Carb'ed & Cammed: low vacuum...
 

Hi All,

I'm starting to tune my newly running hybrid (LS powered Maverick) that has a Torquer V2 cam on a 113° LSA and Holley HP-82751 750 DP. I was having trouble getting it to idle under 1100. The carb was bought here slightly used but completely original. So far, I found the float levels were so low that the adjuster nuts were not even contacting the seat assembly and had to initially be unscrewed with needle nose pliers. Also, all 4 idle mix screws were only turned out 1/2 turn from full in.

After setting the float levels, it now idles great at 800 but is pig-rich and the idle mix screws do absolutely nothing. I hooked up a vacuum gauge and I'm seeing 7" at 800 RPM. Next, I'll try 3.5" power valves (I think it has 6.5"'s now), that should cure the idle mix richness.

Is that low a vacuum reading considered normal for this cam? It thumps pretty mean at 800, but technically, the cam is pretty small compared to other makes/styles of engines...

Russ

add more timing at idle and that vacuum will jump up. If the idle screws don't do anything then I'd say your transition slots are exposed too much or the power valve is blown or too high a rated one.

My curve starts out 12° @ 0 RPM, 22° @ 800 RPM, 34° @ 2400 flat across to 5800 where it tails off to 28° by 7000.

I'm pretty sure the carb has a 6.5 PV that will need to be changed to a 3.5.

Still looking to see if others have low vacuum issues...

Russ

TTT

Still looking for opinions and/or other's setups...

I'm "OK" with my idle, it just seems strange that all 4 idle screws are only 1/2 turn out instead of the recommended 1 1/2 turns. Someone on www.hotrodders.com said it's prolly due to this Holley series of carbs being designed for engines with a tighter LSA. It idles solid at 1000 pulling 10" vacuum. I've had it idling lower, but it's somewhat unsteady. I'll put my 4.5 PV back in and go for a drive!

Russ

2 corner idle system = 1.5 turns out for starting point
4 corner idle system = .75 turns out for starting point

just fyi. My carb is at .5 turns out on my 4 corner system with no issues.

Glad to hear that! I did order up a pair of airbleeds that are .004" larger just to see. Heck, they were only $3.99 for the pair... It does have a stable idle (a little higher than I'd hoped), and that should help with the A/C as I don't have a "kicker" solenoid when it cuts on.

Thanks!

Russ

With the 0.075" idle air bleeds, my screws are out 3/4" turn each. Same idle quality as the 0.071" air bleeds and screws out 1/2 turn. No net gain. Still idles at 1000 and pulls 10" vacuum. Guess I'll put the 4.5" power valve back in, too...

Russ

glad to see the air bleeds had a slight effect at least. Does it idle at 1000 in gear? If so I'd suspect you could add more timing with the map sensor to get it to idle lower. I wouldn't think that 800 would be out of the question.

I've had it at 800, but it's pretty shaky, only 5" vacuum. Doesn't make any difference being in gear with my 3600 stall PI Vig. I've tried sending this "IGN" file to a couple guys on the board, but they couldn't get it to work. I might try 12° from 0 to 500, then jump it straight to 34° all the way out, kinda like the old-school locked advance distributors from the 60's. I'm not too sure on the MAP curve, see what you think...

Russ

236/236 .651/.651 110+4lsa, i saw -6.1 psia with the MSD, about 12.2HG. idle at 900 rpm, i could get it to idle at 600rpm but it shook my teeth out of my head.

with holley 750 vac secondary, 1.25 turns out, with demon 750 dp 3/4 turns out at 4 corner. i ran a 5.5 power valve and it worked fine. I'm a 6 speed though.

I ran 20 degrees timing at idle with 8 degrees advance at 900 rpm. so i had about 28 degrees at idle. it was smooth. I ran about 30 degrees WOT, 38 cruise.

ever thought about trying one of these: http://www.summitracing.com/parts/CCA-5200/. big cams = low vacuum at idle. just a thought.:rock::rock:

I have a similar DP and have the same issue with the idle screws.

Mine isn't the HP, but the next model down mechanical secondary/4 corner idle screw DP. Mine doesn't have the replaceable air bleeds like the HP carbs do. My idle screws are 1/2 to 3/4 turn out.

the Holley book I have says some verisons of DP are set up for more radical cams, thus they build the idle circuits very rich. This book recommended bending a wire into a V and putting one into each idle jet.

I tried a small piece of wire but I don't think the wire was big enough. I have another, slightly larger wire I want to try next.


I am starting to think my solution will be a bigger cam to match the carb :)

I still haven't tried my straight 34° across curve (12° at 500, then 34° from 600 across to 6500)... My plugs got so fouled the other day, it refused to run. Once I cleaned them, it ran fine. What gap are you using on the street? I saw where Billy Flantos was running 0.028", but that's on a race only setup.

No power brakes, and no other reason to need a vacuum can...


BIGGER, yeehaww! :rock:

Russ

the transition slots on the primary or secondary side or both i to also am pig rich at idle with a 750 holley dp to must be a common prob with holleys

if im not mistaken i run 17* of timing from 0-1000 and then ramp my timing up to 29* at 3000 and sustain 29* until redline. Im running a six speed... but ive pulled it all the way to the red with no issues.

Im running a TSP magic stick 3 cam (237/242 .603/.609) and a 750 mighty demon carb. I have my air bleeds 1/2 - 3/4 out. its pretty rich at idle but i pull 10" or more at around 900.

I dont care about vacuum because i run hydroboost bakes. I am going to ditch the demon in favor of a holley hp street soon. the demon has some weird idle issues. its been to a carb professional and i still cant figure out why my idle rpm jumps up all crazy after a drive around the block. if i removed the return spring the idle increases like crazy. its almost like the base plate and butterflys are bent. ive had it.. its gotta go!