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Thread: Tuning help/suggestions
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06-14-2005 #1Registered User
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Tuning help/suggestions
ive got a 67 camaro with a BBC, and ive gotten it running deciently. but i am having problems on hard accelleration. it likes to stumble i guess would be the word.. Instead of nice smooth acceleration upon an idle to WOT smash it bogs down/hesitates a little before taking off. i have tried a few different pump cams but so far it seems best with the pink cam on the #1 position. i have the full cam kit but dont have a chart to compare whats what with. i have gotten it a little better. but can't seem to get rid of it. timing is roughly 25* initial with an advance of around 36*, but it doesnt appear to be timing. i have # 72 jets in the primarys and 80 in the secondarys. plugs look ok, maybe a little on the dark side but not black. im looking for suggestions as to where to go from here. carb is a demon 850 by the way. it almost seems like it runs outta fuel on those WOT smashes because if i roll into it gradually or if i pump the throttle it seems to be fine. thanks for the help and or sugestions
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06-14-2005 #2Try a bigger squirter.
-Matt
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06-14-2005 #3Registered User- Join Date
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Thats the next step. what would the next size be if i have a 31 in there. im not sure how the incriments run, and am not sure where to find out. thanks
after looking around is the squirter # the diameter in .0xx"? if so they seem to go 32 and a 35
06-14-2005 #4is the squirter # the diameter in .0xx"? if so they seem to go 32 and a 35
There's also two different styles of squirters...the normal one that you have, and a tube style. Just like the name implies, the tube style expels fuel out of extended tubes. I dunno what the actual net effect of this style squirter is, but they can be usefull for fine tuning.
If you have 31's now, I would go to a 32pri and leave the secondary alone. If that doesn't do it, bump the sec up to a 32. If it's still not enough, go 35pri...etc.
If you still need more fuel from the accelerator pump after going to 35/35, you'll need to step up your accelerator pumps to 50cc pumps, as opposed to the 30cc pumps you have now. Unless you have a LOT of motor, you won't have to do this.
Also, keep in mind that when you do find a squirter combo that works, they will more than likely work even better with a different pump cam. It's a never ending battle it seems, but it's so worth it in the end.
Allways check your pump actuating lever clearence after changing cams.
I recommend one Advil and two Ibuprofen for sore backs.-Matt
Welders: The only people that think a co-worker catching on fire is funny.
06-15-2005 #5The "dog-leg" boosters are 90-degree and promote a better fuel signal at idle, while the annular style are more for racing (4150 series). Also, what PV(s) are you running? Too much lift at the pump cam will drain the accelerator pump too quickly as Matt said-you will need a bigger kit (50cc) and then go to a larger squirter, then you'll most likely need to go upstairs with the jets, too...say around a 76 and an 84 sounds nice. You definitely have a lean stumble. -Jabin
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06-15-2005 #6Registered User- Join Date
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I believe the demon comes stock with a 6.5 PV. thats whats in there. what needs to be done for the larger accelerator pump kit? is that just a direct swap?
07-12-2005 #7Registered User- Join Date
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well heres where im at now. ive played aroud with it and i have #35 squirters both front and rear. pink cam on the # 2 position and it still bogs. do i try a 74 primary jet, or go for the larger, 50cc pump.
Is it posible that its getting too much fuel. would that cause a stumble like this?
my set up is
402 BBC bored to 414, .060 i believe
AFR magnum 305 heads
236/242 .612 lift cam
11.0:1 cr
26 deg inital 38/40 advanced
thanks for the help
07-12-2005 #8How much vacuum do you have at idle? You need to get the power valves situated before you go any further.
Example: You have 5.5" vacuum at idle. You need 3.5 power valves.
You have 10.0" You need 8.5
Man it's been too long, will somebody please verify this?Last edited by Matt@RFR; 07-12-2005 at 05:39 PM.
-Matt
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07-12-2005 #9Registered User- Join Date
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not sure, ill have to borrow a guage and find out. does the guage just hook onto a vacuum port on the carb?
07-12-2005 #10Yessir. Look in your Demon instructions and it'll show which port to take the reading.
-Matt
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07-12-2005 #11Registered User- Join Date
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thanks matt. by the way what do i want to see for readings?
07-12-2005 #12That depends on all kinds of things, but as just a guess, I'd be looking for 8" ±2".
-Matt
Welders: The only people that think a co-worker catching on fire is funny.
07-20-2005 #13Registered User- Join Date
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matt, i got a vacuume guage and it reads right around 6" at idel. i played around with the jets and bumped up to 74 and 84 and now the stumble is way less noticible but its richer then hell
07-20-2005 #14One thing at a time dude! Never make two changes at once, or you'll have no idea what change effected what. If it's not detonating, don't mess with the jets anymore until you've got everything else dialed.
6" is fine, but you need to change the power valve to a 3.5. with a 6.5 power valve and 6" of vacuum, that means the power valve is constantly open, dumping fuel in that you now can't control.
Fix that, and then re do your idle mixture screws. How many turns out it takes on the mixture screws will tell you roughly where you're at.-Matt
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07-20-2005 #15Registered User- Join Date
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ok thanks. im gonna put everything back and recheck the vacuum. i believe i took the reading with my old set up. but i will double check. thanks again
07-20-2005 #16Change the power valve first.
Is there anybody watching and learning from this? Speak up now if you are because otherwise, I'm gonna work with kman over the phone or IM's or something to speed up the process.
Also, where are you from kman? If there's someone local to you that can help, that'd be a huge bonus. This stuff is difficult long distance.-Matt
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07-20-2005 #17Registered User- Join Date
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matt, thank you. i appreciate your help. i just looked and see power valves ranging from 1.5-10.5. how do you know what to use is there a formula? or is it just years if trying different combos? i will pick up a new P.V. in the morning but might be busy till this weekend. i will get back in touch though. thanks a million
07-20-2005 #18Power valves open when manifold vacuum drops below their rating. So a 3.5 will open (and dump fuel) when manifold vacuum drops below 3.5".
You size power valves two full numbers under what vacuum you have at idle. That way, it's far enough away so that any dips in vacuum won't tickle the power valve, and it also gives the idle circuit a chance to do it's job without dumping extra fuel the instant you touch the throttle.
(you have) 6" - 2 = 4 (round off to 3.5)-Matt
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07-20-2005 #19Registered User- Join Date
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Originally Posted by Matt@RFR
Rubes
07-20-2005 #20We'll keep it here then. No biggie. Feel free to butt in! No farting though.
Last edited by Matt@RFR; 07-20-2005 at 04:10 PM.
-Matt
Welders: The only people that think a co-worker catching on fire is funny.