|05-10-2012 04:51 AM|
How do the plugs look?
Does the car drive OK during normal driving when you aren't into the secondaries?
Try backing the screws out 1/4 at a time and test drive with the secondaries disconnected and wired shut and check plugs.
Once you are confident the primary side is dialed in it will be a combo of accelerator pump shot and spring to get rid of the stumble when you stab it.
|05-09-2012 09:45 PM|
|Coolball||I hate to keep hammering on this but back to the idle screws being 1/2 turn out, wouldn't that indicate a rich condition? Which in turn makes me wonder if the delay/hesitation is related to too possibly much fuel? There isn't any black smoke when I rap the throttle though so maybe not.|
|05-08-2012 05:44 PM|
|cobalt327||If the check ball was left out of the secondary vacuum chamber it'll open too fast in most cases, too.|
|05-08-2012 05:35 PM|
|red406||I'm no expert here, but this problem sounds like something I ran into last year. I was helping a friend tune his truck and had the same issues. I discovered the spring was missing in the secondary. The diaphram was torn too. The secondarys were flopping open under WOT (running lean). It ran like a dog until about 3500 rpm then ran great up to rev limiter. We installed new diaphram and spring, and now runs great.|
|05-08-2012 05:01 PM|
|Coolball||To add to that - what's the cause of having to lean out the idle circuit? My fuel pressure is 6lbs. and I've set the float levels properly. I can only assume it's rich but I'm by no means a carb aficionado and nobody's ever told me otherwise.|
|05-08-2012 04:52 PM|
Why would and or what would cause the idle mixture screws to only be out a half turn? This is where the best vacuum reading occurs for me.
Back to the initial timing perhaps?
|05-08-2012 04:48 PM|
|TommyK||Why are the idle mixture screws out only 1/2 a turn?|
|05-07-2012 08:42 PM|
Basically you want the secondaries to open as soon as they can w/o causing a bog.
A lot will be found in the timing curve I believe. Even fine tuning the accelerator pump circuit to give a seamless transition from idle/low rpm to wfo will help. Get the timing sorted first, then work on the carb tuning.
If the vacuum advance is supplying a lot of timing- timing that is dropping out when you hit the throttle- that can cause a bog, especially w/a stick shift car. An AT car having a correctly matched TC will flash to a higher rpm at the hit of the throttle and that will lessen or eliminate the effect of vacuum advance timing going away, because the mechanical advance is able to compensate for the loss of vacuum advance almost instantly because of the TC stall rpm. In a manual trans vehicle this is delayed from happening until the rpm gets high enough for the mechanical advance to compensate for the loss of vacuum advance- hence a bog/hesitation.
The cure for that is to use more initial timing, and to use ported vacuum for the vacuum advance or less vacuum advance and manifold vacuum. Limit the vacuum advance to 10-12 degrees and use an adjustable can so you can set the tip in point. In most cases the mechanical advance will need to be limited to keep the total (initial plus mechanical) from being too high.
|05-07-2012 06:11 PM|
I know it's tough to give a solution to a problem that you can't address first hand but thanks for the starting suggestions.
I'll try a lighter spring on the secondary and recheck the timing. I set it with a vacuum gauge initially then by a timing light and it's pretty happy where it is. The distributor is the standard GM style from Skip White so it's supposed to be higher voltage. Vacuum advance and set to manifold vacuum. I didn't change the springs or limit the advance.
To answer on the additional details, it's an 82 Camaro with street driven in mind. I'm guessing 3,500 lbs wet all said and done with driver. ST10 4spd, Kevlar clutch and 3.42 gears out back.
Cam isn't really wild, Engine Pro MC1988, 214/224 @ .50 makes power in the range of 2000-4000. Edelbrock RPM air gap, 180cc runner/64cc chamber Aluminum vortec style heads gasket matched with full port and polish , 1.5 roller rockers. It's got stainless 1.60 exhaust and 2.02 intake swirl polished intake valves. Flat top builder pistons with a .045 quench (yeah I know...). I plan to run it on 89 octane if I can get away with it.
So far I've only driven it a couple gears up the block and back last weekend in the almost 3 years I've owned it since it's been undergoing a full frame up (restoration). I just started body work so road tuning can't happen for a while but it's bugging me that gobs of power are lying in the secondaries.
My initial impression was it needs more guts down low and that the transition to the secondaries is HUGE and then the power shows up. How can I pull that power down lower which is where this motor should be. So how soon should the secondaries open?
|05-07-2012 05:53 AM|
|cobalt327||Here's a page w/info (under 10. Ignition advance in the table of contents) on setting up a performance advance curve. The page deals primarily w/the GM HEI, but the info on the advance curve and vacuum advance, etc. holds true regardless of the type of distributor, be it a points type, non GM, aftermarket, or whatever.|
|05-07-2012 05:42 AM|
If the cam has a fair amount of duration/overlap that gives a rough or choppy idle, 14 degrees may not be enough initial timing. Are you using a vacuum advance? If so is it connected to ported or manifold vacuum? Vortec and other fast burn heads can use 34 degrees total timing just fine, other heads that's not going to be enough for maximum power.
Before changing the PV you need to see what the vacuum is while the vehicle is being driven as you do normally. You may find that a 85 PV will be adding fuel even when you're under not much load- and that's not needed. In most cases, a 65 PV will be about right; rarely is a higher opening point a help as long as the jetting is correct.
To set the idle mixture screws use a vacuum gauge to get the highest vacuum at idle.
|05-07-2012 04:12 AM|
You need a few things. First you need more initial timing.
Second you need to adjust the vacuum secondarys on your carb. They are set too tight which is delaying when they come in. First get your timing correct, then take a test drive. If the timing is correct and it still delays then you need to go to a weaker spring on the secondary.
|05-06-2012 05:16 PM|
Hello again hotrodders! I need some help with this carb. It's a street avenger 670 that's been rebuilt fresh and installed. I think there is couple things going but I don't know quite which way to go and would greatly appreciate some direction.
Fresh sbc 350; 9.6:1 compression; 14* initial, 32* all in by 3,000. 4spd manual.
Problem is that when floored from rolling cruise, it's a dog until the secondaries kick in and then it comes alive and will turn the tires off in second gear. I can feel the secondaries open which I know isn't right.
I'm getting ready to change the power valve to an 8.5 from the stock 6.5. It's pulling 17hg vacuum at idle. Jets are at 68 primary and 71 secondary. I've changed the pump cam to blue to get rid of a lean hesitation from idle. I also changed to a 35 squirter which helped a lot.
Last but not least my idle mixture screws are only about 1/2 a turn out from closed which doesn't seem right to me based on the recommended starting point of 1 1/2 turns.
So where am I off at on adjustment?