Have you verified the timing marks are accurate? Rotate the crankshaft to put #1 cylinder at TDC. Use a soda straw or similar (nothing breakable) through the spark plug hole to know when the piston is all the way up. Then make sure the timing mark shows TDC.

The rubber sleeve in the crankshaft pulley can slip, and throw the marks way off.

 

Couple of thoughts/questions…

If the truck ran great for 2 years prior to this, with no surging or stalling, how all of the sudden would there be an issue with the timing? Can timing change over time? I’ve verified many times before this issue and during this issue (when it idles normal) with the vacuum advance disconnected and plugged , that the timing is set at 10 BTDC. Less than a week ago this was verified.

 

Not much, maybe just a few degrees as pieces wear. Usually nothing drastic. But your question raises another. Have you checked for a jumped timing chain? That might account for a sudden change in performance.

 

So just to throw it out there, I actually took a video of me counting the turns of the idle mixture screws last week before I changed them. Prior to that I had never touched them since I replaced the carb 2 years ago. It ran great the way I had it. I did mess with the screws but I put them back exactly where I had them and it was Idling good yesterday before my drive. So I can’t imagine that the idle mixture is the issue. Also, the timing was perfect at 10 btdc with vacuum advance disconnected and plugged confirmed with timing light before I drove it yesterday and there was no surging present. Furthermore, when I did replace the distributor a couple weeks ago, I took pictures of where the old one sat and I matched the new one up perfectly which is why it went in very easily. I didn’t really even need to confirm the timing with the light but I did. Everything seemed great and figured it was solved until I tried to drive it.

I still think it’s a dirty carb. Gonna try spraying it out again and see if that gets it running normal for a short period of time again.

 

No that’s passed my realm of knowledge. I’m learning as I go… if you can supply some instruction or a video link I’d be glad to try to check.

 

It's easy to check for a jumped timing chain. The chain would have been very loose, so that's what you're actually testing.

Remove the distributor cap. Use a socket and breaker bar to turn the crankshaft a few degrees in the normal direction of rotation. This takes up any slack present. Note where the rotor is poining. Now turn the crankshaft backwards a few degrees. The rotor should turn almost immediately. If not, that means the chain is loose and could have jumped.

 

You're welcome to clean or replace the carb. In my over-inflated opinion, though, it's way down on the list of likely culprits. There's no conclusive method to confirm if bad, though. Mostly you rule out other causes until the carb is all that's left, and I don't think you're remotely close to that point.

 

Coach, try searching on line for

"what causes surging at idle with a carburetor"

Several checklists came up when I searched it. Most of the causes have been discussed here but I saw a few that haven't been. It is a more common issue than I thought before. I had a surging idle once on my Chevy 350 and replacing the fuel pump solved it. But check out some of the many articles that come up.

 

That is the first I have seen EGR pop up in this post.
Pull the hose and plug it and see what happens.
You may want to pull the EGR plate off and check it for a "burn through" and make a plat to block it off b4 installing it again.

As for the chain I have not seen one jump and still run but do check for play.
Dave ----

 

My EGR is not connected and hasn’t been since I got the truck. There is no vacuum on the port and I sprayed all around it yesterday (which I’ve done before as well) and really doesn’t seem to have a leak anywhere around it. I’ve plugged the port but makes no difference. Pretty confident it’s blocked off but I’ll verify. As far as I know the intake is stock and I assume the egr is blocked off. There’s no leaks around the base of the carb either.

Also, the PO replaced the fuel pump at some point and seems like an unlikely issue since the fuel pressure at the regulator is steady? Pretty sure it’s electric and in the tank.

 

Ok, so today before trying to start it up I removed the idle mixture screws and sprayed some carb cleaner down there and replaced them where they were. Also sprayed it all over the inside of carb and every nook.

After that I started it and I could at least keep it running today. Even when pulling and plugging the vacuum advance, which I couldn’t do yesterday (immediate stall). I took a video linked below. It shows the timing at 10 BTDC (black line is 10 BTDC, white line is 0) and had a vacuum gauge connected for good measure. You’ll see the difference in idle when I plug the vacuum advance back in. I tried messing with the idle screw and mixture screws once again and didn’t do anything to the surging or how hard the vacuum advance was sucking air. I will also say, the idle mixture screws don’t do much of anything. The timing actually was a bit advanced at first but I think that was from fiddling with it yesterday. Again, the fuel pressure is steady at 5 psi as well. Didn’t get that in the video.

 

Ok no vacuum to the EGR but that dose not mean it still be a cause.
If the surging keeps up I would pull the plate and check it over to make sure it has not burned inside where you cant see it or the spray will show anything.

Electric in tank pump?
Here I and maybe others thought it was motor driven.
You say the psi gauge is not moving, steady at 5 psi lower it to 3 psi to make sure it is not over powering the carbs needle & seat.
Again Edel. carbs are known to have this issue. If you dont want to lower it to 3 psi when surging happens again pull the power to the pump.
You can do this by pulling the relay.

If the 3 psi dose not keep the motor running there has to be other issues as it should run for a bit till the gas in the bowl is used up.
Same goes for pulling power, it should stay running with the fuel in the bowl.
We need to find the cause of the surge and I dont think its a carb issue as it has happened with 2 different carbs but the pump, regulator, EGR are all the same.

If you feel there are too many people helping, I do know this can be a issue, I will bow out and let DaveF or Karl step up.
BTW No hard feeling if you want me to.
Dave ----

 

Nah, you should stick around. I bet you’re hot on the trail. I’m ready to bow out myself. I’ve said my peace…

Something to consider: If the engine is still revving up, not many things can cause that. Plenty of things can drop the RPM, but not too many can increase it. That might trim down the list of potential culprits.

 

Well here’s another question… how do I even know that the engine is revving up? What if it’s the rpm’s dropping and then coming back to normal? That’s what it sounded and felt like today. Maybe it’s not surging?

 

I don’t want anyone bowing out. It is a bit difficult to keep each back and forth straight. I’m trying to do everything that’s asked of me and forgive me if I missed anything.

It’s definitely not a stock motor drive fuel pump. I discovered this as well 2 years ago when you had me chasing the same problem. There’s no fuel pump anywhere under the hood. I traced the fuel lines all the way back to the tank and that’s the only place it could be.