Performance Boats Forum banner

Jet sizes?

Tags
jet sizes
8K views 78 replies 15 participants last post by  gn7 
#1 ·
Regarding my Holley 850dp carb. I am currently running 82 primary and 80 secondary jets with 6.5 powervalves. If I screw the mixture screws in all the way it picks up rpm and likes it. Does this mean I should jet down a couple sizes? the funny thing is after a WOT shutdown the plugs look perfect with the mixture screws out 1.5 turns.. What's going on here?
 
#2 ·
idle mixture screws have virtually nothing to do with your jet sizes. if the motor likes the screws turn all the way it means either your float is too high or your idle bleeds to to be enlarged, but if you don't know what your doing here then just leave them screwed in however far the motor likes. are the power valves in the primaries only? why are your primary jets larger than than the secondaries? 82's plus a power valve? this is way backwards:confused:



 
#4 ·
if you have power valves on the pri & sec then that not nearly as weird as I first thought, I don't usually run PV on the sec side. More like 72 with a pv on the pri side and 80's & no PV on the sec. But none of this effects your idle. Is this a single or dual carb on a tunnel ram?



 
#5 ·
As gn7 said, idle and WOT mixtures are totally separate animals, don't try to connect them.

I run a more "conventional" setup that gn7 is used to seeing in a holley, 67pri, 77sec, and a 6.5" power valve in the primary only. Kind of odd to see dual pv's is all.
 
#8 ·
also is this pri idle only or four corner. you want to rethink your whole jetting and PV set up. Dump the sec. PV and run the sec 7to10 larger than the pri.
72/w PV and 80 sec is safe and then jet down from there. Don't know your combo so it's hard to tell, just basing it off what your running now



 
#9 ·
4 corner idle. Its a 461 12:1 single carb jet. What is the reason for dumping one PV? So to give that a try Ill plug the sec PV and run a 80 and 72 on the Primary with a 65 PV and go out and do the kamakaze WOT shutdown and see how the plugs look? I'll do it if there is a reason why it's better :)
 
#10 ·
if the jetting seems ok then leave it. it has ZERO effect on your idle. NO do not lower your float level, only mentioned it because it can effect the idle, I run my pri at the bottom of the hole and the sec a little higher but the bottom is OK. More than likely you will have to run the screws turned in because of the 4 corner. these carbs were built for engine with pretty lumpy cams and a bad cab signal. If the cams is moderate then it pulls to much fuel at an idle. Just run them turned in and worry about it.



 
#13 ·
#14 ·
another thought...

Check to see if you're exposing the transfer slots under the butterflys. You may have to open your throttle too far to get tnough idle signal, exposing the transfer slots and causing a too-rich idle condition. PV's will not effect idle.
 
#34 ·
Wow this ain't nothing new, I can't believe you guys never had a problem trying to set the idle with an open PV, even worse if it's right at the point where it will open and close all the time, when the PV is open it's dumping fuel in the motor and the idle screws don't do squat, you can close the idle circuit and it'll still run from the fuel being dumped in by the PV!.....
 
#36 · (Edited)
I need you to explain this to me because I'm still not catching your drift.

The way I understand these things, the idle circuit is fed from the main well (unless you are dealing with a "divorced" idle circuit in which it has it's own feed passage in the bowl) but is independant from the main circuit having it's own jet (Idle feed restriction) and it's own bleed circuit (adjusted by the idle air bleed orifice).
How does something in the main circuit affect the idle circuit (unless the main circuit is being activated from improper carb adjustment)???
The only way the fuel from a power valve can get into the engine while it is idling is if the valve is ruptured (leaking down through the power valve referance port) or through a booster (because the throttle blades are open too far from improper carb adjustment/set-up or incorrect float height setting).
What are the other way's you are describing??
 
#38 ·
Just for another opinion. I also have tuned 4150's and 4500's for more years than I will admit.... (the early 70's):D
The incorrect power valve for the amount of vacuum will make the engine run rich at idle. We proved this with a Motec digital A/F meter and wide band UEGO O2 sensor. It had the same results on a boost referenced 4150 on a turbo motor and a normally aspirated engine. I actually did this on a bet. We could change the A/F ratio as much as from 13.8 to 12.8 and more depending on the engine. We put a pv plug in and set the A/F at 13.8, then put in a pv with a HIGHER number than the vacuum reading and the A/F went to high 12's, removed the pv and put in a pv with a number LOWER than the vacuum reading and the A/F went back to 13.8.
We have also tuned many times in the water to check the vacuum reading under load at idle and found the same results.
Just my .02
 
#41 ·
Not sure on a "boost referenced" 4150, but I would bet if you figured out why it was changing the AFR it would run better....because it won't change it unless something else is off...........or the main jet is smaller than the idle feed restriction :)sphss


I hope we know a little more about carburetors today than we did in the "early 70's" :D:D:D:D
 
#39 ·
I really have a hard time accepting this> That would inticate that EVERY time you set up or down you would have to reset you idle, And i HAVE never ever had to do that. Never! A power valve is nothing more than a pressure (negitive) sensitive valve that lets fuel enter the main well BEHIND the main jet thru a predetermined orfice that in the metering block installed at the factoru (unless it is removable as in B.G's etc.) I f the factory installed jet for the PV is roughly 6 jet sizes and I plug it your telling me I am going to have to reset my idle aiir adjustmment? Is that some how MAGICALLY defferant than just going down 6 jets and leave the PV in? Please explain how this is?



