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  #16 (permalink)  
Old 03-02-2006, 10:44 AM
Choctaw Bob's Avatar
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Quote:
Originally Posted by Jmark
You need to get the base timing down to around 14 to 18 degrees before trying to figure your mechanical advance. Then proceed as all the others have said on the idle slot and secondary throttle plate adjustments. Drill the primaries as a last resort.

Personally, on a 2 mix screw carb, I prefer to drill as apposed to opening up the secondaries. The front of the carb will be richer and the secondary side will be fairly lean drawing in the extra air. On a 4 screw carb, its easier to compensate on all 4 corners.

My son's 400 did the same thing, cept we had a Qjet on it. I ended up drilling a 5/32 hole in each primary butterfly and it cured the idle mix problem right up, but its a permanent modification too, short of changing out the primary blades if you drill too big.

Mark
I'd say do not drill. If you start with the throttle plates closed completely, the idle mixture screws 2 1/2 turns out, the base timing at 12degrees, the car (with a stock cam) should idle around 1,000 RPM. Your radical cam may need more air, say another turn. If you get to 4 turns on the idle air, something else is wrong, maybe a blown out power valve or more than 7 pounds of fuel pressure or a stuck check valve(ball) (acellerator pump)
If it was a quadra jet I would say the lead plugs in the bottom of the carb assembly were leaking.

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  #17 (permalink)  
Old 03-02-2006, 10:46 AM
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Quote:
Originally Posted by Tech @ BG
Leejoy,

How big is your camshaft? What is the rest of your engine combination?

That's what I was thinking. 600 to 700rpm is pretty low even for a stock type cam. You may need set it at 800 to 900rpm. Timing might be in the 16 to 18 range at the balancer.
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Old 03-02-2006, 10:59 AM
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the camshaft is new, so are the lifters and springs. Its one of those comp cams extreme magnum flat tappet hydraulic type. I think its part number 11-250-3. It's about .575 gross lift for both intake and exhaust and about 270 to 280 deg duration at .006" lift. not sure on lobe seperation and intake centerline but they are standard I believe. anyway it also has a dual plane intake manifold, MSD ignition and distributor, Holley 650 with vac sec, roller rockers, .060 overbore 454 BB Chevy, internally balanced, stock iron closed chamber heads (big oval ports) with porting and valve job done, domed pistons which make approx 10.5 to 1 comp ratio, 2" headers and so on.
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Old 03-02-2006, 11:31 AM
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Quote:
Originally Posted by leejoy
the camshaft is new, so are the lifters and springs. Its one of those comp cams extreme magnum flat tappet hydraulic type. I think its part number 11-250-3. It's about .575 gross lift for both intake and exhaust and about 270 to 280 deg duration at .006" lift. not sure on lobe seperation and intake centerline but they are standard I believe. anyway it also has a dual plane intake manifold, MSD ignition and distributor, Holley 650 with vac sec, roller rockers, .060 overbore 454 BB Chevy, internally balanced, stock iron closed chamber heads (big oval ports) with porting and valve job done, domed pistons which make approx 10.5 to 1 comp ratio, 2" headers and so on.
You need a lot more carb with this engine and cam. I looked it up, it is 284-296 advertised duration with lobe seperation of 110. The lift is .574 and .578.
This is a pretty big cam for the street. If you run this on an engine simulator, you will see you need a 900 CFM carb to make it run. This may have enough overlap that you may be getting enough reversion to screw up the idle signal.
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  #20 (permalink)  
Old 03-02-2006, 11:57 AM
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I'm running an even larger cam on a 2.5L 4 banger with 10.5:1 compression and it idles at 700 rpm with one hell of a lope- hard to manage on most 4 bangers. I found that one thing that helps a lot is a vaccuum canister, just rip one out of a factory car and hook it up- it really helps stabilize things. Also, I have ahd to drill holes in the plates before, and it does help, though I would start with smaller holes- say 3/32". Timing is very critical on these "not supposed to but we'll make it work for the street anyways" type engines.
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Old 03-02-2006, 01:46 PM
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Quote:
Originally Posted by Choctaw Bob
You need a lot more carb with this engine and cam. I looked it up, it is 284-296 advertised duration with lobe seperation of 110. The lift is .574 and .578.
This is a pretty big cam for the street. If you run this on an engine simulator, you will see you need a 900 CFM carb to make it run. This may have enough overlap that you may be getting enough reversion to screw up the idle signal.
thanks for the advice about the carb and the vacuum can. I am definitely getting a new carb but was waiting for next years budged. I spend about 2K per year on the car and this years money is spent. I carb will set me back at least $350 even if i bought it cheap on ebay. I will try the vacuum can idea and see if that helps. if it does, i will make it a permanent installation.
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Old 03-02-2006, 02:52 PM
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If that's a 650 quadrajet replacement Holley, take it off and throw that made in Japan POS as far as you can. They have a reverse idle circuit to further complicate the recomendations here. Bolt on a 750 Holley and I bet your problems will go away.
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  #23 (permalink)  
Old 03-02-2006, 03:30 PM
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DUDE

we need to talk. I'm starting to think like you now. Is that carb really a POS? why. What do you mean reverse idle circuit? and what do you mean made in Japan? Is that true? you're kidding right?
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  #24 (permalink)  
Old 03-02-2006, 03:31 PM
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Leejoy,
Hook up with one person and follow their lead. As you can see you will get a ton of different opinions or ways to get the job done. Tech@BG knows carbs and if you listen to him and follow "exactly" what he tells you, I'm sure he will get you tuned (Jmark and johnsongrass1 know their stuff as well). There are several guys on this board that can help you. we may go about it in a different order but, the end result should be the same. I "assumed" you had the timing and rest of the tune up nailed down. Even once everyting is dialed in you may still need to drill the primaries to get the idle to lean out. There is no wy to tell until you get the rest of the tune up corrected.

