Has anyone ever successfully done it?

I know Pony Carbs makes a 1.08 that they fit the larger butterflies to the secondary side.  I unfortunatly am without a machine shop, or I would.  I have successfully bored the secondary ventury slightly, as well as slimmed both shafts for significant improvements. 

Thoughts?

Kevin

It was a lean running condition from Idle through cruise.  I could obviously offset this some by putting fat jets in & putting the accelerator pump on generous settings. But even when I did this you could tell that it wasn't right.  It was  over compensating for a lean condition. I tried an entirely different 1.12.  Repeated the process, same basic problem.  The second I put my 1.08 on with it's correct boosters, back to a smooth transition. Idle, cruise, wot.

I tried to use a big block 2bbl years ago, 1993?? (autolite 2100), & ran it for about a month & almost burned valves! I was younger & still learning alot about carbs & was exstatic to get such great mileage & have a healthy carb moan on my then stock 292.  What I soon realized after lean fouling plugs(yes it is possible) was that it was trying to run at something like a lean burn chrysler would.(around 18:1+-)  Remember Stoich is 14.7. So after having my Dad & his master tech chew my A** for tinkering too much I returned it to the stock 2100 small block carb with slight mods to the throttle shaft & a carefully tuned powervalve.

I would love to be able to figure this out, especially since 1.12's are still in fair supply.

Kevin

With your knowledge of carburetor design, I'm surprised you did not see the problem.  The carb is too big.  It takes very little throttle opening to maintain rpm above idle, but the main fuel system is not delivering enough fuel because the venturi vacuum is nearly non-existant, and the idle system is not big enough to flow enough fuel at that rpm.  That's why your 108 works better, it is better sized to your engine.

John

Chuck

Ok, so I guess the large primary's of the 1.12 could be my obstical. I wonder if I could find early carter venturi that could be made to fit the primary side of the 1.12?  I did once try an early Q-Jet on my y block, but couldn't get the vac. adv to run, & then I had to get it ready for a move cross country, so I went back to the 1.08.  The throttle response was ausome! Even without proper Vac. adv.

I did find an interesting tid bit on the Pony Carb site: I think this applies to the Y block as well, so hope no one minds my posting it. The link should bring up a FAQ section on the 4100 1.12 "spreadbore".

http://www.ponycarburetors.com/default.asp?page=faqdetail&id=19

My first thought is a power valve issue.  Bad diaphragm, bad gasket, or vacuum rating too high.

John

What are your thoughts about using a restrictor plate(ala Nascar)with a 1.12 4100 on a stock 272?I agree with you about the 1.12 being too big for Y-Blocks,other than one of the big inch racing engines.

I'll let you know if and when I get it done and on the car.This time of year there just doesn't seem to be enough hours in the day.I retired last january and have been busier this summer than when I worked 90 hour weeks.I should have it going in few days.

I've never tried that, but it would not change the cfm of air going into the engine at less than full rpm.  The way I understand it, using spacer plates under the carb will fool it into thinking the engine is bigger, but I can't remember whether it is the open plenum spacer or 4 hole spacer that does that.  Help, anyone?  We seem to be going through a phase here repeating 50 year old history.  The biggest mistakes made on street engines have always been overcamming and overcarbureting them.

John

This would be the Holley model 4100.  Basically a good carb but never really caught on with the aftermarket crowd.

      Re the question on the restrictor plate. Like John said the issue is the air flow at low RPM. The Restrictor plate won't change that. All it'll do is restrict the amount of air at higher RPM. You have to have enough airflow through the primary side of the carb to create a signal for the fuel circuits to work accurately. If you don't have that because the carb is too big, nothing short of fuel injection is going to help.

And the tall spacer fooling the card into thinking it's a larger engine is right also.I ran into that on a circle track car when the rules required a two barrel for the limited late models.I bought a C&S carb that flowed 925 CFM and it was too much carb for my 383 until I put a 1.25" two hole spacer under it.With that spacer that thing came off the corners like a rocket.

I personally have successfully increased the secondarry venturi on my 1.08.  I know the stone I used is right at 2.0.   With the transitioning work I did to clean them up, (more on the bottom) it could be a fraction more.  The trottle plates for the secondary's are obviously too small for that size venturi. 

Pony carbs claims they reduce the primary venturi on their 1.12 spreadbore to 1".  Do they leave the throttle plates?

Kevin

I ordered a 1.08 from a rebuilder other than Pony for a lot cheaper and what they sent me was a 1.12. when I recieved it I called them and told them it was not what I ordered and wanted the 1.08 and they said they tuned the 1.12 to work on my motor so I decided I would try it. I have been running it all summer and it appears to work great. No black smoke out the exhaust so it is not too rich. maybe I should check the plugs and make sure it is not too lean like you ended up with.

