What causes a surge when you let off the throttle? (1 Viewer)

[fyton2v]

New to me problem, but I was driving in some new conditions over the weekend. Doing 60 + up a long incline (many miles on a decently slopped grade... 3 or 4%) and my engine did something odd. Going up this pass at highway speeds required a lot of throttle ... a lot of throttle for around 40 minutes. It wasn't floored but the secondary was open pretty much the whole way. Running at a normal operating temp, engine felt smooth and normal. When I was on the throttle, all was fine. But the moment I let off a bit, the engine felt like it would surge for a moment. More throttle... fine. Let off, and an immediate surge just before the engine behaved like it normally would at around 3000ish RPMs without throttle. I don't have a tach so it's kinda hard for me to know what the engine was really doing, but it sorta felt like the RPMs jumped a bit. It lasted for at most a second and I could reproduce it after more uphill and persistent throttle. So I've been searching and thinking about what would cause that and I'm just left with some not very productive guesses.

In normal freeway conditions the truck doesn't do this, even when going up fairly steep grades. But I normally wouldn't be climbing for that many miles.

Anyhow, here are some details:
stock 2F with Aisin carb
Vacuum at idle is pretty steady at about 17.5
No backfiring or pinging (anymore) during normal operation.
Timing is set to 7 degrees before TDC.
Fuel pump and fuel filter are new.

Any guesses? After that momentary surge feeling, it runs as if it never happened, until I let off the throttle again to slow down a bit.

 

[Pighead]

What year carb? (Is it a vacuum operated secondary?)
How old is your clutch?

 

[fyton2v]

Stock carb on a federal 2F from 78. Vacuum secondary.... maybe? Must be. I rebuilt it not too long ago following Pinhead's YouTube videos and measurements. The clutch doesn't have too many miles on it ... maybe a couple of thousand and it seems to be fine. No issues with it all the way back down this grade a couple of hours later.

So, if it is a brief surge, how does extra fuel and air all of a sudden make it's way into the cylinders when you let off the throttle? And while going up a hill it's not like the truck could lurch forward at those speeds. I haven't installed the nitrous system yet. It hard to say now but it didn't exactly feel like I smashed the pedal to the floor. So perhaps it was a momentary clutch slip. You'd think that would be more apparent going downhill. This one is confusing.

Edit: Definitely a vacuum actuated secondary. Not sure how I forgot that as a bad diaphragm in there was the main reason why I needed to do the rebuild in the first place.

 

[Pighead]

I've never owned a vacuum operated secondary and have no idea how it works or where it gets a signal from. I do know that my manifold vacuum increases when i let off the throttle. Wondering (wag) if your secondary is responding to a spike in vacuum...

 

[middlecalf]

Yeah, seems like a vacuum spike, and rapid advance. However, any pinging?

 

[fyton2v]

You see, that's where my head was going, but I'm missing the system knowledge to understand how/why. I think I'm going to rewatch Pinhead's videos for more clues.

No pinging. I was having plenty of advancing and pinging issues the other day. With the help of people here I learned that a sleeve on the stop of the dizzy's mechanical advance is supposed to exist, but it disintegrates with age. I replaced that sleeve with an aluminum spacer that must be approx the same dimensions and no more pinging. Man, it runs better with that tiny spacer.

 

[Skreddy]

When tuning my street bikes, I’ll do a long pull and let off. A surge usually indicates a lean condition. Usually.

 

[fyton2v]

So, lean at speed means my power valve or secondary jet could be too small?

 

[Engineer8000]

Is your truck de-smogged? If so, ignore this.

Looking through the '78 emissions manual I see mention of the throttle positioner system that opens the throttle valve slightly more than at idle when decelerating to reduce HC and CO emissions. It could be a mechanical adjustment issue with this or the emissions computer or speed sensor input to the computer. The manual has a pretty good check out procedure.

The ABV for the air injection is supposed to divert the air pump to atmosphere on deceleration, could be something there also but I doubt it.

 

[fyton2v]

Is your truck de-smogged? If so, ignore this.

