When adjusting a holley carb, what does "stumbling" mean?

Dyno was a long time ago. I think I had 14 degrees at idle running off ported vacuum. Now it is 18 at idle 38 all in.

When I gun it, it pulls steady and then stumbles a bit as I get up around 4000 on up.

So yeah, pretty sure it is leaning out.

Not hearing any pinging, but then with the car as loud as it is, I probably couldn't hear it if it was pinging.

But it is still jetted the way it was on the dyno. All I have adjusted is the idle mixture screws, and swapped out the vacuum port.

 

OK.. I'm not sure how adding timing down low could have affected your total number though. The added timing will only lean out the rpm range for which it was added. Only way I could see this mod changing WOT performance is if the mechanical curve was changed over the previous settings and the timing was not all in by 4,000 before. Something's not right here.

Also.. lean misses(which is what this sounds like to me) at highr rpm's will not always result in audible pinging.. and leads to cylinder pressure spikes with additional heat that get out of hand to cause damage to piston ring lands and gaskets. A little here and little there will cumulatively eat away at the gaskets firing ring(and/or piston) until something lets go. Often at times when you're not really even getting on it and masks the real problem until you've pulled the heads off.

I would HIGHLY recommend re-jetting the primaries at least 2 jets bigger to see if the issue clears up to any degree. Either that.. or don't rev the motor up past 3,500 rpm or so.

PS. are you absolutely sure that insufficient pump shots aren't causing a lean hole when going to WOT? Reason I ask is that.. if there's a lean out situation going on here?.. it usually get's worse the higher the rpm goes. Not just over a limited range as you're describing here.

PSPS.. thinking as I reply here.. you could also be dealing with a sudden lean out if the PV is not timed correctly too. Meaning.. suddenly going from moderate throttle.. to heavy or full throttle. The PV will not come in and add fuel fast enough through the boosters during the vacuum's transitional period. Larger and or longer pump shot will usually also fill in that lean spot to some degree.. but you still want a properly timed PV opening too. Just like the timing curve.. it all works together and needs to be tuned to achieve proper fuel curve.

 

I am not "absolutely sure" of anything.

With engine off, front squirters squirt as soon as the throttle moves. when you hit that harder spot where the secondaries are about to open, and move it anymore, all four are squirting.

 

Ok scoop.. you're starting to scare me now. I really don't want to see you hurt this motor on my advice. Can we back up just a bit here?

If/when you have time.. please list all the pertinent spec's for the current carb calibration so we can better look at what's going on here.

Primary/secondary jets sizes?

PV ratings?

Shooter size AND pump cam color/hole # used?

How far out are the mixture screws adjusted to?

 

Dude...I don't remember all of that. 6.5 power valve only in primary. Secondary has a plug.

I knew all the rest a couple days back when i had it torn down, and I wrote it all down on a sheet of paper, but I have since thrown that all away.

I would have to tear it all down again to get those numbers.

 

I guess I got nothing better to do today...

Be back in a minute with all that info.

 

Huh! Cleaning off my workbench and found the sheet.

Primary 68 jets
6.5 power valve
28 squirter
1-1/8 turn out on idle mix screws

Secondary 75 jets
31 squirter
no power valve
3/4 turn out on idle mix screw

Pump cams appear to be a light pinkish-orange color. The screw is in hole #1.

 

I think you know what to do and what your saying. I am of the opinion that beyond basics on a forum is probably not really going to be easy if it even works. I have seen where one thing is said then something else is said that doesn't match the previous statement or findings. Maybe because of sleep lol.. I myself get lost in it all. I do know that if and when I need mine adjusted I will ask you what I might can do, but nothing would be better than having you actually apply your hands to it and see the results. Phone tech service may be better than post after post after post lol
Another note. I personally have experienced my Holley acting different from one day to the next. Baffling lol..

 

Basically what I was getting at was I would rather call you when the time comes haha

 

That's great that you wrote it all down. I'll have to remember to do that..

 

I did not want to have to guess when I put it all back together.

Also, when I tore it down, I did not have the rebuild kit. I ordered it and was supposed to be able to pick it up the next day after work, but just in case it was wrong or whatever might occur to prevent me from being able to reassemble soon, I figured I better write it all down before I forget.

 

I am just glad that the sheet was under all the junk on my workbench. I spent enough time already working on that carb and didn't want to tear it all down again today.

 

Will be out for a few but I'll look over this when I get back in and ppost back in a while.

from quick view.. I see too small a pump cam combined with too small a shooter on the primary side of things. This leads to a long drawn out pump shot which leaves you lean when the throttle opens really quickly. That PV rating is likely too high as well. I'd probably be running a 5.5 instead.

 

Can I just say from a casual perspective, my first thought was why the frack is he running the pink cams on that, those aren't for what he's doing.

Scooper go watch the Holey performance tuning vids on their youtube channel.

And to John Holden an AFR kit is as was already stated an Air/Fuel Ratio Gauge kit. I called it a kit because mine is more than just a gauge. It also has feeds into my computer so I can track my entire engine cycle.

FWIW, with the AFR kit I got, a digital vacuum gauge and a digital tach with external feeds we had a pretty reasonable home dyno set-up for about $300. I mean I wouldn't throw anything with serious horse on it. But it's pretty fun to drive around and then download your data sets and build some sweet graphs. You really get to see what your car is doing then. (home dynyo-ing involves jack stands and danger generally; I should also mention my brother-in-law borrowed the under-wheel thing for measuring the HP and Torque from his employer (Freightliner Engineer) but you don't really NEED that part to tune your engine).

 

Other than the power valve, screw settings, and the jets, this is how it came out of the box.

I don't even have any other cams or squirters.

I had a 4.5 power valve in it when I tore it down. But the rebuild kit only came with 6.5 and 3.5, two of each. I still have the 4.5 and I doubt it is damaged.