rabbit

I am gonna rejet it tommorrow and give that a try. It still has everything exactly like it was before with the exception of the heads and cam. So I am also guessing the jets as well. Also has never had air cleaners because it has velocity stacks on it.

OneBadGMC

For giggles, before you change anything else...

I'd take a piece of cardboard, make a 1/2 circle out of it, and duct tape it to the front of the first velocity stack.

If you went to a wilder cam, you'll have less part throttle vacuum.

Less part throttle vacuum means that it's more prone to turbulent air going over the bleeds and disrupting the bleed air balance.

It could be something as simple as needing to change from the stacks over to air cleaners... or it could be the jets. But making a deflector for the stacks out of cardboard is a heck of a lot easier than changing jets, just as a test.

altune

We found on one car, 66 vette, with a keith black 502 built engine, the surging was coming from, restricted exhaust flow.
he installed dynomax mufflers and all the problems started.
Car accelerates ok, 2500-3000 car seems to surge in power.
removed the mufflers and ran it, problem was no longer there.
Strangely enough.
Just thought you might want to look into it.
more flow more power more exhaust.

olds48

Had that problem once and it ended up being a couple of bad(although brand new)spark plugs.Grab your timing light and see if everything is steady.My next guess would be either vacuum leak/lean condition or LACK OF POWERVALVE.You have a carb(well..two)not an injection system.You need a PV on any street car,And I believe most racecars.There is no way for the main jets to handle every rpm,every change in engine load,every change in manifold vacuum.Remember,the rating on the powervalve is the amount of vaccuum required to hold it closed.When the vacuum drops,from where you step on the throttle,it opens to allow the fuel mixture the richen temporarily to correct the situation.Kinda like a pump shot.Just like Carter cars had stepped needles to handle different throttle positions,Holley's used powervalves.Plugging the back never hurt me on anythinf,but plugging the front won't do ANYTHING but cause you to have to over-jet to add the "lost" fuel.Then your just wasting very expensive gasoline.I never ran a front PV.Always heard they were junk.Got to thinking"then why did Holley even bother?".Started playing around and picked up about 3 mpg.Lots of tunung to be done with those little boogers.Whatever vacuum your engine idles at,get a PV rated 2" below that.Holley reccommends 1-1.5,but with a rowdt cam and uneven idle speed,it will cause the valve to flutter causing rich idle that idle mixture screws won't fix and a wandering up and down idle that nothing will fix.Carbs need powervalves,it not a barrel valve setup.Good luck and let us know something Eric

olds48

Btw,the air/fuel mixture is a ratio.If your moving more air you are naturally moving extra fuel to go along with it.At the same ratio that you were before you recent modsIf your plugs are tan than it's probably not quite lean enough to stumble,especially at one constant speed.Just because you are making more power doesn't mean the your motor is less effeicient and needs a richer A/F mixture to cope.Look at the Vettes making 500+ hp and getting 20-25 mpg+.More power doesn't always mean more jet.Pop the front bowl off and stab in a PV just for the heck of it.Jist my .02c

OneBadGMC

Huh? If you're moving more air, you need more fuel to keep the same a/f ratio. You can't assume that a carb is going to automatically feed more fuel because more air is moving past the venturi.

Besides, the issue isn't at wide open, it's at part throttle. There's going to be LESS air at the same throttle blade position because of the wilder cam/heads, which will cause LESS vacuum on the emulsion tubes, and thus LESS fuel at part throttle.

Just because the plugs are tan when he checked them does not mean it's not lean at part throttle cruising.

He didn't state at what point he checked them. At the point it was surging, did he cut ignition and pull them over to check them, or did he wait until after he idled through his neighborhood and was primarily back on the idle circuit (thus throwing off the reading) to check them???

olds48

If the "wilder "setup moves more air,than the carb will meter the fuel accordingly,but at roughly the same ratio.I'm not saying that he will or will not need to rejet.But more flow does not mean that you need more jet.It is not an injection/throttlebody setup where the air and fuel are introduced to the engine seperately.When the air passes thru the carb it draw the fuel out accordingly.If the engine draws more air ,then it will naturally draw more fuel.He said that wide-open was fine,but surges at cruising speeds,that's why I SUGGESTED that he look into his part throttle tuning.Main jets are for WOT tuning

olds48

Also,one thing some guys get confused on is the throttle controls the amount of air entering an engine.The displacement of any given engine never changes,so the volume of air pulled into the engine is constant for any given rpm.The throttle blade blocks different ports at different positions,and therefore changes the DENSITY,or richness,of the A/F mixture.Throttle plates control the engine speed by changing the density of the A/F mixture,not the amount of air supplied to the engine.

OneBadGMC

I really hope you don't believe this. If you do, you really need to step back and re-think it.

Lets change this a little bit.

You, as a human being, have a given amount of lung volume. We'll equate this to CID.

Unrestricted, you can pull 'x' amount of air (CFM) in per breath in a given amount of time (time = camshaft duration).

If you change this from unrestricted to sucking through a straw, given the same amount of time, you now pull in 'y' CID, which is a greatly reduced volume of AIR.

Density is controlled by weather, such as temperature, humidity, and barometric pressure.

Air volume is controlled by the throttle blades. The motor is unable to inject its maximum CFM based on CID due to throttle blade 'strangulation'.

olds48

I understand what air density is.I also understand that closing the throttle blades restricts airflow thru the carb.I was refering to the density of the A/Fmixture.The richness of the mixture.Engine displacement never changes.Rpm and load does.The faster an engine turns the faster the air will travel thru the venturi and the more fuel will flow thru the main jet,making the mixture richer(denser)and allowing increased cylinder pressure and therefore power output.Engine speed is determined by throttle position,engine displacement remains constant.