Its hard to say but it does feel like it’s surging under throttle until around 2k rpm. But again it’s not easy to feel. No, the fuel pressure is steady around 5 psi. When I went through this before I was sure to dial in the fuel pressure in to what these edelbrock carbs want. I’ll double check but I have not noticed the gauge move when it’s surging.

What’s much more noticeable is how low the RPM’s drop when it’s idling in drive or reverse while also having to turn the wheel.

Again all this is completely intermittent and for periods of time the truck runs and idles beautifully. Other times it surges and stalls out repeatedly.

 

(QUOTE)fuel pressure is steady around 5 psi. When I went through this before I was sure to dial in the fuel pressure in to what these edelbrock carbs want. (/QUOTE)
That 5 PSI might be to high for that carb..
It is known that the edelbrock carbs don't like anything 5 PSI and above.

Next time it surges at idle try and shut the fuel going to the carb. It will run for a bit with fuel in the bowl and see if it stops surging.
Dave ----

 

It sounds like the carb is too far open at idle. The clues to that are you are getting suction on the timed port, and the surging. You can get surging when the carb is open too far, and you are using fuel from the idle circuit and the transition fuel circuit. I am betting when you are adjusting the mixture screws, they are not having that much affect on the idle quality? That is another sign the carb is open too far.

Common things that cause the carb to be open too far to have a decent idle rpm are vacuum leaks, and the initial timing is too low. If you are confident you do not have any vacuum leaks, you can tweak the distributor till the idle rpm raises a little bit, and then you can close the carb up some. Once you get the carb closed up and no vacuum on the timed port, everything will come together I believe.

 

P.S. You can pull the vacuum line of the dist and put a screw in it to block it off and try it as an experiment. Like was mentioned, you can run it forever like this, it doesn't hurt a thing except your fuel mileage. Each time you do something you need to re-tune the carb a little bit, but this experiment should get rid of the dramatic rpm drop. You probably should lower the idle rpm a little anyway, the torque convertor starts kicking in as the idle goes up, you don't want to have two feet on the brake pedal at a stop light. And the torque convertor activating will also make the rpm drop more than it should.

 

I was planning to run a vacuum test today to double check it. Right now the timing is right at 10… can or should it go higher?

I wouldn’t say the idle mixture screws don’t have an affect. If I close them down the idle definitely lowers until it stalls out. I do wondering if I need to tighten them down some. Was thinking they could be a bit too rich and flooding it? I thought I did a good job setting them with highest rpm though. Was gonna try again with vacuum gauge.

I need to double check how it behaves with the ported vacuum plugged, but I’m pretty sure that doesn’t help the idle dropping a lot when putting in gear. Didn’t do that test long enough to see if it stops the surging. Will check on both.

 

Since the surging is intermittent and it runs great sometimes you might want to check your fuel pressure while you're having a surging issue. Which type of fuel pump are you using?

 

I'm leaning fuel issue as your problem because of the intermittent nature. I don't see how a vacuum issue or a carburetor openness issue could be intermittent. A fuel pump, fuel filter or fuel pickup screen or possibly a dirty or sticking float valve in the carb. While it's possible for an intermittent miss on a plug or other intermittent ignition problem those are rare. You can check the plugs and look for a cold one or do a compression test to eliminate missing as a contributing cause.

 

With a carb the only way I can see the fuel psi a cause is if it over powers the needle & seat and this is known to happen with edelbrock carbs even with 5 psi that the mechanical pump puts out.
That is why I posted to pinch off the supply to the pump when this happens to see if it stops.

Yes you can bump the timing up to 12 or 14 BTDC.
The only limits on thst is if you get kick back on hot starts, like the battery is low.
The other is pinging or spark knock when driving and the motor is under a load. Both of them you would need to back the timing off just a little.

I am wIth Dave on the vacuum port having vacuum, idle just a little to high.
If me I would look into why the surging first and fix that then the rest should fall into place.
You can't get the idle lower if it's surging same on timing so fix surging first. (Pinch off the pump supply next time it surges)
Dave ----

 

Ok couple of things…

First of all I just got done messing with the truck and today I could barely keep it running. It's stalling more now than it did. For the time I could keep it running I could verify that the fuel psi was steady the entire time at 5 psi. I did try to turn it down some on the regulator but anything much less and it was acting like it was starved. The truck was surging/erratic idle the whole time I had it running today and the needle really didn’t move on the fuel pressure gauge.

Today unplugging the vacuum advance to the distributor caused an immediate stall. I didn’t do that before so I am a bit confused on that. So driving without it connected is not an option. Couldn’t even check the timing today when I did so a couple days ago.

