Part throttle hesitation

[rhopper]

First post, love the forum. Carb number 17080204, built to recipe #1. Base plate rebuilt by Cliff, along with the big rebuild kit. Large main air bleeds bushed down to .070. Cliffs 42 metering rods, and main jets changed from 71 to 70, per Cliff. I'm outside Colorado Springs at 6500 feet altitude. APT is two turns out from bottomed. CH secondary rods on H hanger. Chevy 350, stock except for 204/214 cam and vortec heads. Performer intake. It's in a 48 Ford pickup with a 5 speed trans.
My idle is fine. 15" of vacuum. Off idle is good with the APT at two turns, further out gives an off idle stumble. Run hard through the gears it pulls fine all the way through the rpm band just like it should. There was a lean pop at higher rpm, but I shimed the H hanger up .010 instead of bending it, and problem solved.
Here's my problem. Cruising in 5th gear, 45 to 50 mph (1500 rpm, approx) if I floor it to pass on a two lane, or if I start up a hill, one to two seconds after I floor it, I get a major hesitation for several seconds. It will work through it after several seconds, then start to pull like it should, or if I lift it will catch up and start to pull. During this time acceleration is weak. I can't see my exhaust so I don't know if it black. My secondary flap spring is set at one turn, but I've had it from 1/2 to 1 and 1/2, and my choke pull off is just under two seconds. By the way, switching from the 71 jets to 70 I've picked up almost three MPG, and it runs sharper. Since it pulls through the gears OK I don't have a fuel supply problem, 3/8 line and a stock pump. Any suggestions?

[omaha]

   Usually when your up at those altitudes, the engine has a tendency to run rich cause naturally the air is less dense (less pressure) that is unless you have the carb set up for it. Also,  you can get away with faster/more timing in the distributer. So, it would be nice to have someone follow  you when this acceleration is being tested. and see if the engine is blowing any black smoke. Changes to correct it might include accel pump and powervalve spring experimanting.   If you could moniter a vacuum gauge, that would be good also. If the vacuum drops too low,  it would seem like the advance curve could be speeded up. the reason is that up at that altitude, . a faster advance might increase the vacuum signal at the slowerspeeds that you are using. THe higher altitude is effectively reducing your cylinder pressure and trying to accelerate in 5th gear and 1500 is something that is tricky to tune for in a carb at those altitudes (this is one area that fuel injection would work better) . I would do the vac gauge test first. I think if it were me I would just down shift in that situation. It really taxes a low compression engine up at those altitudes. These are just my first thoughts, others will chime in on this subject I hope.  (more compression or a small amount of boost would really be nice).
An O2 gauge would be another great tool to use in this situation to determine the A/F ratio in this situation. Just my .02

[Cliff Ruggles]

Probably going rich for a moment.  Does it do the same thing in low gear from a standing start?.....Cliff

[rhopper]

Leaving a stop sign, low gear, works just fine. Either light or heavy throttle it performs fine. Run hard through the gears is fine, also. Increasing speed to 60 mph, approx 1900 -2000 rpm in 5th gear, floor it and it pulls much better, but there is still a faint stumbble a couple of seconds later. I probably wouldn't notice it if I wasn't looking for it. If I use heavy throttle, but stay in the primaries it seems OK. I can feel the throttle pedal start to open the secondaries because of the slight increase in effort, and when I get into the secondaries is when the problem starts. Open the secondaries at 1500 rpm, wait a second or two, and then it starts the hesitation. Other than that, I'm happy with the way it performs, but I may try a 69 jet later on to increase fuel economy once I get everything else done. I forgot to mention, I filed notches in the air flaps, like the picture on page 106 of your book, but mine are alittle smaller. Thanks for all your help on this, just got this truck on the road after six years of work and we're looking forward to lots of miles.

[Cliff Ruggles]

I'm suspecting the metering rod height may be the problem.  The CH rods have pretty short tips, and when you shimmed them up, it made the carb richer at the onset of the secondaries.  Might want to a set of rods with the same size tip, with longer tips and lower them down some......Cliff

[rhopper]

What rods do you suggest, and I'll call and order them?

[Cliff Ruggles]

I can custom machine a pair from the same cores with longer tips, do it all the time.....Cliff