carb help
Thread Starter
Safety Car
Joined: May 2005
Posts: 4,176
Likes: 7
From: Jennings LA
carb help
I have a holley 600cfm mechaincal seondary double pumper it was
running rich at idle, I took it off and the transition slot was almost completely uncovered. I adjusted the secondary throttle plates so it would idle. I read an article that you should square off the transition slot, basicly it should be as tall as it is wide, use this for a starting point. it now seems to not be so rich at idle but the performance is way down. is it possibly to lean. I know this is borderline small for my 383, but it was the carb I had on my 350 I think the primary is jetted at 67. it has a lunati 292 duration 480 lift cam
running rich at idle, I took it off and the transition slot was almost completely uncovered. I adjusted the secondary throttle plates so it would idle. I read an article that you should square off the transition slot, basicly it should be as tall as it is wide, use this for a starting point. it now seems to not be so rich at idle but the performance is way down. is it possibly to lean. I know this is borderline small for my 383, but it was the carb I had on my 350 I think the primary is jetted at 67. it has a lunati 292 duration 480 lift cam
Tech Contributor
Joined: Aug 1999
Posts: 14,370
Likes: 6,364
From: At my Bar drinking and wrenching in Lafayette Colorado
gator -
You're going about your tuning the right way - good job getting the secondary throttle cracked open in order to reduce the primary transition slot exposure.
Now that you have the blades balanced out, you can start doing some tuning to the carb. Your new setup will lean the carb out a hair at idle and just off idle. Don't be afraid to bump your jet sizes up a few sizes and see how the car responds. If you have a little bit of a cam and a set of headers with a free-flowing exhaust, you can easily bump the jetting up a few sizes for a noticable throttle reponse improvement. Sometimes you can even richen up the idle mixture screws a tad for a nice improvement in off-idle response. Don't be afraid to try a few things out - you're not too rich until you puff black smoke out the tailpipes upon acceleration...
You're going about your tuning the right way - good job getting the secondary throttle cracked open in order to reduce the primary transition slot exposure.
Now that you have the blades balanced out, you can start doing some tuning to the carb. Your new setup will lean the carb out a hair at idle and just off idle. Don't be afraid to bump your jet sizes up a few sizes and see how the car responds. If you have a little bit of a cam and a set of headers with a free-flowing exhaust, you can easily bump the jetting up a few sizes for a noticable throttle reponse improvement. Sometimes you can even richen up the idle mixture screws a tad for a nice improvement in off-idle response. Don't be afraid to try a few things out - you're not too rich until you puff black smoke out the tailpipes upon acceleration...
Thread Starter
Safety Car
Joined: May 2005
Posts: 4,176
Likes: 7
From: Jennings LA
Lars, do you think the 600cfm is big enough if I get it jetted right? also should I do primary and secondary jets at the same time? I have headers and 2 1/2 inch true duals. thanks for all the replies. I could not belive how much it reduced my power, looks like I am off to buy jets.
Tech Contributor
Joined: Aug 1999
Posts: 14,370
Likes: 6,364
From: At my Bar drinking and wrenching in Lafayette Colorado
gator -
...you're not setting me up for another "is my carb big enough" discussion here on the Forum, are you...?
just kidding...
I think everyone knows my position on carb sizing: You can make virtually any size carb (within some reasonable limits) run just fine. Your 600 can be set up to produce a nice idle, very crisp throttle response, good drivability, and a good strong feel at WOT. I believe a larger carb will probably get you a little better top end at WOT, and it's something you can consider changing to in the future. But for now, the 600 can be made to run just fine.
My recommendation on jet sizing and jet changing is to bump the primary up 2 sizes at a time and see how the car reacts. Install a set of standard resistor plugs (non-platinum) - I prefer the regular Autolites. The standard plugs are "readable," whereas the exotic plugs burn so clean throughout a varied mixture range that you cannot "read" those plugs with any reliability. If your carb does not have a secondary power valve (most don't), keep the secondary jet sizing 8 sizes bigger than the primary side jets: When the primary side power valve opens, it is equivalent to an 8-size jet increase, so running the secondary side 8 sizes bigger keeps the carb flowing equally at all 4 corners at WOT.
As always, be sure your ignition timing and advance system is set up correctly and operating well. You should be running about 36 degrees total, have a functioning vacuum advance control unit that pulls an additional 16 degrees, and hook it up to manifold vacuum. I suggest you buy the Holley Jet Kit that has all the jet sizes from about 66 through 86 for something like $49 (don't quote me on that but I think that's about right..) rather than buying individual pairs of jets.
Feel free to contact me if you need assistance:
V8FastCars@msn.com
...you're not setting me up for another "is my carb big enough" discussion here on the Forum, are you...?
just kidding...I think everyone knows my position on carb sizing: You can make virtually any size carb (within some reasonable limits) run just fine. Your 600 can be set up to produce a nice idle, very crisp throttle response, good drivability, and a good strong feel at WOT. I believe a larger carb will probably get you a little better top end at WOT, and it's something you can consider changing to in the future. But for now, the 600 can be made to run just fine.
My recommendation on jet sizing and jet changing is to bump the primary up 2 sizes at a time and see how the car reacts. Install a set of standard resistor plugs (non-platinum) - I prefer the regular Autolites. The standard plugs are "readable," whereas the exotic plugs burn so clean throughout a varied mixture range that you cannot "read" those plugs with any reliability. If your carb does not have a secondary power valve (most don't), keep the secondary jet sizing 8 sizes bigger than the primary side jets: When the primary side power valve opens, it is equivalent to an 8-size jet increase, so running the secondary side 8 sizes bigger keeps the carb flowing equally at all 4 corners at WOT.
As always, be sure your ignition timing and advance system is set up correctly and operating well. You should be running about 36 degrees total, have a functioning vacuum advance control unit that pulls an additional 16 degrees, and hook it up to manifold vacuum. I suggest you buy the Holley Jet Kit that has all the jet sizes from about 66 through 86 for something like $49 (don't quote me on that but I think that's about right..) rather than buying individual pairs of jets.
Feel free to contact me if you need assistance:
V8FastCars@msn.com
