Ok, I’m starting a new thread bc I’m very confident that my timing/distributor problem is solved. I’m having an intermittent problem with the 84 460 revving/jerking and stalling. For the short time I got the truck to idle without those issues today I was able to verify that the timing is set to 10* above tdc. Used a timing light with vacuum advance pulled/plugged. Idling in park nicely at around 850rpm.

So the issue is that randomly, the truck will not idle at all and stall out. If I keep the throttle down some I can keep it running but the rpm’s jump/bounce up and down by about 300rpm without doing anything to the throttle. It will also stall out when driving and rolling to stop. Anything below like 800 rpms and it dies. Like I said, this is random and at times drives/runs perfectly.

I’ve had an issue similar to this before and it turned out to be the carburetor. I replace it 2 years ago with a new edelbrock 1406 and it has run great ever since. If you remember, we did a ton of tests to narrow it down. Checked compression, made sure there were no vacuum leaks, and I replaced the fuel filter, fuel regulator, and hoses. Along the way we solved an issue with the alternator as well (pulsing lights) and I went to a 3g setup that’s been awesome and all voltages are good/normal. Also the fuel pressure is steady between 4.5-5psi on the gauge.

The main reason I think this current issue could be carb related is besides the symptoms being similar, the truck sat for about 6 months. During that time I tried to start it a lot without success (bad distributor pickup coil) and gas probably flooded and sat in carb pretty often. That being said all this stuff is as done almost exactly 2 years ago and my buddy thinks the carb shouldn’t be an issue.

Here’s a list of some
other things I’ve done in last 2 years:

New distributor
Cap/rotor/plugs wires
All new big grounds
3G alt setup
starter solenoid
new icm
new ignition coil and plug
new brake booster
new master cylinder
all new vacuum hoses
And lot of other things probably unrelated.

 

I've posted many times about the simple procedure of turning in your idle mix screws gently in to clear any blockage which may have accumulated in the orafices. Count the number of turns to close the jet and open the same number of turns or to the spec number of turns.

I do this any time I have an idle or start issue and especially if there's a chance of fuel system contamination or if the vehicle has been sitting for more than 5 or 6 months. Might not be your issue but it's quick and easy and costs nothing. Do not overtighten the idle mix screws as you may damage the needle valve and the jet. Light thumb and finger snug is good. You just want to clear any blockage.

 

The last thing you did was replace the distributor, right? And these are all new symptoms since then? Makes me wonder if your new distributor is defective.

Three possibilities come to mind:

1) The vacuum advance actuator has a leak. For troubleshooting, you can safely disconnect and plug the line to the distributor, same as when setting the timing. See if the engine behaves.

2) The centrifugal advance mechanism is binding or the return springs are weak or broken. To test, turn the rotor by hand. It should turn maybe 30 degrees and then snap back consistently when released. If good, reinstall the cap. Leave the vacuum advance line disconnected and plugged, as before. Connect your timing light and start the engine. Slowly Increase RPM and the timing should advance smoothly. Return to idle and the timing should consistently return to where it started.

3) The new pickup coil is intermittent. I think your previous thread had details how to test it with an ohmmeter.

 

Your carb may need a kit if you have let ethanol fuel sit in it for 2 years. You can try what BigBlue suggested. You can also spray some carb cleaner around the throat, I know the holleys have small idle air bleeds around the top of the throat that can get plugged and gummed up. Not sure where the edelbrock's air bleeds are located.

 

Hasn't been sitting for 2 years... i did that work 2 years ago and it ran great up until this past spring/summer when the distributor pickup coil died. I'm gonna run the tests mentioned above but I'm very confident that the issue is not the distributor. I actually did have it running with the pickup coil replaced in the old distributor but the timing was off and running really inconsistently and stalling like it is now. It's starting and running much better now with new distributor and timing dialed in but the intermittent revving and stalling is still an issue. I still think it's a fuel/carb issue.

 

Ah, more little clues as we peel back the layers of the onion.

Another possibility is a problem with the supply to the vacuum advance actuator. Typically a special ported connecttion on the carb is used, located such that vacuum is shut off with the throttle plates closed at idle.

