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With warmer weather I’ve been driving the truck more, and noticed an engine surge at idle and at constant speed. The surge is mild. From a cold start, it is quite noticeable. At operating temperature, it subsides at idle. At constant speed, any speed, you can feel it but barely. This is new behavior. No recent changes or maintenance to air/fuel/exhaust.
I inspected the fuel line pre and post fuel pump. Inspected fuel pump. No leaks I can see or smell.
I plan to check and monitor fuel pressure between the fuel pump and carb. It’s a relatively new pump that was pushing 6 lbs when I put it in last year.
I’m on manifold vac right now. Timing set 9° without advance. Looks to be about 37° with total advance.
When I went out this morning to check timing, I also had the pleasure of realizing the distributor is no longer moving freely. Maybe it just sat a little bit over the winter. Hoping a little Kroil will do the trick.
Well. Ported vac setup appears to have cleared up the surge. Not sure why it would make a difference at speed since ported and manifold would be the same. Makes me think I had a vacuum leak on the vac advance.
I also managed to break the distributor free with Kroil, a few days, and a mallet.
I've never been lucky with manifold source. Seemed to work for a lot of GM cars. I added a little cam to a Pontiac once with an automatic and AC, took days of adjustments and driving to get it acceptable. Maybe the reason they had some many mechanical advance only distributors in the day. Ford ported don't care till you open the throttle.
Follow up question. I want to make sure I understand why ported vacuum seemed to work. So I put my setup back to manifold vacuum. I verified base timing at 8 degrees. And total timing at 55 degrees at 2500 rpm. The surge came back. Not bad but there. And a rougher idle. 21 lbs of pressure.
Then I put it back to ported vacuum. Base timing of 8 degrees. But total timing is only 30 degrees. Surge not present. Silky idle. Drives fine - but haven’t goosed it yet.
I was expecting total timing to still be at 55° at 2500 RPM. Can someone explain the difference in total timing? Expected? Or is this problematic (too low on ported setup).
Feels problematic. Note that I have the original emmisions set up. Ported vacuum also ties into the emmisions routings, temp switch, and air cleaner. And now the vac advance. I wonder if I’m just not getting sufficient vacuum to the vac advance….
At cruise RPM there shouldn't be a lot of difference between manifold and ported. I suspect the 30 you're getting is your mechanical and you're getting little or no ported vacuum. you either have the wrong port or as you say your emissions junk is blocking it.
55 might be a bit much, it may require blocking it a bit if it still surges on ported. some engines like 60, but 50 seems to be where I end up.
This picture shows the ported vacuum line on the side of the carburetor. Which supports all my emissions gear. I tied into this line with a vac fitting, which is currently plugged in this picture. When I set vacuum advance up for ported vacuum, I tie the vac advance to this fitting and plug the manifold vac.
I will measure ported vac at 2500 rpm tomorrow — my guess is that I am nowhere close to manifold vacuum resulting in less timing advance.
I’ll also study the vacuum diagram below. Pretty sure it has the vacuum advance pulling from manifold vacuum.
Quick update. I spent an little bit of time, studying the vacuum routing diagram and my current set up. While doing so I found a cracked vacuum hose connector on the ported vacuum switch atop the thermostat. The related line ties into the carb S port.
I reconfigured the vacuum advance to run off of the S port on a carb (ported). Reset face timing to about 8 or 9°. Total timing is now around 55° as it should be.
The cracked vacuum connector may have been the problem from the beginning. I will get it out on the road and test it little bit later today.
