need more low end, secondaries kick in hard, timing or jetting?? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 09-20-2011, 09:22 PM
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need more low end, secondaries kick in hard, timing or jetting??

im having trouble tuning my carb up. i recently replaced the old 76cc heads on my 350 with some 64cc camel humps. ported and polished, springs, studs, all the work. HUGE difference in performance already. im running a holley street avenger 670 on top of it, stock has 65 primary, 68 secondary and .31 spuirter. i upped the spuirter to a .35 which helped alot. rejetted with a 68 primary and 71 secondary. another huge difference. It has vacumme secondaries which i was running the harder black spring. i swapped out for the medium silver. after i did that you can feel the secondaries kick in. right about 2500 rpm on a hard take off this thing opens up and attracts every cop in town. iv always heard if you can feel the secondaries kick in your too lean on the bottom end. so i upped the primary to a 70 and went to piss the cops off again, but it actually made the bottom end a hair slower. then the primaries kick in and off i go. now im kind of stumped... i went to the softer yellow spring and really didnt feel any difference at all. acted the same as far as i could feel. is this a timing issue? im running about 12 initial. not sure on total. my plugs were almost solid white with the 68 primary in there so i figured it was telling me it was too lean as well.. but after it made it run a little more sluggish. i dont know what to do. any advice???

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Old 09-21-2011, 12:10 AM
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Sounds like you need to go back to the 68's, the med. sec. spring, and and try for more adv. with no ping. Might try AC plug range of R44TS. A four hole spacer under the carb will help the bottom end also.
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Old 09-21-2011, 07:47 AM
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If you feel the secondary 'kick' you usually go lighter spring and if you have the yellow spring there's 1 more yellow after that.

A total guess, I think the secondary is too small. You could raise the float level a bit and see what happens but it sounds undersized.
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Old 09-21-2011, 08:29 AM
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Thanks for the replies. I think I might try all of what y'all said, raise the secondary to a 72, put the lighter spring and advance the timing a bit and hope for the best. The only reason I didn't want to play with the timing too much is because it has been running a little warmer than normal while at idle and the color change on my plugs ground strap is pretty close to the threads. I just started reading my plugs to see if I'm too advanced or retarded, is this something I should keep doing or just advance till it runs good with no detonation? Any other advice will be greatly appreciated, thanks
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Old 09-21-2011, 08:44 PM
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I think youre changing too much all at the same time. You said that you changed a secondary spring from silver to black, changed the squirter, changed the secondary jets, primary etc.

First thing Id confirm all my timing. Base, total, and roughly when it starts to ramp up. With all that motor work you did is it possible the timing is no longer set correctly? Easy way to find out in 2 minutes is get the timing light on there and see what you have at 2500-3000 rpm. Likely its good but if its not, all your carb work will be for nothing. Maybe the dizzy moved a bit when you worked on it, maybe you put it in a little retarded or advanced if you removed it?

After that deal with the carb but if you change a secondary spring, dont mess with the primary side just yet. See how it rides. See if you need another spring. Do one side (pri or sec) at a time, not both at the same time.

I think you should focus on adjusting just the primary side first though. What power vale is in there (6.5 is stock, usually a good one for most applications), what is your idle in gear vacuum, are you using vacuum advance on the dizzy?

If you get the primary side tuned you'll have less, if any, transition problems when the secondaries DO kick in. You might just have a band primary side and are changing secondary springs to compensate. You might be tuned perfect though. lol. You might even have a slight dead spot on the accelerator pump (the squirter you changed) and that too will be similar to a stumble and then a huge varooom as the dead spot goes away and the car takes off.

Confirm timing first, everything else will fall into place from that.
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Old 09-29-2011, 09:10 PM
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I run 350 sbc 280d 480 lift cam double hump 461s, 600 holly with alot of work done to it 69 front 74 rear. Drives on street great and turns 7.50s in 1/8. But with 6.50 vac. secondaries to get it to run 7.50s have to run 68 front 76 rear and white spring, which is the lightest and take the little ball out of the vac. diaphragm. but i also run 3000 stall. Think you may be a little lean in back. BUT LIKE HE SAID TRY ONE THING AT A TIME. Hope this helps!
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Old 09-29-2011, 10:05 PM
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Thanks for all the replies, I'm currently running 68/72 set up now with silver spring, still not set quite right but I'm trying. Seems like no matter what I do it still has that sluggish bottom end on it, and the explosive top end. It runs amazing over all. I'm just a perfectionist and this little glitch is really getting under my skin. I guess I will have to buy a a/f gauge just to make sure.
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