Not enough carb transition slot during cruise??

[427dart]

Still working on getting the cruise A/F where the engine likes it in the light 2000 RPM cruise area in 4th gear or 5th gear.
Now again to refresh...this is a 427 cube Windsor with dual 650 Holley carbs in a 2970 lb. '85 Mustang LX with 3.50 gear and it doesn't take much throttle at all to keep it in this cruise range. Could this be the issue of being in the 15 A/F range despite going up to .033 on the IFR and running .070 Idle Air Bleeds.

Problem is to keep the idle speed at no more than 1100 RPM Hot. I have the throttle plates almost closed completely with just a crack of the front primary carb transition slot showing...maybe .015 at most. The secondary throttle blades are just cracked open so as not to stick. The rear carb has no slot showing and there is a small amount of linkage progression before the rear carb starts to open less than 1/4 throttle. There is also a PCV system working and I have a small restrictitor in the Vac. line for that.
Ignition timing is at 19-20 Initial with total at 33 degrees at 2000 RPM.

Does the amount of transition slot open control the volume of cruise Air/Fuel mix available at any particular throttle opening?
Issue is as light highway cruise RPM go up the mix goes leaner to where engine starts to stumble a bit. More throttle gets the mains going and it comes down.
This problem came about with the install of the larger 1 11/16 carb throttle plates to the Holley 1850 carbs.
Anyway to get a bit more slot opening without increasing Idle Speed? Yeah I know I could pull the base timing down a couple degrees then extend the curve a bit and or restrict the PCV flow more.

[pamotorman]

Still working on getting the cruise A/F where the engine likes it in the light 2000 RPM cruise area in 4th gear or 5th gear.
Now again to refresh...this is a 427 cube Windsor with dual 650 Holley carbs in a 2970 lb. '85 Mustang LX with 3.50 gear and it doesn't take much throttle at all to keep it in this cruise range. Could this be the issue of being in the 15 A/F range despite going up to .033 on the IFR and running .070 Idle Air Bleeds.

Problem is to keep the idle speed at no more than 1100 RPM Hot. I have the throttle plates almost closed completely with just a crack of the front primary carb transition slot showing...maybe .015 at most. The secondary throttle blades are just cracked open so as not to stick. The rear carb has no slot showing and there is a small amount of linkage progression before the rear carb starts to open less than 1/4 throttle. There is also a PCV system working and I have a small restrictitor in the Vac. line for that.
Ignition timing is at 19-20 Initial with total at 33 degrees at 2000 RPM.

Does the amount of transition slot open control the volume of cruise Air/Fuel mix available at any particular throttle opening?
Issue is as light highway cruise RPM go up the mix goes leaner to where engine starts to stumble a bit. More throttle gets the mains going and it comes down.
This problem came about with the install of the larger 1 11/16 carb throttle plates to the Holley 1850 carbs.
Anyway to get a bit more slot opening without increasing Idle Speed? Yeah I know I could pull the base timing down a couple degrees then extend the curve a bit and or restrict the PCV flow more.

file a slot in the butterfly in line with the ITS. that is the way some AFB carbs do it

[F-BIRD'88]

Is your pcv hooked to only 1 of the carbs or T'd and hooked to both carbs so that that is shared equallyby both carbs?

If you want the very top of the idle transition throttle position zone to be a bit richer reduce the idle air bleed a bit...

Before you swapped throttle plates what was the idle speed and t slot exposure at idle and the idle off idle transition afr shift?

Do you have the open carb spacers under the carbs now? If so remove them. Put the carbs directly on the manifold or on 4 hole spacers so there is no dual plane plenum sharing. Try that. Idle speed change...
Throttle opening at idle..... Better Primary/secondary idle throttle position balance.....? Better Afr balance?

[wyrmrider]

what manifold again?

[pamotorman]

if you have progressive linkage why not just idle off of just one carb. remember the 3x2 GM engines had no idle circuit in the end carbs.

[F-BIRD'88]

if you have progressive linkage why not just idle off of just one carb. remember the 3x2 GM engines had no idle circuit in the end carbs.

Those were dedicated tripower carbs. These are holley 4 bbls. Holleys are designed to idle on both pri and sec throttles...
The problem is the open carb spacers. Too many throttles available at idle and low speed (8).

Removing the carb spacers cuts that in half for any 1 cylinder. Full divided plenum.

[induction apprentice]

Still working on getting the cruise A/F where the engine likes it in the light 2000 RPM cruise area in 4th gear or 5th gear.
Now again to refresh...this is a 427 cube Windsor with dual 650 Holley carbs in a 2970 lb. '85 Mustang LX with 3.50 gear and it doesn't take much throttle at all to keep it in this cruise range. Could this be the issue of being in the 15 A/F range despite going up to .033 on the IFR and running .070 Idle Air Bleeds.

Problem is to keep the idle speed at no more than 1100 RPM Hot. I have the throttle plates almost closed completely with just a crack of the front primary carb transition slot showing...maybe .015 at most. The secondary throttle blades are just cracked open so as not to stick. The rear carb has no slot showing and there is a small amount of linkage progression before the rear carb starts to open less than 1/4 throttle. There is also a PCV system working and I have a small restrictitor in the Vac. line for that.
Ignition timing is at 19-20 Initial with total at 33 degrees at 2000 RPM.

