83 720 with Z24 bad idle only when warm

[83_720]

Starts right up cold, smooth etc. A couple minutes later when it gets warm, the idle just goes to junk, missing etc. Hoses are new, carb is new. Non-Calif engine. Does have the two egr valves behind carb, 3-hose temp switch on water jacket and anti-backfire valve in front of carb.

Any ideas what to check next and how?

Thanks!

Jim

[synack7350]

sounds like its leaning out when the choke opens. maybe a vac leak or your running your mixture too lean.

[flinterman2000]

More than likely, the mixture is too lean.

[83_720]

I'll give anything a try. How do I make it more rich? The carb is new, and the original carb was doing exactly the same thing. All vaccuum hoses are new as well.

Thanks!

[83_720]

I think I found it. Recessed flat-head screw at base of carb. Not the issue. I did run it in and out at the lowest Idle I get keep it at. Any idle speed less that 1300rpm and it won't run. That mixture screw has little effect if any by the time you are at 1500rpm. It has to be something more drastic. At 2500 rpm the vacuum is rock steady at 22 and the engine is perfectly still. At 1500rpm it is missing and starting to shake, idle up and down. Below 1300 - 1500rpm it won't run. The vacuum reading does drop with rpm. Just feels like a large leak. There is a large vacuum fitting under the carb in the side of the manifold that goes to the vapor canister, anti-backfire valve, and something else. I'm going to buy a plug for that tomorrow and see if that makes any difference at all to rule anything plugged into out. I already did that with the brake booster line.

[heyman421]

I couldn't begin to guess why, but it sounds like your idle circuit is inoperable.

The choke's probably just rich enough to compensate.

[83_720]

But on two separate carbs? The one on there now is new. Same issue as the one that has been on there for 28 years. It is not the carb.

Thanks.

[synack7350]

did you plug off your vac lines yet to see if that helped? I mean there's alot of things that can go wrong with a motor and its ability to idle, ignition can act like a fuel mixture issue, a fuel mixture issue can act like an ignition problem, and a vac leak messes up both simultaneously. usually its a combination of factors that make an engine to the point where someone is scratching their head and saying man I really need a tune up. To be methodical though, I go from vac lines, to ignition, to fuel mixture, in that order.

[83_720]

Finally getting back to this. How can you tell if the computer is OK? I completely unplug it under the driver seat and there is no difference at all. Runs smooth as silk cold. 2 - 3minutes and it gets warm and it just won't idle. shakes etc. and needs to be at 1200 - 1500 rpm to stay running. Hoses are new. Tried this with no hoses ( all plugged) and no difference. Carb, distrib, both coils, wires and hoses all replaced. Exactly the same problem. Compression good on all cylinders ( 160+). Vacuum gauge steady at 21. No smoke.
I would have thought the computer being unplugged would have made 'some" difference. Is is fused?

Thanks, I'm open to anything.

Jim P

[83_720]

Got it.

The ecc unit under the driver seat had no power. The problem was at the ecc master relay just to the left of the battery screwed to the sidewall. One of the wires ( brown one) that powers the relay to allow 12 volts to a number of ecc pins was corroded off. Re wired it and it runs just fine.

One hidden, corroded pin.

Jim P

[drumaniac3000]

Don't ya just love the simple little things...

[synack7350]

ummm odd I thought the "ECU" on these trucks just controlled the cruise control for those vehicles with it equipped. Mine doesn't have one, I have a box that controls the interior lights in its place. Perhaps I'm wrong could someone please edumacate me on this one. Cuz I'm under the impression that all the Z motors before the KA's lacked an ecu, since they weren't fuel injected, dizzy and vac controlled advance? what is there for an ecu to control? Of course the KA;s had an ECU but they came with the hard bodies. The '83 720 z24 was carbed.

[fastboatman29212]

I believe the FSM calls it ECC = electronic controlled carburetor. The are several wires that come off the carb that are bundled and lead back to the ECC unit as well as vacuum switches that are wired. I've never messed with mine, but my '84 FSM has a lot of pages dedicated to it.

[83_720]

Yep. ECC. Wires to Carb (6), temp, vacuum, exhaust gasses...and all with a Carb.

ECC.

[flinterman2000]

The ECC also shuts off the intake plugs under full power so that you can get more advance from the distributor.

[jdroberts720]

if your mixture isnt the cause your distributor may have shifted a little retarding your timing, loosen the mounting screw and play with it a little to tune it by ear and see if this helps any