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  #31 (permalink)  
Old 07-23-2004, 01:40 PM
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jpd37, have you figured out whether the engine needs more or less fuel at idle?

If you have to screw the idle screws all the way in, and yet you still can't get the mixture lean enough to idle well, then maybe I've found your problem.

From the Edelbrock manual:

"Measure the manifold vacuum at idle. If it is below 7" Hg, there is a good chance that the Metering Rods are in the up (rich) position. When combined with a high idle air rate this can cause the Nozzles to discharge fuel at idle. Use a weaker Step-Up Spring (see section on Step-Up calibration) to keep the Rods down at idle. With some cams, a stiffer spring (pink or silver) is necessary. Experimentation is the best way to determine which is best for your application."

This would sound reasonable because since you have a smaller engine than this carb was meant for, it is likely that this carb is allowing too much air into the engine at idle and the engine is not sucking hard enough because it's not a big enough engine.

So, I might expect your "manifold vacuum at idle" to be a bit low.

So, even without a vacuum gauge, if you've found that you need to lean it out more than the idle mixture screws allow, then follow those instructions and use a weaker Step-Up Spring.

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  #32 (permalink)  
Old 07-23-2004, 02:37 PM
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Terje, You have been so helpful thus far that I cannot thank you enough. I had suprise guests drop in from out of town so it will be a few days before I can try all the suggestions and experiments that you and others have made. Be assured, I WILL try all the suggestions. I have been printing all the discussions so I will have them handy when I do the tests. I feel confident I can resolve my problems, it is just going to take a little time to get it done. That's OK with me, cause I'm learning as we go. I'll post my findings in detail after I go thru the process.

Thanks for your ideas and perserverance.
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  #33 (permalink)  
Old 07-23-2004, 11:52 PM
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500 or 600 cfm

you probably heard everything under the sun by now but i was looking at chevy high performance today on line and they had the perfect tech article for you and your problem called,( size matters )they use a 383 sbc 440h.p.440 tor. and test carbs from 390-1050.read it and you'll be surprised.
chevy high performance@primedia.com,,,size matters
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Old 07-24-2004, 12:22 AM
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GoneNova, Is this it:

http://www.chevyhiperformance.com/te...ose/index.html

On page 4 they say, "While we could have altered the carburetor to promote a better power curve and perhaps equal the previous tested power, we would have been bandaging the situation rather than doing ourselves a favor."

I'm not quite sure I agree with them. I mean, they stuck a carb on there that's meant for a larger engine, but even so, before this carb the power and torque were both increasing. I'd really like to see if it would continue to increase if they were to optimize the tuning of the carbs. I'm quite disappointed that they didn't bother to try.

Also on page 4, "It definitely produced a low-speed stumble, and we thought about tweaking the idle-air bleeds but refrained since we had come this far without touching any of the other carburetors."

I find it a pretty lame excuse that just because the smaller carbs were already tuned by Holley to match the engine capacity they happened to be testing on they felt that they shouldn't bother to tune the larger carbs to see what they could do on that same engine if properly tuned for that engine.

Just because a carb that you buy doesn't happen to be tuned for your particular engine doesn't mean that if it was to be tuned properly for your engine it wouldn't perform the same or better, or worse than any other particular carb tuned for your engine.

Also, it would've been nice if they had given us the results of the output from their EGT and O2 sensors so that at least we could judge for ourselves how out of tune these larger carbs were.

Last edited by Terje; 07-24-2004 at 01:34 AM.
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  #35 (permalink)  
Old 07-24-2004, 06:46 AM
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Terge, I remember that articale & thinking thats not a very good comparison if you havent tuned each carb for the application.
Also , I had said earlier that a 500 would work better on the 307 that startd this post., & somebody replied that the 600 should work fine. I shouldve made myself clearer by saying the problem is in the adj/ timing/tuneup/ vac. leak etc., not the carb size. However, I feel that in the end, it would be more responsive with the smaller carb. I.E. 500 CFM

But...it should be able to run just fine with the 600,assuming everthing is tuned properly
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  #36 (permalink)  
Old 07-24-2004, 02:26 PM
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Once I got to page 4 of that article I was pretty disappointed. I had really hoped I would learn something.

