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Location of idle air bypass holes
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Author:  RB1 [ Tue Jul 31, 2012 12:05 am ]
Post subject:  Location of idle air bypass holes

Interesting result here the other day. Had a Pontiac engine in my buddy's hot rod to tune up for him. I put one .093 idle air bypass hole in each primary throttle plate wayyy out near the fuel discharge holes... much further than I normally would do, and I'd be darned if that car had a really hard time restarting. Would be lean and needed a shot of fuel from the accel pump to get it lit back up.

I changed the plates out to a set that have the hole on the inboard side of the throttle shaft, and the restarts were normal again.

My initial thought was that the hole killed some of the signal the idle port would see... anyone experience something like that?

This was a 4779.

Just thought I would pass that one on, and perhaps start a discussion on that topic.

cheers!

Author:  Ken0069 [ Tue Jul 31, 2012 12:32 am ]
Post subject:  Re: Location of idle air bypass holes

Forgive me but I'm a little confused. So why did you put the .093 holes in the throttle plates in the first place? :-k

Author:  RB1 [ Tue Jul 31, 2012 12:40 am ]
Post subject:  Re: Location of idle air bypass holes

When I took the carb off, the throttle plates were well into the transfer slot at idle. I was trying to bring the throttle plate opening angle back down by giving the engine a bit more air thru by putting the holes in the plates.

I had thought that maybe putting the holes out near the idle discharge hole, and transfer slot... it would mix up the fuel a bit... but maybe I went too close?

Author:  Ken0069 [ Tue Jul 31, 2012 8:07 am ]
Post subject:  Re: Location of idle air bypass holes

T slot exposure isn't usually a problem with 4150 carbs is the reason I asked.

Not sure the hole location is all that critical as vacuum will be the same at the idle fuel dump hole regardless of where the hole is in the blade. It's sort of like pressure, ie, pounds per square inch means that pressure is in all directions. Vacuum is the same, just the opposite of pressure but it's in all directions at the same time.

How close was that hole to the T slot?

How much T slot did you have showing with and without the .093 holes? :-k

And just to check, you do have the secondary throttle blades open the same amount as the primaries, right? :-k

Author:  RB1 [ Tue Jul 31, 2012 10:27 am ]
Post subject:  Re: Location of idle air bypass holes

Hmm, I guess the "your mileage may vary" statement is a good one here, as I see the tslots machined too deep quite often on the 4150s, or at unequal depths. But this particular deal was not because of any machining errors from the factory, it was because the engine needed more air at idle and was well into the T-slot which I have never found to be a good thing.

I did crank the secondary throttle plates open a bit more on this, but i find using the air bypass hole can help to lean out the part throttle air fuel ratio at tip in. This may be hard for me to explain, but with the hole in the plates where I usually put them (not way out at the edge), I have seen the air fuel ratio leaner with the throttle plates closed and up to about 20% throttle opening, then after that, the AF ratio richens back up as if there were no bypass holes. I had theorized that this was due to the hole in the throttle plate having less bypass air thru it as it nears vertical.

When opening the secondary plates only, I have found it does not have that effect of leaning out the part throttle fueling, then tapering back richer.

The more we play with carburetors, the more we learn that people size all their channel restrictions to suit themselves...or their theories. Maybe I am creating an issue somewhere up the line with my "tune up" that exacerbates this restart issue. That is the neat thing about carbs, many ways to the top of the mountain.

When working on one type of engine years ago, I found it needed NO idle air bypass but the carburetor came with holes in each plate. I plugged them off and had a pig rich off idle fueling issue, and that is when I found the relationship of how the idle discharge port and trans slot channel pressures are effected by the air bypass holes.

It was such a mystery that I even went to the lengths of outfitting a test carburetor with ports into the idle discharge channel and transfer slot channel, along with a TPS sensor :-k to track the pressures of these channels vs. throttle openings, and with different throttle plate bypass hole sizes and locations. It is a shame that I did not run the holes way out at the edge, which is how this post started.

