4781/850 DP jetting.

[rfoll]

In search of more usable rpm, I have been working on an 850 to rplace my 4780 800 double pumper. Details of the combo can be seen here...viewtopic.php?f=1&t=62825 I ran the carb on Friday and I was down 3 tenths and 3 mph. The 60' was off by a tenth, but the carb did not bog. I reinstalled the 4780 and immediately returned to 11 flat at 12 mph. Stock jetting on the 4780 is 70/76 with a secondary power valve. Without the power valve in the back and as installed I have a jetting of 78/91. The 91 I wrote in pencil on the secondary bowl sounds huge, but the last time I had my afr meter on the car it read WOT in the mid 12s, it doesn't respond well to being leaner. There is a chance one of these jets are off by some, but the car runs hard. Stock jetting for the 4781 is 80/80 with a secondary power valve. I used 84/90 on the 4781 with the secondary power valve removed. If I compare to the smaller carb, this thing is way too lean. That comparison could be apples to lemons. Before I start spilling gas all over the place I would appreciate any thoughts about jetting the larger carb before I install the afr and work on some changes.

[F-BIRD'88]

typical 4781 850 jetting is 80 pri 88 sec (no sec PV)
You will need sec jet extensions..
So go to the track..
Start with plenty rich jetting say 86 pri 96 sec
and work leaner Watch the ET mph

Should have upgraded your 4780 800 to a 750 HP carb (820 cfm) Using the Proform 750 HP upgrade body.
Still can...

Usually those 4781 850's are powerfull carbs...

[rfoll]

In other words you believe I'm lean. From the 4780 jetting it would appear to be so. I have no recollection of how I arrived at the jetting for that carb. I have the Proform body, could never make it do what it is supposed to do, and going down 50 cfm isn't going to get me anywhere.

[1980RS]

Never had much luck with the 4781 carbs. My 830HP works very well as do most of my other 850's of different manufacturers. A 4780 is nothing more than a 3310 with a mechanical baseplate, if you don't believe me measure the ventui openings, I did. Both were the same.

[F-BIRD'88]

In other words you believe I'm lean. From the 4780 jetting it would appear to be so. I have no recollection of how I arrived at the jetting for that carb. I have the Proform body, could never make it do what it is supposed to do, and going down 50 cfm isn't going to get me anywhere.

No I don't believe... You do.. I suggest tune from plenty rich towards lean.. Look for performance not AFR # that looks sexy...

The 750 hp size works. and flows 820. more with throttle shaft,plate, screw areo modes.. Much better fuel metering signal
over a larger range. The Proform 750 hp body swap is easy to dial in. The whole purpose beyond some cfm gain is the adjustability.

[dondon]

not a carb expert by a long shot but my high compression 340, my 454, and my 427 chev all ran the best mph with a 4781 with 83/86 jets with a power valve.

[Tuner]

OP, I think you're right, it is too lean. If you have large intake volume, such as a large plenum with a stack of spacers, it will need more jet than with small intake volume. I have goofed a couple times telling someone to use smaller jet because #95 in a 4781 just seems too big and .... oops. Throw jet at it until it slows down.

Be aware that because the 4781 casting has the venturi ID so close to the choke horn wall the primary acts as if it is a smaller venturi such that if you have PV plugs in primary and secondary it would use 2 to 4 sizes smaller primary jet than in the secondary. But no matter what, jet to whatever sizes give even plug color and best MPH.

The 750 Proform body will definitely not improve either a 4780 or a 4781. We all have our opinions, but the only way I would use one of those is if somebody had a carb with busted boosters and already had the Proform body and didn't want to spend the money to put new boosters in their existing Holley body.

[F-BIRD'88]

not a carb expert by a long shot but my high compression 340, my 454, and my 427 chev all ran the best mph with a 4781 with 83/86 jets with a power valve.

This looks about right to me. The OP should end up very close to this when he tunes from plenty rich, progressing towards leaner to get ET MPH track perf gains.
And not get all wrapped up in AFR guage readings.

The guage may be out of wack or effected by exhaust leak.
These afr guages are best used only as "you're too lean warning devices". Indicating that you need to then look at all the plugs for lean condition.

[piston guy]

I ran a 4781 on my Boss 302 engines and it liked 80 in the front and 88 in the rear with the power valve blocked. It was faster than the 4780 in "my" application. Still have it 40 years later.

[Tuner]

Aren't we lucky the jets are threaded.

[rfoll]

not a carb expert by a long shot but my high compression 340, my 454, and my 427 chev all ran the best mph with a 4781 with 83/86 jets with a power valve.

This looks about right to me. The OP should end up very close to this when he tunes from plenty rich, progressing towards leaner to get ET MPH track perf gains.
And not get all wrapped up in AFR guage readings.

The guage may be out of wack or effected by exhaust leak.
These afr guages are best used only as "you're too lean warning devices". Indicating that you need to then look at all the plugs for lean condition.

The AFR gauge is a comparison tool. If the gauge says 12.5 and the engine likes it, that's what I will shoot for with the 850. A while back i read somewhere people were making more HP with leaner 13:1 ratios. I suppose it worked on the dyno, but on my car it was about 2 or 3 tenths slower at the track. The secondary power valve is history, never to be seen again. With a 5100 RPM converter and 4.56 gears this is not a street car, despite a licence and insurance.

[FC-Pilot]

As a point of reference, we ran a bbc on the dyno with the same carb, but slightly different intakes. One intake was of a much better design and made its best power at 13.0 on the AFR gauge. Swapped intakes and the same engine and carb, it wanted 12.1 on the gauge to make its best power. The design of the second intake had very poor wet flow characteristics so a much richer setting is what made the most power.

Having said that, a different carb size might produce different atomization, and therefore need to show different numbers on an AFR gauge to make the most power. At the end of the day, I let the time slip or dyno sheet tell me what it likes and note what I learned from the info the AFR gauge told me.

Paul

[F-BIRD'88]

I will repeat it again. A AFR guage is at best a "you are just getting too lean" warning device.. It warns you to look at all the plugs for a lean situation. Helps keep you out of trouble.

[rustbucket79]

An AFR gauge doesn’t know if it’s failing rich or lean, doesn’t know if there is a header or collector gasket leak, or if the manifold has fuel distribution issues where there are rich and lean running cylinders. (Not that a carb can correct the latter)

Trial and error with an accurate yardstick is a good method for determining the best jetting.

I ran a very old 850 8162 Holley years back, IIRC 83’s square with plugged power valves worked the best for me.

[rfoll]

Because it's been said a number of times, I want to be clear that I have no leaks. I mentioned afr gauge and some people are now beating me with it. What I have is the Innovate Motorsports LM2 . It records readings from launch, through each gear, shift point rpm etc. It has been very useful in the past. If I had taken the time to hook it up when I went to the track last Friday, I would have known I was very lean. The car ran perfect at my initial jetting, but as I said in my 1st post, it was slow. FC, it did occur to me the carbs might behave differently. 1980RS, You are correct about the 3310/4780 similarities. I was looking to make the car launch harder and installed the double pumper to see if the 60' would improve. I literally copied the setup from the 3310 and found a half tenth. What I was really looking for here was suggestions for a starting baseline. Fbird and Tuner, thank you for that, I would have never thought the jets would need to be that big. .