 
#40 ·
i remember the thread well on HB. If you look at the path of the fuel through the PV, you can see there's no way it should have any effect on idle. It's physically impossible. I've never seen it happen, purely due to the PV. My guess is that when tests are performed that indicate such an effect, there's other things changing besides the PV. If you could mechanically open and then close a PV while the engine was running, with a properly adjusted carb, and nothing else changed, there should be no difference in idle mixture. If there is, then something else is changing as well. It's really a very simple system. If someone can explain how the fuel is getting through the PV enrichment system at idle in a manner that effects idle, I'm listening. "I don't know, it just does" won't cut it, either.
 
#43 · (Edited)
If you take a properly running motor with a properly set up carb, you take the main jets out and put them in your pocket and fire the motor and it will idle perfectly. If you plug the threaded part of the power valve port to block it off from the vacuum, leaving the PV restrictors exposed, it will idle perfectly. I have done this for those that like losing their money :D
So what happened when BD "conducted his test"? Have no idea, but like Steel said " theres other things changing besides the PV. Can't tell you how many times I've seen the wrong gasket type on the wrong PV. Did the gasket leak fuel directly to the vac area behind the PV. Maybe the diaphram leaked a very small amount but will open and close with a vac.Warped metering block. The float level changed? I don't know, but as Steel said, it is impossable for a PV or the absense of one to effect the idle,EXCEPT, where 058 stated if you get some really crazy and fast vac swings and then the PV turns into a diaphram fuel pump and will actually pump fuel UP the main well to the booster and, but this very rare and requires some very fast and wide vac swings. I have had this problem but only when I tried to build an IR manifold using 4 Holley 2bbl in a ManiFre type setup, and the vac swings were insane. Ten years later it dawned on me what was happening. ART IF YOU WOULD LIKE ME TO DEMO THIS FOR YOU.........
BRING MONEY, I'LL USE MY MOTOR. :)hand



 
#44 ·
I should not post here.

Hi Bob. Good to here from you.
All of you fellas out there are confusing me with big fancy terms. I think the original problem was that his engine rpm would increase as the idle screws were screwed all the way in. All the idle screws do is shut off an air flow. His condition would suggest that there might be a vacuum leak in his system, such as carb base or intake manifold. Check it out. Al
 
#49 ·
Hi Bob. Good to here from you.
All of you fellas out there are confusing me with big fancy terms. I think the original problem was that his engine rpm would increase as the idle screws were screwed all the way in. All the idle screws do is shut off an air flow. His condition would suggest that there might be a vacuum leak in his system, such as carb base or intake manifold. Check it out. Al
Actually Rocket, you have it a little backwards there. except on newer (read smog) carbs the idle screws adjust the fuel;);)
OK lets get back to SKM problem, if it's not due to him having a carb that Holley designed for a lumpy ass cam ( the real reson for having 4 corner idle)
or his throttle blades aren't to far open and taking the carb "off" the idle circuit and on to the transfer which literally kills the screw adjustment capabilties of the idle circuit, there is one more thing. If the carb is BRAND NEW or resently rebuild by someone that knows, then take off your scoop/air and squirt the living hell out of ALL the air bleeds with carb cleaner, if these get blocked the idle and/or the main will dead rich. My money says he has a cam or maybe a dual plane manfold, that is sending the carb a stronger vac signal than Holley had intended and he will just have to run it with screws turned in. So what.



