Z22 81 datsun 720 high idle

[datsunranger]

Hi, new to the site and to the datsun 720, but my friend asked me to fix his 81 720. Found out he had a bad head gasket, replaced that without a hitch, but the truck idles high. Maybe around 1100 or so, i tried looking into the idle adjustment screw but it didnt do a thing, the fast idle cam(think thats what its called, sorry not a carb guy) looked like it operated perfectly, and the choke opened as it warmed up.

 

Is there anything you guys can think of to check or mess with, the truck has plenty of power now, cleaned the valves and head up, but the idle things pretty annoying.

 

Thanks for any help you can give me

[metalmonkey47]

Have you checked ign timing?

 

Vacuum leak?

[datzenmike]

Throttle cable too tight.

[datsunranger]

I did check the throttle cable, it wasnt the problem. I didnt check timing though, it looked like it hadnt been touched since 81, so i figured it was good. Not to mention it still has to pass smog so i figured it was close to spec, since they check that and it passed not to long ago.

 

Anyone have the vac specs?

[Jon-Doe]

you have a vac gauge and your positive nothing is wrong with the fast idle? the butterflys close fully?

 

if so ....

id start with timing and mixture first, its most probably running really rich and too advanced.

turn the mixture/idle down untill you achieve 750-800 rm. go from there with a vac gauge and if you can find one a timing light (the vac gauge should get you correct timing anyway)

 

for most engines when using a vac gauge, good vac should be around the 18-21 mark @ 750-800rpm (anywhere around there is good)

lower = vacuum leak, low idle, lean mixture, retarded timing

higher = rich/timing too advanced, high idle

stutter = burnt valve/spark miss

 

thats just my general rule of thumb im sure you can find a vacuum chart some where on the interweb with alot more detail

(also a big cam will have a lower vacuum and a small stutter)

[datzenmike]

Secondary throttle plate may not be closed fully or sticking

Primary throttle shaft may be worn and throttle plate cannot close.

Fast idle cam is sticking, shouldn't be on any of the steps.

Dash pot badly adjusted, if equipped.

 

Even if the timing was advanced or a vacuum leak you should be able to turn the idle screw down.

 

Is vacuum advance hose connected properly? Disconnect hose and place thumb over end of hose. Does the idle change?

[datsunranger]

Thanks for all the info, my friend is going to bring the truck over tonight and i'll take a look at it. Hopefully its something simple i overlooked.

[datsunranger]

Turns out the secondary throttle plate had been sticking the slightest bit open because of some dirt build up and the dizzy vac line had a pretty big crack in it. Cleaned and replaced the vac line now the trucks all back to normal, idles at 800rpm well, pulls about 21 of vac.

 

Thanks for your help, without a FSM or a lot of carb experience, Ratsun can be very helpful.