Help getting Holley 3310-2 w/Proform body running better

[fairlaniac]

I typically run a Holley 750 DP onm my '66 Fairlane 390. It works great at the track but it's too rich on the stret. So instead of always messing with it I figured I'd run my 730 vac 3310-2 for street use. I bought a Proform body without choke for it. Here is what it has

72 primary jets and I have the upgrade to a metering block (plate is long gone) and it has 84 secondary jets. It also has replaceable air bleeds which I don't understand much but here are the current sizes.

Primary
Idle, main, main, idle
75 26 26 75

Secondary
Idle, main, main, idle
45 26 26 45

While the 750 DP runs perfectly on the same engine the vac 750 has an off idle stumble. I kind of feather it (giving gas) as I leave the clutch out. If I don't it kind of bogs down. The other day after a short drive to get warmed up I let it idle for a minute and stabbed the gas and it backfired through the carb. Now the 750 DP will not do this, just the vacuum 750. It revs up pretty sweet. Any ideas on which way to go on the 750 vac to get the off idle stumbe gone? I also have a 28 primary squirter on both carbs and 30cc pumps.

Thanks,
Doug

[raceman14]

Sometimes the idle circuit channel does not line up right with the baseplate, you need to make sure that the holes in the baseplate gasket do not cover up any of the holes in the Pro-Form as they are not cast with the same precision as your holley.

If this is not the case, do you have a secondary squirter on the Vac carb? It most likely needs some extra fuel from the squirters if it is not a vacuum problem. Also that +10 on the rear jets to make up for lack of power valves is not really correct for a street engine, I would drop the rear jets 5 right off the bat and see what that does.

Does your vacuum secondary work???

[F-BIRD'88]

The primary idle air bleeds are a bit too big creating a lean off idle flat spot.

Solder up the #75 primary idle air bleeds and drill with a .062" 1/16th drill bit.

It will now be just slightly too rich, using the 1/16" drill bit hand hone the primary idle air bleeds a bit at a time to around .064 ish. Use a Air fuel ratio gauge to get the idle and off idle afr just right. +/-14.7:1 AFR

Look at the stock 750VS carb body for the size of the stock idle air bleeds.
The new carb body will want the same idle air bleed size. ( or real real close)

The idle , off idle and throttle response will be much better.

There is no reason you cannot them fine tune the idle and off idle and primary cruise jetting on your 750DP carb
so its not so "rich" as well, using the AFR gauge and a vacuum gauge as a guide.

Assuming the primary carb jetting @WOT is correct now:

When you make the primary jetting a bit leaner you want to increase the size of the PVCR to compensate so the WOT AFR stays the same but the part throttle cruise AFR is a bit leaner closer
to 14.7:1.

Try to end up with 70 to 72 pri jetting and 80 to 85 sec jetting on either-both carbs.
You'll find they both work pretty good once each is dial in.

what is the primary and sec jetting you are using now in each carb?

What is the size of the idle air bleeds on your stock 750VS carb body?

While a wide band AFR gauge is sweet you can get this all done using a simple narrow band type afr gauge
(heated type O2 sensor best) makes dialing in the idle afr and air bleeds real easy.

The idle air bleeds you get in the carb body kit are only rough start points.
Again the stock Donor carb body air bleeds size is your start point The final best size will be real close
and the AFR gauge will show you the way.

Use common electronics flux core solder to solder up the hole in the air bleeds to make your own custom ones.
use a electric soldering iron or a propane torch to heat up the air bleed and solder.
very small changes in idle air bleed size make a big different in the AFR.
hand hone using the 1/16" drill bit a bit at a time to get it just right.

If you want you canthen do pretty much the same thing to the secondary side idle air bleeds.
The process is the same Look at the stock donor carb secondary idle air bleeds size for your start point.
adjust the .045" air blleeds you got untill they are just right.
this will really sharpen up the vacuum secondary side opening response. Work the sec throttle opening independent of the primary throttle while testing usign the afr gauge to get the sec idle and off idle afr just right too.

Final check with the air cleaner installed. Ya it matters.
Automoatic transmission:
The power valve must be closed when the motor is ideing in gear.
a big cam create low manifold vacuum at idle in gear and requires a low rated power valve 3.5 or 4.5" usually.
A big racey cam like lots of initial timing at idle that does not drop off when put in gear.
The distributor advance curve probabily needs recurving. This really cleans up the idle. "rich" smell exhaust.
What cam? Auto or 4 speed trans?

You should end up with two very good 750 carbs.

