2 x 4 Quick Fuel Carb Tuning

[Cyclone03]

I'll admit I stopped reading when the IAB change moved you in the right dirrection.
Put the as delivered IAB back in and start leaning the IFR's.

With 2 carbs you have the potential to dump a lot of fuel at light load/cruise. You can drop the IFR .001" at a time but I think you will end up in the .026 range on the primary side. .028,.030 secondary. IAB v.IFR is about 3:1 a .073-.070 IAB change is like a .033-.032 IFR change. But the IAB really  tunes WHEN the transfer system starts and stops it really does move the point it works,and you can see it with an O2 indicator,tach and vacuum gauge in back to back testing.   

I may have missed  it,are the carbs 4 hole idle?

If it's primary only,2 hole, try to get the mixture screw out 1 turn  at idle ,if you find it just stumbling right off idle when you drive it turn the mixture screws IN 1/4 turn,if it gets better make the IFR bigger ,by .002.

As Ross's chart shows you will find yourself driving around in the transition area the most,with dual quads you may not pull into the mains at all until 3000rmp and 80mph.

Being you have so much fuel flow available I think your secondary will "just" show the transition slot,call it barely opened,you dont want the blade sealing the bore closed. work on getting the primary at the "optimum" .020 or square slot shape. You want good strong transfer fuel flow just off idle.

When you get the IFR right you will see idle O2 AFR changes with 1/4 turn of the mixture screws. Work to have all the mixture screw set at the same point out also,when setting start at a base,say one out,then turn each in 1/4,continue around 1/4 turn at a time at some point the idle rpm should drop if it starts running rough but doesnt die at full closed lean the IFR. You may (most likely) find when you lean the IFR to the point you can kill it the transition is too lean,go back up on the IFR .001 and call it good enough because the drivability is more important ,work the mixture screws to clean up the transition.

It really is a back and forth process .

In tuning a Demon 850 on my stroker 390 I put 90 mains in all four corners so I knew when the mains came in and found with a 3.70 rear gear in 4th gear 1:1 I didn't see any AFR change until 3500rpm and over 1/3 throttle, had to get the vacuum in the 8-10in range .
In OD the load went up so the turn on point was much lower at 2600rpm but still 8-10in vacuum. In normal cruise driving as high as 70 mph a big carb,or two, works in the transition zone almost 100%.

I will say with my above test the mains do pull at mid to high engine load,call it more than just tipping it.
Sometimes a test brings clarity and can help you understand the carb circuit you are working in.

Focus on one circuit at a time to get the best drive-ability.

[The Real McCoy]

Cyclone03

Thanks for the feedback.  You make some good points that match up with what my engine seems to be doing, especially being on the idle and transition circuit while going down the road.  Ross has also been a huge help on this.  I have been sidetracked by other things going on so have not been able to devote a ton of time to this.  I will get some IFR's coming and go with your recommendation.  Probably will take some time to get the IFR's and get them in but will post results when it does happen.

Carbs only have mixture screws on Primary side.

[The Real McCoy]

Just an update......

I ended up ordering some .025 and .028 IFR's for the primary side.  Seems that QF has .020, .025, and beginning with .028 they start in .001 increments on IFR's.  (Secondary IFR's are .040 and are part of metering plates so did not make them smaller). 

I changed the IFR's from the original .033 to .025 and the IAB's from .076 back to the original .073.  The secondary IFR's are still .040 (original) and IAB's are .045 (QF's initial recommended change from .033 to .045).

The above change made some progress bringing the cruise O2 reading up to about 13 and idle about 12.  It sometimes will drop down to the 10.5 to 11 at idle and noticeably load up but usually work it's way back up to 12 if it is left idling for a few minutes.  It will also sometimes lean out a lot, to 20+ while driving a round town at low speeds with light throttle, giving it just a little throttle will bring it back down quickly.  I put about 300 miles on it over the weekend and O2 readings were very stable around 13 going down the road at 2000 to 2400, this was a definite improvement but still a bit rich.  It did pickup at least 2 MPG over any previous MPG data, I do record miles and gallons when ever I fuel up.  The best before was maybe 11, usually between 10 and 11.  The trip to where we went on 2 lane roads, 55-65MPH netted 13.3 MPG.

So moving on to the rest of story....., during this whole ordeal I have noticed raw gasoline in the vacuum lines when disconnecting them.  Wasn't really sure if it was in the ported line or the manifold vacuum line until just recently but it appears to be the ported line based off recent observations.  I have about a 6" vacuum hose stub with a nipple union for each port which goes to the ported and manifold vacuum nipple on the throttle base plate of the primary carb.  I have also noticed plenty of gasoline under the carburetors after they have been removed during some of the recent exercises with the carburetors. It is very common for raw fuel to be puddled up in the primary bore of the intake after the carbs are removed.  One day after the primary (front) carb had been off overnight I measured how much fuel was in the front and the  back bowl.  I got approximately 7/8 ounce from the front bowl and 2 3/4 ounce from the rear bowl. 

Last night when we got home I immediately pulled the carbs off and set both carbs up on blocks on the garage floor so they were level.  It wasn't long  before there was a puddle under the rear carb.  The front carb took a little longer but it started dripping gas out of the ported 6" stub vacuum line.  It's puddling of the gasoline was not as sudden nor as severe as the rear carb, it only puddled under at the vacuum hose and not under the carb. I guess I can't say if the leaks are actually from different leak points or not.  The rear carb does not have vacuum ports to set them up the same way.

 This morning I measured the fuel in each bowl, the carbs had been off the car for about 16.5 hours.

Front Carb                                                           Rear Carb
Primary Bowl 2 1/4 ounce                                     Primary Bowl 1 7/8 ounce
Secondary Bowl  3 1/8 ounce                                Secondary Bowl 3 1/4 ounce

This fuel leakage all seems to be internal.  There is no raw gasoline dripping externally on the carburetor or intake manifold, other than if the ported vacuum line is disconnected it will dribble a little out.

So I guess my questions are.....
1.  Is this fuel migration out of the primary bowls normal?
2.  If it is not normal what would the root cause be?
3.  If this fuel migration happens when it is not running, is it possibly the main reason for the rich conditions when it is running?

Appreciate any ideas on this.  Thanks

[The Real McCoy]

Hey 601 hp

You sent me an inquiry via PM back in early February as to what happened with my Quick Fuel carb problems.  I did not see the PM until a few days ago and responded via PM.  Check you PM and advise if you have any other questions.

Thanks.