Jump to content

Dies when warm and drops back down to idle...


S1SSYSPACEK

Recommended Posts

Hi! I'm new to the forums but have been lurking for some time now. 

 

I recently bought my first Datsun. An '83 720 King Cab 2WD with 70K original miles. The guy I bought it from flooded the engine and killed the battery right before our eyes while he was trying to start it so we got it for a really good price and towed it home. I replaced the plugs on the left hand side (I couldn't access the ones on the passenger side easily) threw in a fresh battery and it fired right up. A little smokey at first but after a minute or 2 the smoke disappeared.

 

Took it around the block a couple of times and it was great. Shifts like a dream, quiet, and everything seemed perfect. Then I took it about 3 miles down the road to meet up with some friends. Parked it for about an hour and then went to grab some food when all the sudden it died out as I was taking a U-turn and dropped it back down to idle. Started up easily again, which makes me think that's not necessarily vapor lock, but it kept dying out at every red light and stop sign until I got it home. 

 

Again, started right back up when I'd turn the key, but just wouldn't idle. 

 

Now I'm experiencing that same issue every time it's warm. It'll run fine at first but when it goes back down to idle after the engine is warmed up it'll die when I come to a stop. 

 

I have extensive experience with 2-stroke mopeds, motorcycles, and other small carbureted engines but I can't figure this one out. Is there some type of switch that changes the idle point when the engine is warm? Should I just turn the idle screw in a little bit so that it idles faster? That seems like a band-aid, but I'm willing to try it. Also, where is the idle screw? Do I need to remove the air cleaner to access it? 

 

Thanks in advance for your help! I'm super excited to drive this thing more when it doesn't die out all the time!

Link to comment
  • Replies 12
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

One thing... Have someone turn the ignition on off on off on off (don't turn to start) just on and off while you listen bear the carb. You should hear a soft clicking from the carb. Does it???

 

If yes

Warm it up and turn the idle speed up enough to keep it running.

  • Like 1
Link to comment

Idle cut off solenoid like mike said. If still dies coming to a stop,clogged jets or maybe fuel level too high? . Try gettin revs up and cover carb to choke it out. High vac might suck the shit thru,otherwise need to pull carb apart. Be sure any fuel filter has been changed, commonly overlooked item.

Link to comment

I'll try to see if I can hear the light clicking sound coming from the carb as soon as I get home from work. 

 

Say everything is cool and I have to turn the idle up. Would I pry off that cap in this photo and turn the screw in? Or would I adjust this "idle compensator" that's pictured here

 

Thank you again for your help!

Link to comment

I can hear the light click from the carb when turned to the "on" position. Rode it to the gas station. Filled the tank up. Died the whole way there and back once it was warm, per usual.

 

Took it home and parked it. Wouldn't stay idling so I adjusted the idle screw in a half-turn at a time and would fire it up and try to get it to idle and still no luck. Someone had removed the factory seal that blocks the idle adjust so that tells me it's been tampered with in the past. I turned it in a total of 2 and a half turns and saw no effect, so I set it back to its original setting (backed it out those 2.5 turns) and threw in the towel.

 

Not sure what to do now. Le sigh.

Link to comment

The screw that is sealed adjusts the mixture of air to gas making it richer or leaner. Don't mess with it yet.

 

The screw you want is the idle speed screw. This picture's descriptions are not good. The idle speed screw is labeled' throttle adjusting screw' below. It turns the engine idle speed up and down.

 

carb110.jpg

 

 

OK it would seem that the idle circuit is possibly plugged. Gas is not getting there. Here's what to do.

 

Remove the idle cut solenoid labeled the 'anti dieseling solenoid'. Have a care as there is a spring and small pointed rod inside and it can fall out. Carefully save them and put aside. Now unscrew the idle adjusting screw and remove it. Count the turns so it can go back to original position.

 

Get a can of carb cleaner, one that comes with a red plastic straw. Stick the straw into the idle cut solenoid and spray it out. Then do the idle mixture screw hole. Go back and forth. Again, have a care as these two places are connected and spray may come out into your face and that shit stings. Do the spraying several times to flush out any dirt and replace everything.

