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Discussion Starter · #1 ·
I have my engine running now since I swapped cams, I have changed the power valve as needed. I just can't get it to idle right. old cam was .244/.252 @.050" on a 110 lsa, new cam is 266/274 @.050" on a 106 lsa. carb is a 750 mighty demon. i know, i know, most of you do not like the demon carbs, but i have had good luck with this carb after I dialed in. it idled perfect with my old setup, now with my new cam, i have problems idling. i get it to idle good, but then if i put it in gear, it dies, or when the electric fan comes on it dies. seems a little rich at idle now. i have tried adjusting the mixture screws, best idle and vaccuum is about 3/4 turn out. maybe it's just the "nature of the beast" with a cam with this much duration. It wants to idle at 1200-1300 rpm, does this sound about right or a little high for this cam . I have not driven it yet, but other than idle quality, it seems crisp and responsive. any idea's? do i need to experiment with different size idle bleeds or idle restrictors?
 

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I have had great luck with my Demon too, some people just have not gotten the hang of tuning them in my opinion.
That is a pretty big cam, on a tight lsa, it may be a challenge to get to idle under 1000 rpm, but it should.
Did you just throw the carb on and go from the old combo and it ran poorly? How are you adjusting it? I have had best luck setting thethrottle shaft screws at about 1/2-3/4 turns (evenly!), then adjusting 4 corner for best setting. what are all the current adjustments? And are you running vac advance?
 

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Im usually a big vac advance fan, but with that much cam, I'd try either disconnecting it, or putting it on ported. Dial in as much initial advance as you can and still crank the motor hot. check your advance curve, make sure your total is still ok and you are not getting ANY mechanical advance at idle in nuetral. Then tune the carb.

Guessing you have vac advance on full, which we usually recommend. When you drop it in gear, your vac goes to the floor with that cam, the can pulls 5 or 10 degrees of timing out, and she quits.
 

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Discussion Starter · #4 ·
I have no vacuum advance on my msd dist. timing is locked at 36 degrees. I put the carb the carb on from the old combo and immediately had problems, my first realization was much lower vacuum at idle than my old cam, about 5.5, and i had a 6.5 power valve in it, so i figured that was the problem. switched to a 4.5 power valve and i still have problems. to begin with, i had to turn the idle speed screw in 1 1/2 turns from where it was with the old cam just to get it to idle. turned the 4 corner mixture screws in 1/8 turn at a time until idle fell off. i ended up at only 3/4 turn from being all the way in. i sprayed around the carb base and the intake to check for vacuum leaks, none. i need to get this ironed out because i am going to KCIR on march 10. I really didn't think i would have this problem after this swap. i have never ran any cam this big before so I don't know what to expect as far as idle goes.
 

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1 1/2 turns is too much I think, I'll bet you are out of the transfer slots. Does it respond to the 4 corner adjustment, like will it bog down and try to die if one is screwed all the way in?
This may be stuff you already know, but I'll ask anyway, did you adjust both idle speed screws exactly the same? This makes a big difference.
If you are not adjusted equal, try baselining back to 3/4 turn each on idle speed screws, put the 4 corner at 3/4-1 turn at all 4, and try starting it (Mine runs best with the idle air at 3/4). You may need to keep it running manually, if so, bring the idle speed screws out about 1/8 at a time on both, untill it idles by itself. Does that make sense?
That cam is definately going to need to idle a bit higher than the old one, it's pretty agressive. Before you mess with air bleeds, you might try leaning out the primary jet 2 sizes and see what it does, I doubt it will help because you lost carb signal with lower vacuum, but you never know and it's easier to try.
 

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I agree, you should open the secondaries a little so you can close the primary blades. If that doesn't help try drilling small holes in the blades on the primarys first.
 

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Open the sec. at idle or drill some holes in the throttle blades. How much convertor do you have that is a lot of cam unless your combo is a very serious one.
 

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1200-1300rpm idle sounds about right for that cam. If it idles fine in park but won't idle in gear, I think oger has it right - the converter might be too tight. That motor/cam will want a 4500-5000 convertor.
 

