750 to 850 carb swap

I'm going to swap my 750HP for a 850DP with a milled choke horn tomorrow night. Right now there is 78/88 jets in the 750 with a 5.0PV in the front, PV plug in the rear, nice tan color on my plugs. Should I go with this jetting to start in the 850 or go up a bit?

Assuming the pv thingy is equal, I'd think that a bit more airflow would require a bit more fuel as well. To be sure, though, I'd go to Holley's site and check out the factory jetting for starters. Every combo will be different but that's probably a safe baseline to begin tuning.

The guy who suggested my current jetting and nailed it perfectly said he would start with 80/90 and the same PV setup. He rebuilt this carb this past winter.

What's the model number on your 850?

I'm not sure, I'm picking it up tomorrow morning. The seller lives about 1.5hrs away so I left the money with my sponsor at his speed shop and the seller brought it there when he came to the city today. He always goes there for a visit when he comes in...LOL

When you get the number, let us know.

I'm not sure what the jetting is in it or if the seller changed it. Mike said it was bone stock when he got it to rebuild, very nice condition. I'll post the numbers in the morning.

PV is a function of engine vaccum at idle, and where you want the PV to come in to enhance the fuel curve over the jet surface area.  

A 10 jet split front to rear is a little odd.  The jet size split is to compensate or correct for the power valve, and its normally 6 jet sizes, as thats the equivilent fuel pull surface area the power valve gives you.  Your not increasing the rear by 6, your decreasing the primary by 6.  You get to run a leaner primary circuite because the power valve comes on and covers the WOT fuel demands of the carb.  The secondaries, because all single carb applications unless modified, run a progressive linkage are tuned without power valves as the secondaries are used for high RPM/WOT performance only.  The primaries with smaller jet surface area make low RPM and off idle tunning easier and more fuel efficient.

All that said, the change from a 750 to an 850 on the same engine, the jets in the 850 I would suspect at least a 6 jet jump over the 750 to compensate for a decreased vaccum signal on the fuel circuits.   Easier flow of more air, means less pull on the fuel.....

And nice tan color on the plug. If your looking at plugs with 20k on them is fine. Ideally a plug is read with a 10x magnifying glass and a small light.

http://www.autorepairinstructions.com/?p=623

Pretty fair 101 on spark plug reading.

There were no numbers on the carb now that the choke horn was milled off. I might call Holley to get their take on my setup and see what they suggest.

No numbers on the side of the main body? That number will give you a lot of info, as Holleys that are rated in the 850 cfm range can vary WIDELY where jetting is concerned.

Take a look here. www.mopartech.net/showthread.php?t=31

Factory jetting and pv's listed.

I spoke with holley, the numbers on the metering plates didn't identify it either. 1-3/4 bore, down leg boosters, 4 corner idle, 6.5PV front and rear, 78/89 jets in it. Notched float and extensions in the rear.

When I ran an 850 on my 460 88/93 seemed to work best using 93 octane pump gas.

What is your altitude? We are very close to sea level at the track. Last race the DA was 2200ft on Sunday. The carb is a 4781 according to my buddy.

Factory jetting on a 4781 is 80 square with 6.5 pv's.

I'ts no wonder the guy said it was too much for his 350, 89 jets in the rear without a PV block. 89 and a 6.5 PV would be a lot of fuel would it not?

With the secondaries open, that would likely be too fat on anybody's engine. Holley's factory jetting may not be just right for everybody's stuff, but they don't usually miss by that much. I'd go back to the factory setup as a starting point for fine tuning.

I'll go 80/89 and block the rear PV, track only car - not street legal anymore so I'd rather opt for rear PV delete like my 750 which works awesome.

Yeh... for track use only, I'd drop the pv's too.

kim wrote:The jet size split is to compensate or correct for the power valve, and its normally 6 jet sizes, as thats the equivalent fuel pull surface area the power valve gives you.  Your not increasing the rear by 6, your decreasing the primary by 6.  You get to run a leaner primary circuite because the power valve comes on and covers the WOT fuel demands of the carb.  

Monitoring and adjusting the power valve enrichment circuit will adjust the amount of fuel the power valve circuit delivers. Therefore if your carburetor is without removable air bleeds you can adjust the idle quality by adjusting the power valve circuit accordingly in conjunction with the main jet and idle air adjustment.

BigRigTech wrote:I spoke with holley, the numbers on the metering plates didn't identify it either. 1-3/4 bore, down leg boosters, 4 corner idle,  6.5PV front and rear, 78/89 jets in it. Notched float and extensions in the rear.

If a four corner metering block and circuit is Incorporated in your carburetor, operating without a power valve in the secondary position may effect the quality of idle in regard to proper WOD air/fuel ratios.

Again, if removable air bleeds are available, you can tune the idle quality without the above mentioned ideas.

Just another idea,
Dave.

BigRigTech wrote:What is your altitude? We are very close to sea level at the track. Last race the DA was 2200ft on Sunday. The carb is a 4781 according to my buddy.

We are about the same,I think about 200'.

Did you have power valves front or rear?

It had a 3.5pv in the front, blocked in the back. I no longer run it but a t ram & 2 800s on the same motor, it`s 4 tenths quicker.

How much cam was in that combo?

If its a drag only car, get rid of the Pv's and jet up about 4-5 jet sizes. It helps to make tuning a little easier IMO, you don't need part throttle response in a drag car.