Secondary Tuning Question

[davis95]

I've been experimenting with different secondary rods and hanger heights the past few days to get the best overall performance from my near stock 350 with a few bolt-ons (headers, peformer intake, and mild performance cam). The carb is a 1977 750 with large main air bleeds, 77 jets, 52K primary rods, and CH secondarys. I did add a .135 inlet and drilled the mixture orifices to .087. The primary side was easy to tune and responds very well to both the ATP and mixture screws. The secondary side took a little while. I started with the CH rods and factory hangar (not sure which letter hanger) and at WOT from a stop made a nice launch and shift to second. Still at WOT shortly after the shift to 3rd it would start to loose power (lightly popping). At this point I couldn't tell if it was lean or rich. I pulled the CH rods, bent the hanger to 41/64 and put in a set of DA rods. I had to bend the hanger down about 1/8" to get it to 41/64. I tested it again and it actually ran better in the lower rpm's, but did start cutting out again at about 1000 rpm higher. At this point I'm thinking too rich, so I swap the CH rods back in with the 41/64 hanger. Doing another test drive I was pleasantly surprised I was going the right way with the rich vs. lean thing. The car pulled hard all the way up through ALMOST the top of 3rd before I felt a lack of power and slightly rich pop. I did 3 more tests with the CH rods (tightening the airdoor spring 1/8 turn each time until I got it perfect). Now it pulls great out of the hole and also has nice power at 90mph+. One question I have pertaining to secondary rods is: Can using a rich rod and a tight spring get you the same results as using a lean rod and looser spring? I ask this because I still have a hard time distinguishing between a rich or lean condition when tweaking the secondarys.

[omaha]

    I have always been told that it is better to be rich  on the secondary side. I have always be told and have read that a lean condition will cause misfire at WOT, therefore, run it on the rich side. Just kinda "sneak up" on the spring tension/pulloff rate but make sure it does not run lean. Dont know the best way these days to tell if the mixture could be too rich except by using the drag strip or wideband A/F gauge. Gas today makes it hard to read the plugs (from what I've heard.) It would be interesting to see what some of these racers {like Cliff} do in order to tune the secondary side. Probably have to make a note book and make lists for spring tension, pullof rate. I would think that when you get the rod/hanger combination correct, then the only thing that you would have to tinker with would be the spring tension and the pulloff rate (as long as no other parameters change). course im talking the average street-strip car. Im sure some racers have a bunch of different rod/hanger combo's for various track, elevation, barometric pressure etc etc. Old school technique: when you "stomp it" look in the rearview, there shoud be a bit of black smoke exiting pipes.........aahhhh, just right.....(course if the guy behing you keels over from choking, then you might have gone too far) LOL!!    Hey good luck with your tuning, enjoy!

[Cliff Ruggles]

"at WOT from a stop made a nice launch and shift to second. Still at WOT shortly after the shift to 3rd it would start to loose power (lightly popping)."

This symptom is from insufficient fuel delivery, and has nothing to do with secondary hanger, metering rod, air door spring tuning, etc.

Once you figure out how to keep the carb full on hard runs, the tuning process starts all over again, as it's impossible to tune effectively if the carb isn't full at all times.  It doesn't go empty all at once, so as the fuel level in the bowl is lowered during your testing, the A/F ratio is changing regardless of which metering rods or hanger is being used.....Cliff

[davis95]

Nice to get some direction on this. I did finally get it running good, but the air door spring is at about 1 1/4 turns now. One thing I forgot to mention was that I removed the filter in the carb (It has an inline one also before the carb) between test runs. I only made one more run since then, and it pulled good throughout the test run. When I get some time off again I'll go through the secondary testing again. Hopefully the fuel filter was the restriction. I'd still like some clarity on on how to distinguish between lean and rich on the secondarys without an afr meter. Is secondary rod selection based more on engine demands or what kind of seat of the pants peformance one is personally looking for such as more power down low, top end response, etc. Sorry for all the questions, but I really want to get a good handle on all of this. Thanks again, Doug

[Toronado]

 ;Drods and hangers are for adjusting
the secondaries fuel to air ratio
and rate, also the spring on the air flap
controls vacuum and air, then there's
the dash-pot that restricts the air flap
door for correct amount of time so it
gets enuf fuel before the air flap opens (so it does'nt pop open).

But I think all your problems could
be accelerator pump not giving a good
 shot when your secondaries open
there lean it pops. It may work now cause you
locked out you air flap by tightening it,
and you loose power if air flap cant open  all
the way, and stay open at high rpm.

Is your vacuum dash-pot working correctly?

[davis95]

Pull-off is new and completely opens in less than 2 1/2 seconds. Fresh rebuild on carb. I'll be doing some more test and tuning today. I haven't been back in the car since I first posted this. I'm hopeful that the inline filter was my fuel restriction. Will update. Thanks, Doug

[Toronado]

Simple sec. not getting enuf fuel
& closing down the air flap by tighting spring is helping
by restricting air. The air flap
spring should not have much resistance.
The accelerator pump shot needs to be
checked for adequate fuel. Check
my earlier post similar problem,
it was never the rods and hanger
wrong direction just trying to save you hours.

[davis95]

Will check the acc pump. Thanks.

[davis95]

Just got back from 2 more test runs. I backed the spring off 1/8 turn and the popping came back. I tightened it back 1/8 turn and it pulls hard and never misses a beat from dead stop to 125 mph. The spring is 1 1/4 turn past flap closing. Could this spring be that weak from age, or does is still sound like I could have a fuel delivery problem? It pulls great, but I wonder if I can get it even better.

[Cliff Ruggles]

Spring could be weak, and the cam worn down/loose on the shaft.

Our HP kits come with those parts, we always replace them here.  The cam and spring are also available separately off the website.........Cliff