Carb Jetting
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Mountain Pass
Joined: May 2011
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From: Gillette
Carb Jetting
My dad and I rebuilt a 400 last summer and put it in my 1978 F-250. The engine had to be bored 60 over. I put a Comp Cams XE262h-10 cam in it. Also has a 670 CFM Holley Truck Avenger carb. Only other thing of note is the Machinist milled the heads to get the compression up for me. Off hand I cant remember what he said he got it to but im thinking it was right around 9:1. I may be mistaken.
Problem im having is the engine runs really rich. On warm day it does a bit better but still can tell its rich. When i noticed it was last winter when it was about 20 degrees out and it acted like the choke was shut. I checked and it wasnt. It has #68 jets in it atm.
I know the 670 is too big. Its already paid for though and there so i need to if possible figure something out. I also know that the reason it runs better when its hot is the air is warmer and thicker as apposed to being cold and thinner. Just trying to save someone the trouble of typing that part out. I just hope theres something I can do to get it to run a bit better. Any help is much appreciated. Thanks.
Forgot to add the elevation here is around 6000 feet
Problem im having is the engine runs really rich. On warm day it does a bit better but still can tell its rich. When i noticed it was last winter when it was about 20 degrees out and it acted like the choke was shut. I checked and it wasnt. It has #68 jets in it atm.
I know the 670 is too big. Its already paid for though and there so i need to if possible figure something out. I also know that the reason it runs better when its hot is the air is warmer and thicker as apposed to being cold and thinner. Just trying to save someone the trouble of typing that part out. I just hope theres something I can do to get it to run a bit better. Any help is much appreciated. Thanks.
Forgot to add the elevation here is around 6000 feet
Post Fiend
Joined: Mar 2011
Posts: 8,501
Likes: 4
From: Lynchburg, VA
Welcome to FTE.
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Mountain Pass
Joined: May 2011
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From: Gillette
Im sorry I misspoke. Apparently i misunderstood. For some reason i was always under the understanding that as air got colder there was less of it. Therefore if jetting didnt change it would run richer. Then after reading your post I sat back and remembered my snowmobiles. Up on the mountain as it got colder it required more fuel.
That being said im even more confused as to why when it got colder outside it ran richer or seemed to. Course a friend of mine years ago told me that running super lean can have same effect as running super rich. Same effect as in how the engine acts. It felt like the engine was loading up really badly. I drove it when the weather cleared up and it ran a lot better. I guess i just assumed that since the carb was a lot bigger then i needed I must be running way rich. Maybe im too lean.
That being said im even more confused as to why when it got colder outside it ran richer or seemed to. Course a friend of mine years ago told me that running super lean can have same effect as running super rich. Same effect as in how the engine acts. It felt like the engine was loading up really badly. I drove it when the weather cleared up and it ran a lot better. I guess i just assumed that since the carb was a lot bigger then i needed I must be running way rich. Maybe im too lean.
Hotshot
Joined: Feb 2004
Posts: 10,750
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From: The Shenandoah Valley
FWIW .... 670 cfm isn't "too big" for a 400. Vacume secndaries would be best, and with a stiff enough spring in the vacum diaphram housing the back half will open slow enough to let the motor keep up and avoid a bog.
Many a 350-400 cubic inch motor has lived quite happily under 650-750 cfm carbs. It's when people do little tricks to manually or otherwise open the secondaries earlier than needed with no accelerator pump shot, often at low speed .... when the "big boggs" are found.
BRUUaaaAAAAAHHHHH
Has this Holley Avenger been reworked, overhauled, rejetted or otherwise had metering altered?
Many a 350-400 cubic inch motor has lived quite happily under 650-750 cfm carbs. It's when people do little tricks to manually or otherwise open the secondaries earlier than needed with no accelerator pump shot, often at low speed .... when the "big boggs" are found.
BRUUaaaAAAAAHHHHH
Has this Holley Avenger been reworked, overhauled, rejetted or otherwise had metering altered?
Tuned
Joined: Sep 2010
Posts: 279
Likes: 8
You need a friend with a jet selection or try to take a best guess if you have to buy them, the only way to know is to just go down in jet sizes until it gets better, I would try and borrow a pair of 63 or 64 for starters for the primary barrel, stay out of the 2nd barrel drive it and see how it does, lightly anti seize or grease the bowl gaskets and you can get alot of on and offs out of it before having to replace them.
That should have 89 jets in the 2nd barrel, I would put the 68s in there just to see how it responds, but be carefull you dont want to lean it out too much.
Just do a internet search on "Holley tuning guide" you can get decent info that will help, pull a plug that will tell you if it is rich, it will be really dark or black, too lean will be white, start reading some tech articles, you can get that carb running well.
That should have 89 jets in the 2nd barrel, I would put the 68s in there just to see how it responds, but be carefull you dont want to lean it out too much.
Just do a internet search on "Holley tuning guide" you can get decent info that will help, pull a plug that will tell you if it is rich, it will be really dark or black, too lean will be white, start reading some tech articles, you can get that carb running well.
Post Fiend
Joined: Mar 2011
Posts: 8,501
Likes: 4
From: Lynchburg, VA
BTW, when you do a plug chop to check the main fuel circuit, you need the rpm to stay above idle until you cut the engine off. Allowing it to idle will mess with the results. Since your at 6000ft, I wouldnt be at all surprised that your running rich.
Compare your plug to these:
Compare your plug to these:
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