Carb questions

[Mark Bliss]

Back story:

Car has been sitting. A long time.
Pulling the cover off and examining, it looks like a long lost barn find.

So I am thinking, clean things up, see if I can get it running and primarily, see if I can pick up where I left off as far as tuning and tweaking.

First order of business, re-plumb the fuel system, clean out the carb, fresh pump and filters, etc.
The car was built in the 70's when steel braided hose and AN fittings were all the rage. It's been previously re-hosed for the common hose deterioration reasons multiple times over the decades, but I am thinking this time around the dated looking blue and red fittings and stainless hose gotta go.
Modern black synth lined hose and black fittings ordered. Filters, still researching. But the old Fram HP1 has to go.

So I pulled the carb expecting the usual deposits of dried sludge crud, but it's not too bad! A little clean-up and freshening should do it.

Sorry- heres my question, or maybe just info to discuss. I was running a 70's vintage Holley 780 vac on a warmed over 67' 390 GT.
My memory is that the last time I messed with it I had bumped the primary jets one size up for testing and decided to go back. (Looking at the original spec data sheets from Holley, they list the original sizes as 71/76). Upon inspection, I find I had last installed 72/80.
My question is about the secondary. I recall it being common to maintain approx. spread of 5 or 6 sizes, and the original spec sheet bears this out. Was I a little out to lunch here? Or am I over thinking this?
It's been a while, but I have messed with Holleys quite a bit "back in the day".

Any thoughts from the mind-hive welcome.

(I have more questions, but this this gotten long for starters.)

[Phil Brown]

I always start with jetting the way that Holley built them then see what the combo wants for the fine-tuning. Holley typically built them pretty close to how much fuel is needed for how much air they will pass

[GerryP]

Jetting depends upon how the carburetor was built and how the engine is configured.  As above, always put the carburetor back to the way it came out of the box.  Don't fiddle with it until the engine tells you what it wants.  Jetting is not always square as the secondary is assumed to be used under high load when the mixture needs to be a little fatter.  The jet spread depends upon whether there is a power valve in the secondary.  Kind of ballpark, but no power valve usually increases jet size up 6 to 8 sizes.  And of course, metering depends upon signal.  On an open plenum intake, a Holley will always want more jet over a dual plane intake.

In your case, just because the carb looks good doesn't mean it is.  You should always replace the soft parts on any carb that was previously used, then has been in storage for a long time.  You can expect the accelerator pump to be shot, the one-way diaphragm for the pump to be cracked and hardened, the O ring on the needle and seat to be hard and cracked.

[Mark Bliss]

Thanks to both of you, and so far- agreed.
And my vague description of "cleaning and freshening it up" I did mean a soft parts kit.

My intention is to return to the original jetting to start- per the Holley specs. ("out of the box wouldn't specifically apply, as I got it used sometime in the 70's)

As I recall it, the general consensus (and my own experiences) left me inclined to think "universal" models were often jetted safe, and I often ended up leaning them out 1 or 2 jet sizes. While OE specific models were often closer.
This is a (I think its 33100?) that research describes being originally used on a 69' Chebby 396, not that it means anything.

Another related question- When I last messed with this I was planning a track day and wanted to try a 1" spacer. I have since decided that was likely either a mistake or wasted effort on this dual plane manifold and not likely to give the slight top end MPH bump I might have hoped for.
When setting that up, I recall checking the throttle for fully opening at WOT. Not paying close attention, I made an adjustment that as it turns out- causes my lead foot to open the throttle past WOT by perhaps 15% or so, as there is no real built in stop at full open.
Is it common to acquire or fabricate something for this condition?

BTW, it may be common knowledge to others, but on these vac secondary carbs, it is quite possible that opening the primaries 115% and having the secondaries open fully the link rod can drop down in its slot and upon throttle release, jam the secondaries wide open.
Guess how I know...

[Joe-JDC]

Thinking has changed from the '60-'70s on many things fuel related.  First, you may have to enrichen the carbs primary jets to get the benefit of the 10% ethanol gasoline used today.  If you get a pinging on acceleration, then you need to slow down the advance curve or jet up the primary jets to compensate for the ethanol, or both.  Nearly every carburetor I have dyno tested over the last 10 years or so have needed the jetting increased on the primary side.  That includes QF, Holley, Edelbrock, and Pro Systems carbs in every size from 500 cfm to 1250 single carbs.  Joe-JDC

[fryedaddy]

not trying to hijack your post but i have a new 850dp on my 428,had it apart many times till i was ok with it.i had a early 80s 850dp on it that i was very happy with,great throttle response,but i sold it and my new one just aint the same.UPDATE my buddy i sold the old one to is going to let me have it back.i dont know why the old one is so much better,but it is.

