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Old 03-18-2014, 09:40 PM
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4 hole spacers & Jetting

I have a 300hp crate 360 magnum in my 91 half ton dodge power ram 150 4x4 truck. Intake manifold is a Mopar Performance dual plane M1 and carburetor is a quick fuel super street series 680cfm vaccum secondary carburetor.
I was running a 1 inch 4 hole phenolic carb spacer and recently bought and installed a 2 inch 4 hole phenolic carb spacer. This 2 inch spacer did something to my power curve, my truck seems to pull way harder from around 1500 to 4000 rpm. It pulls so much harder and it literally feels like I piked up around 50 more ft lbs of torque.
I know my idle is a little lean with this spacer and I ordered some idle air bleeds to richen it up a little since adding this spacer. But I cant tell if my main and secondary jets are now lean as well. I have read that when adding bigger spacers it's a good idea to re-jet at least one or two sizes richer, is this true? My carburetor came with 68 main jets 78 secondary jets. But I live 4000 feet above sea level and the carb tech I spoke with at quick fuel when I was buying my carb told me to jet down 1 size for every 1000 feet increase in elevation, he said these carbs are jetted for sea level. So I bought some 64 main jets and 74 secondary jets and installed them right away when I bought the carb so I wouldn't be too rich at my higher elevation.
I also had to lean down my idle circuit a considerable amount as well. But I was thinking since I just added a 2 inch 4 hole spacer it might be a good idea to get some 65 main jets and 75 secondary jets and install them.
Does adding bigger spacers lean down the mixture? Am I wrong in thinking it might be a good idea to jet up a size front and back since adding this 2 inch 4 hole spacer?

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Old 03-18-2014, 11:13 PM
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spacers and jetting are difficult to get perfect with out a dyno. a 2 " spacer might increase intake velocity enough to change the power curve and require jetting and or air bleeds
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Old 03-20-2014, 05:44 PM
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Quote:
Originally Posted by pjc360 View Post
I have a 300hp crate 360 magnum in my 91 half ton dodge power ram 150 4x4 truck. Intake manifold is a Mopar Performance dual plane M1 and carburetor is a quick fuel super street series 680cfm vaccum secondary carburetor.
I was running a 1 inch 4 hole phenolic carb spacer and recently bought and installed a 2 inch 4 hole phenolic carb spacer. This 2 inch spacer did something to my power curve, my truck seems to pull way harder from around 1500 to 4000 rpm. It pulls so much harder and it literally feels like I piked up around 50 more ft lbs of torque.
I know my idle is a little lean with this spacer and I ordered some idle air bleeds to richen it up a little since adding this spacer. But I cant tell if my main and secondary jets are now lean as well. I have read that when adding bigger spacers it's a good idea to re-jet at least one or two sizes richer, is this true? My carburetor came with 68 main jets 78 secondary jets. But I live 4000 feet above sea level and the carb tech I spoke with at quick fuel when I was buying my carb told me to jet down 1 size for every 1000 feet increase in elevation, he said these carbs are jetted for sea level. So I bought some 64 main jets and 74 secondary jets and installed them right away when I bought the carb so I wouldn't be too rich at my higher elevation.
I also had to lean down my idle circuit a considerable amount as well. But I was thinking since I just added a 2 inch 4 hole spacer it might be a good idea to get some 65 main jets and 75 secondary jets and install them.
Does adding bigger spacers lean down the mixture? Am I wrong in thinking it might be a good idea to jet up a size front and back since adding this 2 inch 4 hole spacer?
Kudos to you for experimenting with spacers. This is an unknown science at best and can be optimized only by experimentation in my experience. Fiddling with the air/fuel ratio is best accomplished by closely inspecting your spark plugs with a magnifier/light such as this one....
Longacre 50886 Spark Plug Viewer with Holder - Speedway Motors, America's Oldest Speed Shop

Plugs are checked by making a full power pass, then moving the gear selector to neutral, chopping the ignition and coasting to a stop. If the motor is operated at an idle after the power run, the results on the plugs will be masked by the idle mixture, not the full power mixture.

Here is a good tutorial to begin with. Do your own google searches to find others that make sense to you.....
Spark plug reading
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Old 03-21-2014, 12:51 AM
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If you want to spend a little money you could buy a wide band 02 sensor and see what the A/F ratio is doing in real time. The price has come way down on these in the last few years.
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