Finally got my carb comparison done Friday night...

[BattlestarGalactic]

I've had a pair of 735's that I converted to single feed a few years ago.  I had a few issues trying to get them working, like float issues that I finally sorted out(was a real "duh" moment).  I tried them back in spring before I loaded the car for Beaver.  I knew they ran okay but wanted to get a new baseline so I put my 600's back on for first pass at Beaver.  Well, my weekend ended quickly with transmission issues so I finally got some time to get to a TnT night.

I made a pass with the 600's.  Car went 11.05 @ 122 mph.  I bolted the 735's on and tried again.   Went 11.08 @ 122.   I have a wideband in the car just for this purpose and my A/F went way rich.  Was 12.5, now 11.2 so I need to lean them out a touch.  I think there might be maybe a tenth in it if I get them dialed in.  I wasn't expecting anything real drastic, but was curious and that is why I wanted to try them.

In doing the conversion, I just used a pair of 600 rear plates since that is what I had laying around.  I need to investigate what the jetting is so I can take some out.   The 600's I run are basically blueprinted stock, with only a jet change to 67(66 stock), the lightest secondary springs, and larger squirters.

The one big thing I noticed was the drastic change in throttle response.  Wow, completely different then the 600's.  Real snappy, like having an alum rod motor snappy.  I'm guessing it has to do with that stepped booster.  The 600's work well, but no where near this.   I found it harder to keep rpm in the burnout due to any flinch of your foot would zing the motor instantly.  I don't run any 2 step.  Never have.  Never an issue.  This would just be something I would have to adjust myself for after 23 yrs of running the 600's.

Not sure when I'll get another chance to try the carbs.   At my stick races you don't get any extra time for playing.  Two time trails and you race.

[Drew Pojedinec]

Stepped down legs are pretty awesome.

[427LX]

How would same boosters work in the 600's?

[Drew Pojedinec]

Stepped down legs promote atomization so your best gains are from larger carbs.
The carb will flow more with them compared to normal downlegs as the internal
Hole is larger.
I keep a few dozen around to install and play with. Old 4779’s really like them.

I hadn’t done enough testing on 600’s to see the gains there. If Larry wants I could put a set in his 600’s. He’d have to retune to get the best use out of them.

[RJP]

Stepped down legs promote atomization so your best gains are from larger carbs.
The carb will flow more with them compared to normal downlegs as the internal
Hole is larger.
I keep a few dozen around to install and play with. Old 4779’s really like them.

I hadn’t done enough testing on 600’s to see the gains there. If Larry wants I could put a set in his 600’s. He’d have to retune to get the best use out of them.

Are the 'stepdown' boosters you refer to the same as skirted boosters used in a 735 Cobra Jet Holley? If not can you provide a pic or 2 of 'em? Thanks.

[BattlestarGalactic]

RJP, yes these are 735 CJ carbs with that special looking booster. 

Drew, thanks for the offer but I think in the end I will end up just running the 600's.  Guess after I get the 735's dialed in I'll decide for sure.  I don't see any drastic ET improvement once I lean them out, but time will tell.  I do have a couple questions if you don't mind.  What is the jet size for the 600 secondary plate and what is the jet size for a 735 secondary?  I haven't researched enough to see what I did with using the 600 plate(It was what I had handy).  If I don't change plates I may work on air bleeds.

As I mentioned, the throttle response is VERY touchy and personally, in my situation, I don't really care for the feel of it. 

[Drew Pojedinec]

No, not skirted, no not like the ones on the -U CJ replacement

Sure I can post photos.....


First picture are the stepped boosters on the left, and stock downlegs with a .140 fuel feed on the right.  These are what you'd find in common Holley downleg carbs, same boosters as in the early 3310's, etc.  They work well.
IMG_3144 by Drew Pojedinec, on Flickr

Here are some .152 feed leg stepped downleg boosters.  Some high end carbs use them, I use them on 3310's and also 4779 carbs.  Note they are the same external size, but larger internally, also the machined ring makes a ledge which shreds fuel much better as it leaves the booster.  I especially like to use these on racier deals, especially a really cold intake where the enhanced atomization makes the biggest difference.  Sure sure, we could just use annulars, but if you are trying to get more flow out of a smaller carb like a 750cfm, this is a good way to do it.
IMG_3147 by Drew Pojedinec, on Flickr

[Drew Pojedinec]

Larry, a stock 1850-2 looks like this:

List number         1850-2
Type            4160
Primary
Float            Side Hung
Booster            Straight leg .136
Cross channel         .136
Pump nozzle         .025
Pump type and cam      30cc/orange
Idle air bleed         .067
High speed bleed      .029
Metering block#         8485/1947
Main Jet         66
Power valve         6.5
PVCR            .043
Emulsion         two .028
Kill Bleed         none, emulsion tube feed
Idle Feed restriction      .026
Needle and seat         .097
Venturi size
Throttle plate size      109

Secondary
Mechanical or Vacuum?      VS
Spring color         plain
Diapghram length
Float            Std
Booster            straight leg
Pump Nozzle         na
Pump type and cam      na
Idle air bleed         .028
High speed bleed      .026
Metering block/plate#      9
Idle Feed Restriction      .031
Jet size         .067
Needle and seat         .097

For a drag race setup, like yours or for 2x4 usage, I often reduce the secondary Main Air bleed to .026, and drill the metering plate to have a .035-.-040 IFR with a .078 main feed.

