Off-Idle Hesitation

[rdorman]

Do check with the builder. Depends on many things. You will probably find the total should be in the range mentioned, 32-34. Maybe as low as 30 but doubtful as high as 36 with a modern alloy head. My lazy old open chamber iron cleveland heads likes bunches! Initial, probably in the 16-20 range. If you bring it back to 8, it will be a slug by comparison off the line and you have to open the plates more which MAY aggrevate the stumble. Get the timing right, adjust the carb and go from there. Many a builder does not take the time to recurve the distributor. So, they set it for total timing, max power and don't worry about the initial. If is starts and idles, fine. They are looking for max power. But, that does not work so well for drivability. I tend to run as much initial as I can. So long as it will start when hot (and heat soaked for a few minutes) and does not rattle, it is good. I crank it in until I can't get it to turn over and back it off two degrees and that is where I leave the initial. Have it curved for total and forget about it. I chased that darned off idle issue until I was blue in the face and it was all about the timing and the effect the timing has on the carb. Happy tuning!

[CWizard]

Hey guys -The title pretty much says it. I can't read the numbers on the damper. Rechecked my timing while totally awake and the real numbers are 22 initial, 34 total. Exactly the same as when the engine was dynoed by the builder. Rdorguy, I tried the small tap on the primary accelerator pump and to my amazement the engine almost died! Recovered immediately, though. So I'm either too rich or right on the hairy edge. Since it idles fine and the idle mixture screws are about 1 1/2 turns out, I'm thinking the latter. But I may be too close to the edge since I tuned it with the air cleaner off.

I think timing is OK so I'm going to retune the carb (this time with the air cleaner on), and maybe try smaller primary pump nozzles. Will probably check the idle transfer slot unless a dramatic change occurs from retuning. It'll be a day or two, since I'm wrapping up another science project - I'm measuring the pressure loss caused by my remote oil filter setup.
Thanks to everyone for all the help.

[rdorman]

Originally Posted by CWizard

Hey guys -The title pretty much says it. I can't read the numbers on the damper. Rechecked my timing while totally awake and the real numbers are 22 initial, 34 total. Exactly the same as when the engine was dynoed by the builder. Rdorguy, I tried the small tap on the primary accelerator pump and to my amazement the engine almost died! Recovered immediately, though. So I'm either too rich or right on the hairy edge. Since it idles fine and the idle mixture screws are about 1 1/2 turns out, I'm thinking the latter. But I may be too close to the edge since I tuned it with the air cleaner off.

I think timing is OK so I'm going to retune the carb (this time with the air cleaner on), and maybe try smaller primary pump nozzles. Will probably check the idle transfer slot unless a dramatic change occurs from retuning. It'll be a day or two, since I'm wrapping up another science project - I'm measuring the pressure loss caused by my remote oil filter setup.
Thanks to everyone for all the help.

Sounds like you are on the right track! Give your air bleeds a little squirt of cleaner. I do it everytime I have my aircleaner off. Check the transfer slots first and get them right. Then use the tap trick and set the idle mixture. You don't tap the pump hard, just enough to get a drop out. If the idle gets better, turn the idle mixture out, worse, in. About 1/8 turn each and try again. When you are close, an 1/8 out and tap will make it worse, 1/8 in and tap will make it better. The difference will be subtle. Four corner idle? Repeat on the secondaries. Have about 2/3 of the air fuel coming from the primaries. Repeat tapping both pumps and turn all four equally about 1/16th of a turn.

I don't recall, but you did check the accelerator pump to be sure that fuel comes out immediately didn't you?

Take a look at a plug. Soot around the top of the body (near the threads) is a good indicator of a rich idle (which leads to a lean transfer curcuit). Ideal is about 1 turn out.

Change the cam and squirter back to stock. Rarely, do they need tuned. If this doesn't do it, we will go from there!

[CWizard]

1. Lots of carbon/soot on the plugs (checked 2).
2. Idle transfer slot, as found: About .060 uncovered primary, completely covered on the secondary side.
As left: About .030 primary, completely covered secondary (no change made)
3. Idle mixture screws: As left: About 1 turn out, primary; about 1/2 turn out secondary.
Results were very encouraging but not 100%. Off-idle hesitation is much less severe, but still present. During the mixture screw adjustments, at no time did manifold vacuum improve when I (gently) tapped the accel pump lever. Adding fuel was always a detrimental effect. So it wants more air. I'm not finished tinkering with the idle mixture just yet (I'm going to try the technique advocated by Don Gould at www.4secondsflat.com, which is very similar to the advice given here), but I 'm thinking about a small hole in the primary butterflies.
Thanks for all the suggestions and guidance, and please keep it coming.
CWizard