Six Pack Idle Mixture Screws Don't Do Much???

[PurpleBeeper]

Well everyone.... I re-installed my carburetors after rebuilding them again very, very carefully & using "bone stock" settings. It runs like a new engine....smooth idle, 12" vacuum, no smoke out of the tail pipes nor smell of fuel.... and I haven't even adjusted the carbs yet! The idle fuel mixture screw on the center/driver that didn't work was some "oily sludge" that came out of the metering block. As far as the "pig rich" condition, that appears to be related to over-jetting and/or an incorrect carb gasket.

 

[biomedtechguy]

Awesome news! Where did you get your rebuild kits from, what part numbers? What jet size did you go with for the center carb? Same ? for the carb to manifold gaskets. Stock metering plates in the outboard carbs? How many turns out are your idle screws on the outboard carbs?

 

[Polara1974]

Great news Purple!!! I've been following this thread and I'm happy to know you sorted things out. Impresive how much your Vaccum read has changed!!! Good!!!

It will be of great help to other if you can post your carbs calibration for reference.

 

[PurpleBeeper]

Awesome news! Where did you get your rebuild kits from, what part numbers? What jet size did you go with for the center carb? Same ? for the carb to manifold gaskets. Stock metering plates in the outboard carbs? How many turns out are your idle screws on the outboard carbs?

As I mentioned, I have not tuned the carbs yet (haven't even set float levels), but here's what I have right now:

Holley 3-396 trick kits (I think that's the number)
#62 main jets
Outboard plates = Quick Fuel, but I copied the stock 34 front & 35 rear metering plates ('70 carbs). Front carb jets are 80/80 (stock is 79/79) and rear is 82pass/77driver. Also, I got 0.029" idle feed restrictors (QF plates come with 0.039").
Power Valve = 4.5
Idle Mixture - started 1-1/2 turns out center & 1/8 turn out all 4 corners of outboards.

FYI - I just limited the mechanical advance on my distributor (16-degrees BTDC initial timing) & my next step will be to tune the carbs.

 

[62MAX]

You are too lean on the center carb,original was #64,you could probably go to a #65 with no problems,also,I think you will find that # 81 on passenger side front and rear and # 83 on driver side front and rear will work quite well.If you have 12 in hg,go back to the 6.5 power valve.

 

[biomedtechguy]

Thanks to all posters.

 

[PurpleBeeper]

You are too lean on the center carb,original was #64,you could probably go to a #65 with no problems,also,I think you will find that # 81 on passenger side front and rear and # 83 on driver side front and rear will work quite well.If you have 12 in hg,go back to the 6.5 power valve.

Thanks again 62Max. I think you're right on all counts, but since I've been chasing this horrible carburetor issue for several months, for now at least, I'm going to tune the carbs in near-stock configuration and let my 1/4-mile time slips lead me towards the perfect tune. Today I will start to "dial in" the ignition timing & carburetors and see where that takes me. Thanks to everyone for all your help!

Hey, does anyone know anything about buying & installing a cost-effective and reasonably accurate oxygen sensor and associated hardware to tune in the fuel mixture? I think the fact that I don't have a "H" crossover pipe in my exhaust may help me.

 

[dla4567]

I believe mine is from Innovate engineering, found the best price on Amazon but don't remember how much. It does take a lot of guess work out of tuning.

 

[biomedtechguy]

Set your initial timing at 15*,idle speed at 900 rpm,center carb screws out two turns,end carbs 1/4 turn and do what the second paragraph of the attached form tells you,adjust each screw one at a time until there is no change in idle speed then go back and re adjust center carb screws and idle speed.As for metering plates,69 1/2 use plate #6178 front,#6180 rear,70 uses #34 and #35 rear,340 uses #41 both front and rear.Wondering what main jets you are using and be "sure" you have no vacuum leaks and also the correct metering block and metering plate gaskets.

The picture you attached said to put your finger on EITHER of the outermost idle bleed holes. After turning both idle mixture screws out 1/2 turn out from all the way in on both outboard carbs, I adjusted the idle mixture screws on the center carb (wound up at 1 3/4 out on both sides of the center carb using my digital tach on my timing light). If I put my finger on the idle bleed hole one at a time on either outboard carb, the idle doesn't change, which SHOULD be good? Now if I put my fingers on BOTH (instead of EITHER) bleed holes at the same time, the idle drops...just asking if the EITHER (one at a time) method is correct? Thanks. Seems to be running/idling/starting/WOT really good.

 

[62MAX]

View attachment 338891
The picture you attached said to put your finger on EITHER of the outermost idle bleed holes. After turning both idle mixture screws out 1/2 turn out from all the way in on both outboard carbs, I adjusted the idle mixture screws on the center carb (wound up at 1 3/4 out on both sides of the center carb using my digital tach on my timing light). If I put my finger on the idle bleed hole one at a time on either outboard carb, the idle doesn't change, which SHOULD be good? Now if I put my fingers on BOTH (instead of EITHER) bleed holes at the same time, the idle drops...just asking if the EITHER (one at a time) method is correct? Thanks. Seems to be running/idling/starting/WOT really good.

One at a time,once you get no change covered or uncovered it is correct.

 

[biomedtechguy]

One at a time,once you get no change covered or uncovered it is correct.

Awesome! Great. I couldn't hear or see a difference, but I will hook up my timing light digital tach and watch the display to see if there is a variance. It's idling great and doesn't smell like a refinery

 

[PurpleBeeper]

I was trying that finger over the outer air bleed hole trick, but wasn't seeing too much difference. However, when I ran the idle down really low, like 700rpm, I could just "hear the mixture" with noticeable changes in idle speed when I turned the outboard idle mixture screws in/out. I went one at a time & every time I'd get the idle to go up, I'd turn the idle screw out on the center carb. When I maxed out my idle at the front/rear carbs, I re-adjusted the center carb the same way, then set the idle screw on the center carb to where I idle at about 950rpm. I plan to go back and try to tweak the mixture screws again with the finger-over-hole method to see if I can get it closer, but the engine is running like a new crate motor and I have other fish to fry (brakes) at the moment.

 

[Polara1974]

Purplebeeper were did you got the 0,029 idle feed restrictors for the Quickfuel plates? They are 4-40 thread and I cant find them at Sunmit.. Do you have a part number?

 

[PurpleBeeper]

Purplebeeper were did you got the 0,029 idle feed restrictors for the Quickfuel plates? They are 4-40 thread and I cant find them at Sunmit.. Do you have a part number?

I just called Quick Fuel on the phone and told them what size I needed. I think I paid $3 for all four.

 

[biomedtechguy]

Resurrecting this thread. Found this process in the picture helpful, again. Thanks

 

[440+6]

Resurrecting this thread. Found this process in the picture helpful, again. Thanks
View attachment 538207

I have that same article somewhere, that's how I've been tuning my secondaries.