carb tuning
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Senior User
Joined: Jan 2003
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carb tuning
I have a problem starting my truck no matter when i try to start it i have a 390 with an "S" code intake, 600 holley vac. secondarys, a 901 cam, it will turn over and sputter kind of and then sometimes it will blow back up through the carb and it takes a while of turning over and playing with the gas to get it running, it will even do this a little right after i turn it off and try to start it agian. I think it is in the carb somewhere but i am not sure, any suggestions? Do u think i might need to tune it alittle?
Post Fiend
Joined: Jan 2003
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From: Iowa
carb tuning
Is it a brand new installation, or has it been working then quit? You might try adjusting the idle mixture screws. There are two on each side of the primary metering block. Turn them to the right until they stop, do not tighten them. Then turn them back two turns. This should allow the engine to idle right, then tighten them back down until it sounds right. Mine are at about 1 & 1/2 turns out. If this doesnt work, you might tighten the idle speed screw a little. If that doesnt work, spend the weekend and rebuild the carburetor. Make sure to use compressed air to clean out all the passages, and get a book on it if you dont already have one or know how to do it. Good luck! 
Edit: Didn't see the part about blowing back up through the carb, I think your cam timing may be off, someone else can help you on that, I havent done one yet...
Edit: Didn't see the part about blowing back up through the carb, I think your cam timing may be off, someone else can help you on that, I havent done one yet...
Last edited by rusty70f100; May 8, 2003 at 10:46 PM.
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Joined: Mar 2003
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From: Corvallis oregon
carb tuning
Pull out a few spark plugs, if they are black you need leaner jets, if they are white you need richer ones. You should first try the idle mix sarews, but it sounds like your carb needs a little more then that. It can be hard to tell if you have too much or too little fuel sometimes. Just make sure that your choke isn't stuck on.
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carb tuning
I screwed with the idle misture screws alittle today it made it run alittle better but it stil doesnt start like it should it seems like the choke doesnt kick on or something like that im not that big into carbs so any other suggestions would be great.
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Joined: May 2003
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From: LAKE JACKSON, TX
carb tuning
did you ever get a good back fire through the carb? if so you need to rebuild it. holly carbs are very sensitive one good back fire tears it all up. summit sells a kit to put in a relief vale in the plate somwhere not quite sure how it works but it comes with everything you need and a drill bit to put the valve in. sounds like the problem to me.
Post Fiend
Joined: Jan 2003
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From: Iowa
carb tuning
I'd go to the nearest Auto Zone, or comparable store, and get a trick kit. Then do a full rebuild on it, and follow the instructions to the letter. Make sure you get plenty of carb cleaner, clean everything off, and use compressed air on all the passages. Have fun!
Post Fiend
Joined: Feb 2001
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From: Missouri
carb tuning
While it could be the carb, this has ignition timing written all over it. Try and advance your timing a few degrees.
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Lead Driver
Joined: Apr 2001
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From: Texas
carb tuning
I'm with Sean on this one. Back off the timing, and see what happens. Put a light on it if you get a chance. But, the actual timing means less to me when using a group of non-stock equipment. You have a different, intake, carb and cam. With that, you may need to test it by ear for a bit to find the sweet spot.
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Thread Starter
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Senior User
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carb tuning
hey thanks guys for the suggestions i will try screwing with my timing a little and if that doesnt work i will rebuild my carb if neither of them work i will write back. In the mean while any more suggestions will help.
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carb tuning
Originally posted by pach7498
did you ever get a good back fire through the carb? if so you need to rebuild it. holly carbs are very sensitive one good back fire tears it all up. summit sells a kit to put in a relief vale in the plate somwhere not quite sure how it works but it comes with everything you need and a drill bit to put the valve in. sounds like the problem to me.
did you ever get a good back fire through the carb? if so you need to rebuild it. holly carbs are very sensitive one good back fire tears it all up. summit sells a kit to put in a relief vale in the plate somwhere not quite sure how it works but it comes with everything you need and a drill bit to put the valve in. sounds like the problem to me.
Thats not it. You are talking about the power valve. If you have a blown PV, It will richen the mixture at all vacuum readings.
Popping through the carb is either a lean condition or timing. My bet is timing.
The carb kits will contain all gaskets needed for the carb and needle and seats. They run about $20-$30 depending upon where you get it and what carb # you have. I suggest only using a genuine Holley kit, or a Barry Grant kit, they are not much more $ but they are much better.
Post Fiend
Joined: Jan 2003
Posts: 8,600
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From: Iowa
carb tuning
I just paid $26.74 at the local Advance Auto Parts for the Holley Trick Kit. It comes with all the gaskets, an assortment of power valves, check *****, and other little things that are easily lost. I would suggest using everything new that came with the kit that you can. If you're running lean, you might buy a jet set, too. Be aware, however, that if you're gonna be changing the jets a few times to find the correct one, and you have the fuel transfer tube, you could have a problem. There are these annoying little o-rings for the fuel transfer tube. It is almost impossible to get these things to reseal right after the initial installation. While you're at the auto parts store you should stock up on these, if your carb has the fuel transfer tube. Have fun!
