Light throttle stumble, Holley carb.

Hey folks, I've been having a light throttle stumble issue on my Holley Street Avenger 670. It happens at pretty much any speed, but only at really light throttle. Unfortunately, my engine only requires light throttle to keep the car up to speed, so it can get annoying. I have tinkered with the carb just a bit by increasing the primary jets to 72's and the secondaries to 70's. I pull it apart for a good cleaning every spring, and the problem has persisted. Other than this stumble, the car pulls pretty hard and the plugs look to be just about spot-on. I'm borderline too small on the carb, I feel. I went with the 670 to keep the throttle response crisp, and I shift it at 6000rpm due to the production bottom end pieces (did use ARP bolts in the bottom end, though). The tech at Holley confirmed my selection, for whatever that's worth.

Background on the car:
Home built 350 SBC, full roller (Vortec block, crank and rods, Keith Black 10.8:1 pistons)
Comp XR294HR cam http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=191&sb=2
Edelbrock Performer RPM heads, exhaust ports touched up, upgraded comp springs, valve guided relieved (stuck an exhaust valve a couple years ago, my machine shop said the edelbrocks tend to run tight), scorpion 1.5 rockers
Performer RPM intake
****** Super Comp headers
2.5" exhaust, dumped after mufflers
Full MSD ignition (6al box, blaster 2 coil, pro-billet distributor i tuned with light mechanical advance springs and a delco low-vacuum advance can, to compensate for the 9" of idle vacuum, 12* timing initial with the vacuum disconnected, 36* all in by 3000 rpm)
TH350 trans, 2500 stall, shift kit
55 rear housing, 57 center section, Yukon 3.55's and an Eaton posi (already blew one center section apart, it's on the list to upgrade to something stronger).

My initial thoughts are I need a lower # power valve and to try changing the accelerator pump cam. This is the first carbureted car i've really tuned by myself. I've owned other carbed cars, but they were stockish for the most part.
Thanks in advance for any help!!

 

Run a va***m check at idle, divide the va***m by 2 and that is the size power valve you want.

I haven't had a lot of luck with the Street Avenger carb. That doesn't mean that yours is bad but most of the time when I have had to fiddle with one it is on someone elses car and they just haven't wanted to work correctly. I have had pretty good fortune with a 600 CFM vac secondary holley on most mildly built street applications.

 

I have the same carb on my 36'. I had to play with the accelerator pump cams to get it to quit doing the same thing. Buy the set and change them until you find the one that works for you.

 

thanks PNB.
i did a check before and got 9" from the small port next to the PCV port on the base of the carb. no leaks that i can tell and my power brakes work well enough, which i find a bit odd. i would think they would either be really firm, or maybe i didn't do the vacuum check right and it's making more than i thought. i've been weighing a decision to go with a 700 or 750dp. you think that would be overkill?

 

it came with a couple. do you remember which color cam you used?

 

I am checking in to keep track of this. I am having similar issues with the 2G on my 283.
I will ***ume the fixes would be the same?

 

I want to say I was running a pink but can't say for sure. I can check tonight.

 

Try bumping up base timing and see what it does

 

if it's no bother.

 

it happens less, but it still happens. i was running 16* base, but the gas around here lately wasn't agreeing with it, so i backed it off. it idles better at 12* if that means anything.

 

Bigger shooters

 

I had a close problem with some different carbs I ran.

I had a 600 vac sec that ran fine with a 4.5 pv, then built a 700 dp with the stock 6.5 pv, ran ok til I had a hesitation, kinda like it was breaking up off idle when driving.

Not a problem with the squirter. Then swapped a 750 vac secondary same 6.5 pv, and it did the same thing, took the 4.5 pv out of the 600 and put it in the 750 and no problems

 

would the shooters being too small show on the plugs?

 

i'm definitely going to swap power valves.
what i get is just a quick cut-out, like i snapped the throttle shut, then it come back. all happens in less than a second.
i've also been told i may be exposing the transfer slot and pulling fuel from the primaries at idle. it doesn't smell TOO bad, but it is a little rich.
plugs are black on the base, but tan on the porcelain and electrodes.

