Surging at light cruise

[chilly460]

Ran the Merc up to a car show in PA on Friday, 210mi run.  As I was driving, noticed it started surging at light cruise, such as following someone and just maintaining 45mph on secondary roads, anywhere from 1500-2500rpm.  Any bit of throttle and it would clean up.  Idles perfectly. 

I’m guessing carb is dirty and I’ll pull it apart and clean it, but just thinking through it.  Wouldn’t think it’s the idle feed as it idles fine. I’m thinking it’s something with the power valve as it happens at transition and cleans up with any drop of vacuum.  Main jet, I don’t think so as it was perfectly clean any time I was at light throttle like running 60mph and up.  I’ll also check for a vacuum leak.  Any other ideas?

Thought about timing as well but the on/off of the surge was immediate, no change in rpm at all was needed, just squeezing the throttle, so doesn’t seem like spark scatter

[allrightmike]

     Surging at low power demand followed by a cleanup with a bit more throttle opening usually indicates a lean condition for whatever reason. My guesses are either a fuel restriction in the transition circuit or possibly the rear barrels not closing completely. I can tell from your post that you know carburetors and will figure it out.

[Drew Pojedinec]

Small vac leak?
Timing dropping out at that rpm?
At that rpm you probably aren’t fully on the mains so some idle fuel is in play.

Rule out stuff and narrow it down

[chilly460]

Drew, it did it anywhere from 1500-2500 so don’t think it’s timing related. 

[galaxiex]

Probably not your issue... but...

My 68 Mustang 390 with a CJ 735 cfm carb would surge at 60-70 mph light throttle cruise.
Tip in throttle and surge was gone.
Ran perfectly in all other aspects.

Had the oval open element air cleaner and a 68 Shelby hood with open scoops.

If I pulled the (converted to) manual choke out "ever so slightly" the surge would clear up.
Choke NOT pulled enough to make it richer. "Just barely" moved the choke blade less than 1/16".

My theory...

The open air cleaner and hood scoop disturbed the air flow into the carb.
Pulling the choke such a tiny amount changed the airflow thru the boosters such that the surge went away.

I meant to fashion a shield in front of the air cleaner to block air from the scoops to see if that helped.
Sadly I sold the car before ever trying this...

[hbstang]

check your intake manifold bolts to make sure they are tight,i had 2 engines that were built 10 or more years ago and one with alum heads and one with iron heads,were loose and most were finger tight.replace intake gaskets on both and they were perfect,vacuum back to normal.both were aluminum intakes as well.also check for bad vacuum hoses as well.and if a vacuum advance distributor, make sure plate moves freely.

[My427stang]

I'd clean the high speed air bleeds, if one is blocked it could pull over early, although seems unlikely that significant and repeatable

I'd unhook the vacuum advance and try it

I'd block the PCV and try it. 

I'd probably even look real close at all the wires, maybe one starting to fail.

Look to see if a spring broke in the mechanical advance

Do each one at a time

[Stangman1]

I have the same thing I’ve been slacking but I’m gonna richen up to idle bleed by making them smaller. I recently went down on jets and now have this problem. When it was jetted higher it was fine. No hesitations on tip in but at steady 22-2500 like little surges.

[chilly460]

This is a situation where it was fine and has popped up in a long drive, so I'm guessing it's a blocked feed.  Was messing around setting up garage equipment this weekend so I didn't mess with it, will dig in this week. 

[Mark Bliss]

I am finding pump gas is even more "iffy" than usual lately. Even my modern EFI vehicle seems slightly unhappy. Especially in hot weather.
I would agree with others that the described symptom sounds like a slightly lean condition. And giving it a little throttle probably opens the power valve enrichening the mixture.
Traditional experience is that the easy fix is to add a little primary main jet. But honestly, I always felt that was often a bit of a crutch, only covering up the problem.
If the carb has a modern or upgraded emulsion tube circuit, you might try raising the primary float level three flats and re-evaluate. This usually shifts the fuel curve slightly richer and can cure a mildly lean curve.
Just taking a stab.

[Drew Pojedinec]

Mark, gasoline 100% has been more troublesome since April.
For me at least it is iffy right off and when a week old it will begin to act leaner due to burn characteristics.

[chilly460]

This was happening on a 500mi round trip, there were several stops for gas involved so I doubt it was an issue with fuel. 

Still haven't had time to mess with it, got the 352 on the run stand and started last night so that's taken priority.

[6667fan]

Sounds like a potential transition from off idle to mains being in fully. Surging often indicates lean condition.  You could experiment with mixture screws. Back ‘em out 1/4 turn at a time. Even if this compromises a lean quality idle it could confirm you are lean at cruise.
If confirmed then you could re-set screws and take air away via idle air bleeds, ( if carb is so equipped). Also experiment with taking air away from HSAB,( again, if carb is equipped with them).
I have found that from engine to engine, the time that the HSABs become impactful varies. At the rpm you are having the problem it could be either.