Like I said, I think that is the theory. 

Anyone?

I found some reducer ends of 3/4 pvc that I happen to have & cut them up. I experimented & cut a groove in the wanna be venturi reducer to fit around the brass tube that sits in the primary side. (I think it is an equalization tube.)  I cut these pvc pieces short enough to clear the throttle blades on the bottom & to barely stick up on the top.

I measured them right at 3/4" ID.  The booster cluster fit nicely & the veturi piesces were snug.  I couldn't get the stupid fragil holley jets out of this particualr carb that I have had sitting since 2004 so I just put it together & tried it.

Of course imediatly the acccelerator pump was shooting out the side of it's housing so I found an un-opend kit & replaced it & the power valve.

when I fired it up, it idled real smoothly, just a bit high.  as soon as I craced the throttle it responded, but imediatly started to roll a rich coughing blurb, blurb, blurb,.  I reached back & with my left hand opend the secondarys while opperating the primary's with  my right.  I cleared the rich  mixture by rapping it a couple of times. still the same . Way too rich! 

off with the carb!   A quick trip to the local parts house revealed that you can not always find Holley jets on the shelf any longer.  I did find a wide tipped screwdriver to try to get the stuck ones out.  Moments later I scrounged & found a pair of 46's  to replace the 57's that were  previously stuck.    While there I decided to replace the needle & seats .  Back on the truck it fired up & idles a bit lean.. A good sighn! I adjusted the idle screws  & reved it a bit.. Definetly alot smoother. With an idle that sounded crisp.  I took it for a quick drive... Still a mosquito killer. !! good only once the secondary comes in.  I took it off, & removed the pvc inserts. I found in my box of carb stuff that little tin piece that bolts to the booster cluster that blocks a portion of the venturi.  (came factory on some 2100's & some 4100's)  I hoped that it would make the carb think the primary's were smaller, & gibe a sronger signal to the booster cluster. I put the bigger jets back in, & tried again....

No more smoke, had to open the idle screws about two more turns from previous.  .. Out on the road, noticebly less throttle application to maintain speed.  Great once in wot.  however, when stopped & take off audiable pre-ignition. This is with 57's on the primary side, & 63's on the secondary.  Back to square one... The Airflow in the primary's at low speed does not suffice on my 301 y-Block with e-4 cam, Cite Heads.  Sadly, I don't think it will work properly without alot more cubes.  It works great above 2000.  The y block could really scream with an engineered spreadbore like Pony Carbs sells.  I put the old 1.08 back on, after freshening it's accelerator pump, power valve, & Needle & seats. Once the plugs get nice & hot, It will be back to it's old self. 

Once again, my rudementary tests do not show the 1.12 working on a y block without running improperly.  I imagine that with a specific booster cluster & jet I could find a medium.  Not sure what the trade off might be though.

Kevin

One other thing to think about with these carbs is the accel pump linkage is driven by the spring coiled around the throttle shaft boss.  This has to be tight against the drive link so that the pump squirt has no lag.

Doug T

(I am NOT a newbe)

Sounds like a tough motor!  I am curious what cam specs did they grind for you?  I currently still have the Cuise-0, but plan on putting a t-5 in soon to help with my 4:10 gears.

On the Autolite carbs, You are correct about the throttle shaft spring.  The great thing about the majority of the Autolites is that you can bend the accelerator pump linkage to get full stroke of the diaphram in most cases.  I haven't tinkered with trying a modified squirter for my issues, simply because the mixture issues I have are more at moderate throttle opening vs poping the throttle quickly.  It is a signal to the booster cluster issue.  I wish I could pick the pony carb people's brains to see what they do to make this carb compatible with a bone stock 289.. I know that they have to do some air bleed calibration, just not sure weather or not it is in the booster cluster, Or the carb body.  Looking at my 1.08 I have two 1/16" or so holes that are directly in front of where the booster cluster goes in. These are not in either of the 1.12's I have.  These holes I belive go into the area where the cluster sits.  If I remember the siphoning process these "bleeds" calibrate the pressure difference between the venturi & the chamber that the cluster goes into.  Also, the holes on the sides of the brass tubes from the cluster vary from carb to carb...

The 2100's are great.  I experimented alot on the motor before rebuild.  I ended up with the stock 2100 with the throttle shaft slimmed, hand pollished venturi, & some misc. fileing on the upper carb body. 

Kevin