FBO ignition system, 6 bbl carbs, and constant manifold vacuum

[biomedtechguy]

Ok. Because of the way FBO designed my distributor, for my motor, I have to connect a constant manifold vacuum source to the FBO distributor's vacuum advance can. I found one under the center carb, where the choke vacuum diaphragm is connected. I assume that there are no other constant manifold vacuum connections, (are there?) and that I will have to use a Y to have both lines connected, unless I can connect the choke vacuum diaphragm to the ported metal nipple higher up on the center carb?
The other issue is my 440 now has gone from 850 rpm idle to 1100 rpm, and I don't see any way to adjust the idle down further. The "anti run on solenoid" is no longer active (it used to work) and the screw that would normally contact it doesn't touch it. Maybe I have too much fuel "leaking" in through the outboard carbs? They are "fully closed" as far as the linkage goes, and I don't know how to close them further even if they are slightly open or allowing fuel to come in. I could REALLY use some help as tomorrow is my last tuning day before leaving Friday afternoon for a 2 1/2 hr each way trip to Mopars at the Battleship in Mobile, AL.
The distributor is designed to have 18° advance at idle with no vacuum, the constant manifold vacuum source is designed to add 12° advance at idle (full manifold vacuum) and 16° mechanical advance which is all in at 3,200 rpm. So at WOT I have 34° total advance all in at 3,200 rpm, but if I lift or am at part throttle, the constant manifold vacuum kicks in additional advance.

 

[dla4567]

I have a vacuum port on the back of my manifold. Choke needs to be on manifold vacuum. Was the center carb on high idle when you adjusted the linkage to the secondaries? That will give you a high idle. Disconnect linkage to outboards and push them closed while idling. Most don't use the idle solinoid, you should be fine with out it. For what it's worth, my timing works best at 22 at idle, 37 all in at 2200. Your settings sound pretty close to get you started. Also if not using vacuum port on carb be sure to plug. Good luck.

 

[dla4567]

If your having trouble, you could run it with out vacuum advance until you have more tuning time.

 

[biomedtechguy]

Only "trouble" is too high an idle. I appreciate the reply. I will test drive it tomorrow morning. I may make some mixture screw adjustments as well. Any ideas on how to further properly reduce the idle speed? Is there any other constant manifold vacuum source for me to reconnect the choke vacuum diaphragm to or do I need a Y? How can I check and adjust the outboard carbs at idle?

 

[dla4567]

Idle speed- unscrew idle speed screw on center carb, disconnect linkage to outboard carbs and push them shut. If it's still idling to high you need to check for a vacuum leak.
Other manifold vacuum source- some manifolds have a plug on the back, if you do not, you will need to use a Y connector.
Only the fuel/air mixture can be adjusted on outboard carbs at idle. (We all know what a pain that can be). I use a a/f gauge, some adjust to best vacuum or rpm. (Mine are best at 5/8 turn out, but every engine is different).

 

[biomedtechguy]

Awesome! Great thorough answer. I really appreciate it!

 

[biomedtechguy]

I was able to reduce the idle speed. Also, when I disconnected the rod from the rear carb it slowed the idle a bit. Problem is the rod slides in/out of the rear carb hole easily, but the clip must be slightly pulling it open. I will tweak the clip if I can (maybe make the hole in the clip very very slightly larger) It's not feasible to adjust the rod AFAIK because it's less that 1 turn off. FOB system works good.
I gently bottomed both idle mixture screws on the center carb and the engine actually smoothed out some-any advice? I didn't leave them that way, I opened them about 1 full turn. Went on a test drive, seemed to run ok in a variety of different situations.

 

[440beep]

Hmmm if you "bottomed out" the center carb idle mixtures screws meaning the center carb is fully lean now and the idle improved leads me to believe the outboard carb idle screws are over rich.

The outboard mixture screws should only be an 1/8 turn out. Front one is easy to adjust, the rear or center carb needs to come off in order to reach those rear idle mxture screws.