Looking through the '78 emissions manual I see mention of the throttle positioner system that opens the throttle valve slightly more than at idle when decelerating to reduce HC and CO emissions. It could be a mechanical adjustment issue with this or the emissions computer or speed sensor input to the computer. The manual has a pretty good check out procedure.

The ABV for the air injection is supposed to divert the air pump to atmosphere on deceleration, could be something there also but I doubt it.

Hey. Thank you for that info because you just connected two disconnected brain cells. I'll have to review my high idle and TP adjustments. This is something I recently did after doing a bunch of work on the engine and there is a chance I goofed. My choke has been finicky since then and I just put that into the "I'll check that out later" bucket. Also, I now recall a similar a similar surge at lower speeds and lower RPMs. Not sure why I didn't connect those dots. So I do think my TP isn't adjusted correctly. Not sure it would cause the same surge at higher RPMs, but it does give me something to monkey with versus just wondering. Thanks for this!

 

[numby]

The way the TP works is by holding the throttle open until your speed drops below a certain mph. Can't remember right now but I'm thinking 17mph. Or 22 maybe? Do you tend to brake hard and fast or coast to stops? Fast braking seems to make the TP response more noticeable as you decelerate quickly down the mph range and the throttle holds open until your speed drops below the threshold. Tire diameter will affect when it releases as it's designed for OEM tire sizes. Worst case is a fawked emissions computer. Check the rpm adjustment for the TP which is somewhere in the 1200-1600 range. It's not a critical setting. It's just there to prevent afterfire from the leaned-out '78 emissions set up. If you've got no issues coasting down the RPM range with your foot off the pedal then you can pretty much eliminate the vacuum lines through the VSV. I think Jim C's desmog instructions turn th TP into a simple choke pull off.

 

[Engineer8000]

TP VSV should turn off around 14 MPH. My experience testing these computers tells me they are not that accurate WRT vehicle speed (or RPM for later years). I would apply at least +/- 20% tolerance band for these, especially on a computer that has not been cal checked lately.

The book states the engine should run at 1200 rpm for federal with the TP set.

 

[fyton2v]

I started digging into it a bit today. Thought I would just revisit and baseline idle mix, TP and fast idle settings. I pulled off the air cleaner cover and monkeyed a bit with the set screws while it was warming up. It stalled out, which was odd because I didn't change the idle or idle mix enough. Turned the idle up a bit and I could get it to run, but when turning the idle back down, RPMs were dropping too fast. It was odd. I thought maybe my idle mix was way off so I started doing a lean drop and the mix screw wasn't affecting RPMs or vacuum. More odd. So I did what anyone would do and looked down the carb horn, as if somehow that would show me what the hell was going on. And I did see a decent amount of fuel dibbling out of my secondary venturi at idle. I wasn't expecting to see that. It also looks like my bowl is flooded. Not sure how/when this happened. I'm unsure this caused the surge, but it's something I'll need to fix first before I do more testing. I don't think my secondary linkage is stuck. Maybe it's gunk. My hard fuel lines are as old the truck, but the filter is new and the carb was recently rebuilt. I blew out the lines not long ago and they let air move freely, but who knows. There could have been some crap in there.

 

[Engineer8000]

Integrated circuits and fuel injectors make this easy and dependable.

 

[Pighead]

Integrated circuits and fuel injectors make this easy and dependable.

Until they don't

 

[Engineer8000]

They "don't" a lot less than points and carburetors.

 

[fyton2v]

I’m having fun in an analog world and I’m going to stick with the old stuff for a bit. Eventually, I’ll get bored and replace it with EFI.

 

[fyton2v]

Took off the horn of the Aisan carb, disassembled it and gave it a couple doses of carb cleaner and compressed air. Also checked to make sure the float clearance was correct. Also did some cleaning and airing of the carb portion left behind on the manifold. I noticed that my secondary venturi was a bit loose. Not sure why, but probably the fault of the dummy that did the rebuild. Also replaced the blinker fluid while I was in the area. For what it’s worth, previously my TP was set too high. Disconnecting vacuum caused the idle to immediately jump. Now it comes on sorta nice and easy.

Anywho… seems to be running fine, now. The secondary is behaving. Will take it on a bit of a drive tomorrow and see how she does.

Thanks again.