Holley 870 cfm? How to tune?
04-06-2003, 02:54 PM
#1
Race Director
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Holley 870 cfm? How to tune?
After many days of procrastination......I finally replaced the spark plugs.....The plugs indicated a very rich condition...The Hardcore 870 cfm carb is a double pumper with four corner mixture adjustment screws.....I'm afraid to set the lean/rich mixture since setting too lean can cause detonation and possibly blow the top of pistons off......The screws were set by Bill Mitchell during the dyno.....Can these screws go out of adjustment? Should I leave them alone and adjust the float levels? How do you know when you have the mixture in proper ratio? Do you set floats first, then the mixture screws or visa versa? :flag :seeya Cappy
04-06-2003, 03:36 PM
#2
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Re: Holley 870 cfm? How to tune? (8T1-7T9 BIONIC VETTE)
any carb that is jetted on a engine dyno is going to be rich because of the higher under the hood temp in the car unless you have outside cool air going to the carb. you need to know what were the atmospheric conditions were when the carb was dynoed. also you need to lean out 2% for every 1500 ft in alttitude above where the carb was dynoed. :chevy
04-06-2003, 03:42 PM
#3
Safety Car
Re: Holley 870 cfm? How to tune? (8T1-7T9 BIONIC VETTE)
Cappy:
75% of all muscle cars run in a rich condition (or so a wise man has told me ;) ). By running your motor rich you are missing out on power and washing the cylinder walls down...which is very hard on rings.
I ran into the same thing with my 468. Driving on the highway it was fine, but whenever I took it in town it would start fouling plugs at low RPMs and run crappy.
The easiest way I have found to tell if your motor is running at the proper air/fuel ratio is the spark plugs. I don't run any fancy platinums, you can't read those the same as a standard spark plug. I run regular R45 AC Delcos. Your plugs should have a nice tan color to them whenever you pull them out.
De-jetting the carburetor has a very slight effect on how rich it runs. Leaning out the idle circuit is the only way to get your motor running properly and lean. You want to open the primary and secondary throttle plates an equal amount...but not so much that the transition slots are showing. If the transition slots are showing it throws the entire idle circuit out of whack. Depending on how big of a cam you have, you may need to drill a small hole in each of the primary throttle plates. You want to drill the hole on the front side of the primary throttle plates closest to the wall.
In most instances opening up the secondary throttle plates to allow more air at idle is more than sufficient enough to lean out a motor unless you are running a hairy cam. Once you are certain that the idle circuit is where it needs to be then you need to start messing with the four corner idle circuit screws. You want to start by cranking the screw the entire way in until it bottoms out. Now you want to SLOWLY start cranking it back out. You continue cranking it out until you hear the idle start to drop off (this can also be done using a vacuum guage). Repeat for the remaining three screws.
This will get you right where you need to be. My car used to burn my eyes if it sat in my garage and idled....no longer does it do that. I no longer foul plugs around town and my spark plugs look brand new just like I installed them nearly a month ago. My motor is also noticably more powerful than before, it's like a night and day difference.
I'm sorry about my spelling but I am in a bit of a hurry to get to my girlfriends. Hopefully you can get your motor leaned out and pick up some free ponies :) .
*EDIT*
You shouldn't need to mess with your float adjustment. You can verify correct float adjustment by taking your sight plugs out while the motor is running. The fuel level should be even with the bottom of the sight plug hole. If it isn't then you need to adjust your float level accordingly.
[Modified by bence13_33, 2:44 PM 4/6/2003]
75% of all muscle cars run in a rich condition (or so a wise man has told me ;) ). By running your motor rich you are missing out on power and washing the cylinder walls down...which is very hard on rings.
I ran into the same thing with my 468. Driving on the highway it was fine, but whenever I took it in town it would start fouling plugs at low RPMs and run crappy.
The easiest way I have found to tell if your motor is running at the proper air/fuel ratio is the spark plugs. I don't run any fancy platinums, you can't read those the same as a standard spark plug. I run regular R45 AC Delcos. Your plugs should have a nice tan color to them whenever you pull them out.
De-jetting the carburetor has a very slight effect on how rich it runs. Leaning out the idle circuit is the only way to get your motor running properly and lean. You want to open the primary and secondary throttle plates an equal amount...but not so much that the transition slots are showing. If the transition slots are showing it throws the entire idle circuit out of whack. Depending on how big of a cam you have, you may need to drill a small hole in each of the primary throttle plates. You want to drill the hole on the front side of the primary throttle plates closest to the wall.
In most instances opening up the secondary throttle plates to allow more air at idle is more than sufficient enough to lean out a motor unless you are running a hairy cam. Once you are certain that the idle circuit is where it needs to be then you need to start messing with the four corner idle circuit screws. You want to start by cranking the screw the entire way in until it bottoms out. Now you want to SLOWLY start cranking it back out. You continue cranking it out until you hear the idle start to drop off (this can also be done using a vacuum guage). Repeat for the remaining three screws.
This will get you right where you need to be. My car used to burn my eyes if it sat in my garage and idled....no longer does it do that. I no longer foul plugs around town and my spark plugs look brand new just like I installed them nearly a month ago. My motor is also noticably more powerful than before, it's like a night and day difference.
I'm sorry about my spelling but I am in a bit of a hurry to get to my girlfriends. Hopefully you can get your motor leaned out and pick up some free ponies :) .
*EDIT*
You shouldn't need to mess with your float adjustment. You can verify correct float adjustment by taking your sight plugs out while the motor is running. The fuel level should be even with the bottom of the sight plug hole. If it isn't then you need to adjust your float level accordingly.
[Modified by bence13_33, 2:44 PM 4/6/2003]
04-06-2003, 03:49 PM
#4
Race Director
Thread Starter
Re: Holley 870 cfm? How to tune? (bence13_33)
Thanks Clem and Bence......I've printed your post and will use the info you provided to make the adjustments.....
I just did a very slow parade with this car and that could be why the plugs are fouled. I won't do any more parades....
Run her for awhile and check plugs again.....if they are fouled....I'll do the four corner and drill plates as instructed....Thanks again :flag :seeya Cappy
I just did a very slow parade with this car and that could be why the plugs are fouled. I won't do any more parades....
Run her for awhile and check plugs again.....if they are fouled....I'll do the four corner and drill plates as instructed....Thanks again :flag :seeya Cappy