 

[biomedtechguy]

Hmmm if you "bottomed out" the center carb idle mixtures screws meaning the center carb is fully lean now and the idle improved leads me to believe the outboard carb idle screws are over rich.

The outboard mixture screws should only be an 1/8 turn out. Front one is easy to adjust, the rear or center carb needs to come off in order to reach those rear idle mxture screws.

Thanks!!

 

[dla4567]

I think 1/8 turn out would be to lean. Every engine is different, mine are best at 5/8-3/4 out.

 

[62MAX]

Thanks!!

Do this then go back to the center,when you get it right the ends will probably be between 1/8 and 1/4 and the center around 2 turns,you have to get the ends right before the center.Like said above, if the ends are rich you will have little or no center adjustment.It is a pain but the easiest way to get the rear adjusted is to switch them front to back,adjust and then put them back.

 

[biomedtechguy]

OUTSTANDING!! Thanks again!!

 

[biomedtechguy]

Update: I used a piece of vacuum tubing with a small flat blade tip inserted into the end, and I was able to adjust the rear carb idle mixture screws withoutremoving the carb. All 4 outboard carbs idle mixture screws are 1/2 turn out. I started at 1 1/2 turn out on the center carb. The center carb idle mixture screws have a significant effect now on the idle. Still playing with the center carb idle mixture screws, but around 1 1/2 to 1 3/4 seems good. Idle speed set to appx 900 rpm.

 

[440beep]

Can you post a picture of the vacuum hose tool please.

 

[biomedtechguy]

Sure. Tomorrow morning. It's just something I came up with. After "sweating" having to remove the rear carb to only set the screws where others suggested and then reinstall, even if I switched the front to back carbs, I could still only adjust whichever one was in the front. So I was thinking about it for a week, and about 30 seconds after I woke up one morning the idea came to me. Not as good as having the angled aftermarket plate I'm sure-it's tedious, but doable. I will post a pic in the AM. While I'm thinking about it, would "tuning" the front carb screws, then swapping front for back give the same/similar result? I would rather be able to adjust BOTH front and back carbs during the same "session".

 

[62MAX]

Switching back to front,adjusting and then reversing is as good a way as any and it works,just a PITA with what is involved.Did it since the early 70's with no issues other than time involved until I bought a Promax plate for the rear carb.When I bought mine they were somewhere around $100,today I think they go for $175 if you want to spend the money .

 

[biomedtechguy]

Thanks for the reply. I'm sure I will get the promax plate eventually. This is a "solution" for those who have the stock setup. Pic of my "solution" coming up in a bit.

 

[440beep]

Yea, I'm on the verge of spending the money for the Promax setup, but the Six Pack is running pretty damn good now, so not sure it's worth it. Not like I am constantly dicking around with the end carb idle mixture.

 

[biomedtechguy]

What prompted this round of carbs tweaking was the new FBO ignition system and I knew my idle was too rich...made my eyes water there were so many gas fumes. The center carb idle mixture screws all the way closed making the rpms increase and the motor smooth out pointed to an obvious problem that the outboard carbs needed to be leaned out. Big improvements so far, probably going to "dick around" (lol) with them a bit more, but I am really close IMO to 6 bbl idle nirvana. I will install the jetable aftermarket metering plates in the outboard carbs next.

 

[62MAX]

What prompted this round of carbs tweaking was the new FBO ignition system and I knew my idle was too rich...made my eyes water there were so many gas fumes. The center carb idle mixture screws all the way closed making the rpms increase and the motor smooth out pointed to an obvious problem that the outboard carbs needed to be leaned out. Big improvements so far, probably going to "dick around" (lol) with them a bit more, but I am really close IMO to 6 bbl idle nirvana. I will install the jetable after market blocks in the outboard carbs next.

http://www.quickfueltechnology.com/search?query=metering+plate

And if you buy the jetable plates,get them from Quick Fuel ,both cast and billet available and 1/3 the cost of Promax.You have to ask for the cast plates as they don't have them listed on the web site.Here is a picture of both.