View Full Version : Tuning help/suggestions

06-14-2005, 04:45 PM
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

06-14-2005, 06:22 PM
Try a bigger squirter.

06-14-2005, 07:06 PM
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, 07:31 PM
is the squirter # the diameter in .0xx"? if so they seem to go 32 and a 35

I've never measured one, but yeah, I believe that's how they're sized. They're available in 28, 30, 31, 32, and 35 (for gas).

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. :)

06-15-2005, 09:15 AM
The "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

06-15-2005, 12:26 PM
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, 05:11 PM
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, 05:34 PM
How 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?

07-12-2005, 05:38 PM
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, 05:40 PM
Yessir. Look in your Demon instructions and it'll show which port to take the reading.

07-12-2005, 06:25 PM
thanks matt. by the way what do i want to see for readings?

07-12-2005, 07:15 PM
That depends on all kinds of things, but as just a guess, I'd be looking for 8" 2".

07-20-2005, 02:02 PM
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, 02:31 PM
One 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.

07-20-2005, 02:43 PM
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, 02:49 PM
Change 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.

07-20-2005, 03:01 PM
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, 03:40 PM
Power 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)

07-20-2005, 03:51 PM
Is there anybody watching and learning from this? Speak up now

Yeah, I was, but didnt want to butt in. I'm going thru the same off idle stumble in my 455 Pontiac. I've been steping thru things like Kman is, just not as diligently. I'd like to hear how things progress, but if you guys take it off line thats fine...I'll get thru it.


07-20-2005, 03:53 PM
We'll keep it here then. No biggie. :) Feel free to butt in! No farting though.

07-20-2005, 04:15 PM
Thanx Matt, thats :cool:
So would the basic procedure be something like;
Idle mixture (primary side only)
Accellerator pump (primary and secondary, since I have mechanical secondaries)
Jets (I have the Holley adjust-a-jet setup on the primary side only)

I know there would be some overlap, so I may need to go back and forth a few times right?

my idlle screws are just barley out (cant remember the exact, but less than 1/2 a turn. the power valve is 6.5 and my idle vacc is ~11. and yet it seems way to rich at idle. I'm thinking of getting the Innovate A/F meter to help with tuning. Since I got many cars, it would be very useful I think.

Kman, dont mean to highjack the thread, can we share??


07-20-2005, 06:46 PM
ok thanks matt. just curious, plus i find that knowing a reasoning behind doing something helps me remember for the next time.

Rubes, feel free to step in. I am glad this is benefitting someone other then myself.

07-20-2005, 11:30 PM
i find that knowing a reasoning behind doing something helps me remember for the next time.

I'm exactly the same way. :)

Rubes, this is what I do when I get my hands on a new carburetor:

- With carburetors of unkown history, check the throttle shafts for excessive play, check the base plate (and the manifold) for flatness, and check that all the screws are tight.

- Pull the bowls off and do a general visual check, and note power valve ratings and jet sizes. Also make sure that both needle & seats move freely. (Ok, the seats won't move, but you know what I'm saying, damnit!)

- Flip it upside down and set the curb idle screws to where there's .040" of the idle transfer slot showing on the primary side. Adjust the secondary curb idle so the butterflies just start to move, and then back them off to closed, but no further. Note each screws' position. Wire the throttle to WOT and check the accelerator pump linkage-to-accelerateor pump actuator clearence. (That terminology isn't right, sorry). I think it should be around .010", but in reality any clearance is ok, allowing for thermal expansion. Set the idle mixture screws to 1 1/2 turns out from closed.

- Put it back together. Take a permanent pen and write all the numbers you noted on the carburetor somewhere that it will make sense to you later. Also, write down squirter sizes, high speed and idle air bleed sizes.

- Put it on the motor anc start it. Timing will be the first thing you need to verify. After the motor is up to temp, shut it off. Wait 30 minutes and start it again to let everything heat soak. If the motor won't idle on it's own at this point, don't change anything!!! Have someone keep a steady idle going. Set the float levels. Check idle vacuum and compare that to what power valves you have in it. Change as necessary. Mess with the idle mixture screws until it wants to idle. On a good carburetor (read that as a Demon or a professionally prepared Holley), you want to adjust the idle mixture screws 1/8 turn at a time, moving all four equally. You can use the manifold vacuum as a helper when adjusting the idle mixture screws: The best quality idle will have the most vacuum.

- At this point, your goal is to move the curb idle screws the least amount possible. If the thing is nowhere near idling at this point, you need different sized idle air bleeds. You should be able to get a perfect idle without ever touching the curb idle screws, but by balancing idle air bleeds and idle mixture screws. The mixture screws control the amount of fuel going into the airstream, and the idle air bleeds control the amount of air. When everything is just right, you'll still have between .030" - .050" of the idle transfer slot showing (not that you can see it), and the idle mixture screws will be between one to two turns out from closed.

- After all that is when you get to see if you need to mess with squirter sizes and pump cams. That's simple...you mash the throttle and change stuff until it scares you. :)

OR, you can pay $800-$1,000 for a professional built carburetor that you bolt on and go. I've never seen any pro built carburetor that was more than 4 jet sizes too fat (they never ship them lean), two sizes on the high speed air bleeds, and maybe 1/16" turn on the curb idle.

07-21-2005, 03:14 AM
Thanx for the time Matt. All makes sense. This was an out of the box Holley.


07-27-2005, 01:49 PM
matt, i tried the new power valve, but havent really been able to test it because i gernaded a rear universal and broke the yolk on the rear diff. The cars at the tranny/driveline place as im too busy to fix it. hopefully it will be ready for this weekend or beginning of next week. i will be back in touch though. again i appreciate your time and help.

07-27-2005, 02:18 PM
Atleast it's making power! lol

08-07-2005, 05:40 PM
a little update. im going to a 50cc accelerator pump. talked to a few racers at the track on wednesday. i was running a 14.2 best at 105. but i had to pump the throttle to keep it going. they said the way it sounded on the track and what i was telling them made them think lack of fuel. picking it up tomorrow. i also ran a few 15.oxx at 104.xx. im leaving the line at an idel then rolling into it. i also rechecked the timing and it was around 42 total. i backed it down to around 32-34 total. seems to start and run a little better now.

08-10-2005, 02:22 PM
ok i installed the 50cc pump, and drove it a little. not too bad. changed the carb cam and it got a little better. installed 74 jets in the primary and it feels a little better yet. gonna play around with the cams some more though.