I don't have the money to go out and buy 7 different size carbs and dyno the vehicle for each one just so that I can learn something new, but they had the carbs and the equipment and took the time to get 99% there. That extra 1% of effort would've made for a great article where I could've learned something new.

Instead what I learned is that a properly tuned carb that's large enough for an engine performs better than a non-tuned carb that is larger than it needs to be.

I already knew that a smaller carb would not perform on the high end, which is pretty obvious.

I'm guessing that Holley applies that formula to calculate carb size from engine displacement and RPMs, then tunes each carb for the size engine they are targeting, and that's why the carbs that were targeted for that size engine performed best on that engine. So, I believe that the numbers the article is showing is more of a matter of carburetor tuning than carburetor size. (Except for the 390 CFM carb)
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Old 07-25-2004, 03:08 AM
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Here's a really cool page about tuning carbs: http://www.bob2000.com/carb.htm

It was posted in another thread by DTL504.

It's all about Holley carbs, but most of the info there applies to any carb. Also, I believe that to properly understand your carb, you should understand other carbs. You should know what other carbs have that yours does not, and what yours has that others do not, as well as what your carb has in common with others.
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  #38 (permalink)  
Old 07-25-2004, 08:51 AM
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Quote:
Originally posted by Terje
.....I'm guessing that Holley applies that formula to calculate carb size from engine displacement and RPMs, then tunes each carb for the size engine they are targeting, and that's why the carbs that were targeted for that size engine performed best on that engine. So, I believe that the numbers the article is showing is more of a matter of carburetor tuning than carburetor size. (Except for the 390 CFM carb)
IMHO, Holley's intent was to simplify choosing a given carb cfm for the average guy. Using the formula (including the VE portion of the equation), a customer can get a carb that will work adequately with a minimum, if any, tweaking. In this case I'm referring to a stock or mild build in your average street car.

If you want to get the optimum peak performance from a highly modified street/race setup, you need to consider more factors than the equation allows for.

Patience is one virtue you must have (along with knowledge) to dial in a carb to achieve the absolute best performance.

Access to a dyno don't hurt either.
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  #39 (permalink)  
Old 07-26-2004, 06:38 AM
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Here's an update from tlast weekend's efforts. I purchased the calibration kit for the 1405 and with the aid of the charts in the manual started by changing the rods one stage leaner. After that I started trying differant springs to until I achieved a reasonable idle. I tried 8 variations of spring and rod combinations each time writing down the vacuum readings off the carb and the idle rpm and the engine temperature. The most suprising thing was when the engine temp reached 180, the secondaries began to bleed fuel while idling. I figured the floats were set too high. The setting was 7/16" as recommended by the manual. I increased the setting to 15/32" and reduced the flooding some. I increased the setting to 1/2" and the secondary bleed seems to have stopped. I'm not totally convinced I have solved the problem entirely, but I have made a nice improvement so far. I measured the angle of the carb to see how close to level it is. It is 2 degrees tilted back. That may have something to do with it. I also disassembled the entire carb and blew out every canal, tube and opening I could find with compressed air and I did it in both directions. Never found anything unusual. I also tested for leaks using the starter fluid test. Did not notice any change in the way it ran. Cutting off the fuel and running the bowl empty simply proved it was running rich because it revved a bit before it died. As I got the carb to meter less fuel I noticed the engine would restart quicker, but not instantly. The only time it would start instantly was when the motor was shut off and then immediately restarted. I have to work all week so I won't have another chance to tinker more until next weekend. Thanks for the help and I really do appreciate the help and discussion.
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  #40 (permalink)  
Old 07-28-2004, 06:16 AM
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OK,first off,is this 307 equiped with the stock 72 Q Jet intake? Next,where do you have your distributer vacume line hooked to? And last,are you sure you have the right pvc valve?any other sorce of a vacume leak,brakes,etc? It sounds like you do have some carb issues,but sometimes focusing on one area can make you feel as if your a dog chasing his own tail,as a foot note,I put a 525 road demon jr on my freinds 69 307 stock camaro with a square bore performer eps intake from summit for about 400 bucks,together, and it runs killer,that was about a year ago and its his day to day driver,tried and tested,did no jetting.
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Old 07-28-2004, 06:36 AM
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The intake manifold is an Edelbrock Performer aluminum unit. I removed it and replaces all the gaskets. It was leaking oil at the front and back.

