View unanswered posts | View active topics It is currently Thu Aug 17, 2017 7:48 am



Reply to topic  [ 7 posts ] 
 Location for holes in butterflies 
Author Message

Joined: Mon Nov 30, 2015 3:55 pm
Posts: 47
Location: Jupiter, FL
Is there an ideal location for holes in the butterflies? Just curious, because I noticed that the four holes in my butterfly plates are oriented at the inside of the bores on the opposite side of the transition slots. Should I bother to flip them around?


Fri May 12, 2017 3:55 pm
Profile

Joined: Mon Oct 21, 2013 12:15 am
Posts: 226
Location: San Diego, CA
If they are necessary, the locations are fine.

How much exposure do you have on your transition slots? By looking at your engine specs, timing, and carburetor settings, you shouldn't require bypass holes in the throttle plates. If you are able, I would recommend getting a new set of plates without the holes, and see where you end up with your curb idle screws, mixture screws, etc.

If your carburetor will tune properly without them, your throttle plate angles will be corrected; which in turn should provide the proper slot exposure, pump cam orientation, etc.

I always try to tune without the holes, if at all possible.


Fri May 12, 2017 4:17 pm
Profile
Site Admin

Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
ToddGrossman wrote:
Is there an ideal location for holes in the butterflies? Just curious, because I noticed that the four holes in my butterfly plates are oriented at the inside of the bores on the opposite side of the transition slots. Should I bother to flip them around?


The throttle plates can't be turned around because they are asymmetrical, more so on the secondary. The throttle shafts are offset slightly and the screw holes in the blades are off-center a bit so there is more area on the upstream portion of the throttle blade to provide a closing force, so air flow and manifold vacuum will pull the throttle closed.

Whatever may be an "ideal location" could depend on individual circumstances, but I think it makes sense to locate the hole near the curb idle discharge and T-slot so the high velocity jet of air issuing through the hole in the butterfly impinges on the fuel discharging from the CID and T-slot at idle and low load. A carb calibration which needs IABP holes in butterflies is probably on a radical cam engine which is inclined to have low intake vacuum and the ragged idle resulting from exhaust dilution and poor fuel vaporization. When it comes to vaporizing fuel at idle and low load, every little bit helps.


Fri May 12, 2017 6:26 pm
Profile

Joined: Mon Nov 30, 2015 3:55 pm
Posts: 47
Location: Jupiter, FL
The carb was originally from PS and that's what they built for my combo specs. I never noticed the holes were inboard, and I don't know I've ever seen it recommended to put them there, either.
I got my carb back together and will try this again on Monday.


Fri May 12, 2017 6:40 pm
Profile
Site Admin

Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1744
As an example look at the IABP situation in some Dominators where dry air enters the center of the plenum area about as far from idle/T-slot fuel entry as it can, because for some reason some people seem compelled to put the IABP bleeds in the center of the carb around the air cleaner stud. I have moved the same IABP area from the central location to holes in the butterflies near the T-slot and curb idle discharge and found the result is a much smoother idle and steadier A/F in off-idle cruise.

Even a rocket surgeon can realize the streaming jet of air coming through the hole in the butterfly will more readily and thoroughly mix with the streaming jets of fuel issuing from the curb idle port and T-slot if the streams of air and fuel impinge on each other so the dry air can mix with fuel as soon and vigorously as possible.


Sat May 13, 2017 12:36 am
Profile

Joined: Thu Mar 28, 2013 6:09 am
Posts: 728
Image


Sat May 13, 2017 6:09 am
Profile

Joined: Mon Oct 21, 2013 12:15 am
Posts: 226
Location: San Diego, CA
The location of the hole is absolutely best at the front of the plate, near the transition slot.

My point for his particular tune is to try a set of blank plates, and then work from there.


Sun May 14, 2017 12:34 pm
Profile
Display posts from previous:  Sort by  
Reply to topic   [ 7 posts ] 

Who is online

Users browsing this forum: No registered users and 2 guests


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Search for:
Jump to:  
Powered by phpBB® Forum Software © phpBB Group
Designed by ST Software for PTF.