Author:  Ken0069 [ Tue Jul 31, 2012 10:44 am ]
Post subject:  Re: Location of idle air bypass holes

Yeah, whatever works for a particular application is what is best! ;-)

Author:  Beretta [ Thu Aug 09, 2012 5:48 am ]
Post subject:  Re: Location of idle air bypass holes

After reading this a few times :- I am willing to try a few things with my E85 carb as I am having issues with idle and some cold cyd's threw out the run.
I will post a picture later today of where the holes are and these are .095 in each blade as this is a 1250 HP body that has T-slot showing with the blades fully closed.
What I was thinking of doing is put a small hole where it usually goes and another hole on the other side of the shaft. Not sure what that will do and never did it before and also never seen one done like that either so it will be a test.
If you have something to add to this good or bad post it =; Thanks

Author:  Scott Smith [ Thu Aug 09, 2012 6:12 am ]
Post subject:  Re: Location of idle air bypass holes

I was going to show Illustration but my photo gallery is down [-(

Author:  Beretta [ Thu Aug 09, 2012 8:34 pm ]
Post subject:  Re: Location of idle air bypass holes

Can't edit my post but here is the hole location in the throttle blades..
Image

Author:  Scott Smith [ Fri Aug 10, 2012 4:43 am ]
Post subject:  Re: Location of idle air bypass holes

Image

Split the difference between the idle fuel discharge hole and the Tslot

Author:  Eric68 [ Sat Aug 11, 2012 4:29 pm ]
Post subject:  Re: Location of idle air bypass holes

rarely do I use holes in the throttle plates in any of my e85 carbs. the reason being is that I can usually add enough air by opening the secondary side more -- this allows the primary to be closed back down to where the blade angle is better (right amount of tslot showing)

A key factor with this too is timing at idle. The more initial advance you have the less blade angle the engine will need. If the distributor has a big advance curve in it the blades will have to be opened more exposing more tslot. I like locking the timing out on e85 stuff as this seems to really work well with just about everything and keeps me from having to drill any holes.

Keep in mind that the larger the throttle blade is the more air can be passed by it so with a big Dominator it becomes even less likely that I need holes in the blades. I have seen a couple right from Holley though that had the tslots cut too low and if I hadn't fixed that issue I probably would have had to put holes in the blades.

With real little carbs (like 390 4bbl or 500 2bbl) IMO they often need holes in the blades if used on a restricted class engine.

Author:  ajcasini [ Sun Aug 12, 2012 12:41 am ]
Post subject:  Re: Location of idle air bypass holes

Eric how do you fix the issue of T-slots cut too low?

Author:  jmarkaudio [ Sun Aug 12, 2012 9:56 am ]
Post subject:  Re: Location of idle air bypass holes

With the inconsistencies of the Dominator main bodies you HAVE to run idle bypass somewhere in almost all the Dominators. I put small hole in all of them, usually around .060 just to have a small amount, it's easier to drill more if you need with starter holes once the carb is together.

Author:  SGDCXCVI [ Sun Nov 25, 2012 7:58 pm ]
Post subject:  Re: Location of idle air bypass holes

jmarkaudio wrote:
With the inconsistencies of the Dominator main bodies you HAVE to run idle bypass somewhere in almost all the Dominators. I put small hole in all of them, usually around .060 just to have a small amount, it's easier to drill more if you need with starter holes once the carb is together.


So Mark, where do you drill the hole? Is it as Scott suggested above?
Cheers Colin

Author:  Ken0069 [ Mon Nov 26, 2012 8:40 pm ]
Post subject:  Re: Location of idle air bypass holes

I'm not Mark but I can answer this. Yup, the location that Scott shows is known as the "preferred" location although the original poster seems to have better results with it in another place. :-k The ones that I've had to drill I've put in the "preferred" location also.

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