 

Now I'm going to assume everything is going to work now so as someone messed with the idle mixture we should try to set it right. Warm the engine up thoroughly.

 

If idling fast when warm turn it down as much as you can with the idle speed screw. 

Now turn the idle mixture screw in until the idle quality drops, it may do so immediately so try turning out. Find the fastest smooth idle. I assume the idle speed increased so turn it back down with the idle speed screw. Turn the mixture screw in until idle quality drops then turn out until idle quality drops. There will be about a full turn where the idle is very good so set roughly in the middle. (not critical just do your best) Again turn the idle speed down. Continue to do this and as many times as needed until you cannot improve the idle quality, and the idle speed is around 700-800 RPM. That's it.

Link to comment
  • 4 weeks later...

Thanks Mike! This has been a huge help! I've been fiddling with it the past few weeks and can't seem to get it to idle any lower than around 1200 but it seems to be alright. 

 

The air mixture screw doesn't seem to like being turned in too much, but doesn't really start idling any rougher when I back it out...so I'm just kinda working towards where I think it should be when I get that rough idle turning it in.

 

That said, it's tricky to get it to the right spot when it's cold vs. when it's hot. I can tune it all day when the engine is warm but then when I find that magic lowest idle spot and let it cool down it's too low, so I turn it up and then it idles fast once it's warm. Tough to find that happy medium. 

 

Gonna keep playing with it this weekend and see where I can get it at it's best. I'll try to take some video and see if maybe you hear something that I don't. 

 

Again, thank you! You've gotten me leaps and bounds farther than I'd ever be able to on my own! 

Link to comment

1,200 seems to be the cut off point for the idle circuit. If you can't get it lower maybe the idle cut is not letting gas to the idle circuit. I had this recently on a carb swap. Turn the idle down near 1,200 and it would quit. I took the idle cut solenoid out and removed the pin inside, put the solenoid back and Bam! I had idle down to 600.

Link to comment
  • 3 weeks later...

Hmm. I'm going to try that and see if it works. 

 

I'm seeing less and less consistency on the idle mixture screw. I can get it to run super choppy if I turn it in too far, but once I back it out and it's running better I can keep on backing it out and there is virtually no change. 

 

I'm also seeing the RPMs jump around at idle. It's like the truck is trying to work it's own issues out. I made a video today of what it does. This is me sitting with the engine warm, not touching the gas or anything. 

 

Also, I'm not sure if this hose (that seems to lead from the intake) is supposed to be connected to anything or if it just is a vent hose. I don't feel any air flowing out or sucking, and when I cap it with my finger it doesn't have any noticeable effect on how the engine runs. Figured I'd post it and see if it raised any red flags with y'all. 

 

241yalv.jpg

Link to comment

1,300 RPMs isn't idle, around 750 is. This is too high for the idle mixture screw to have much effect. Above 900-1,000 the centrifugal ignition advance will be adding more timing which will alter the idle too. That slow loping may even be from EGR thrown in.

 

Try adjusting the BCDD. Turn the screw in the center counter clockwise a couple of turns. This decreases the sensitivity of the BCDD and it turns off sooner. The BCDD is like a small carb inside the carb that comes on during fast speed deceleration and is supposed to shut off before idle. Sometimes they stay on and increase the idle speed.

 

That hose goes to the T V V.  There are three hoses to the T V V. One for EGR and one to the Vacuum advance on the distributor. When the engine is cold there is a vacuum leak so that there is no EGR or vacuum advance. To keep dirt out, the air leaked in comes from the air filter. When warmed up the vacuum leak closes off and under certain conditions, EGR and vacuum advance can happen. Look on your air cleaner housing for the other end.

  • Like 1
Link to comment

Probably not directly related to your current problems, but I see that your float bowl vent line is capped off.  That's probably not ideal, since it doesn't allow the pressure in the float bowl to equalize with atmospheric very easily.

Link to comment

I believe that is the bowl vent that is only used on California emissions carbs. It collects fumes from boiling gas after shut off on hot days. There would be a hose from the carb to the charcoal canister where they are collected.

Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.