Does the amount of transition slot open control the volume of cruise Air/Fuel mix available at any particular throttle opening?
Issue is as light highway cruise RPM go up the mix goes leaner to where engine starts to stumble a bit. More throttle gets the mains going and it comes down.
This problem came about with the install of the larger 1 11/16 carb throttle plates to the Holley 1850 carbs.
Anyway to get a bit more slot opening without increasing Idle Speed? Yeah I know I could pull the base timing down a couple degrees then extend the curve a bit and or restrict the PCV flow more.

.070" seems very large to me for a primary idle air bleed? high idle could be associated with excessive valve guide clearance. But of course more carbs, bigger dia. throttle plates and p.c.v. do aggravate the issue....

[427dart]

what manifold again?

The Price Motorsports

[427dart]

Made a few adjustments. Found that there was some play in the rear carb secondary end link and I adjusted the link to keep the secondary plates closed against the throttle stop screw just like the front carb. This helps keep the idle at a fixed speed. You want it just to stay closed with a very slight wiggle not jammed tight.
Also found the timing was closer to 34 degrees Total and backed the Initial down 2 degrees to 19. This combined with secondary adjustment gave me a 850 RPM idle at same throttle setting so I adjusted carb idle speed slightly to give 1000 RPM idle which should give me just a bit more slot opening.

Swapped in .067 IAB's in front carb only and made a short drive and it has helped the A/F reading at 2000 RPM with low to mid 14's which is where you want it if the engine will tolerate it.

An .070 IAB size is much the norm for most Holley carbs as an out the box setting and allows a range you can go up or down with available Number drills.

More road testing this evening as the weather cools to low 70's.

The weather over the last month has been hot and very humid and now we have a week long break of 70-80 temps and lower to normal humidity..perfect for my carb tuning!

[wyrmrider]

agree with f-bird on the open spacers
get it working with no spacers first

[Tuner]

Agree. I wouldn't use open spacers on that intake and I would use 4-hole gaskets so the only cross-talk between bores is through the small channels in the centers of the lower surfaces of the carb bases. Open spacers make the distribution can-o-worms have about four cans open at once.

[427dart]

I put the 4 hole 1.5 inch spacers back on before the test run this morning and it is smoother than my modified open to 4 hole setup.

[Geoff2]

You only have 0.015" of T/slot showing at idle?
Lucky you!
That leaves more of the TS available for transition onto the main cct, a good thing. The part of the TS slot above the blades act as an air bleed, & as the blades open more, less air is bled in & more slot exposed below slot, enrichening the mixture. A very ingenious design. A performance carb has a shorter TS than an emissions carb.
Sounds like you have transition/cruise dialed in pretty well, so if it ain't broke, don't......

[427dart]

You only have 0.015" of T/slot showing at idle?
Lucky you!
That leaves more of the TS available for transition onto the main cct, a good thing. The part of the TS slot above the blades act as an air bleed, & as the blades open more, less air is bled in & more slot exposed below slot, enrichening the mixture. A very ingenious design. A performance carb has a shorter TS than an emissions carb.
Sounds like you have transition/cruise dialed in pretty well, so if it ain't broke, don't......

Yeah it's fairly close to being as good as a carb gets. Just got back from a drive and it's in the 13.9-14's now when cruising around at light throttle.
When you let off the throttle during a coast situation it goes into the mid 12's from vacuum I guess pulling fuel out the idle passage.
Hard to have it all with a carb but that's how it is!

[F-BIRD'88]

Try it as you got it but T the PCV hose and connect to both carbs so that the PCV flow rate is shared by both carbs.
Reset both carbs for better balance with that and then road test. it is the same amount of PCV air flow bit is now shared by both carbs. As both those generic 1850 carbs are designed around a working PCV system. Now you can tweek that and tweek the fuel curve +/- in different driving modes. Find the PCV flow that is right at idle,,, The PCV flow that is right at cruise and the PCV flow rate that is right at WOT, and the PCV flow rate that is right on DE-celeration. All within reason.
Now build/hack a custom dialed in PCV valve that does this stuff. Now that you seem to have the carbs close.


You can do a cylinder balance test to see what each cylinder is doing (leaner/richer than...) stronger weaker than of the 8 cylinders. using the AFR gauge and the vacuum gauge and the tach by temp/momentary disabling the spark on each cylinder to find the strong ones and the weak ones of the 8 cylinders . The weak/leanest cylinders will show the least change in the man vac and AFR when disabled. At idle and at off idle.. Just looking for weak offenders of the 8.
"Cylinder balance".. EG: if you got a lean cylinder at idle/off idle and you ID it and correct it a bit so the AFR is now not so lean/weak on that cylinder, now you can lean all the cylinders overall a bit, as you are always tuning fr the worst of the 8 cylinders
and the AFR gauge and the vacuum gauge is showing you a average of the 8, at best.
You don't really need to know the exact AFR, just isolate and ID the weakest/leanest cylinder of the 8.

Does the vacuum drop/afr shift test result from the simple running "cylinder balance test" agree with the spark plug appearance differences? Is that weakest/lazy-ier cylinder running leaner than or running richer than -- all the rest of the 8?
Lots of ways to tweek the carbs a hair here/there to correct for that. Including tweeking the PCV flow bias/balance on the front and back carb. If they both share that job.

As they both were designed as a "primary carb" with a working PCV attached. Best to let them both run on this motor as equally is possible. You got 8 points of fuel flow with the 8 carb venturii.