Idle feed restrictor size

[turbotbirds]

What size are most of you running?  Did you put the restrictors in the metering blocks or in the main body?

Helping a friend out with his car and its got good a/f at wide open, but part throttle and cruising is way rich (10 and below).

If you have pics, that would be great also.

Thanks, John

[Boost Engineer]

While Idle feed restrictions do add some fuel under the conditions you describe, Jetting and
fuel coming through the transfer slot passageway typically control the areas that you are describing.  Jetting feeds the main circuit boosters in the venturi and the transfer slot feeds
the engine until the boosters are activated.

Many install a restriction device (pressed in or a set screw) in the base of the main body to control the transfer slot fuel flow. I typically open up the size of the passage in the main body
to a tap size, use steel set screws, and then heat them with a torch to remove the majority
of the hardness so I can drill them.  If you use brass set screws then that set will not be necessary BUT you might strip one out on occasion.  Make the size around .080" initially.

Tom V.   

[turbotbirds]

Thanks for the input!

Yeah, the transfer circuit seems to be the culprit since we have .024 high speed air bleeds in it with only the siphon break in the emulsion package and its bringing the main metering in to soon.  With this set up the fuel curve is super flat, but just way rich going into boost and cruising.

I'm gonna build another BRPV for it later, but that will not cure the rich cruise issue.  I like the idea of just reducing the transfer slot orfice.

Does the .080 help lean it a lot over the stock?

I have never messed with this circuit before, since in my last car all I was worried about was WOT! hahaha

Thanks,

[tuner]

What carb are you dealing with and have you determined it’s not that the PMJ is too big?  Does your carb have changeable bleeds?

Of course, the IJ controls the fuel to the T-Slot but many times when the IJ is big enough to give the curb idle adjustment the best sensitivity the off-idle is too rich.  When that’s the case, as when there is low vacuum/long overlap valve timing, a jet in the T-slot passage is the cure and then you tune the off-idle with the TSJ and IAB.

The most convenient way to jet the T-Slot is with 10-32 x 3/16” brass socket setscrews in the metering block face of the main body.  In that location they’re easily accessed for tuning.  The Idle Jet in the metering block can be done with a 6-32 x 3/16” brass setscrew.  There are pictures in post #45 in this thread http://www.innovatemotorsports.com/forums/showthread.php?t=4958
of a 4500 carb with the deal but the 4150 is the same program.  The orifice sizes are very application specific, so the sizes used in that carb in that thread won’t apply to your situation.  It’s very likely the sizes used in the 870 Avenger in this thread will be closer. http://www.innovatemotorsports.com/forums/showthread.php?t=3134

If you don’t already have Innovate’s LogWorks software it’s a free download from this page. http://www.innovatemotorsports.com/support.php There are a number of threads detailing T-Slot tuning in the Innovate Forum with before and after data posted in the LogWorks format.

[turbotbirds]

The carb is a proform main body.  The front jet is 75 with .093 PVCR's.  The rear jetting is 95 without a pv.  The carb has downleg boosters and I did the vent tube mod to it.  Idle air bleeds are 75's front and rear and the high speed air bleeds are 24's.  The metering blocks are stock with two emulsion holes per side and a siphon break.  This has given us a perfect flat air fuel with 20psi once the boost is up.

There is some great info over on the innovative site.  I think we will try to tun the transfer slot restrictor first to see if that helps, if not then maybe enlarge the "siphon" hole in the metering blocks a little larger to help delay the main metering system.

If just the siphon hole is enlarged some, if the 24's are kept in the high speed bleeds, will the fuel curve still be flat, or will it cause lean out in the upper rpms?

Thanks agian!

[tuner]

You’re welcome.  The ProForm body has 1.400” (or nearly so) venturi, doesn’t it?  If that’s the case a #75 MJ is usually fairly generous for the part load.  Have you tried smaller?

Yes, enlarging the kill bleed (I call it) will raise the starting RPM of the main some.  Careful with that though, it’s hard to drill it smaller.  What size is it now?  .026 - .028 is usually good.

If just the siphon hole is enlarged some, if the 24's are kept in the high speed bleeds, will the fuel curve still be flat, or will it cause lean out in the upper rpms?

The answer to that is edgy because to describe bleed changes in absolutes is folly.  You just have to try it.  It might in one engine and not in another.  The difference in carb venturi sizes makes it so the range of metering pressures (the venturi generated pressure the jets and bleeds deal with) is such that a small carb may have the same working metering pressure at 1/4 power that a large carb has at 100%.  In other words, top end for the large carb is low end for the small one in terms of the working pressure of the flow through metering orifices.  That’s one reason why many times the effects of a change are counter-intuitive and not as described in the enthusiast media.
 
