Holley jet question [Archive] - Team Camaro Tech

: Holley jet question


SS69Cam
Apr 28th, 09, 09:18 PM
I have a 350 with a mild cam (don't know specs), 10-1 CR, 1:94 heads,headers and an HEI ignition. I have a holley 600 with vacuum secondaries. I am rebuilding it and find that it has #66 jets. Is this enough for the combination I have or should I try something else?

dyno jonn
Apr 28th, 09, 09:28 PM
Unless you have an engine with low vacuum, your carburetor should work with jetting the way it came from Holley. Once the cam gets so large that the vacuum drops below 16 inches you get to re-engineer everything. Before that, leave it alone.

dubs68camaro
Apr 28th, 09, 10:34 PM
The only other thing to take into consideration is the elevation of the city you live in. Holley jets all of their carbs for sea level. I live in a town that's about 3,500 ft. You really have to jet things down to get the A/F ratio right here.

victimizati0n
Apr 29th, 09, 06:27 AM
did you have any problems with the carb before?

if not, i would say leave the jetting alone, as im pretty sure that carb comes with 66 jets from the factory (assuming this is a holley 1850, 4160 carb)

DjD
Apr 29th, 09, 08:19 AM
Every combination is different, Holley does fairly well at making their carbs bolt on adjust the idle and go. I have found they tend to jet the primary side of the carbs they advertise as "street carbs" a bit lean. I saw a large performance improvment jetting up several sizes with an Avenger 670 (4150 vac sec) on my 383. I did this after a recomendation from someone running one on a 327 that had also found the carb too lean. Of course my gas mileage dropped a bit even when cruising but I was also able to lean out the idle more after the jet change.

On the other end of the spectrum I have been playing with an HP series 750 and it's stock jetting (72 and 84) is too rich on the road. Gas mileage has dropped off drastically and my buddy following me said he could follow me with his nose.

My point here is they sell jets for a reason, always start at the factory settings but once you get beyond setting the idle and getting rid of a flat spot or hesitation and start looking for more performance you will want to jet up or maybe down a size or two to see what improvment you can make. You can always put it back but you don't know if it's really right until you make a change or two or have a way to read your air fuel ratios...

fatblock
Apr 29th, 09, 05:19 PM
Anytime you find the need to go up more than 2 numbers on the primary jet cruising on the main ckt..you are better served playing with power valve opening points and enlarging the power valve channel restrictors to enrichen the main well vs just swapping in bigger main jets and flushing mpg down the drain.

DjD
Apr 29th, 09, 06:43 PM
Anytime you find the need to go up more than 2 numbers on the primary jet cruising on the main ckt..you are better served playing with power valve opening points and enlarging the power valve channel restrictors to enrichen the main well vs just swapping in bigger main jets and flushing mpg down the drain.

I think I understand what is being done, with the PVCR worth about 6-8 jet sizes in it's stock form, use a higher rated power valve to allow that 6-8 jet sizes to pull in sooner instead of jetting up. The power valve is the switch that opens or closes based on vacuum allowing fuel to flow through the PVCR, the higher number PV the sooner it opens.

Where I think this get dicey though is enlarging the PVCR. I wouldn't recomend this to anyone unless they are comfortible enough to start drilling holes in their carb's metering block. Any change to inlarge the PVCR is permanent where as a jet is a replaceable screw in item. Do you have information showing what drill size to use to get a pre-determined flow through the PVCR? How does this help a lean condition before the PV opens? How do you address being too rich at WOT?

Anyway, hope this isn't confusing the issue, the op just wanted to know if his jetting was right for his engine combo!

dyno jonn
Apr 29th, 09, 07:20 PM
Power valve restrictor channels should only be opened up about about .002 (2 thousandts) at a time, and only with a drill in a hand held fixture, not any type of power tool. Seat of pants, A/F ratio gauge, drag strip or stop watch accelerating from a fixed speed and distance is about the only way to see the difference. Yeah, if you go too far, there's no going back. When I start I check the original channel size using the back end of a number drill and then procede with caution.

fatblock
Apr 29th, 09, 07:47 PM
I think I understand what is being done, with the PVCR worth about 6-8 jet sizes in it's stock form, use a higher rated power valve to allow that 6-8 jet sizes to pull in sooner instead of jetting up. The power valve is the switch that opens or closes based on vacuum allowing fuel to flow through the PVCR, the higher number PV the sooner it opens.

Where I think this get dicey though is enlarging the PVCR. I wouldn't recomend this to anyone unless they are comfortible enough to start drilling holes in their carb's metering block. Any change to inlarge the PVCR is permanent where as a jet is a replaceable screw in item. Do you have information showing what drill size to use to get a pre-determined flow through the PVCR? How does this help a lean condition before the PV opens? How do you address being too rich at WOT?

Anyway, hope this isn't confusing the issue, the op just wanted to know if his jetting was right for his engine combo!

Dennis..like you stated,its Marcs call if he is interested in modify carbs beyond a simple jet change.OP rules imho.:beers:

fatblock
Apr 29th, 09, 07:59 PM
Power valve restrictor channels should only be opened up about about .002 (2 thousandts) at a time, and only with a drill in a hand held fixture, not any type of power tool. Seat of pants, A/F ratio gauge, drag strip or stop watch accelerating from a fixed speed and distance is about the only way to see the difference. Yeah, if you go too far, there's no going back. When I start I check the original channel size using the back end of a number drill and then procede with caution.

Yep..modern science has not figured out how to drill a hole smaller..no going back.I beg to differ.Ever inserted a wire restriction in an orifice and seen flow increase vs decrease?Two thou is ifr typical,enlarging the pvcr by .010 to .015" is common when the main jet is downsized but the total flow into the main well remaines the same with a properly selected pv opening point.

lluciano77
Apr 29th, 09, 09:28 PM
First off, you need to set your jets for cruise. Drive at steady speeds in 10 mph increments up to 70 mph. The jets come in with the mains. This point can be hard to find. Lean down until you feel a surge and then go back up two sizes.

As far as the mains/idle they are connected so a change to the jets can affect the idle circuit. Always reset your idle mixture after a jet change.

The PVCR (powervalve channel restrictors) can be changed easier by drilling and tapping for brass restrictions. It is not exactly easy to do. You have to drill to the right depth so that the brass restrictions will thread safely down below the PV. I don't recommend it unless you are familiar with machine work. The PVCRs are realy close to the PV. I use 3/16" 8/32 brass setscrews to vary my PVCR area. You can get the from McMaster Carr. I also use a bottoming and pass through tap. Start the holes with a pass through and then tap your depth with the bottoming.

I disagree with the powervalve opening point versus jet change to get proper mixture. The jets are for cruise. The powervalve only opens under load where the engine would otherwise lean out. The PVCRs are not a variable restriction, they are either open or closed. I don't see how bringing them in earlier will help. They jets are timed and curved via the HSABs (high speed air bleeds). The PVCR is on off only.

To answer the question, you are probably pretty close where you are with the jetting now. Are you experiencing lean surging at cruise? If not, you may even want to jet down to see where your lean limit is. Once a surge is felt jet 2 sizes up.