I have a Holley 850 ultra hp carb that is running very rich at part throttle cruising. Specs are 84 jets front and back plus 4.5 power valves front and back.

Engine is a 493 with 10" vacuum at idle in gear.

Car is mostly street driven and I know I have a race carb but was wondering if I can do anything.

I was thinking about knocking down the front jets about 5 sizes and increasing the back 5 sizes to make up for the loss.

Thoughts?

"I was thinking about knocking down the front jets about 5 sizes and increasing the back 5 sizes to make up for the loss."

Only for a test.

If you decide to keep the smaller primaries, drill the PVCRs out to get the mix back. I find that even their street carbs are a little fat and need smaller primary jets and bigger PVCRs. But the race carbs are much, much too fat at cruise and in the transitions for the street.

If this is a double pumper, they are way fat for the street and require several changes to get street friendly. It can be done, but does require some mods.

Whats the cruise A/F readings

This is a race double pumper and I don't have a Air/Fuel ratio gage other than my nose and fuel gage. It is amazing how fast it burns through gas just cruising around.

It idles great and runs great at WOT.

I have never modified a carb other than replacing jets, PV's and accel pump cams and squirters.

Can the mods you are talking about be undone? Kind of worried about the effect a overly rich condition would have on an oxy sensor while trying to tune carb. I have heard they don't last long if they mixture is to rich. So I didn't think I should buy a A/F meter.

I may sell the carb and go with a more street friendly one if I have to.

84s both front and rear with a pv doesnt seem right to me?

The new 850 Ultra HP seems to be the one carb in that lineup where Holley really missed on the calibration from everything I've read. I was ready to drop the $$$ on one, but couldn't find a positive review of anyone who had tried one (that wasn't associated with a magazine article ).

I run a standard 850 DP on my 63. I had to jet up alot with the 493 and I found that if I went to much on the primaries it would also be to rich at cruise RPM. There is nothing fancy on my 850 as it has all the stock metering blocks and all. So I had to limit the jet size on the primaries as I still run a power valve. I ended up jetting the secondaries more as I found that the limit on the primaries with my setup was about 88 jets and I ended up with 94's in the secondaries. It runs great like that and if I fatten it any more I will up the secondaries. Yea this 493 wants alot of fuel. Ron

Well, I got it for 1/2 price brand new I figured I could make it work. I had reservations about buying it because of my mostly street driving. Like I said it is great at idle and WOT which I guess is all you need if you are only using it for racing.

Don't forget that all that extra fuel is washing down your cylinder walls and fouling your oil. Pull your dipstick, I bet your oil is black as diesel oil after only a little drive time. Not good!

Without a wide band, a good reference book and a little experience the real fix will be a bit of a problem.

You will need small drill bits for the PVCRs. Something in the .060 range +/-. If you over drill them, inserting wires can get you back close.

But the balance between the mains and the PVCRs are just part of the issue. The transition slots are way fat as are all the metering block metering holes. This is a race carb and is metered as such.

All these things can be changed/adjusted, but it is too much hassle for most people.

Every carb I have messed with has needed at least tweaked to the particular engine. And some are not even remotely close. The WOT is usually not too bad. It is the idle, transitions and cruise that typically need the most attention.

Some of the changes can be made seat of the pants. But cruise and some of that really needs to be done with a wide band. And a good Holley book. It takes time and experiments.

The cruise should end up in the 14.7 range +/-. But most of the other adjustments are not so clear cut.

I do not mean to make this sound difficult, but there is a lot going on with the carb from idle to cruise. And all the circuits overlap which makes targeting the issue that much more problematic.

It is too small for your application, but believe it or not, the carb that consistently needs the least amount of mods to get really close to most engines I have messed with is the good old 3310 (750). From 340s to mild 400s, it is one of the best street and street/strip carbs I have seen.