[levisnteeshirt]

off idle stumble can be corrected a few different ways ,,, first , do you have mixture screws on the secondary plate ? did somebody put a screw in the secondary linkage to pull it in manual ? power valve too high can cause this , does it have a power valve in the secondaries ?

sometimes the constant idle port in the base plate of the secondaries will get clogged , its a small hole below the transfer slot , what secondary metering block do you have ?

where are the mixture screws set ?

if you have mixture screws on the back block , the constant idle port needs blocked and take the secondary idle air bleeds from 45 to 75's ,,,

sounds like its lean , how long has this carb been sitting before it was placed back in use ?

try a 31 squirter with the orange pump cam , i'd buy a trick kit for it , with the secondary spring kit , and go through it ,,, read the directions , they work

those carbs are a little lean compared to a DP , but pretty sure the power valve channel is slightly larger to offset that ,

[F-BIRD'88]

As said check the location and the depth of the locating dowel holes in the new carb body.
A mis match or lack of dowel hole depth can create a pesky carb throttle base to body vacuum leak.

[rfoll]

I could be wrong here, but I ran into a problem with the Proform 750 double pumper body I bought second hand. Your secondary idle air bleeds look too big. When I talked to Proform, I was told some of the bodies are set up with bleeds for adjustable 4 corner idle. Without 4 corner idle the bleeds need to be close to the same size in the rear. That solved much of the backfire issues. I never did get this carb to work right, but at least I could drive it after I changed bleeds.

[fairlaniac]

Sometimes the idle circuit channel does not line up right with the baseplate, you need to make sure that the holes in the baseplate gasket do not cover up any of the holes in the Pro-Form as they are not cast with the same precision as your holley.

If this is not the case, do you have a secondary squirter on the Vac carb? It most likely needs some extra fuel from the squirters if it is not a vacuum problem. Also that +10 on the rear jets to make up for lack of power valves is not really correct for a street engine, I would drop the rear jets 5 right off the bat and see what that does.

Does your vacuum secondary work???

No issues on hole alignment.

No squirter on secondary, it looks like it is plugged.

Secondary vaccum does work. Used paper clip trick and put it on the plunger shaft, drove it and checked. The clip was moved.

Thanks,
Doug

[fairlaniac]

[quote="F-BIRD'88"] Thany for the tips. I have replied in bold print and underlined my responses to your questions.

The primary idle air bleeds are a bit too big creating a lean off idle flat spot.

Solder up the #75 primary idle air bleeds and drill with a .062" 1/16th drill bit. I have some .036 bleeds that I can use to drill out to .063 to replace the 75’s.It will now be just slightly too rich, using the 1/16" drill bit hand hone the primary idle air bleeds a bit at a time to around .064 ish. Use a Air fuel ratio gauge to get the idle and off idle afr just right. +/-14.7:1 AFR I don’t have a AFR installed. I have bungs, two narrow band O2 sensors however it is really tough to install the bungs in my headers while on the car and if you’ve ever put header in an intermediate chassis with FE you’ll know I have no desire to pull the headers. I could install the in my “X” pipe 2-3 inches past my flange/ball connection???Look at the stock 750VS carb body for the size of the stock idle air bleeds.
The new carb body will want the same idle air bleed size. ( or real real close) Eyeballing is about all I do on this.The idle , off idle and throttle response will be much better.

There is no reason you cannot them fine tune the idle and off idle and primary cruise jetting on your 750DP carb
so its not so "rich" as well, using the AFR gauge and a vacuum gauge as a guide. I’ll play with that at another time.Assuming the primary carb jetting @WOT is correct now:

When you make the primary jetting a bit leaner you want to increase the size of the PVCR to compensate so the WOT AFR stays the same but the part throttle cruise AFR is a bit leaner closer
to 14.7:1.

Try to end up with 70 to 72 pri jetting and 80 to 85 sec jetting on either-both carbs.
You'll find they both work pretty good once each is dial in.

what is the primary and sec jetting you are using now in each carb? DP has 72Pri-84 Sec and Vac has 72Pri – 84 sec. I copied the Vac to match the DP for starters.What is the size of the idle air bleeds on your stock 750VS carb body? I never could find what Holley had as OEM size.While a wide band AFR gauge is sweet you can get this all done using a simple narrow band type afr gauge I have a goofy color meter red-yellow-green from Eqqaus (heated type O2 sensor best) makes dialing in the idle afr and air bleeds real easy.

The idle air bleeds you get in the carb body kit are only rough start points.
Again the stock Donor carb body air bleeds size is your start point The final best size will be real close
and the AFR gauge will show you the way.

Use common electronics flux core solder to solder up the hole in the air bleeds to make your own custom ones.
use a electric soldering iron or a propane torch to heat up the air bleed and solder.
very small changes in idle air bleed size make a big different in the AFR.
hand hone using the 1/16" drill bit a bit at a time to get it just right.

