Dead spot on tip-in regardless of throttle speed; Holley 4150

[65ShelbyClone]

This is something I have searched and read quite a lot about, but none of the 100+ articles, books, and threads I scoured directly addressed this. I even have the Haynes Holley rebuild/tuning book and it doesn't either.

Anyway, my mechanical 650dp wasn't holding a steady idle; it would slowly climb from ~600rpm cold and hang around 1000-1100 once it was fully warm. I baselined the butterflies and idle screws according to a post on here by IIRC D.Hearne because I thought maybe the transfer slots were open. The idle cleaned up a bit, but now there is dead spot right off idle. It's totally dead, especially on slow and medium tip-in speed. This leads me to think that it is not the accelerator system. If I slap the pedal down, it shoots past the dead spot and the car responds immediately.

Any recommendations?

 

[10secgoal]

Take compressed air with the idle mixture screws out and shoot it in the holes. The idle circuit runs at idle, and ends right about tip in. Sometimes junk gets lodged in there that can't be removed with carb spray, causing it to go lean on one bank. Could explain the pump shot covering it.
See if the timing chagnes during warm up. Seen springs beome weak after warm up and advance the timing at idle. Could lock it out for a quick test.

 

[65ShelbyClone]

Thanks. I'll try clearing out the idle circuits like you say. The carb was used and I put a rebuild kit in it, but I've always been suspicious that it could have problems later on. Fortunately I have another 650 and two 600dps to borrow parts from for troubleshooting.

The distributor is a rebuilt Duraspark that might have all of 5000 miles on it, so I kind of doubt that the springs are sacked out(unless they get reused[?]).

 

[D.Hearne]

Check for slack in the accellerator pump linkage. If there's too much, that'll cause your problem.

 

[65ShelbyClone]

Yeah, I did that too. There again, I can very, very slowly apply the throttle and it will hit that dead spot and start stumbling. It even does it on engine braking. I can be on a slight downhill with my foot completely off the pedal and it's fine, but when I just breathe on the pedal I can hear it missing in the exhaust(Shelby-style side exits....under my seat).

 

[D.Hearne]

What jets and powervalve are you running?

 

[10secgoal]

Vac leak? Know you probably checked, but tip in can have a gap if the floats are low.

 

[65ShelbyClone]

Jets are 65p/76s and PV is 6.5.

I pulled the idle screws and blew out the passages. That alone seemed to help, but I also leaned out the idle and adjusted the butterflies and it seems to have cleared up quite a bit.

 

[pabear89]

Use some carb cleaner on the air bleed holes.
when they get plugged it will have this effect on idle and off idle responce.