carb jetting

[fryedaddy]

i have a stock 850dp on my 428,i changed from cj heads,pi intake to bbm heads,rpm intake.normally would you have to change jets after the head,intake swap

[Falcon67]

850 DPs are normally pretty lazy, I would not change from stock jetting until I had some reason.  Specifically, from road or track testing. 

[fryedaddy]

i guess i need to run threw the gears about 6000 and see if it lays down or keeps on keepin on.i bought a used 950 ultra hp off a used carb salesman,he had put 72s in the primarys and 76s in the seconds with no powervalves in it.i took off down the road and it fell flat.i kept going up till i had 92s and 96s in it before i was done.i had it apart 6 times.it would get up and go but got 5mpg or less

[My427stang]

Almost as a rule, I start with bone stock configuration to include PVs when I install an unknown carb.  Rarely does it need much changing.  PVs are good too unless it's race only. 

If it is a standard 4781 (normal downleg not annular booster)  I would run 80s front and back with 6.5 PVs, if you have fear of the 6.5s, you could go 5.5s if you have some cam, but generally stock works well and early (bigger numbers) is safer

If you want to eliminate the rear PV (which is not a bad idea, because it can come uncovered under acceleration) then start with 80 primary and a 6.5 PV and set up the rears with an 88 main jet with a blocked PV .  If you do that, watch carefully for the PV plug and clearance in its well, some of them hit the body of the carb and can cause you fits for gasket seal

If it is not a 4781 and some odd type of 850, those jetting recommendations don't apply

[Falcon67]

Same as 427 - first thing I'd have done with the 950 is put it back in Holley stock configuration.  Those have a rep for running very well right out of the box.  I'd love to have one on the dragster.  That is race only and I still use a power valve.  IMHO running without PVs gets what you saw - a fat carb with poor fuel metering that tends to load up.

[machoneman]

Yep, Chris nailed it. No PV = loading up a lot. Put it back to stock jetting and settings and start from there but with a PV.

[fryedaddy]

i think the 950ultra came with 92s front and back.i ended up leaving the 96s in the back and 84s in the front with a power valve.i may use that carb at the track but my 850 thats on it now has 80s all around still stock.i use it on the street to save gas and run proper

[Tobbemek]

i have a stock 850dp on my 428,i changed from cj heads,pi intake to bbm heads,rpm intake.normally would you have to change jets after the head,intake swap

Jet size  fore a specific carburetor ( 850 dp is  around 80 ) is mostly related to venturi size, air speed/velocity making a depression in the booster/main fuel well. If you have to goo more than 2-4 numbers from Holley std jetting you have the wrong size carb fore your engine ore a really
screwed up combo of parts.
If you get a secondhand carb,  a good ting to do is look at" Holley's numerical list"  to identify your carb and std jetting.
Holley sometimes changed jetting over the years on there versions  of the same carb but mostly on the PRI side and then often related to power valve channel restriction.
If you"over carb" your engine you have to go way up in jetting because the to big venturi in the to big carb for the combo, the engine has a harder  time getting the air velocity going in the venturi. " Mr moor's LAW. If bigger is better to big must be best"  Bigger jets in a combo like that gets the boosters going but with very poor atomization big droplets of fuel entering the intake tract and a very sluggish lazy performance and  down in Torque/ power.     

850 DPs are normally pretty lazy    Falcon67

I learned the"   Mr moor's LAW"lesson as a kid in mid 70is switching a 2300 Holley 500cfm back an forte between a std. 68 Fairlane 302  and a Volvo 4cyl 110 ci.
The Volvo needed over 10 numbers bigger jets just to accept throttle movements. By the way the 500 cfm carb was to big even fore the std 2brl
302. No response on seat of the pants power just taking a lot of gas. 
Fore a hotted street driver big block, i really like  the the "old style" calibrated Holley 750 with a choke its hard to go wrong with them.
 The idle/ transfer calibrations is often very close to optimal, and preferably with vacuum sec. The old style 715-735-750-780 VS "loved baby has meany names" carbs with a signal tube into the right side PRI venturi can be sett up so the SEC throttles is following the PRI throttles like if it was a 1:1 linkage mechanical carb when "pedaling "  super crisp on gas response and with the same power attribute as the DP.
One might say "the 850 and larger carbs make moor power". Shore does, but i will gladly give up " high rpm pump losses" those extra horses on the street,for a better manner/crisper engine easier on gas, usually your tires are up in smoke any way before you reach them extra horses.
Look at Barry R dyno carb test he did fore a customer prompting on a Holley 3brl VS carb.
The 850 better suits 550+ HP engines.I just calibrated ( 84 different single moments drilling and treading to get fully adjustable ) an old Speed Demon  850dp on a 572"/620hp  C-brand crate engine in a 65 Impala great carb fore that combo.

Talking about carbs there is definitely certain carbs to stay away from, that have design issues, and i am not talking about  prettiness.
The Edy 1407 750 cfm performance carbs that i have com a cross, hade the shorter PRI booster clusters  from the smaller venturi 1406 carb.so that the booster didnt reach down ( about 3/8 to short ) in the smallest/narrowest part of the venturi and by that not getting a proper "signal to the boosters. The situation becomes  like a "over carbing problem" the manufactures  solution to this problem is drilling out the emulsion tube like a Swiss cheese. And moor,the throttle hight position in relation to tranferslot and ported vacuum hole in the boore is off.
Like that you get a super overly rich part throttle situation fouling plugs and on. Then one goose and by the "eminent" Jet, rod and spring assortment tuneup kit, and starts following the instructions but nothing you find in that box will sort out your problem, then people think they are stupid that cant follow a instruction properly
There are others to like som of the old Holle/summit copy ford 4100carbs.It could have been a great carb, with annulars, holley pump membrane, PW  and no gas spilling working on them, some of them are not right in hight wear the cluster goes  to the main body  and if you raise the float level enough fore compensate, you get a driving ,corner, brake "spillover situation.
There are others to but getting way of topic and my bad English spelling takes to darn much time i better end this now.

 


 
 
 

[fryedaddy]

great info,i understand.i looked up the 950 ultra hp and it came with 92s square,the guy had 72s and 76s in it with both powervalves blocked.it fell on its face at 3000 rpms.i didnt have any 92s so i put 88s and 96s in it. with a power valve in the front, no fouled plugs,no smoke.it may have been more carb than i needed thats why i bought a new 850.but i didnt have any trouble and it didnt stumble when you hit it from a dead stop

[Falcon67]

I would just say 850s are usually lazy because they have a large venturi area in relation to the throttle bores.  So it takes more air flow in that configuration to get a good signal to the boosters.  I also use the Holley listings to guess at base jetting for home brew stuff - most of the ProForm  "750" main bodies seem to run around 800~830.