[thatdarncat]

Thanks to both of you, and so far- agreed.
And my vague description of "cleaning and freshening it up" I did mean a soft parts kit.

My intention is to return to the original jetting to start- per the Holley specs. ("out of the box wouldn't specifically apply, as I got it used sometime in the 70's)

As I recall it, the general consensus (and my own experiences) left me inclined to think "universal" models were often jetted safe, and I often ended up leaning them out 1 or 2 jet sizes. While OE specific models were often closer.
This is a (I think its 33100?) that research describes being originally used on a 69' Chebby 396, not that it means anything.

Another related question- When I last messed with this I was planning a track day and wanted to try a 1" spacer. I have since decided that was likely either a mistake or wasted effort on this dual plane manifold and not likely to give the slight top end MPH bump I might have hoped for.
When setting that up, I recall checking the throttle for fully opening at WOT. Not paying close attention, I made an adjustment that as it turns out- causes my lead foot to open the throttle past WOT by perhaps 15% or so, as there is no real built in stop at full open.
Is it common to acquire or fabricate something for this condition?

BTW, it may be common knowledge to others, but on these vac secondary carbs, it is quite possible that opening the primaries 115% and having the secondaries open fully the link rod can drop down in its slot and upon throttle release, jam the secondaries wide open.
Guess how I know...

Holley has made at least a dozen versions of the “3310”, they are up to at least 3310-11 so far, plus more sub variants. The early ones had a secondary metering block, then Holley went to secondary metering plates, without a power valve. Add into that differences between down leg boosters & straight leg, internal passage differences, air bleed changes, etc. Always best to be sure of the exact version you have. And of course only the original version with date codes during car production were used on the actual Ch**y applications, so the vast majority, even of the early ones were just aftermarket.

Any 3310 SHOULD have a positive stop to keep from going past full throttle. I’ve included a picture showing the arm and how it contacts the base. Check yours over carefully - it maybe has been modified, or is broken. But that could be very dangerous. Replacement throttle arms are available if there’s a problem with yours. You can get add-on mechanical throttle stops too, I think Moroso still makes one. They’re not a bad idea to keep from bending/breaking the throttle arm on the carb. We use them sometimes when bracket racing to adjust ET, but they’re not a bad idea just from the safety aspect. 

[Tommy-T]

I like the red/blue fittings and steel braided hose.
Haven't grown out of my Pro Street days.

Can't add a lot to the carb discussion, but a downleg 3310 can be a really sweet carburetor. Many have gone very fast with one.

[Mark Bliss]

not trying to hijack your post but i have a new 850dp on my 428,had it apart many times till i was ok with it.i had a early 80s 850dp on it that i was very happy with,great throttle response,but i sold it and my new one just aint the same.UPDATE my buddy i sold the old one to is going to let me have it back.i dont know why the old one is so much better,but it is.

No worries, the post and question is intended as a conversation starter.

[Mark Bliss]

Thinking has changed from the '60-'70s on many things fuel related.  First, you may have to enrichen the carbs primary jets to get the benefit of the 10% ethanol gasoline used today.  If you get a pinging on acceleration, then you need to slow down the advance curve or jet up the primary jets to compensate for the ethanol, or both.  Nearly every carburetor I have dyno tested over the last 10 years or so have needed the jetting increased on the primary side.  That includes QF, Holley, Edelbrock, and Pro Systems carbs in every size from 500 cfm to 1250 single carbs.  Joe-JDC

Thanks Joe, I hadn't really fully considered the effect of the ethanol in pump gas as it pertains to this, but you are obviously correct.
Back when this engine was first built, the primary fuel issue was lead content and valve seats of course. I initially thought I would be running either some leaded race gas blend or additives, but as time went on and I learned more, I ran more and more pump gas and never detected any signs of issues. The valve lash (solid flat tappet) certainly never wandered a bit.
I did run some pump gas after 10% ethanol became common, but this thing never seemed to care much what I put in it as far as I noticed.
But the stoichiometry and its effect on jetting considerations has me re-thinking things a bit. Thanks again for the suggestion.

