tuning help
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tuning help
i'm having some trouble tuning my new 390. it has 9.5-1 comp, edelbrock rpm heads, rpm intake, msd dist and 6a box, 750 demon mech carb, comp xtreme energy cam with 284in/296ex dur. and 584in/588ex lift. i have an adjustable valvetrain, i'm running a 2500 stall with 4.56 gears. the problem is when i hammer on it, it seems like it has a flat spot. the trucks goes better if i slowly give it gas. i have the timing set at 15 degrees btc, i have a mechanical advance dist, so i have all the timing coming in at aroung 3400 rpms. the jettings close, a slight brown. any suggestions ? i'm sort of stumped ! 
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Joined: Jul 2003
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From: iowa
to much fuel at once! I don't know what do fix it. I thought mech sec were recommended for manuals not autos but you have a good stall so its hard to say. check with ratsmoker on the msd. I think he runs 19btdc and 19 mechanical.
Posting Guru
Joined: Jul 2003
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From: Colville, Washington
Does the engine rev quick under no load? It could be the accelerator pump circuit-either not enough or wrong timing of the pump shot. How does it accelerate from say... half throttle on up?
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From: Planet Earth
Sounds to me like your accelerator pump is shooting to much or not enough fuel. Can you get a accel pump cam kit like they sell for a holley. I had the same problem... if you were being easy on the throttle, taking off from a stop sign, it would go briefly, then crap out, then jump again. If you just floored it, you would fly.
I bought this kit from holley, it consisted of six or seven different colored, different sized, lobed nylon bushings. You would install the different pieces to give you different accel pump charactoristics.
I put the biggest one in, and I havent' had a hesitation problem since...
Good luck.
Sam
I bought this kit from holley, it consisted of six or seven different colored, different sized, lobed nylon bushings. You would install the different pieces to give you different accel pump charactoristics.
I put the biggest one in, and I havent' had a hesitation problem since...
Good luck.
Sam
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i just got in from tinkering with the truck, i put a different curve springs in it, now i'm fully advanced by 3800 rpms. i was reving it up slightly and the carb seemed to gurgle when i snapped the throttle fast. building the engine was fun, now tuning it is a pain !!!!
Fleet Mechanic
Joined: May 2004
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From: Montana
carbs suck man especially holleys but there where its at. I think you need a carb guru in your town. IT almost sounds like a metering problem or jetting......Im no pro though. Heck my carb had a flat spot in it and it was just dirty i soaked the body in gas and blew it out and it was all good.
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changing the advance springs helped a lot !! now when i get into the secondaries it sort of gurgles and goes, i assume the secondaries are too rich? would the valvelash contribute to any performance agains or loss? i think my lash needs adjusting, again !!
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From: Corpus Christi, Texas
Led sled,
Got a couple of question?? when you are idling the truck do a "snap test" on your throttle. If it seems to studder than pick up than your squitter are to big and to rich, if you do this and it tries to back fire than your to lean on your squitters. But what you are describing sounds like your to rich I got a 390 with a holley 750 mech sec. I am running 76 jets in the front and 83 in the rear, I have a 25 squiter in the fron and 35 in the rear. because you advance your ignition to come in soonner and your trucks seems to run better it sound like you have a very rich problem. another thing how does it idle?? just make sure your timming is good and than setting your carb to your engine is not to hard just time consuming!!
another thing try stalling your engine out in gear if your rpms don't come fast while you acclerate on your pedal than you have a rich problem, if your rpms come up but seem to fall of you have a lean problem, try this I hope it helps it took me four months of playing with mine to learn some ways to tune it in!!
Got a couple of question?? when you are idling the truck do a "snap test" on your throttle. If it seems to studder than pick up than your squitter are to big and to rich, if you do this and it tries to back fire than your to lean on your squitters. But what you are describing sounds like your to rich I got a 390 with a holley 750 mech sec. I am running 76 jets in the front and 83 in the rear, I have a 25 squiter in the fron and 35 in the rear. because you advance your ignition to come in soonner and your trucks seems to run better it sound like you have a very rich problem. another thing how does it idle?? just make sure your timming is good and than setting your carb to your engine is not to hard just time consuming!!
another thing try stalling your engine out in gear if your rpms don't come fast while you acclerate on your pedal than you have a rich problem, if your rpms come up but seem to fall of you have a lean problem, try this I hope it helps it took me four months of playing with mine to learn some ways to tune it in!!
Last edited by polcat; Jun 18, 2004 at 09:30 PM.
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when i do the snap test it does studder. the truck idles decent considering the cam i have. when i break torque the truck the rpm's don't shoot up, and the tires barely spin. the more i push on the pedal , the same results i get.
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From: Corpus Christi, Texas
Sounds like you have leaned it out to much go up on your squiters to about a 30. your squiters work from idle to just off idle, after that your engine starts to pull vacuum through your intake and your jets take over, slowly brinn up your rpms if it does not stumble but starts to gain power that your jets are ok if it still stumbles you might want to go up on your jets. before you do this try screwing in your front air mixture screws all the way end and back them out tell you here your engine just begin to histate do one at a time same way on both. you air adjustment screws might be to far out to this could cause this problem. tunning a carb is hard believe me!!!!
Last edited by polcat; Jun 19, 2004 at 03:58 PM.
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ii know ! but whn its all said and done its worth it. i put a 28 squirter in the front, and a 79 jet. i kept the back the same as you mentioned. it has a very lite flat spot but is 100% better then before ! i do i tune the secondaries?
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From: Corpus Christi, Texas
Led sled, Sorry have not been able to post for a while, Sounds like your truck is running better. the way i tune the secondaries is by making sue my total timing is no more than 40 degrees and go out and stand on it, if it does not ping than I know i am good , if it does than i need to go to a larger jet, if you have air mixture screws on the secondaries, screw them all the way in and back them out about a turn maybe a turn and a quarter. Just takes time, I just got a knew carb so when I get mine bakc running I will have to tune my truck all over again!!
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i just got it running nice, i went to a cruise night and it starting running funny, it was hard to start . my buddy on the way home in his 70 fairlane pulled up next to me and wanted to run, well..... i smoked him!!!! but now there iss a noisr that sounds like someones taking a spraypaint can and rattling it in my engine ! i pulled the valvecovers off, everything up there looks good, got oil pressure. any ideas !!! i'm tried of this stuff !!!!!!!!!!!!!!!!1