I tried looking spraying around for vacuum issues and couldn’t find any. I messed around with the idle mixture screws and the idle screw some more to no avail. Put them back where I had them. I also put a vacuum gauge on it but the idle was so erratic it was pointless.

I have braided metal lines to pinching them isn’t an option. I also tried to advance the timing by rotating the distributor some but that had no affect on the erratic idle so I put it back where it was.

 

Putting on my Sherlock Holmes hat and lighting my pipe… Are you saying the engine stalled as soon as you disconnected this line, before you could even plug it? I think that’s what you meant, but want to be positive.

With this new info, My thoughts are you’ve got a vacuum leak elsewhere. Adding an additional leak (unplugging the distributor without yet plugging that line) was enough to kill the engine.

Have you tried plugging that line with the engine off, and then starting it?

How are you checking for vacuum leaks?

 

Yeah it stalled before I could plug it. And yes, I tried to then plug it and then start it but it would not idle without my foot on the throttle. Im
not sure what changed because this wasn’t happening this drastically over the weekend. Obviously I was able to check the timing before today…

I was spraying all around with carb cleaner to look for leaks. I went through this before I replaced my last carb and I replaced just about every hose under the hood. I even replaced my brake booster, pulled the valve covers and replaced the gaskets. I couldn’t find a leak today.

The biggest impact on the rpm/idle by far is turning the steering wheel.

I can also verify that the voltage was steady under all circumstances today.

 

I also played this this idea some and took a video. Also I closed the plate thingy (name?) the idle got higher and improved to an extent. When it got close to being closed it got worse. When it returned to where it had been without me manipulating it, it stalled. I’m not positive about other vacuum leaks but I do know the timing is set correctly. I did with a light and rechecked it many times when it was idling well. The timing was right at 10 and even when I tried to advance it some today it did not stop the erratic idle. I can’t completely rule out a vacuum leak but would that cause the idle to surge so much? It bounces by like 400 rpm.

 

You may be in uncharted territory here. You may have more than one active fault, too. You could find and fix one problem but the engine still misbehaves. You're left scratching your head, as you just fixed something. And then you've got all sorts of well-meaning but varying advice, all over the map. Your mission, should you accept, is to sort it all out.

I'd suggest forgetting about the Hail Mary pass, that one thing that will suddenly fix everything if you could just figure it out. Ain't gonna happen at this point. Best you can do is work through some potential causes, eliminating them one by one. Do the easy stuff first, to cut down on your efforts.

Back to the centrifugal advance: Did you ever check that? Details in a previous message. Five minutes with a timing light, tops. Super easy.

Vacuum advance: Disconnect and plug the line. Take the system completely out of the equation for the remainder of troubleshooting. Again, super easy.

Other vacuum leaks: The carb spray method only works with a steady idle speed, so is not applicable right now. I've had good results with a hand-held vacuum pump, pulling a vacuum on the lines connected to the manifold and carb. Make sure the vacuum builds up normally and then holds after you stop pumping. A drop-off indicates a leak. You can pinch off the various branches to isolate the leak(s).

Another option is to disconnect and cap any sources feeding non-essential loads such as HVAC, speed control, etc. See if the engine runs better. You can even plug the supply to the brake booster, but don't drive like this, of course.

I'd also suggest leaving the carb alone for the moment, even if it's not perfect. You want to try things one at a time and look for any changes. Tweaking the carb at the same time might mask any improvement caused by isolating a vacuum leak or other fault.

 

One more thought, along the lines of making only one adjustment at a time. You’d want to set a baseline for the carb while testing/eliminating other possible causes.

We’ve no idea how the carb is presently adjusted. Maybe you could do a basic rough adjustment of the idle mixture, as previously detailed by OOTD (One Of The Daves). Even if not perfect, it should be reasonably close for the engine to run consistently while you test other things. There are plenty of misadjusted carbs running around out there, still performing reasonably well.

 

If you could hook up a timing light with it idling, you would probably see your timing is way advanced. When you unplugged the vacuum advance, the timing went back to base which you said was 10 BTDC. You need to get it running with the line unplugged to the distributor. I am not sure what your problem is, but it looks like the advanced timing at idle is covering it up some and making it run, but barely running.

Are you running the stock intake? If you are, do you still have a EGR plate under the carb? The engine running better with the choke plate partially closed is a sign it's getting too much air somewhere. Those EGR spacer plates are known to be leakers when the exhaust eats out the plate where the gasket goes.