 
#45 ·
Well ***** I have a hard time explaining stuff typing so this probably won't make sense either but here goes!. The "Power Enrichment" curcuit is totally separate from the idle and main circuits and only works when vacuum drops enough to open it up then fuel flows, you can be at any rpm and if the vacuum don't drop down enough to open the PV you'll never get the extra fuel, at the same time you can be at any rpm(idle to WOT)and if the vacuum drops low enough to open the PV, you'll get the extra fuel. So, this has nothing to do with the idle cirduit or it's adjustment, you can change/block the PV and the idle "circuit" does not change it has nothing to do with it, and the "idle circuit mixture" stays the same. When you're at idle vacuum holds the PV closed and you're on the "idle circuit" only, if vacuum drops enough to open the PV or you have a ruptured diaphram, fuel will flow into the motor through the "Power Enrichment" circuit, not the idle circuit or the main circuit and the fuel mixture is still the same in the "idle circuit", but what you're getting is you could say is a leak and it fattens up the motor at idle, but idle mixture in the "idle circuit" still remains the same but with the excess fuel being dumped from the open PV, you can close off the idle circuit and the motor will still run on the fuel being dumped by the open "Power Enrichment" circuit!.. Once you set the idle mixture in the "idle circuit" it will never change unless you readjust the mixture screws, no matter what you do/change in the rest of the carb, but adding fuel at idle from another circuit regardless which one is bound to fatten the idle up, but not through the gawd damn "Ilde Circuit"!.... And if fuel could not flow at idle through the "Power Enrichment" circuit, there wouldn't be any need to worry about what PV you have and how much vacuum it takes to hold it closed!... Ok I'm done fuk it!...

BTW Bob, you kinda answered the question there with a leaky PV gasket!...:D:)hand
 
#59 ·
bet when you put on the single plane, you'll have to back out the screws a little, yeah it makes a differance. and you are prolly going to have to rejet too. The dual plane just creates a stronger signal at the carb.



 
#67 ·
I think it has been very well explained but to add my recent experince to this. I just built a 350 ci motor for my chevelle and the car would not idle for beans with the same carb and set up that was on the stock 350 motor. It was always dead rich at idle and cruise on this new motor, and I tried every combo of jets I could imignine. Then thought of the PV and measured my vacuum. Turns out I had only 6 inches at idle and the PV was a 8.5 Bought some PVs lower than 6 and walla, brought the air fuel ratio both at idle and cruise right on in according to the wide band O2 meter!!
 
#68 · (Edited)
The fact that it fixed your cruize is exactly what the PV is suppose to do. The fact that it fixed the idle isn't. If the power valve is wide ass open the main well/circuit can only see this as a jet increase of 6-10 sizes, the PV channel goes straight from the PV to the main well. So your saying that jetting up will affect your idle. Thats never happened to me. So I have to believe some thing else is affecting the idle when made the change, gasket leak or small pin hole in the diaphram?



 
#71 ·
you'd be asking the wrong guy here. I have no need for single plane except they are cheaper. Have yet to find one that I could get to work worth a shit without way more work than it was worth, only to find out I could make more power, over a wider range with a decent tunnel ram and do it way easier. You just know I opened a can of worms with that statement!
But have just given up on them, just don't have the patients for em



 
#73 ·
I had the same problem on my 850 carb, my idle was too high & it was dripping out of the booster sometimes, it was just on the edge of the idle circuit & main jet curcuit .when i blew my pv it ran way fat at idle ,missing etc. could have been because it was running past the idle curcuit being on the edge. . i set the idle down . here is some info that helped me, they also helped me boost refrence my 850 myself,i am running 2 power valves also.
from the holley site:
(The idle system supplies the air/fuel mixture to operate the engine
at idle and low speeds. Fuel enters the main well through the
main metering jet that is screwed into the metering block. Some
of this fuel is then bled off to an idle well where it is mixed with
air from the idle air bleed hole. The idle well leads directly to the
idle discharge port and the idle transfer system where this air/fuel
mixture is discharged.
Most Holley Street Performance, O.E. Muscle Car, Competition
and Pro-Series HP carburetors utilize idle mixture screws, located
on the sides of the primary metering block. These control the volume
of the pre-mixed air/fuel coming through the idle well.
Turning the screws clockwise will “lean” the idle system.
Conversely, turning the screws counterclockwise will “richen” the
idle system.
The initial adjustment is made by turning the mixture screws in a
clockwise direction until they lightly bottom. Back them both off 1-
1/2 turns. Connect a vacuum gauge to a carburetor vacuum
port that will have access to full manifold vacuum at idle. Start
the engine and allow it to warm up. Once the engine has
warmed up and the idle stabilized, the choke should be disengaged.
Adjust the idle mixture screws to obtain the highest vacuum
reading. Each screw should be turned an equal amount so
that the system is balanced.)​

From a holly rebuild book:
(No more than .040 to .060 of transfer slot should be showing on the primary side. if opening the secondary idle stop doesn't corrrect the problem you may have to drill a 1/16" or 1/8" hole in the primary butterflies .)

By the way, your power valves are 6.5 ,what is your vacuum? they say choose a pv that opens 2-in (hg) below your lowest steady cruise reading for cars & 2in(hg) below idle reading in gear for auto trans. on race engine with hi overlap cam, high idle deal hope the info helped.
 
#76 ·
IMHO, if a person has trouble tuning a carb on a dual or single plane intake, then that person will have more trouble with a tunnel ram.

Engine Parts and correct installation is only part of the equation for driveability and hp. Tuning is the other part.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top