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I dont know about your carb...
On a holley,
If the primary idle screw is too far in, then the engine is idling on the sec circuit.
If the prim idle screw is to far out, there is not enough air passing the sec for good idle mixtures, and is idling on the power ciruits

Adjust the primary idle screw to middle of the std setting.
Then adjust the sec idle screw to bring the idle down....adjust mixture screws as you go.
Then do final idle tune on the primary which maybe -/+ 1/2 turn.

you mention got the dizzy locked at 36 degs....the advance has influnce on the mixture...with such a hi advance at idle and that cam it could well be there is not enough adjustment in the carb to compensate

I used to have a 268/274 cam @104 I had a 'lumpy' idle thu smooth @ 550 rpm with 10 deg advance, no vac advance and 5 to 7" vac. I never liked that cam.

What is the use of the car?..if primaraly street use it is way over camed and way over carbed
 

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My cam is 264/274 @ .050 with .630 lift in a 355 with an 825CFM race Demon and it idles at 1,100 rpms in neutral. Shortly after buying the Demon I bought a book from Barry Grant on how to tune them.

If your carb is fairly new it should have the idle-eze feature. If it does you don't need to drill the throttle blades.

If your best idle is with all 4 mixture screws set at 3/4 turn, you need to go with smaller idle feed restrictors in the metering blocks. As I remember Barry Grant reccomends the mixture screws be set between 1-1/2 and 3 turns out. If they are not in that range you need to change the idle feed restrictors. I also suggest a 2.5 power valve.

You should get it to idle nicely at 1,200 rpms in neutral.
 

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According to Barry grant and what has worked for me, open primary and secondary idle speed screws equally, and no more than .020 of transfer slot exposed. I think at 1 1/2 turns you are beyond that. If not sure just pull the carb and verify, again, they should be equally open.
Also according to B/G, your 4 corner should be between 1 - 2 1/2 turns out total, some respond well to less (mine is right at or just under 1, if I recall).
Bottom line, all motors act differently, but regardless of what your final rpm is, it should idle smooth (relative to the cam obviously), not drop way off in gear, (that converter should be fine I think), and have clean crisp transition with off idle throttle throttle response. It may want to idle 1200-1300, but I wouldn't give up getting it lower yet.
 

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Highest idle speed is too lean for a typical drag engine with a large cam. Idling at 1200-1400rpms is fine. Mine is at 1200, makes life much easier to have it idle just a touch fast. I see no reason to have it idle as low as possible, it only makes cold starts harder, and longer warm ups before you can cruise around the pits to warm it up.

I set the idle screws with a method where i turn the driver side primary in until the engine starts to surge, then back it out 3/4 turn. Reset all the idle screws to this setting then repeat until the results are consistant. Shouldnt take more then 3 times at the most.

I fine tune it with seans method, clean off-idle response, and a reasonably smooth idle.
 

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67 Freak,

A couple of questions:

1) What intake manifold are you using?
2) Have you double-checked your fuel pressure, and float levels yet?
3) Does your carburetor have an Idle-Eze in the baseplate?
4) Have you checked to see how much of the transfer slots (primary and secondary) you have exposed at this point?
5) When you put the car into gear how much was your RPM changing?
 

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Discussion Starter · #19 ·
intake is a vic jr #2999 (1" taller than 2975)
fuel pressure is 7 psi
float levels ok
carb does not have idle-eze
usually a 200 rpm drop in gear

I started all over with the butterflys 3/4 turn open primary and secondary, and adjusted from there, seems better but i didn't have a lot of time to play with it. Iwill see how things go at the track this weekend and let you know. I have a busy schedule durring the week and don't get time to mess with it.
 

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Freak,

You really should pop the carb back off, and reset the butterflies so that all four are open to expose about .020" of the transfer slot. Then open or close both the same amount to get your idle speed. There is no way to do it accurately from the top without knowing where you're starting from. A 4000 Stall shouldn't drop that much, so you're most likely still too far into the transfer slots.
 
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