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Author Topic:   Carb Questions...
MrXerox
Gearhead

Posts: 326
From: Oklahoma City, Oklahoma
Registered: Nov 2000

posted 03-09-2001 09:04 PM     Click Here to See the Profile for MrXerox   Click Here to Email MrXerox     Edit/Delete Message   Reply w/Quote
Hello, I am in the middle of my first major rebuild of a Holley 850DP and had a couple questions for you guys. First off I noticed that the jets on the rear have some sort of metal tube around them which extends them about 1 inch longer into the bowl and does not looks like a factory piece, what is the purpose of this? I also noticed that the power valves on both ends are dummy power valve plugs...why is this? The motor this will be installed on is a street 351C 4V quench with a mech. roller 244 @ .050 and .623 with an xcellerator intake with an auto trans with approx 3400 stall. Ignition is complete MSD with a curved dist running 20 inital and 18 advance all in by 2500. Does anyone have ballpark figures I can use in reguards to jet sizes, carb cam selections, squirters, etc... I have a holley trick kit ready to go Thanks gang!

[This message has been edited by MrXerox (edited 03-09-2001).]

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Dave_C
Gearhead

Posts: 744
From: Gadsden, Al
Registered: Aug 99

posted 03-09-2001 10:37 PM     Click Here to See the Profile for Dave_C   Click Here to Email Dave_C     Edit/Delete Message   Reply w/Quote
The rear jet extensions are a good thing. Leave them in. In a hard leaving drag car the fuel will slosh to the back of the bowl on launch. When it does this it uncovers the jets as the fuel sloshes away from them. The extensions make sure that the jets stay covered with fuel all the time. Don't have this problem in front as the fuel sloshes toward the jets,not away.

The plugged power valves are a common drag race thing also. The power valve is there to add fuel when needed. A race carb is running at wide open throttle all the time anyway, so just jet it accordingly. The rear power valve also gets uncovered just like the jets do, but no way to extend it. When running a pv plug you jet up by 8-10 jet sizes. It will run very rich putting around the pits, but will run good on the track.

For a baseline on the carb: With pv plugs: jet it around 84-85 on all 4 corners, 35 or 37 squirters to start, may need bigger if no transbrake. Biggest pump cam you have.

Later,

David Cole

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MrXerox
Gearhead

Posts: 326
From: Oklahoma City, Oklahoma
Registered: Nov 2000

posted 03-09-2001 11:27 PM     Click Here to See the Profile for MrXerox   Click Here to Email MrXerox     Edit/Delete Message   Reply w/Quote
Since this is primarly a street driven car, would it not be better to run with power valves and smaller jets so as not to be running rich all the time? How much should the jets/squirters change if power valves are to be used? what about just using a power valve in the primary side and a plug in the secondary?

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Dave_C
Gearhead

Posts: 744
From: Gadsden, Al
Registered: Aug 99

posted 03-09-2001 11:50 PM     Click Here to See the Profile for Dave_C   Click Here to Email Dave_C     Edit/Delete Message   Reply w/Quote
Oops, went back and re-read. It is for street.

You can put the pv 's back in. I would put them both front and rear. Running pv's will be be around 8 sizes smaller on the jetting. Jet it around 76-77 to start. You may need to go up or down a size or two after looking at the plugs. Leave the jet extensions on. I would leave the squirters alone to begin with. With that cam I would think that you will need about a 4.5 pv.

later,

David Cole

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MrXerox
Gearhead

Posts: 326
From: Oklahoma City, Oklahoma
Registered: Nov 2000

posted 03-10-2001 11:52 AM     Click Here to See the Profile for MrXerox   Click Here to Email MrXerox     Edit/Delete Message   Reply w/Quote
Sounds good, should I still use the biggest pump cam that I have in my kit? Also, is there something I should coat my blue bowl gaskets with to get better wear out of them if I am going to be in there changing jets/pvs often?

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Dave_C
Gearhead

Posts: 744
From: Gadsden, Al
Registered: Aug 99

posted 03-10-2001 01:00 PM     Click Here to See the Profile for Dave_C   Click Here to Email Dave_C     Edit/Delete Message   Reply w/Quote
I would still use a big pump cam. That's a big carb and you still have 351ci. Start out bigg and then you can go down if it runs ok.

Coat the gaskets with "Pam" cooking spray and they will still seal, but will come off without sticking. Also been told that chapstick works well too.

You shouldn't have to change jets that often once you find the setup. Since it's a street car you might want to have a summer setup (leaner) and a winter (richer). Once you find the right power valve you won't have to mess with it. I based the 4.5 on the fact that I run a 3.5 with a little bigger cam in my 383W stroker. It's also a solid roller, .624/.630, 262/272 @ .050. I pick the pv by the vac the engine pulls at idle in gear. Take the vac, subtract 1 for fluctuations due to rough idle, then divide by 2, round to the low side. Mine pulled 9" vac. Subtract 1, divide the 8 by 2, got 4, round down to 3.5, since pv's come 3.5,4.5, etc.