Royce
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  #25 (permalink)  
Old 03-02-2006, 03:38 PM
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Quote:
Originally Posted by camaroman7d
Leejoy,
Hook up with one person and follow their lead. As you can see you will get a ton of different opinions or ways to get the job done. Tech@BG knows carbs and if you listen to him and follow "exactly" what he tells you, I'm sure he will get you tuned (Jmark and johnsongrass1 know their stuff as well). There are several guys on this board that can help you. we may go about it in a different order but, the end result should be the same. I "assumed" you had the timing and rest of the tune up nailed down. Even once everyting is dialed in you may still need to drill the primaries to get the idle to lean out. There is no wy to tell until you get the rest of the tune up corrected.

Royce
Great advice!
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Old 03-02-2006, 06:54 PM
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thanks royce. good advice.

I should add that the engine seems to run rough at higher rpm also, even if i play with the timing (up and down). when the car is in park and idling (rough and smelly) when I manually open the throttle linkage the motor runs faster but not very smooth. the timing light flashing is not super smooth either. it appears that it's not just an "idle" problem. I solicited the bulletin board for help. anybody in chicago area (that's where I live) will get free food and drink if they come over and help me out.
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Old 03-02-2006, 07:44 PM
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Quote:
Originally Posted by leejoy
thanks royce. good advice.

I should add that the engine seems to run rough at higher rpm also, even if i play with the timing (up and down). when the car is in park and idling (rough and smelly) when I manually open the throttle linkage the motor runs faster but not very smooth. the timing light flashing is not super smooth either. it appears that it's not just an "idle" problem. I solicited the bulletin board for help. anybody in chicago area (that's where I live) will get free food and drink if they come over and help me out.
If you would make it Food, Drink and Women, I will drive up from Dallas

After reflecting on that statement a bit, I will have to review resume's and photos of the women and a menue of the food and drink
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Old 03-03-2006, 09:08 AM
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Originally Posted by Choctaw Bob
If you would make it Food, Drink and Women, I will drive up from Dallas

After reflecting on that statement a bit, I will have to review resume's and photos of the women and a menue of the food and drink
ok - done

I'll grab a few gals in the neighborhood and see if they are willing to donate some of their time to our cause.
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  #29 (permalink)  
Old 03-03-2006, 09:25 AM
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Carb Tuning

Leejoy,

You may have written down, and I've just missed it. What intake manifold are you using? That is a pretty healthy cam for a street engine, but you should be able to get it to run properly. I'd aim for about 900 RPM idle. I'd start out with between 15 and 18 degrees initial timing, and shoot for about 36 total. Normally with a camshaft of this size you and an idle RPM below 1000 you shouldn't have to drill holes in the butterflies if everything is setup and tuned properly. Agreed a 650 is small for this combination, and you would get better performance with a 750 carburetor. The smaller carb will have higher air velocity, and will be richer at idle. I'd start out with your butterflies set so that you can just see the bottom of the transfer slot on the secondary side, and see about .020" on the primary side (a little square as noted by others). Now set your mixture screws to about 1 turn out from seated. Set the float levels to the bottom of the windows, and make sure your boosters are not dripping any fuel. That should put you in the proper RPM range as a starting point. If it's not doing what you want at that point you need to see if the engine wants more or less fuel. Assuming that everything else is OK, this is pretty simple. With the engine running, press on the accelerator pump arms without moving the throttle linkage. This will add fuel without changing the amount of air going through the carburetor. Smoother idle = wants more fuel come out on the mixture screws. Idle worse = you're already too rich go in on your screws. Since this is a new package you might want to borrow a carburetor off of a known working vehicle to eliminate a faulty carburetor from pulling your hair out.
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Old 03-03-2006, 09:32 AM
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Quote:
Originally Posted by Tech @ BG
Leejoy,

You may have written down, and I've just missed it. What intake manifold are you using? That is a pretty healthy cam for a street engine, but you should be able to get it to run properly. I'd aim for about 900 RPM idle. I'd start out with between 15 and 18 degrees initial timing, and shoot for about 36 total. Normally with a camshaft of this size you and an idle RPM below 1000 you shouldn't have to drill holes in the butterflies if everything is setup and tuned properly. Agreed a 650 is small for this combination, and you would get better performance with a 750 carburetor. The smaller carb will have higher air velocity, and will be richer at idle. I'd start out with your butterflies set so that you can just see the bottom of the transfer slot on the secondary side, and see about .020" on the primary side (a little square as noted by others). Now set your mixture screws to about 1 turn out from seated. Set the float levels to the bottom of the windows, and make sure your boosters are not dripping any fuel. That should put you in the proper RPM range as a starting point. If it's not doing what you want at that point you need to see if the engine wants more or less fuel. Assuming that everything else is OK, this is pretty simple. With the engine running, press on the accelerator pump arms without moving the throttle linkage. This will add fuel without changing the amount of air going through the carburetor. Smoother idle = wants more fuel come out on the mixture screws. Idle worse = you're already too rich go in on your screws. Since this is a new package you might want to borrow a carburetor off of a known working vehicle to eliminate a faulty carburetor from pulling your hair out.
Dear Mr. Tech

I already did the first part of your message, now I can test the accelerator pump idea and see what that does. good idea. I may have a blown power valve. I need to help to explain. If it is blown I can see why i'm having these problems. I think when the idle mixture screws are turned all the way in there is little or no change to idle if I recall......indicating a blown (open-ruptured) power valve right? what I don't understand is if the power valve is not blown, but open, how would that affect idle? At idle it's just drawing through the idle ports anyway and so what if the power valve is open. I don't understand.
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