I have found that a lot of the Holley race carbs. are not good street carbs., especially at part throttle You can fix them by limiting the fuel feed on the idle and transition circuits by drilling them and tapping them for #6 brass set screws and then increasing or decreasing the size of the hole in the set screws. You reallly need a wide ban 02 sensor system on the car to be able to get the AFR dead on You can do that by the hit amd miss method, that will take a little longer I did a older Holley 1050 Domintaor 9375 non HP that way, I reduced the idle fuel feed hole size down from around .080 to .045 and then reduced the intermediate fel feed by similar amount percenatge wise, that took the AFR at very light part throttle cruise between 2000 and 2500 RPM from 11.0 to 11.6 to 13.5 to 13.8 AFR I should have reduced them even more and did some more fine tuning but I didn't You can also open up the idle, intermediate and high speed air bleeds on the top of thw carb. to lean the mixtures out also, lots of choices on fine tuning. You really need to make sure that the carb. does not go lean at wide open throttle also while fixng the other circuits on them, all of the circuits can and do affect the wide open throttle mixture some Good luck The main jets affect all of the mixture, I try and fix what is not what I like first, idle, transition, part throttle and then W.O.T.

What if the rich condition is prior to the mains activating?

Pin gauge or find out what the IFR's sized and go from there. If it's larger than ~.032, I'd find a way to get it sized around .032, reset idle mix, and try from there. The ultra should have adjustable bleeds for all the stuff you need to mess with to clean it up.

I wouldn't step down 5 in front and up 5 in rear to balance, you're asking for distribution issues doing that.

I do have and indexed drill set and an old holley tuning book. Not sure how much it would have in common with this carb. But the carb does have the screw in air bleeds at the top which I can play with first. I think I will read my holley book first and get myself familiarized with all the concepts you guys are talking about.

There is some very good reading on yellow bullets wed site. Go to the naturally aspirated section and look for the 4150 metering thread[it is a few pages in]that thread will keep your head spinning for awhile.

Thanks, I'll check it out

Just out of curiosity. Why would you run a 4.5" power valve when you have 10" of vacuum at idle?

Poor man's version of what cab said with easy instructions:

Big motor has plenty of cruise power with carb barely cracked open =running on idle circuits and transition slots. Choking the primary jets might not get it done. Corner idle circuits can be choked off with the idle mixture screws, T-slots are not so easy- they run pig rich.

T-slot and idle discharge port mixture is decided by the idle feed restrictions (IFR) and the low speed or also called idle air bleeds. The easiest way to attack it is by sticking little v shaped pieces of wire into the IFRs in the metering blocks. You will see that there is the IFR hole and a little angled recess next to it that can be used to hold the V of wire.

I think I ended up using guitar B string wire (.017?) and now my double pumper is really close. I used to have to clean it out by running up the revs constantly but now I can just ease the pedal a crack and putt along behind grandma in her toyota until I get some open road.

Important: the 'corner' idle screws do not 'mix' air and gas, that job is done by the IFR and air bleed working together to make an emulsified mix. This mix is delivered straight to the T-slot and has to go past the corner screw before it can come out the idle discharge port (little hole below throttle blade). Make sure at idle you only show a square of T-slot below the blade.

As you lean out the emulsion coming out the t-slot at cruise you also lean out the idle, so you'll need to spin the corner idle screws to keep the idle happy. Some initial timing is crucial for all this to work too.

A sign this is what you must do is a carb that idles well with the corner screws barely open from screwed all the way in snug. As I tried bigger wire diameters to restrict the IFRs my corner setting went from roughly 3/4 turn out to 1 1/2 turns out. Fine tuning can be done with air bleeds. Some folks will drill the throttle blades- same effect but much harder to undo.

Good luck
Radar

Well for one thing I sure don't have to worry about them opening early and really richening things up. They were in the carb when I got it and I always read that you should have the power valve number 1/2 of the vacuum number.


Thanks for the easy to understand writeup radar

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