If your brain hasn’t been numbed in a while, study the physics of “flow through two orifices in series” and to that add “compressible flow through two orifices in series”. There’s probably something in the NASA Technical server.  “Two-phase flow” describes what happens after the air and liquid are mixed.  There are some papers in there on that subject.

But… if you don’t change the MAB it should stay the same on the upper end.  I don’t think it’ll go nuts.  In general terms, the e-holes and kill bleeds affect the low end and the MAB’s influence is more toward the top.  If there is e-hole area below float level, enlarging the MAB will richen the bottom at the same time it leans the top. 

If you plug the lower e-hole that should help lean the lower part of the main circuit flow in the range where the bleed air is encouraging the flow.  The kill bleed adjusts the start of the main flow and the e-hole air causes more fuel flow in the low range right after the main starts.  A bit of lead or a toothpick tip is easily removed from the e-hole so playing with that isn’t such an un-doable deal if it isn’t what your situation warrants.

[turbotbirds]

I just reread what I wrote on my description of my metering blocks.  What I meant to say was the blocks have two emulsion holes per side and a kill bleed.  The only thing left open is the kill bleed.  We epoxied shut the two emulsion holes per side.

With a .036 high speed bleed and the emulsion holes shut, the air fuel was pretty flat but had a gradual leaning from 12.0 to about 12.5 a/f.

By changing the high speed bleed to .024 it is perfect flat at 12.0 through all the rpm once boost starts.

But when we reduced the high speed down to get the a/f in check, this is when the cruise and part throttle got really rich.

You are helping a bunch!

Sounds like we may need to unblock the emulsion holes and keep the high speeds at .024's and check out the fuel curve!  What do you think?  Or should I try to lean the transiton circuit first?

Thanks

[tuner]

Good on the e-holes.  If you open one it will probably get richer.  Emulsion air encourages low throttle main delivery.  I’d jet the t-slot.  Have you tried a slightly larger MAB?  You might raise the MAB to the same size or .001” smaller than the kill bleed.  What size is the kill bleed?  It’s bigger than the .024 MAB, right?  .026 or .028?

On the t-slot deal, jet it so lean it’s stupid and come up one drill bit at a time while playing with the IAB’s until it’s just rich enough to run good when it’s fully warm.  Probably start at 1/16” just to get a taste of real stumble and work up.  Did you read about it in the 870 Avenger thread in the link?  That gentleman is getting 18 MPG with a 471” RB Mopar in a `69 Charger.  It took him a few months and he has a data system, but …. it keeps us out of mischief, eh?  What kind of WB are you using?

[turbotbirds]

Wow, there is a ton of great info on your site!

I don't know the size of the kill bleed hole.  I will check that out and get it within spec this week once I get to work on the car again.  I got all ideas your right though and the kill bleed is .026-.028!  I'll take the main air bleed to that level and check the WOT a/f and see how that effects the lower rpm rich condition.

Then I will do the transfer slot restritions on the main body if its still rich.

Right now we are using a friends chassis dyno with their wide band.  It seems to smooth a lot, so we may have to get one to road test.

I'll let you know what happens, thanks a ton!!!!

John

[turbotbirds]

Wow is all I can say, thanks for the help guys!

I couldn't find any brass set screws so I got some steel ones and did like Boosted engineer said do them and drilled them.  After they were heated up they drilled pretty easy.  I drilled the set screws to 1/16. 

The cruise stumble/flooding and boost transition stumble/flooding is completley gone!!!!!  It has a slight hessitation if you mat it, so I think its a little lean right there and may be able to be cleared up with a larger squirter.  Now it only has 31's for the squirters or go up to 5/64's on the restrictions.

Drivablility is 500% better since we installed these and going into boost is much easier without any flooding!!!  Going on the dyno today or tomorrow to get the tune exactly right for cruise and to make sure the transition circuit restrictions didn't lean out WOT!

Thanks again for all the help!

[tuner]

Greetings John,

What were the results of the dyno session?

[tuner]

????????????

[turbotbirds]

Thanks for the help, sorry its been so long.  He has taken it to the track and busted the tranny so we didn't get it on the dyno to see exactly what the change made to the a/f.  I know it leaned it a lot, but don't know exactly how much.  Once he gets it back running we are going to put it back on, hopefully.  Thanks  John