AE82 4AC idle intermittently drops/stalls

[xetv]

Hi guys,

as the title implies, on my 4ac, it starts up fine when cold but idles high, around 1500rpm, and adjusting the idle speed screw doesn't change anything. Also intermittently the idle drops to 500-600rpm and will stall if it's not in neutral (automatic) (maybe running too lean?). I took off the air cleaner, sprayed some carby cleaner, and checked for vacuum leaks, but it didn't help, the fuel filter is clean and I haven't had any problems with the fuel pump before. This problem hasn't happened before (in the past 4 months), it only started a couple weeks back after the engine shut off and wouldn't start till 30 mins later (I suspect it was a vapor lock) but i don't know why that would cause this idling issue. Any help would be much appreciated.

[altezzaclub]

" it starts up fine when cold but idles high, around 1500rpm, and adjusting the idle speed screw doesn't change anything. "

Does it have a separate cold idle screw, working on the choke quadrant? I've never dealt with a  4AC, but that system of one idle screw for cold and a different one for hot is pretty common. It may be the idle jet is blocked, or something is wrong with the hot idle screw system.

[Banjo]

Might be time for a carby removal & complete clean ?   Idle jets & passages are small, & it does not take a lot of "gunk", to close them up.

Just to remove your suspicions, about a vapour lock, & the fuel pump; remove the fuel cap lid, whilst idling, & see if that makes any perceptible difference.

As a further thought; if your fuel filter hasn't been changed lately, that would also be a good idea, so that you have covered all possibilities.

Cheers Banjo

 

 

[xetv]

Thanks for the replies, so update, today I tried messing with the choke, I used a paperclip to hold it in place (I'm thinking it could be an issue with the auto choke system), which fixed the problem where it would idle too high, but it still intermittently drops down.

It does have a fast idle screw and an idle speed screw, but it was working perfectly before and I haven't touched them, so i don't think those could be the issue.

When it does idle too low, holding the choke slightly closed brings it back up. Could be an issue with the fast idle cam.

One of my other ideas is that it could be a vacuum leak in one of the vacuum systems, like the hot idle compensator, so the vacuum leak only happens when that system is active, but this seems unlikely.

If anyone has other ideas, I just wanted to check all other possibilities before I do a carb rebuild.
Thanks,

[altezzaclub]

"When it does idle too low, holding the choke slightly closed brings it back up. "

"This is a valve to admit extra air at hot idle.."

They seem to match. Can you disconnect and seal the hot idle compensator line and see if it fixes it.

 

[Banjo]

Quote

When it does idle too low, holding the choke slightly closed brings it back up. Could be an issue with the fast idle cam.

Holding the choke slightly closed (which fixes it) indicates the mixture is too lean; as closing the choke (less air) would make a little richer.

When the engine is up to temperature the choke should be fully open.  Get the engine up to temperature & leave the choke fully open. Increase the idle mixture screw, to get the same result, as you did, by closing the choke off slightly.  If adjusting the idling screw does not have the right effect, then time to strip carby, & give it a fully descale, with a good carby cleaner.

Cheers Banjo 

[Tymm_1]

Hey,

Since adjusting the idle speed screw didn't help and you've checked for vacuum leaks, it could be a few things like fuel delivery issues, a dirty idle air control valve (IACV), or faulty engine sensors. Considering you're new to troubleshooting, it might be a good idea to get a mechanic's opinion for a more precise diagnosis and fix.