Link to comment
J-Grams Posted November 26, 2015 Author Report Share Posted November 26, 2015 Update for my truck! I replaced the the spark plugs, wires, distributor cap and rotor. Everything has been running great for the past week and I got the idle to an acceptable level. The plugs and the cap were especially crusty, definitely needed replacing. Thank goodness I didn't need to get into the mixture set screw! Thanks again for all the help and Happy Thanksgiving -Jensen Quote Link to comment
DrvnDrvr Posted November 29, 2015 Report Share Posted November 29, 2015 Excellent!! Not going to lie... I had to google map Mosier, Oregon. And I've lived here for 23 years. Welcome to Ratsun. The Memaloose Rest Area/State Park Area is there. Rich in historic significance. Welcome to Ratsun Jensen. BTW DatsunMike just gives a damn. Thanks Mike! Quote Link to comment
blackbomber Posted November 30, 2015 Report Share Posted November 30, 2015 Hey man,, the black bomber had this problem too a while back..mine was a air leak around the base of the carb. where the gasket is. is was sucking air in there and killing it... make sure all your hoses are hooked up to the carb too..check for cracks....there is a lot of hoses running off the carb.....on my 84 any way.... Quote Link to comment
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