65 degrees where I am today so good time to fiddle with this. I will say that today the surging was more prevalent than it has been. In the video below the vacuum advance line is plugged and you’ll see the rpm’s dance by about 100rpms. In the video the mixture screws are at 2 turns on each side (turned down from before). FYI, these screws do very little to the idle. Also quadruple checked that the timing was at 10 BTDC. I had the vacuum gauge on it just for giggles and you’ll see that all over the place. However, it was steady at about 20 when the truck first started and the choke was on with no surging.

In the video it’s between 800-900 rpm in park and when I put it in gear with the vacuum advance disconnected and plugged the idle only dropped to around 650 which was great to see. There was still a lot of surging and hesitation but it was good to see it not drop 500-600 rpms like before. I could also turn the wheel without it wanting to stall as bad (still dropped rpm though). Not sure if the idle screws made a difference or if it was all a fluke and will be different tomorrow.

Still not very happy with how it sounds or how much the idle fluctuates.

 

Here is another video from inside the cab with the vacuum line disconnected and plugged and mixture screws set at 2 turns each. You’ll see it stall after hitting the throttle once and the surging. But on the good side you’ll see the rpm’s stay closer together from park to rev and it stay more steady when turning the wheel.

 

I agree with you, it sounds like we are trying to cover up another problem. The vacuum should be steady at idle. This is a very long thread and I have forgotten have you ever done a compression test on this engine on all cylinders? That would eliminate any mechanical problems or valve train adjustment problems.

 

Did you ever check the accuracy of the timing marks? If the outer sleeve of the pulley has slipped (not uncommon), the marks will not be accurate not matter how carefully you set the timing.

To check, manually rotate the crankshaft to bring #1 piston to TDC. Use a soda straw or similar (nothing rigid that could break, don't ask how I learned this) to determine when the piston is at the very top of travel. Your timing marks should show 0 degrees. If not, the marks are not accurate.

Not sure inaccurate timing would cause the surging, but it one of those easy little things to rule out before loading an expensive carburetor in the parts catapult.

 

I have not done a compression test. That might be beyond my capability and don’t have the tools for it. Vacuum does hold steady at times. Like I said, today when I first started the truck and the choke was on the vacuum was very steady at just about 20.

 

I actually did do this. Everything lined up fine.

 

Going from 3 turns to 2 would not do much is true because you're going from way way open to way open. But rpm did go down in park. Keep turning them in 1/4 turns at a time alternating with resetting the idle to 650 in R OR D. You are getting there it seems.

 

“Capability”? If you can change spark plugs, you can do a compression test. Nothing complicated at all.

“Don’t have the tools for it”? Go stand in front of your house and wait for me. I’m hopping on the next flight. When I get there, hand me your man card.

Criminy, kids these days. All I need is the flimsiest whim of desire for some obscure tool, and next thing I know it’s in my Amazon cart. My kids are going to have fun one day cleaning out my shop…

A compression gauge is a very basic diagnostic tool, part of any automotive tool kit. If you don’t want to order online, put a paper bag over your head (cut eye holes) and pick up one at your local Harbor Freight. Get the thread-in style, not the kind with a rubber tip. $30 or so. Some auto parts stores even have free loaner tools, so you’re going to have to work harder on your excuse.

Does your engine have a compression problem? Who knows, but a compression test is basic troubleshooting, one more thing to rule out.

 

My bad I had the wrong idea of what a compression test was. Going on vacation next week so I’ll check bad in with you guys in a week or so. My gut still tells me it’s the carb. Seems like a fuel issue is the only thing that really explains the intermittent surging issue outside of a vacuum leak. The fact that every symptom matches what happened to me 2 years ago says a lot. Truck sat for awhile, now there’s a surging and stalling issue.

 

As pointed out do a compression test so we know if the motor is worth putting any more time in on.

I almost want to say there is a vacuum leak some where on that motor.
With the choke on and it holding vacuum steady is why I say this.

Next time it is up to temp, choke off and it is surging try and cover the carb intake with a rag little by little and see if it stops.

I did have an older Holley that had an internal vacuum leak and even rebuilding it did not fix it.
Now when I rebuilt it I matched the gaskets up to the old ones so if someone used the wrong gasket(s) I just followed the same path.
I ended up replacing the carb with a used one off EBay and swapped the choke over as need and it runs great.

After the compression test maybe we need to do a smoke test for vacuum leaks?
Dave ----

ps What intake is bolted to this motor?
Has it even been off the motor before?

 

Time to cancel your vacation. You can't leave us hanging like that. You wouldn't have had that much fun anyway.

Another free bit of troubleshooting is a cylinder balance test. With the engine idling, even if not perfectly, disconnect the plug wires one at a time. Use insulated pliers and wear heavy gloves so you don't get shocked. As you disable a cylinder, the engine should run a little rougher and slower. If no change, that means that cylinder wasn't producing enough power compared to the other seven.

A cylinder balance test is a good precursor to a compression test. If a cylinder is down on power, pay close attention to its compression. Lots of factors can reduce power, not just compression, so look at all possibilities if a balance test doesn't pass.

I know you've got your heart set on a new carb, and that may well be what is needed. Not about to stop you if you'd like to try that expensive gamble. But as cheap as I am, I'd sure like to rule out other possibilities before prying open my wallet.

 

Here's some interesting pictures to study. https://www.autoequipment.com.au/vacuumgauge/vac1.htm That link has a button at the bottom with more gauge readings on a 2nd page.

I hate to send you in too many different directions, but I wonder what would happen if you don't use the timing light, but loosen the dist bolt so you can just turn the dist with some hand pressure, and get it running and turn the dist just a little bit so the idle speed is a little higher. I wonder if that helps it out.

 

That vacuum bounce is pretty rhythmic, then for a second or two it smooths out then back to a drop. Kinda looks like the bad valve bounce shown on the link Franklin posted.

Compression test will sort that out and point to the cylinder. If it's a sticky intake valve maybe it shows up after sitting? A good romp on the freeway might loosen it up?

 

Take carburetor apart and step by step make sure all parts are there and very clean, ethanol gas is quick to go bad and causes all sorts of sticking parts in carbs...carb gaskets shrink & go bad , then set throttle plates & float etc...etc everything on carb make right, needle & seat could have dried gas or dirt........ then take fresh non-ethanol gas in can and run straight off it. Install new spark plugs, check rotor & cap...........double check timing and start truck, then drive it..........have on clean underwear in case your n a accident, and stop for little ole ladies.........Happy New Year