Wrong jets?
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From: East Coast
Wrong jets?
I have a 302 with a holley 600 cfm 4160 on it. It has #66 jets. It runs ok, but to get it to run good the idle mixture screws must be turned nearly all the way in. What would cause that? Thanks
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From: Billings MT
Wrong jets?
Ok, you have to turn the air mixture screws almost all the way in, thus restricting the air, making the air/fuel ratio greater.
I would go with bigger jets. I'm not real familiar, but I would think 2 sizes bigger would help a lot.
This is assuming that everything else is working correctly like the choke, and there are not any vacuum leaks.
I would go with bigger jets. I'm not real familiar, but I would think 2 sizes bigger would help a lot.
This is assuming that everything else is working correctly like the choke, and there are not any vacuum leaks.
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Joined: Dec 2001
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From: Louisville,Ky.
Wrong jets?
It depends on what the LIST # of the carb is. By turning the mixture screws all of the way in, you are cutting off the fuel, not air. There are a few list #'s out there that have reverse idle circuits, but most are traditional where the screws control the fuel. Will it die if you turn them all of the way in?
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From: East Coast
Wrong jets?
The carb is a 4160, list # is 0-8005. I believe jbalestri is right, it says on the metering block that counter-clockwise leans the mixture, so the screws tap into air, not fuel.
Like I said, it runs best with the screws all the way in, it dosn't stall. It runs rougher as you turn the screws out.
And it is correct that I would want larger jets, not smaller, right? That would increase the amount of fuel so I could use the screws to let more air in accordingly by backing them out. Any bad side effects to that? (changing jets)
Choke works fine and I'm not aware of any vacuum leaks.
Thanks!
Like I said, it runs best with the screws all the way in, it dosn't stall. It runs rougher as you turn the screws out.
And it is correct that I would want larger jets, not smaller, right? That would increase the amount of fuel so I could use the screws to let more air in accordingly by backing them out. Any bad side effects to that? (changing jets)
Choke works fine and I'm not aware of any vacuum leaks.
Thanks!
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From: Billings MT
Wrong jets?
So, counter clockwise is backing out the screws, correct? If so, backing out the screws decreases the air/fuel ratio. The only way to make that smaller is to increase the air because the jets control the amount of fuel and that is a constant, based on jet size.
I would think that you should be able to call any performance shop that has a clue and they should be able to tell you what size jets you need. Altitude plays a big part, but I always thought that carbs were sold with "sea level" jets in them.
Really you want it so that the screws are about 2 turns out. That leaves plenty of adjustment. I had a jet boat with a 455 Olds with a Holley on it. One lake that I used to go to is about 4000 ft elevation, and another is about 5500 ft. I had to tweak the mixture screws everytime I went to one of the lakes, because the carb was that finicky. I would not want a set up that didn't have a little adjustment in it.
I would think that you should be able to call any performance shop that has a clue and they should be able to tell you what size jets you need. Altitude plays a big part, but I always thought that carbs were sold with "sea level" jets in them.
Really you want it so that the screws are about 2 turns out. That leaves plenty of adjustment. I had a jet boat with a 455 Olds with a Holley on it. One lake that I used to go to is about 4000 ft elevation, and another is about 5500 ft. I had to tweak the mixture screws everytime I went to one of the lakes, because the carb was that finicky. I would not want a set up that didn't have a little adjustment in it.
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From: Louisville,Ky.
Wrong jets?
As a general rule, only change about 2 sizes at a time according to Holley. I would try 68's first. A little is alot on these things. I have worked on Holley's for many years and have also found this to be true thru my own experience. Also, check for small vacuum leaks before you change the jets. If your only problem is at an idle, your drivabilty may suffer from being too rich if you start jetting up the carb. check all vacuum lines, around the carb gasket and intake gaskets. If you do have to change the jets, be sure to remove all of the old gasket material from the metering block and thouroughly clean all debris out of all of the port holes. Spray some silicon spray on the new gaskets before installing them, it will make them come off easier if you have to open it up again, also, make sure that the o-ring goes in properly on the fuel transfer tube that feeds fuel to the rear barrel.
Jimmy
Jimmy
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From: Corvallis oregon
Wrong jets?
I personally only change one jet size at a time. ou can spray wd-40 on the intake and carb gaskets to see if there are any leaks, the more will respond to the spray.
And keep a close eye on your plugs.
And keep a close eye on your plugs.
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Posting Guru
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From: East Coast
Wrong jets?
Carb dosn't have any vacuum lines on it, just two off the back of the manifold and their all new. Sprayed 2+2 around intake and carb and didn't notice any difference, so I don't think there is a vacuum leak. Rebuild the carb about a year ago and it was always like this with the mixture screws, but I put vasaline on all the gaskets so it won't be too hard to get it back apart.
