yet another jetting question
Thread Starter
Burning Brakes
Joined: Mar 2006
Posts: 1,036
Likes: 2
From: Huntingburg Indiana
yet another jetting question
I have a Holley 3310-4 750 on my 350. I know, some people think it's too much, but it's been done before. I have 9.8:1 compression with domed pistons, lunati cam with 262/268 duration and 468/ 489 lift. I am using hooker comp headers and a low rise weiand dual plane (that's all that will fit for now). The carb comes with 72 primary jets and a 6.5 power valve. I have gradually moved up to 80 primary jets. I tried reading the plugs, but they always look white. Does today's gas make it hard to read plugs? I always run 92 octane. I do not have the money for an a/f meter right now, so would anyone with a similar combo be able to give me an idea on what jets would be close? I just don't want to hurt the engine. It seemed to be more responsive with the factory jetting, but the white plugs made me think it was lean. Thanks for any help.
Former Vendor
Joined: Apr 2009
Posts: 766
Likes: 3
From: Charlotte North Carolina 704-394-5150
Do yourself a favor and find a chassis dyno and spend some time on it. Instead of the "hit or miss" method you are currently using, The cost of spending time on the dyno to get your components "dialed in" will be more than worth it. There should be one around you somewhere. Even if you had to drive a while to get to a shop with a dyno to get it right...don't you think that would be worth it.
Just a bit of advice from a guy who deals with customers who are trying to do what you do right now. And I end up putting it on a dyno and get it tuned to peak performance. A
A guy in Boulder Colorado will have a different set up than a guy in Miami Florida even with the same exact engine. Altitude, air density, and other factors change the way the engine may need to be set-up. So using someone elses jet sizes may or may not work. I am taking this to the extreme, much like a drag racer who travels will have to change things to get the car the best for the current conditions.
Look into it, dyno testing is really not that expensive in relation to you damaging your engine due to testing without proper equipment which can often be a dyno.
"DUB"
Just a bit of advice from a guy who deals with customers who are trying to do what you do right now. And I end up putting it on a dyno and get it tuned to peak performance. A
A guy in Boulder Colorado will have a different set up than a guy in Miami Florida even with the same exact engine. Altitude, air density, and other factors change the way the engine may need to be set-up. So using someone elses jet sizes may or may not work. I am taking this to the extreme, much like a drag racer who travels will have to change things to get the car the best for the current conditions.
Look into it, dyno testing is really not that expensive in relation to you damaging your engine due to testing without proper equipment which can often be a dyno.
"DUB"
Drifting
Joined: Mar 2001
Posts: 1,395
Likes: 6
From: Indiana
Most likely, the 72's that it came with will be pretty close to correct if not a bit rich. Anything below 2500 rpm is controlled by the a/f idle screws in the primary metering block. As many people that have tuned with wideband a/f setups have found, the slightest move with those screws can make a huge difference.
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Mine were white too but I know I was cruising in the 13:1 range as well as idle. So I think it is the gas, I was using Sunoco 94 I have since changed and will look at the plugs again 
Save up and get wideband so you know where you are at. 80 primaries is good for 555 nitrous motor
Save up and get wideband so you know where you are at. 80 primaries is good for 555 nitrous motor
Race Director
Joined: Apr 2000
Posts: 17,676
Likes: 201
From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Thing is as posted above, your idle mixture screws have more affect on cruise than your main jets, believe me I know. Just a slight turn on one screw can make your idle mixture jump from 13:1 to 14:1 and this carries on into your cruise as long as you are below about 2500RPMs.
So until you get a wideband and can "see" what is going on I suggest putting the 72's or 74's back in and tune the idle mixture screws to get you highest vacuum reading. That's about all you can do for now and make sure you are running the right plugs and you timing isn't too far advanced
So until you get a wideband and can "see" what is going on I suggest putting the 72's or 74's back in and tune the idle mixture screws to get you highest vacuum reading. That's about all you can do for now and make sure you are running the right plugs and you timing isn't too far advanced
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Thread Starter
Burning Brakes
Joined: Mar 2006
Posts: 1,036
Likes: 2
From: Huntingburg Indiana
I have my timing set at 36 mechanical and 52 total as the experts recommend. I increased the compression from 8.5:1 to 9.8:1 so I went with one step colder plug than the motor used originally.
Pro
Joined: Mar 2009
Posts: 703
Likes: 2
I've had some pretty stout small blocks in my days but never have I ran one more than 41.. The motor in my Vette right now was dynoed at 452hp with 10-1 and a BG Mighty Demon 750 and I have my timing locked out at 36 with the weights tied down and the vacuum can unplugged.
Le Mans Master
Joined: Jun 2004
Posts: 6,231
Likes: 65
From: Seattle WA
St. Jude Donor '14
I agree that something approaching 1970 LT-1 factory jetting (also with a 3310) is probably right in there. I have an Innovate...and it's saved me untold dyno dollars as well as teaching me a bunch about carbs. Reading plug color is a bit of a lost art these days, but I would step back from your colder plugs right now.
Your timing is right on, and you definitely want both mechanical and vacuum advance for a street-driven SBC.
Your timing is right on, and you definitely want both mechanical and vacuum advance for a street-driven SBC.
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
52 total
I've had some pretty stout small blocks in my days but never have I ran one more than 41.. The motor in my Vette right now was dynoed at 452hp with 10-1 and a BG Mighty Demon 750 and I have my timing locked out at 36 with the weights tied down and the vacuum can unplugged.
I've had some pretty stout small blocks in my days but never have I ran one more than 41.. The motor in my Vette right now was dynoed at 452hp with 10-1 and a BG Mighty Demon 750 and I have my timing locked out at 36 with the weights tied down and the vacuum can unplugged.
Burning Brakes
Joined: Aug 2005
Posts: 1,164
Likes: 1
From: Europe, France
Most Holleys ( let's say 90% ) are already jetted on the rich side right out of the box.
With a 750 cfm carb, you're probably running most of the time on the cruise and idle circuits, so changing the main jets has only little or no effect on the spark plugs color.
Big carb = big venturis = small air speed and carb signals with small engine displacement.
I'm pretty sure your carb is already rich enough with the stock jets.
The speed of incoming air is simply not sufficient to create enough vacuum to suck fuel from the openings... This is probably why you're running lean.
With a 750 cfm carb, you're probably running most of the time on the cruise and idle circuits, so changing the main jets has only little or no effect on the spark plugs color.
Big carb = big venturis = small air speed and carb signals with small engine displacement.
I'm pretty sure your carb is already rich enough with the stock jets.
The speed of incoming air is simply not sufficient to create enough vacuum to suck fuel from the openings... This is probably why you're running lean.
Last edited by 73StreetRace; Aug 8, 2009 at 04:05 AM.