Later,

David Cole

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Gary Ferringer
Journeyman

Posts: 59
From: Kennerdell,Pa.
Registered: Feb 2001

posted 03-10-2001 01:01 PM     Click Here to See the Profile for Gary Ferringer   Click Here to Email Gary Ferringer     Edit/Delete Message   Reply w/Quote
Which list number is your 850? Most of the ones that I have run across (including the ones that I own) came from Holley with 80's in both ends. I will have to look at my list for the newer ones,they might have different sizes. I have list number 4781-1's. Also,you definitely need to run a front power valve on the street to get any decent idle quality. I don't know what your idle vacuum is,but I would recommend just from looking at your cam specs,that you would want to use a 6.5,or maybe even a 5.5 in the front. If you let me know what your idle vacuum is ,I could give you a better recommendation. The back power valve would not be as critical to use on the street,but if you use it probably a 6.5 or a 7.5 would work. By the way,the jet sizes that I gave you are with the power valves in.

The 35 and 37 squirters should be a good starting point.

------------------
'85 Capri
Cleveland powered
carburetted

'72 Pinto
CARBURETTED,draw thru turbo 4 cylinder. Runs good.
http://pub43.ezboard.com/blowdollardragracersassociation

[This message has been edited by Gary Ferringer (edited 03-10-2001).]

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MrXerox
Gearhead

Posts: 326
From: Oklahoma City, Oklahoma
Registered: Nov 2000

posted 03-10-2001 04:01 PM     Click Here to See the Profile for MrXerox   Click Here to Email MrXerox     Edit/Delete Message   Reply w/Quote
It is a List 4781-2 and came with 80 and 84s in it. I do not have the cam installed yet so I do not know what the vac is going to be yet. Just trying to get a feel for a range to try out. Thanks for your help guys!

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Moneymaker
Administrator

Posts: 19704
From: Lyons, IL, USA
Registered: May 99

posted 03-10-2001 07:09 PM     Click Here to See the Profile for Moneymaker   Click Here to Email Moneymaker     Edit/Delete Message   Reply w/Quote
That is a lot of jet for a power valve equipped carb on a 351 CID engine. I also prefer larger discharge nozzels on big port heads.

------------------
Alex Denysenko Co-Administrator and Moderator NHRA/IHRA/SRA member
Fleet of FoMoCo products
Moneymaker Bio
US Class Nationals link

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jkilroy
Gearhead

Posts: 1719
From: Vicksburg, MS
Registered: Dec 99

posted 03-10-2001 08:24 PM     Click Here to See the Profile for jkilroy   Click Here to Email jkilroy     Edit/Delete Message   Reply w/Quote
I'm with Alex, thats a lot of jet for a 351. With that big a carb and cam I would expect more jet to be required to offset a weak vacuum signal but thats over 10 sizes bigger than what I have in my 390, and its still rich!

------------------
Jay Kilroy
68' Fastback GT 390
"No such thing as a cam thats too big"

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MrXerox
Gearhead

Posts: 326
From: Oklahoma City, Oklahoma
Registered: Nov 2000

posted 03-10-2001 09:38 PM     Click Here to See the Profile for MrXerox   Click Here to Email MrXerox     Edit/Delete Message   Reply w/Quote
Sounds like I have conflicting opinions on this...If 80's with the PVs will be too rich and 70's on a 390 CID is too rich what should I start with? the squirters that came on it are 25s and I dont think those are large enough. Alex, are you saying go with a smaller jet size and bigger squirters? How about the pump cams? should I put the same ones on there that came on it for starters? kid vishus if you read this, what combo do you run on your street cleveland with your 750DP?

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Gary Ferringer
Journeyman

Posts: 59
From: Kennerdell,Pa.
Registered: Feb 2001

posted 03-10-2001 10:02 PM     Click Here to See the Profile for Gary Ferringer   Click Here to Email Gary Ferringer     Edit/Delete Message   Reply w/Quote
The best that my 850 ran on my Cleveland was with 81 jets all around. Also it was with a 35 primary squirter and a 40 on the secondary if I remember right. I have quite a lot less motor than what he has.

Most of the 850's come from Holley with 80's all around and 2 power valves. I do think that some of the newer list numbers might come with 78's in the secondarys. The 4781-2 that MrXerox has came with 80 and 80 with both power valves. I can't imagine going with 4 or 6 sizes smaller unless you were using a very small intake and was getting too much booster signal at high RPM's. I ran an xcellerator for a while,and 80's worked best for it. 81's worked best for both my portosonic,and strip dominator.

I am currently using a 4010 series 750. With the correct spacer height combination I am running nearly as good as the 850,with a lot better driveability in the pits. Once this season starts and I get to play around with it a little more,I am confident that it will deliver as good or better performance. I was glad that Holley came out with this carb. Almost like the Autolite 4100,only with a CFM rating closer to what I need,although I do have one of the rare Autolites with the 1.19 venturies. These came with Holley jets which makes things a lot easier. They were only around 600 CFM though,still a little small. I only went 1 1/2 tenths slower with this carb though. Its too bad that the 4010 series didn't take off. They suffered from a lack of understanding I think. One good thing though,everyone thinks that they are junk. I have three of these now and the most that I have paid for any of them is $50. Their loss is my gain.

------------------
'85 Capri
Cleveland powered
carburetted

'72 Pinto
CARBURETTED,draw thru turbo 4 cylinder. Runs good.


http://pub43.ezboard.com/blowdollardragracersassociation

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