A 735 Quickfuel looks like this:
List number      Quickfuel 735cfm SS
Type         4150 VS
Primary   
Float         Center Hung Nitrophyl
Booster         Stepped Downleg .152
Pump nozzle      .031
Pump type and cam   30cc, blue cam #1
Idle air bleed      .070
High speed bleed   .035
Metering block#      NA billet
Main Jet      68
Power valve      6.5in 4window
PVCR         .052
Emulsion      3 @ .028
Kill Bleed      In Low angle channel .028
Idle Feed restriction   .032
Needle and seat      .097
Venturi size
Throttle plate size   1.685

Secondary
Mechanical or Vacuum?   VS
Spring color      Adjustable pot
Diapghram length   NA
Float         Center Hung notched
Booster         Stepped Downleg .152
Pump Nozzle      none
Pump type and cam   none
Idle air bleed      .070
High speed bleed   .035
Metering block/plate#   na
Idle Feed Restriction   .032
Jet size      78 with jet extensions
Needle and seat      .097



Ironically, both of these calibrations came from my notes on carbs that I sold to Mark "turbohunter" on this FE forum, so I'll leave him to comment on them being ok, great, or needs improvement

Ok for the CJ replacement carb, here are my notes:

List number         4609
Type            4150
Primary
Float            Center hung brass
Booster            Staight with bell
Pump nozzle         .028
Pump type and cam      #1 White
Idle air bleed         .076
High speed bleed      .026
Metering block#         5673/4609
Main Jet         66
Power valve         6.5
PVCR            .052
Emulsion         2 @ .028
Kill Bleed         emulsion tube feed .026
Idle Feed restriction      .029
Venturi size         
Throttle plate size      172

Secondary
Mechanical or Vacuum?      VS
Spring color         Plain
Diapghram length      Short 2.1 overall
Float            Center hung brass
Booster            Downleg with cutouts
Pump Nozzle         na
Pump type and cam      na
Idle air bleed         .028
High speed bleed      .025
Metering block/plate#      Block 7003
Idle Feed Restriction      .040
Emulsion         2 @ .028
PVCR            .067
Jet size         72

(low jet size, but massive PVCR as there is obviously a PV in the secondaries)



Sorry If I hijacked your thread Larry..... my bad.

[BattlestarGalactic]

My carbs are factory 735 CJ carbs, not QF versions.  It has the odd bell shaped booster in it.

It's been so long since I actually looked inside these carbs(last winter), I thought the primary jetting was 72?  See, my memory has gone to crap.

Going by your chart, the 600's have .67 secondary jetting, the 735 are 72.    Since I put 600 plates in it, it should be comparable? 

Drew, we're good.   Getting good information out here for others to see.

[Barry_R]

Downlegs of some sort are almost always good. 
Stepped ones are really good.
Bell booster works pretty good too, but knock out some airflow.
Have you seen some of the super small cross section annulars the race guys are using these days?
I have seen some with about eighty zillion discharge holes in multiple rows around the ID...

Back when Holley cheapened the 3310 to satisfy the big box discount retailers desires for lowest possible prices, a couple of the engineers at Holley bemoaned the loss of the downlegs and said it turned the carbs to junk compared to the originals.

[Drew Pojedinec]

When considering jetting, remember these 735 jets are really small.... but they also had 10.5 power valve with a huge pvcr. The secondary pvcr is almost as big as the jet itself.

[BattlestarGalactic]

So it basically leans it out really hard at cruise speed, but fattens it up under power.

Remember, I took the secondary PV out when I converted it to single feed.  Thus it has 600 plates in them.  That is why I was curious as to the fuel settings between stock 735 and the 600 secondary plate it has now.

I know there is HUGE amounts of variation due to booster size, venturi size, air bleeds, etc, but since I ran the 600 plate would it be close to a 600 carb jetting?  Apparently it is getting more fuel(bigger venturi, different air bleeds)?

I like my 600's, so likely will continue to run them unless I find 2 tenths in these 735's?  Not that ET is that critical to me, but hey who knows?

[Drew Pojedinec]

I’d think you’d want larger fuel restrictions in the metering plate.
Given that you probably don’t want to swap out a ton of plates, I’d start with a
.040 ifr and .082 main feed. If you want the main fuel feed can be drilled
And tapped. I often use 8-32 set screws, but given the size you may want 10-32. This way
If you go too large at least you can go back.

If you are willing to drill and tap those I’d be happy to send you
Some predrilled restrictions.

[RJP]

Thanks Drew. I've heard about stepped boosters, years ago a fellow did that to a pair of 600s and claimed that it improved fuel shear and better atomization. Only difference was his step was below the discharge hole. 

[BattlestarGalactic]

Thanks for the offer Drew.   I do have all the facilities to machine anything I need, plus a decent selection of pin drills.   I blueprinted the 600's with brass set screws and drilled the holes trying to get them equalized.  One carb(forget which one) would always draw more fuel then the other.  We basically figured it had to be intake design(Tunnelwedge) as it didn't follow the carb, it was the location.