If you want you can then do pretty much the same thing to the secondary side idle air bleeds.
The process is the same Look at the stock donor carb secondary idle air bleeds size for your start point.
adjust the .045" air bleeds you got until they are just right.
this will really sharpen up the vacuum secondary side opening response. Work the sec throttle opening independent of the primary throttle while testing using the afr gauge to get the sec idle and off idle afr just right too.

Final check with the air cleaner installed. Ya it matters.
Automoatic transmission:
The power valve must be closed when the motor is ideing in gear.
a big cam create low manifold vacuum at idle in gear and requires a low rated power valve 3.5 or 4.5" usually. I have a 6.5 primary power valve. The secondary has no provision for power valve but the block does have mixture needles.A big racey cam I have a Comp 282S cam. like lots of initial timing at idle that does not drop off when put in gear.
The distributor advance curve probabily needs recurving. This really cleans up the idle. "rich" smell exhaust.
What cam? Auto or 4 speed trans? 4 speed ToploaderYou should end up with two very good 750 carbs.

Thanks for your help, I appreciate it very much. I’ll be giving some of this a try this weekend.

[levisnteeshirt]

who put the secondary idle mix screws on it ? did they drill the hole required in the base plate for them to work ? ,, measure the power valve restriction on the 3310 block , and the 4779 block ,, the 3310 should be bigger , the jetting will be a close comparrison for the 2 carbs , but it won't be exact

if the 750 dp doesn't have sec idle mix screws , and if your not sure the baseplate for the VS carb was drilled for 4 corner idle , take the dp sec block and try it on the vs carb , keep the 45 idle air bleeds in it

[fairlaniac]

See my responses in bold - Thanks!

off idle stumble can be corrected a few different ways ,,, first , do you have mixture screws on the secondary plate I do have mixtures screws in the secondary metering block (conversion kit ftom 415=60 to 4160 from Holley.? did somebody put a screw in the secondary linkage to pull it in manual ?No power valve too high can cause this , does it have a power valve in the secondaries ? No secondary PV

sometimes the constant idle port in the base plate of the secondaries will get clogged , its a small hole below the transfer slot , what secondary metering block do you have ? verified it's open with carb cleaner

where are the mixture screws set ? about 1-5/8 turns out (all 4)

if you have mixture screws on the back block , the constant idle port needs blocked I guess I need to identify the constant idle portand take the secondary idle air bleeds from 45 to 75's ,,,

sounds like its lean , how long has this carb been sitting before it was placed back in use ? it sat three months

try a 31 squirter with the orange pump cam , i'd buy a trick kit for it , with the secondary spring kit , and go through it ,,, read the directions , they work

those carbs are a little lean compared to a DP , but pretty sure the power valve channel is slightly larger to offset that ,

[fairlaniac]

who put the secondary idle mix screws on it ? did they drill the hole required in the base plate for them to work ? ,, measure the power valve restriction on the 3310 block , and the 4779 block ,, the 3310 should be bigger , the jetting will be a close comparrison for the 2 carbs , but it won't be exact

I am the only owner of the carb since new - I installed the Holley kit to convert to secondary metering block, the OEM plate is gone. Mixture screws were already in the block with the new kit.

[levisnteeshirt]

does the 750 dp have sec mix screws ?

[fairlaniac]

does the 750 dp have sec mix screws ?

Yes, 4 corner but I'm not concentrating on it right now.

Thanks,

[levisnteeshirt]

without the baseplate drilled for 4 corner ,, its not going to work right with that metering block ,,, ya need to get a Holley book , it shows you how to do it , or find a metering block that doesn't have the screws for the back ,, the secondary block kits i've bought in the past , doesn't have the screws ,, so ya have 2 choices , get a metering block , or have the plate drilled ,,

[fairlaniac]

without the baseplate drilled for 4 corner ,, its not going to work right with that metering block ,,, ya need to get a Holley book , it shows you how to do it , or find a metering block that doesn't have the screws for the back ,, the secondary block kits i've bought in the past , doesn't have the screws ,, so ya have 2 choices , get a metering block , or have the plate drilled ,,

I had the conversion kit on my 750 VAC for 5 years and it ran awesome. My Fairlane ran a best of 12.22/113 with it. The DP ran a 12.17 best so I found my vac to run fine. I just wanted to play around with it and get a chokeless body. So my baseplate seemingly worked well the past 5 years. All I changed on a successful carb was the Proform body which gave my replaceable bleeds and a chokeless body. I would expect it to perform similarly once I got the bleeds and jets squared?