Holley Ultra HP 750 tuning tips not tricks

[vector1]

I did a major retune on a couple Holleys, had a hesitation on two engines using the hp 750 and wasn't so sure there couldn't be a better tune.

Engine 1 was a 408, afr205, 10-1, vic jr., 11" idle, cruise 15". Nothing special, pretty familiar with the cobra world.

Engine 2 was a 351w fms crate engine, afr185, 9.5-1, Z cam, rpm air gap, 15" idle vacuum.

The carbs were identical when purchased and are identical now except for idle adjustments and main jets.

I started with the 408 installing a set of old metering blocks that were recommended by a few and mimicking the settings on the main body of the old carb and using the newer float bowls. I ended up frustrated as I don't think I could end up with the fuel curve I wanted because of lack of adjustability. My impression was this carb was designed for heavier cars, and I was operating in an area at cruise that was not intended and ending up with a richer mixture than I wanted. I couldn't tune this out unless I made some mods, and then would probably still be a sacrifice.

So I did some net research, leafed through some books, and made some observations from carbs I had sitting around. Most of the carbs stay within a certain range so this is where I tried to stay, using little nuances to fine tune what i think would work. There were some fine people on another forum that provided helpful advice also.

I came to the conclusion that transfer slot restrictors were going to be needed for the adjustability. This is an easy mod and could possibly relieve a lot of headaches for people.

I did the first carb and installed it on the 408 and it was wonderful. Cruise afr is 15.5-16, and on acceleration never gets above that then it starts to dive with more throttle and the pv opens. No hesitation, great acceleration.

There is some drilling and 4 holes to tap in the main block but all is easy to do if you have the patience and tools and parts.

Here is how the carb is initially set up from Holley

IFR .035
PVCR #53 or .0595
PV 6.5
JETS 76 x 4
IAB #49 or .073
HSAB .036
SQUIRTER .031, suggest you check these with a drill bit.
EMULSIONS top-bottom .028,.028,.028,blank,.028

Here is what I did

IFR .031
PVCR no change
PV no change
JET no change
IAB #51 or .067
HSAB no change
SQUIRTER no change
TRANSFER SLOT RESTRICTORS, PRIMARY #55 or .052, SECONDARY #53 or .0595, figured a range of .046-.0595 from my research.
EMULSIONS top-bottom .026,.028,.0292,blank,.031, note the ultra hp uses the 5 hole emulsion wells.

This worked so great I set it up exactly as this for the 351 and installed it, felt it was a little rich at 76 main jets so dove for 72 and that was too lean and settled on 74's. Still ran great and no hesitation.

So what you need are an 8-32 tap for the t slot restrictors and associated #29 bit, 8-32 brass set screws, 6-32 brass set screws for the emulsion wells and ifr's, be able to solder and drill some of the air bleeds or use new ones. I used an in/lb tq. wrench for the drain plug at 3 lbs and the bowls at 2 lbs although 3 lbs is recommended.

I can go into more detail if someone has questions or wants to try it. I don't know if it will work with other carbs unless the 5 hole emulsion wells are used, I think that is a big part of success with the new tune, and possibly part of the downfall of disappointment when they don't work as purchased.

The idle eze makes it really nice because when the t slot restrictors are installed the base plate comes off, and you can adjust the throttle blades to within a hair of the bottom of the t slots. I did this to both carbs, I did have to raise the blades 1/4 turn primary & secondary because I could not get the 408 to idle with the idle eze all the way open, once I opened the blades 1/4 turn I could turn the idle eze down for adjustment, the 351 setting remained and I was able to adjust with the idle eze. Note: when the idle eze is used it also effects the afr.

I did have an afr and vacuum gauge to verify the 408, the 351 is not equipped.

I thought this was a real success and hope others can experience the same.

So with this setup, you are actually idling on the idle circuit (13.5), at cruise you are on the t slot (15-16), mild acceleration is on the main jets (16), and max acceleration should be in the pv (12.5-13.5). -(approx afr range)- The afr curve I ended up with was very good.

