Need Tuning after dyno run

[Cliff Ruggles]

If the engine slows down when you turn in the mixture screws then you are able to lean it up.  Make sure that you are completely on the idle system, no nozzle drip at idle, this tells you that you have enough idle bypass air for what you are doing....Cliff

[leeklm]

Went for my first test drive yesterday.  Overall, the carb is working well, good throttle response and pulls hard up to 5K, but do need to adjust secondaries, which are coming open too soon, especially when bogging the motor somewhat when in too high of a gear.

Question...  Once I have my choke pull-off opening after the suggested 2 seconds from loss of vacuum (currently about 1 sec), what is the overall relationship of adjusting secondary opening rate with the choke pull-off compared to the flap spring adjustment?  It seems that the two adjustments overlap with each other, at least somewhat.

Are there scenarios where one should be focussed on more than the other?

[Cliff Ruggles]

The pull-off just needs to dampen the opening to keep the flaps from "whipping" open and requiring too much spring tension to slow them down.  For most set-ups with a decent power to weight ratio, about 1 to 2 seconds will be a good place to be for release time.

There are other players at work here, so it takes some tuning time to figure out what each application wants.  Thise include accl pump shot/duration, POE set-up, hanger height, and metering rod taper.

Keep in mind than in actual use, the release time using real engine vacuum is slightly slower than simply compressing the pull-off by hand and releasing it.

Rule of thumb for most set-ups is your pull-off is too fast if you have to wind the spring past apprx one full turn to keep the engine from stumbling when you go quickly to full throttle.

Don't forget to put some grease on the pin where the spring rides so it can open smoothly.....Cliff

[leeklm]

Hey Cliff,

Carb and new motor are running pretty well, but after installing a new posi rear, I am getting a bog/stumble when punching the throttle from a rolling start.  With the one wheel wonder, I would usually break the tire loose, the motor immediately winds up, and no bog or hesitation.  With the posi, my improved traction is keeping the tires from breaking free, and takes a little longer to get things rolling.  Once it does, no issues.  If I give it a little rpm before popping the clutch and punching the throttle, again, no issues.  Everything winds up quickly and away we go.
 
I have tried different piston springs, and the dark blue seems best.  I have also tried the two pump rod holes, and the inside hole seems best.  For good measure, I also did the tape trick around the secondary holes to be sure fuel slosh was not the problem.

One of the core issues I believe is that my muncie currently has the 2.20 1st gear along with a 3.31 rear axle and 255/60/15 tires.  I have to believe a switch to a 2.56 1st gear would solve some of my issues. 

What would you suggest at this point, or am I expecting miracles from my carb based on the total 1st gear noted above?  The motor discussed through this thread is a 455 with mild cam, 18" vacuum at idle, dyno at 500 LB & 400 HP.

Thanks!

[Cliff Ruggles]

Everything changes for secondary tuning once you find traction.  The engine is loaded longer at lower rpm's, so you basically need to slow things down some.  Primary side tuning would have little if any effect here, go after the secondaries instead.....Cliff

[leeklm]

Since it felt like I was chasing my tail around somewhat, I decided to start from scratch with tuning.  I wired the secondaries shut, and did some testing with primary jets, pump rod position, and apt settings.  Once everything was feeling good on full & part throttle, and looked good on the A/F readout, I am still a little boggy on the rolling start.  The new setup performs better with pump rod in outer hole, plus I downsized from a 73 to a 72 jet.  The A/F is reading around 10.2 during my boggy start, which seems too rich.  My idle is reading at 13.5 with 18" of vacuum. 

Based on the above notes, would it make sense to try to lean the accel pump?

Thank you!

[Cliff Ruggles]

Full throttle start?.....Cliff

[leeklm]

Correct, full throttle start

[leeklm]

Found a nice remote parking lot for a little more testing today...

Regardless if I mash the throttle or just give it an instant 1/2 throttle from an off-idle 700 rpm rolling start, I still have the bogging for a few yards.  I tried from a faster rolling start, say around 1,200-1,500 rpm, and no problems at all.  The engine winds right up, breaks the tires loose when the TQ peaks around 3,500 rpm.

It feels so close to having the carb almost perfect, but am looking for that "instant on" switch when I punch the throttle... To recap, this is with a 2.20 1st gear, 3.31 posi rear, and 255/60/15.

[Cliff Ruggles]

If it's acting up from a dead stop try installing factory baffle plate behind the choke housing, or filling the "slot" behind the choke housing with Marine Tex.  If  you can't tune out the stumble, it may be throwing some fuel out of the carb on hard launches....Cliff