Vacuum Spring Problem?

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69signetv8

69signetv8

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Guys, I seem to be having a problem getting the most out of the signet when I mat it around 60 MPH. For starters, I'm pretty sure my kickdown is adjusted correctly. When I mat it I get a burst, but for a very short time (maybe 3-4 sec) then it drops off like the secondries have closed. (my best guess) I did confirmed that the kickback rod is all the way back to the firewall and there is nothing stopping the secondries from getting fully open. I have changed out the springs before as I have the kit,however I can't seem to get it right. I do understand that I don't what them opening just crossing the steet or even fully open going 60. Due to it being vacuum sec, what is the best spring size to use to assist in holding the sec open "until" I let off on the gas?
Thanks.

I'm sure everything else matches pretty well.

273 Performer intake,
Carb: Holley 600 w Elc Choke
Stock cam
 
I'm not sure if I understand. If they are working properly, you should not feel any openning at all. They should just move. The spring resists the vaccum caused by airflo. So as the secondaries open, if the vaccum present drops (like the sec are openning before the engine is really ready for them) then the secondaries will be forced closed by the spring. Thsi happens immediately, so you would never notice it. So some questions..Did you change springs already? If so, was the small check ball in teh secondary vaccum pod re-installed (it must be, dont run without it)? If you did, what color is in there now? Witht he small cubes, I would think the 600 needs a very heavy spring to operate properly because of the limited airflow the carb primaries see. (basically, the secondaries may not open fully at all on a 273 unless it really revving)
 
moper said:
I'm not sure if I understand. If they are working properly, you should not feel any openning at all. They should just move. The spring resists the vaccum caused by airflo. So as the secondaries open, if the vaccum present drops (like the sec are openning before the engine is really ready for them) then the secondaries will be forced closed by the spring. Thsi happens immediately, so you would never notice it. So some questions..Did you change springs already? If so, was the small check ball in teh secondary vaccum pod re-installed (it must be, dont run without it)? If you did, what color is in there now? Witht he small cubes, I would think the 600 needs a very heavy spring to operate properly because of the limited airflow the carb primaries see. (basically, the secondaries may not open fully at all on a 273 unless it really revving)
Click to expand...

I'm running with the Purple (med light spring). The ball is still in there, however I've heard a lot of guys say get rid of it (not required).

I'm sorry if I'm confusing you. In my mind.....If I snap the throttle going 50 or 60MPH to the floor boards and hold it there, I beleive that my secondaries are opening fully.. (4 b kicking in), however I don't think there staying open for very long, just a few secs or so. Shouldn't they stay open until I let off on the gas? Theres that loss of power pass kickdown feeling?
 
What that tells me is go back to the unpainted spring and try it. The secondaries are starting to open, then the airflo stalls (too big a carb w/4bbls open) and the purple spring does it's job and closes the secondaries. As I said, you may find the next stiffer spring from natural wil be the best, but one step at a time. The check ball allows the secondaries to open smoothly. When it's removed, there is a lean situation that can develop because there is not enough airflow right away to draw fuel out the secondary venturies, and the 30cc acc pump in the primaries isnt enough to cover it. it's an "old school" thing. It might work on a larger V8, it wont help on a 273. What rpm are you at when it studders like that?
 
I'm running with a 8.75 - 3:91 gears, sooo I would say about 3100 RPM or so @ 60
I like your advice "up one step and try med"
 
well just as an example, here is the formula off Holley's website for sizing. Assuming 100% efficiency, you use this: CID x RPM(where 100% efficiency occurs)/3456

Now, generally, no engine will get to 100% efficient, but you can use you max rpm instead of efficiency rpm. Its not a perfect formula, as most street engines are way below 100% (Holley site says 85%, I tend to think a well tuned street engine will do a little better due to headers, better intakes, ignitions, etc.).
I plugged in the 273, and 6000 rpm, and came out with 474cfm. now normally you'd multiply that by the 85%. according to the site, you need 403CFM. I'd say a more accurate size would be 450.
now here's the kicker...Most Holleys are rated by dry flowing an example of them. Thats the cfm rating you see, like 600, 750, 800, etc. Except, they dont work by moving only air. The fuel in the airstream reduces the total amount of air slightly, so the carbs generally dont flow the rating when on an engine. The Avenger line, and HP lines, are wet flowed, and the cfm ratings are accurate. The same is true for Demon carbs, BG carbs, and every custom carb shop. They all wet flow test.
So, you 600, really flows around 590cfm, but the engine really only needs about 75% of that, and that's assuming you're revving to 6000 rpm. This means you wont be needing the secondaries much to run well with it. As I said, try using the natural spring, and then try the next color stiffer. preferably at the drag strip, so you have a good idea of what it runs. Sorry for the ramble, just thought i'd try to catch the questions before they get asked...lol
 
69signetv8 said:
When I mat it I get a burst, but for a very short time (maybe 3-4 sec) then it drops off like the secondries have closed. (my best guess)
Click to expand...

To me, this sounds more like a fuel related problem than a vacuum secondary problem. Things I would check are float level, fuel filter, fuel pump, and of course all the hard lines and hoses. Maybe get a fuel pressure gauge to make sure the pressure doen't drop off when you floor it.
 
Hmm, Yes it all adds up. Thanks for all your information. I'm going to do some spring changes first off (after I pick up that new holley spring changer adapter) Just to let ya know, I was also told I don't have the nicest setup as far as engine matching go's....(and I thought it wasn't bad) ask questions, live and learn :book:

1. My intake is the older performer (streetmaster) single plenum...bad for torque and overall performance for these days standards. (get rid of it)
2. Due to the rebuild and good compression, headers, crossover I was also informed to go for a 340 cam "or in that area", that more lift would run much better with the 3:91 gears.
3. Maybe a "little" port job at the same time to max airflow. And last, perhaps a little more converter stall.

I was informed this would more then likely be the way to go to max this little 273 engine I then should carry out the information that you guys suggested for added tuning and it would make me a very happy camper. Would you agree?
 
I would do a couple of things, if it were me. First, get your hands on a 318 port sized intake. A dual plane, take your pick, just so long it's for the 318 size port. I'd also replace the heads before I had port work done. I'd swap on a set of "302" 318 heads. They are a closed chamber, with much ports much better suited to torque from a smaller engine. A cam, the stock 340 would work, but I'd probably go for a small Comp Extreme Energy. The XE256H would be #1 choice. But, I think by careful tuning, you can get it to run ok the way it is. Maybe not the best, but certainly not so bad it cant run well.
You may also, as it was said, want to check the float levels and pressure, just to be sure.
 
Thanks "Mopar" great advice, I'll will keep my eyes open and see what comes up. Thanks again and have a good July 4.

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