I haven't had a chance to check full throttle afr's, not enough concrete yet, but I have had it up through 3/4 throttle and into the pv and the curve was real nice.

[vector1]

These are the holes you want to tap for the transfer slot restrictors.

[Gaz64]

Interesting results.

Myself, I would have changed the IFRs only first.

[vector1]

Changing the IFRs and IABs changes the air/fuel ratio of the fuel entering the idle circuit, why it is recommended to only change one so much before the other needs to be changed. By keeping these constant and at a good ratio you can restrict the slot itself which measures at about .082 in area to get the desired result, plus you can use a smaller restriction on the primary side for lean cruise afr and something a little larger on the secondary where it is not needed.

Here is the full story for those time inclined:

http://speedtalk.com/forum/viewtopic.php?f=1&t=36018

[vector1]

Also made an interesting observation I would like to add. With the old metering block the cruise afr was anywhere from 13.5-14.5/15, depending on the IAB number, couldn't get it any leaner without adjusting the IFR, and it was fixed. Also the IFR was at the lower portion of the metering block and resulted in a smoother afr reading than if the IFR is located at the top. At this richer cruise setting, the car would accelerate real well from cruise, but the mpg was lower and I didn't want to cruise at the richer afr's. So people buy a carb, and it has been my impression most of the carbs made by the aftermarket tuners can be on the rich side, probably for safety. People get one of these carbs and think they have something really hot, what they don't realize is they are cruising at the richer afr's. So with the t slot restrictor installed you can adjust this to wherever you like. jmobservation.

[vector1]

The emulsion package is going to need some work. Had a chance to put a full throttle run on the 408 and there is a big ol ugly lean spike right before the pv comes in. I believe the .031 was a bad idea on the emulsion stack and is going to need some tweaking, likely at least a .028 as original maybe smaller. Everything else was still good. also moving the IFR to a lower position in an effort to smooth the afr at idle and cruise, I'll update the thread when changes are made.

[vector1]

Okay, this is my rereiteration, many things learned. One, easy to go round in circles, almost like trying to align a car, one thing effects another effects another and so on. Best thing for me i realized was to go back to the original calibration and take another avenue.

Emulsions--.028-0-.028-0-.028. I felt there were too many or too much area so in the end just removed the second from the top.

IFR--I moved this restriction to the bottom of the well, smoother afr is supposed to be the result, so doing this necessitated a recalibration in this circuit as it effected the afr at idle and cruise.

.033/.070 with t slot restrictor of .043 on the primary side. If you can do the t slot restrictor it makes things so much easier in adjusting the lean cruise. On the secondary side the t slot can be larger as you are not cruising on it, just used for transition to the mains, .055 used here. .031/.067 was used with .046 restrictor on the primary also and worked good.

PV is now 9.5 on the primary side, 8.5 secondary. This adjustment can help alleviate the lean afr climb as you accelerate, bring the PV in at the highest setting without undue opening, idle is at 11". Also something I learned, the PV starts opening at the rated depression, it will take a bit before it is fully open, so if our light cars accelerate at a fast pace, you might want to lead it a little.

PVCR .0595, no change, possible change as the observed afr was 12.5 at full throttle.

HSB .035, dropped from .036

Squirter .031, no change

Accelerator cam white primary, pink secondary.

Recommend afr gauge and vacuum gauge for all adjustments.

[vector1]

This morning for an experiment, I switched the idle circuit around, IFR .031, IAB .067, t slot restriction to .046, just to if there was much difference. Didn't appear to be much, highway cruising uses so little throttle that it seems like it is in the bottom of the t slot along with the idle adjustment and a little richer than I would prefer, someday maybe fix that.

The PVCR was reduced, originally .0595 was 12-1, .055 was 12.5, and .052 where I ended up at was 13-1. So each size drill bit was good for .5 on the afr gauge.

[vector1]

Here's a pic of condensation on the bottom of the carb, the top of the carb/air cleaner is sealed at the hood opening. temp was 73 with 59 humidity. Pics didn't come out, maybe try again.