750 to 850 carb swap

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750 to 850 carb swap

Post  BigRigTech on June 26th 2013, 5:56 pm

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?
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Re: 750 to 850 carb swap

Post  maverick on June 26th 2013, 9:40 pm

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.
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Re: 750 to 850 carb swap

Post  BigRigTech on June 26th 2013, 9:58 pm

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.
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Re: 750 to 850 carb swap

Post  maverick on June 26th 2013, 10:01 pm

What's the model number on your 850?
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Re: 750 to 850 carb swap

Post  BigRigTech on June 26th 2013, 10:40 pm

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
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Re: 750 to 850 carb swap

Post  maverick on June 26th 2013, 11:00 pm

When you get the number, let us know.
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Re: 750 to 850 carb swap

Post  BigRigTech on June 26th 2013, 11:03 pm

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.Very Happy
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Re: 750 to 850 carb swap

Post  kim on June 27th 2013, 2:17 pm

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.

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Re: 750 to 850 carb swap

Post  BigRigTech on June 27th 2013, 3:05 pm

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.
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Re: 750 to 850 carb swap

Post  maverick on June 27th 2013, 3:53 pm

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.
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Re: 750 to 850 carb swap

Post  maverick on June 27th 2013, 3:59 pm

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

Factory jetting and pv's listed.
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Re: 750 to 850 carb swap

Post  BigRigTech on June 27th 2013, 7:08 pm

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.
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Re: 750 to 850 carb swap

Post  460vega on June 27th 2013, 10:45 pm

When I ran an 850 on my 460 88/93 seemed to work best using 93 octane pump gas.
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Re: 750 to 850 carb swap

Post  BigRigTech on June 28th 2013, 11:49 am

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.
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Re: 750 to 850 carb swap

Post  maverick on June 28th 2013, 1:39 pm

Factory jetting on a 4781 is 80 square with 6.5 pv's.
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Re: 750 to 850 carb swap

Post  BigRigTech on June 28th 2013, 1:44 pm

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?
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Re: 750 to 850 carb swap

Post  maverick on June 28th 2013, 1:48 pm

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.
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Re: 750 to 850 carb swap

Post  BigRigTech on June 28th 2013, 1:55 pm

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.
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Re: 750 to 850 carb swap

Post  maverick on June 28th 2013, 2:06 pm

Yeh... for track use only, I'd drop the pv's too.
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Re: 750 to 850 carb swap

Post  LivermoreDave on June 28th 2013, 3:31 pm

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.

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Re: 750 to 850 carb swap

Post  460vega on June 28th 2013, 6:15 pm

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'.
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Re: 750 to 850 carb swap

Post  BigRigTech on June 28th 2013, 6:28 pm

Did you have power valves front or rear?
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Re: 750 to 850 carb swap

Post  460vega on June 29th 2013, 9:40 am

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.
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Re: 750 to 850 carb swap

Post  BigRigTech on June 29th 2013, 10:52 pm

How much cam was in that combo?
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Re: 750 to 850 carb swap

Post  dfree383 on June 29th 2013, 11:17 pm

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.
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