A few issues

[jxu109]

I am at #3 in the book...  76 jets and 54 rods, Tubes are at .036, Down channels are at .059

My idle screws aren't effecting the idle enough.  I have them about 6 or 7 turns out -- Is it normal to take the down channels to 60's or 70's?

My cruise AFR is in the mid 14's.  When easing into the throttle from a cruise, I get a lean spike (16-18) before it starts to enrichen.  Would a stiffer power piston spring fix this, or is there something else I should do?

I would also like to try to lean out the cruise a little more, but the only way I can figure to do this is to drop down to a 75 jet -- and of course, if I don't get my lean spike to go away, I would imagine smaller jets would make the spike worse.

Thanks

[Cliff Ruggles]

Need quite a bit more info.

What is the carburetor number?

What size are the upper and lower idle airbleeds?

What size are the main airbleeds?

What is the secondary stamp on the metering rods, L, P, K, M, etc?

What are the engine specs, cid, compression ratio, cam specs, etc......Cliff

[jxu109]

17083222 - 800CFM tear-drop shaped main bleeds in the air horn...

lower bleeds are at .070, upper are .067

Idle tubes are .036

Down channel is now at .064

Main air bleed is at .085
Jets are 76, rods are 54M

350, 10:1   
Comp 270H Duration: 270/270 @ .050": 224/224 Valve Lift: ..470/.470
Performer RPM heads, Performer intake
Idle at 750 with 15-16" vacuum.
it's in a 68 C10 pickup.  3:73 gears, super T-10 4 speed with 26.5" tires

By the way, I got the choke pull-off and other things last week, it's all very nice! -- Is the restriction inside the pull off a VERY tiny hole?  I poked around in there with a #60 drill bit, and couldn't seem to figure out if I was in the right place or not.

[Cliff Ruggles]

Take an old drill bit and make a reamer out of it by carefully grinding it on 4 sides to a very sharp point.  Easy to locate the hole and make very small changes the diameter.


You should have enough idle fuel.  Double check the secondary throttle plates to make sure they are well seated and not adding any additional air?

Open up the idle tube to .038", and test it again.  It should have PLENTY of idle fuel at that point, or at least you should see an improvement?.....Cliff

[jxu109]

I've fixed a few bugs, and here's where I am:

lower bleeds are at .070, upper are .067

Idle tubes are .038

Down channel is now at .059

Main air bleed is at .089
Jets are 76, rods are 54M

I think it's almost there, but while cruising, when I ease into the throttle, it's going lean, which I believe is causing a bit of a ping (the pinging didn't start until this latest calibration...).  My timing is 16 initial + 20 mechanical all-in at 3,000.  I have 10 deg. of vacuum advance starting at 4", fully deployed at 10".

At low speed steady cruise, it holds a low 16.x:1 AFR.  When I start easing into the throttle, for the first 2-3 seconds, the AFR spikes up into the 17's, before dropping down to 13.5-14.  I have to go about an inch into the pedal from cruise before it starts to enrichen.

Where should I go from here?  Is the lean surge a normal condition, and I just need to tweak the APT until The surge isn't as bad?  I was thinking about buying some stronger power piston springs to try, but figured I should ask here first.  Or, maybe it's just that I've been running to much intial timing with the vacuum advance all this time?  It's probably close to 30 degrees at a part throttle/low speed cruise.

Thanks in advance

[Cliff Ruggles]

Lean "tip-in" is not power piston spring related, as the vacuum is really high at light load.  Raise the APT and see if it helps.  If you end up more than 5 turns up from seated, try going to .077" main jets.

Did you verify the position of the metering rods in the jets during the rebuild?  We end up bending the primary hanger arms in at least 90 percent of the carbs built here to get the metering rods correctly located in the jets......Cliff

[jxu109]

OK, will try the APT --

Where do I find info on positioning rods?

Thanks

[Cliff Ruggles]

The positioning of the metering rods is done by leaving the float out and installing them.  Never assume the hanger is exactly the right height, verify by looking at the metering rod location in the jets in the up and down positions. 

It's pretty common for hangers to be bent from prying them out of the carburetor.  The arm also comes up in the piston and may need re-seated.  When correctly set, the tips should be in the smallest portion of the jet, up high enough as to not obstruct the entrance.  In the down position the fattest portion of the metering rods should be in the smallest portion of the jets, but not so low as the fattest portion of the rod is obstructing the entrance to the jets.

APT carburetors allow us to custom postion the upper sections in the jets externally, which is a really nice feature......Cliff

[jxu109]

Good to know.  I'll check it out.  Turning out the APT seems to have tamed the surge, but not eliminated it.  I think I'll just have to call it good where it is, and if it's still pinging, I'll try to take away some initial timing.