 

OK here is a dumb question for you. How long have you been running the carb?

I was reading some info from Holley the other day about the ethanol in our fuel. They were saying that they had cured the problem with accelerator pumps and alcohol but going to the alcohol pumps on thr newer carbs but that with current technology they couldn't cure the problem with degradation to power valves. It could be that if your carb is old enough that the rubber in the power valve is degraded enough that it no longer functions properly.

Just a thought.

The other thing I am thinking is that you are showing pretty low va***m numbers. My 400+ HP 355 makes 14 inches at idle.

 

First I am very surprised the tech at Holley thought that was enough carb, even their own calculator would recommend a 750, probably vacuum secondary like a 3310, also known as a 4160 series. 750 CFM, Dual feed, w/ vacuum secondary. The cam you have with 294 Advertised duration and I think it said 244 @ .050", the stock power valve if its a 65 will be way rich at idle. I'm in between welding for someone, but i think you said you pulled 9 vacuum, divided by 2 that's a 45 or 4.5 P/V.

 

you're a smart cookie!! this is a replacement carb from holley, because they warranted my first 670. my particular trouble was a defect in the metering plate casting process, that would basically begin oxidizing the plates as soon as fuel touched them. they did the right thing by me.

the cam i'm running is decent sized and i'd imagine i'm somewhere north of 400hp myself. i patterned this engine off of this guy: http://www.edelbrock.com/automotive_new/mc/crate_engines/performer_rpm_410.shtml but with more compression, more cam and cleaned up heads. the 670 holley probably makes more power than a 750 edelbrock (damn sure feels like it having tried an edelbrock 750 on it). what do you think? i've never dynoed it and probably never will.

 

i can only imagine it's because i said it won't see anything over 6000rpm (where i shift it) and it's a street car. i'm going to try a 750vs a buddy has and see how it likes it.

 

I'm the wrong guy to ask about Edelbrock carbs. I switched from a 600 Holley to a 750 edelbrock carb on an old 390 Ford I had and never did see the performance from the eddy I got from the Holley.

You may just look into the power valve for sure. If you are only pulling 9" then you want a 4.5 for sure.

My thought on builds is this you and I can build the exact same engine from identical pieces and have the same dyno guy tune them both and won't come up with the same numbers. I don't understand why it works that way but it does.

 

Yes sir, I understand, just giving my 2 cents from 30 years of dyno tuning. I sincerely hope you get things worked out. You do know that Pork-n-****** is going to send you a bill right????????????????????

 

A 600 vac sec holley will feed that thing if jetted properly. I'd check the acc pump and clearance between pump arm and screw. Lippy

 

i've seen that happen. some engines are just spiteful!!

 

Put it on my tab. hahahahahahaha

 

Hey I'm fresh out of bills. I may send him an IOU.

I had an old Ford once that I could have figured out what was wrong with it if I just could have gotten the rind of it.

 

it's unchanged from the factory, but i would imagine closing the gap up slightly would be the way to go?

 

i'll have to send you an iou on your iou. lmfao

 

Ditto! I've drilled those out .001-.002" bigger to eliminate that problem... It works!

 

Well, gotta jump in here. Cam with 240-250 at .o50 in a 350c.i. engine. Your vac seems close to me. Problem is a street avenger isn't designed to work with a cam that big in a 350. The idle fuel circuit is set up lean for smaller cams in a 350. You might cover up the problem with pump cams and squirters, but ya really need more idle fuel. IFRs need to be bigger. Or, go to a carb set up for the engine/combo.

 

so what are you thinking as far as stepping the carb up? would a 750dp be overkill? gas mileage is already atrocious (10mpg or so).

 

Yep, double pumper for sure. But ya might not have enough converter to go very big. Personally, I would use a 650dp on it. Does your converter actually stall around 2500?