The car has manual brakes.

The carb has a manual choke.

The PVC valve is not new but it is loose when I shake it. It flutters and the stopper is pulled into the housing when the engine is running. It is connected to the large diameter vacuum port located in the front center of the carb. I do not see any evidence of holes or cracks in the connecting rubber tube.

The HEI distributor has a new vacuum advance on it that has been tested and holds vacuum nicely. It is connected to the small diameter vacuum port on the driver's side of the carb. I do not see any evidence of holes or cracks in the connecting rubber tube.

The small diameter vacuum port on the front passenger's side of the carb is covered with a ribber cap.

The trans has a vacuum connection to the intake manifold behind the carb.
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Old 07-28-2004, 08:20 PM
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Well,If it were me,I would look at two more things,float needle and seats,but the other,as I have had this happen,fuel pressure,I once bought a car that had a high pressure pump installed before me,it did work fine,but over time developed a overflow condition because the needle could no longer maintain a seal,it also bled after shutoff and smelled like gas, something else to ponder,I put a regulator on mine as the pump was a nice high volume edelbrock,I set it at 6 psi from over 12,Im not sure what yours will tolerate,but I would be Leary over 8.
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Old 07-29-2004, 01:57 AM
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jpd37, It sounds like you're making a ton of progress and learning to be an expert with your carb. I know how nice it feels to start understanding how everything works together so that you can plan and make changes that produce expected results.

I'm sure you'll have that thing tuned perfectly in no time.
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Old 07-30-2004, 11:35 AM
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Well here is the latest. I replaced the rods with one stage leaner and changed the springs to orange (5hg). I reset the pressure regulator to 5.5 lbs. I found a bunch of junk in one of the fuel inlet strainers, so I removed and cleaned both of them. I reset the floats to 1/2" (the recommended setting is 7/16") and set the idle mixture jet screws to 4 turns out. All vacuum hoses have been pressure tested for air leaks and test OK. The engine timing was checked and set at 10 degrees w/o vacuum. The ignition package (wires, distrib., coil, etc) are all new. Plugs are older but set at 0.045". The choke butterfly setting was corrected to the recommended 1/16" gap fully closed and all the linkage was checked and is working very smoothly.

Now, the engine starts much better and so far is starting quickly when warm and after being off for over 3 minutes. The engine seems to have a bit of sputter (something new) at idle. I'm going to go back to the stock spring to see if that will eliminate the sputter. I do not have a tach so i have set the idle by ear using my Chevy truck's 350 cid as a reference for rpm. I have not taken the car out on the road yet. I have been doing all this work in my garage in the evenings and I don't want to be out after dark trying to work under a hot hood on the street. Tomorrow I will take it out for a test drive.

I read the post about H vs: X pipes. I'm wondering, since I don't have a crossover pipe, if that would help the motor to idle better because of balanced exhaust gasses.
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Old 07-30-2004, 01:48 PM
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Those older plugs might be the source of your grief,especially being run a little out of wack? Also,Im sure there are differing opinions,but for a stock coil I run my plugs at .040.

Stock hei that is.
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