[Mark Bliss]

Thanks to both of you, and so far- agreed.
And my vague description of "cleaning and freshening it up" I did mean a soft parts kit.

My intention is to return to the original jetting to start- per the Holley specs. ("out of the box wouldn't specifically apply, as I got it used sometime in the 70's)

As I recall it, the general consensus (and my own experiences) left me inclined to think "universal" models were often jetted safe, and I often ended up leaning them out 1 or 2 jet sizes. While OE specific models were often closer.
This is a (I think its 33100?) that research describes being originally used on a 69' Chebby 396, not that it means anything.

Another related question- When I last messed with this I was planning a track day and wanted to try a 1" spacer. I have since decided that was likely either a mistake or wasted effort on this dual plane manifold and not likely to give the slight top end MPH bump I might have hoped for.
When setting that up, I recall checking the throttle for fully opening at WOT. Not paying close attention, I made an adjustment that as it turns out- causes my lead foot to open the throttle past WOT by perhaps 15% or so, as there is no real built in stop at full open.
Is it common to acquire or fabricate something for this condition?

BTW, it may be common knowledge to others, but on these vac secondary carbs, it is quite possible that opening the primaries 115% and having the secondaries open fully the link rod can drop down in its slot and upon throttle release, jam the secondaries wide open.
Guess how I know...

Holley has made at least a dozen versions of the “3310”, they are up to at least 3310-11 so far, plus more sub variants. The early ones had a secondary metering block, then Holley went to secondary metering plates, without a power valve. Add into that differences between down leg boosters & straight leg, internal passage differences, air bleed changes, etc. Always best to be sure of the exact version you have. And of course only the original version with date codes during car production were used on the actual Ch**y applications, so the vast majority, even of the early ones were just aftermarket.

Any 3310 SHOULD have a positive stop to keep from going past full throttle. I’ve included a picture showing the arm and how it contacts the base. Check yours over carefully - it maybe has been modified, or is broken. But that could be very dangerous. Replacement throttle arms are available if there’s a problem with yours. You can get add-on mechanical throttle stops too, I think Moroso still makes one. They’re not a bad idea to keep from bending/breaking the throttle arm on the carb. We use them sometimes when bracket racing to adjust ET, but they’re not a bad idea just from the safety aspect. 

Ooh! Thanks for including the pic! I appreciate the extra effort!

I am pretty sure this thing is a 3310-0, but I would need to go out to my workbench and re-check.
And I need to look at the base pate assy. I don't think it looks like that one at all! Might have to dig out some other old example for inspection as well, this is intriguing.

[Mark Bliss]

I like the red/blue fittings and steel braided hose.
Haven't grown out of my Pro Street days.

Can't add a lot to the carb discussion, but a downleg 3310 can be a really sweet carburetor. Many have gone very fast with one.

Of course its a matter of taste, which is subjective.
I think I overdosed on the red/blue/stainless braid over those years.

And yeah, this specific carb has been a good one. Worked well, no problems.
The only thing I ever really did was go to a light sec. spring (Relatively light stick car with some gear) and bump the sec. stop open a tik due to a little cam to keep the primary idle circuit where it wants to be.

[Mark Bliss]

Update-
First, I was mistaken, the carb is a 3310-1, not that it matters.

Kevin nails it. I don't know who or when, but somebody modified the throttle stop.
I don't know how that slipped by all those years, but I knew something seemed odd.

So now I have to find a solution for that issue. Throttle shaft, base plate, aftermarket stop, not sure yet.

[Drew Pojedinec]

Secondary jetting is not larger due to a greater demand of fuel.

They are larger due to no power valve enrichment many times.
Also in the case of a 780cfm carb, the venturi are larger than the primary.

[Mark Bliss]

Thank you Drew,

But my question wasn't about why there is a spread between pri/sec, but as to any ideas why this one might be jetted so far from original calibration.
But its only 4 sizes in reality. It just struck me as odd, and I have no recollection whether it was like that (or close) when I acquired it, or if I felt there was some reason to...
I dunno, it's been a long time and the reality is I have to accept that I was once young and knew it all, now I am far older and know far less. But bigger/more is always better, right?

I am in the process of ordering a throttle shaft and some parts, and I am inclined to roll back closer to its original calibration and go from there.

Thanks again though, I believe it was some of your articles that I read recently that were very knowledgeable and in depth, especially as it concerns some old misunderstandings and practices. If so I recall your comments on power valves and thinking "finally, someone explains it clearly"
It aint rocket surgury...