If the base timing and carb isn't adjusted in proper sequence, you can end up with vacuum advance active at idle, or right on the line. Same thing if manifold vacuum is connected, not ported. Ignition advance (centrifugal and/or vacuum) has a HUGE effect on idle speed. Because the behavior is so erratic, ignition timing sure seems like a prime suspect.

Definitely try the tests I suggested, if only to humor me. I highly suspect you will find the engine behaves consistently once you disconnect and plug the vacuum advance supply line. If so, the next step will be to determine if the distributor itself is at fault, or if the supply is the problem.

 

Just noticed another possible clue in this picture, that might help explain the random nature of the problem. You're running an open air filter, with no heat stove to help prevent ice accumulation in the carb throat. 30-50F is prime weather for icing. The last time the engine behaved, was the weather outside that temperature range?

 

Saturday and it was above 50

 

Thanks for giving him permission to try what BigBlue said. That simple trick has solved more idle and start issues than Carters got pills. Try it...you like it!

 

Today I did as suggested with the idle screws and some carb cleaner. The revving seems to be not as bad but leads me to my next issue.

The sticker under the hood for my 460 says it should idle in N at 800 rpm and 650 in drive. However my truck is idling at 1000 and it drops down below 600 in drive. I spent an hour adjusting the idle screw and mixture screws and can’t get it to stop dropping so much RPM. When it idles down below 600 I’m worried it will stall.

 

I believe you have a L and a H idle mix screw on your carburetor. Try opening the low idle mix slightly or raise the idle speed till you meet 650 in park. Then in neutral try closing the high idle screw down slightly. If you can't get the N idle down towards 800 then lower the idle speed a bit. Then go back and check rpm in park. You might open the L mix slightly. In other words use an iterative process to hone in the best you can.

Also the clearing of the jets by turning in the screws and by using carb cleaner may require that the process be repeated as there may be another blockage occur with the closing and opening and cleaners dislodging another bit of dirt or grime causing another blockage. So one and done might not be enough but often is unless the carb is truly ready for a rebuild and interior cleaning.

The engine should be warmed to operating temperature to fine tune the idle speed. Also does your carburetor have a step down idle position when the vehicle warms up?

 

I would also suggest reviewing a service manual for your engine and carb. Follow the procedure in the service manual for fine tuning. The above post is based on general carburetor adjustments and is not specific to your carb. That's why I recommend you follow your service manual.

 

Your carburetor may have two idle speed screws. If it does follow the service manual for the initial idle adjustments.

 

If you are still running the edelbrock carb, there are no low or high mixture screws. There is one idle mixture screw for the left side of the carb, and one for the right. You turn them in and out several times till you get the highest idle speed.

With the extreme drop in idle speed, one thing to check is the vacuum line to the distributor. When it's fully warmed up and idling at about 1000 rpm, pull the vacuum off the distributor and put your finger over the hose. If the rpm drops and there is a slight suction on your finger, the hose is plugged into the wrong port on the carb, or you have the idle set too high and it is starting to put vacuum on the ported vacuum port.

If you want a steady strong idle when it's put in and out of gear with a automatic trans, I recommend using ported vacuum for the distributor. When it's setup properly, you will have no vacuum on the distributor at warm idle.

 

Now I think we’re getting somewhere. I just went out to check the carb and distributor vacuum hoses and noticed some odd stuff. I did replace the carb 2 years ago but it’s the same carb (edelbrock 1406) that was on it before. Therefore I installed it exactly how the previous owner did.

Above is a diagram of the carb. Note the timed vacuum port and the manifold vacuum port.




Above is 2 pictures showing the timed vacuum port plugged (RED) and the manifold vacuum hose (YELLOW) running to a metal port in the back passenger side of the engine near firewall down to the transmission.

Next, above you will see the vacuum port on the distributor runs to a port on the intake manifold (ORANGE).

Lastly, you see an unplugged metal port (PINK) that goes into the intake manifold on the passenger side. Just under the electric choke of the carb. I can’t believe I’ve never noticed this before!!

Now I’m not very knowledgeable with this stuff but this doesn’t seem right at all. The only thing that does seem right is that PVC port which does go to the PVC valve on passenger side valve cover. I think I’ve got some mixed and matched vacuum hoses! Thoughts?!!