Got problems with my 870 fouling plugs think I´m at a crossroad. Going to try another carb cheap way get an used 800 dp(heard that 850 dp is running rich?) or an 3310 750 vacum to try and see if things works better? More expensive way have a q-jet build after my specs or a pro system 4150? Will a 750 or 800 dp hurt my performance alot not chasing the last tenth? The car is 95% street and 5% strip combo is 489 cui with 10.3 in comp hyd roller with 621 615 lift and 242 248 @ 050 (248@050) brodix rr ovals and current carb as mentioned is a 870 sa.

Marcus

Marcus, I think you should get a book and learn about tuning. Swapping carbs because of FA ratio is NOT the way to go. It's all about jetting and the systems around the jets.

Marcus, I think you should get a book and learn about tuning. Swapping carbs because of FA ratio is NOT the way to go. It's all about jetting and the systems around the jets.

Maybe, but I´ve tried a lot of things. Think I missed in my first post that it´s under idle/cruise it´s running rich. First to much of the transfer slots was exposed opened the rear blades but it was´nt enough so I ended with drilling the primary blades. Changed to smaller jets and got a stumble wich means the jets are to small? and how much are the main jets affecting at idle/cruise? To get it idle good in gear I need to open the idle mixture screws 2 1/8 wich indictes something is wrong? So guess next thing is to drill and tap the ifr for drilled set screws with holes? So before I destroy the carb more I think it´s better to try something else besides it seems to be a common problem with 870`s and fouled plugs most with performance cams seems to end with somethiong else?

How many inches of vacuum do you have in gear while idling? What # power valves do you have in the carb?

Pv´s are 4.5 front and 3.5 rear 7 hg inch at idle with gear and 13 in p or n and it fouls plugs it both p and n and in gear.

Marcus

Pv´s are 4.5 front and 3.5 rear 7 hg inch at idle with gear and 13 in p or n and it fouls plugs it both p and n and in gear.

Marcus


You probably already know this but, Holley recommends dividing you your vacuum by 2 and using the power valve with that number. So with 7in in gear, both valves should be 3.5 or #35's. I'm no carb guru, but I would take Holley's word for it and use 35's front and back.

That front power valve is opening sooner than the rear one and that could very well cause some problems.

Marcus..as you well know,the carb is a passive device.responds to what is above it and below it.Start with your ignition system first.When you have the spark timing fall into known combos that are close to your application..then you can start fine tuning the carb for desired a/f ratios.

Another thing about pv and your vacumn, you need to have enough to open them and keep them open at WOT, otherwise they will close on you and make you think your main jets are too small. Cruise with main jet, run wot with your jets and pv's. A simple vacumn guage will help you with those decisions. This is the issue when trying to run a carb too large. Use what your motor needs , not what you or anyone else thinks it needs. Do the math.
I was tuning my AF with an hp 850 on my motor and I got into this pv issue, in reality, I really needed my 3310 back on. Lowest vacumn at WOT kept the 65 Pv open, closed on a cruise. I had 45 pv's and wot vac creeped up to 5" and the shut off..

[quote=zdld17;1042596]Another thing about pv and your vacumn, you need to have enough to open them and keep them open at WOT, otherwise they will close on you and make you think your main jets are too small.

Hmm...actually,the power valve is in flow mode until suffiecient manifold vacuum can overcome the preset installed spring rate and close the pv.They are open to flow at rest and only close with vac applied.Vacuum does not open a pv..it closes it and stops main well enrichment.5"hg of vacuum at wot points the finger at at a restricted induction.Starts at the air cleaner and ends at the intake valve seat.

Hmm...actually,the power valve is in flow mode until suffiecient manifold vacuum can overcome the preset installed spring rate and close the pv.They are open to flow at rest and only close with vac applied.Vacuum does not open a pv..it closes it and stops main well enrichment.5"hg of vacuum at wot points the finger at at a restricted induction.Starts at the air cleaner and ends at the intake valve seat.

You've got that backwards. The power valves open when the vacuum drops low enough to relieve the spring pressure. The #45 valve will open at 4.5in of vacuum or less and so on for all of the other number power valves.


edit: go HERE (http://www.holleytv.com/featured_landing.php?reset=true), click on CARB TUNING, then click All 4 and on the right side, click the 1st video called Holley carburetor power valve. Watch that video.

Are you running any Ingnition Amplification? I STRONGLY Reccomend MORE SPARK to Burn that extra Fuel. Also, lower your front jets, and make up for it on rear jets!
What about your timing? is it advanced enough? There is alot of math involved with tuning your carb, study and learn........That 850 should be on time with the right tuning.
P.S. Check your mixture at idle.

Just my two cents

You've got that backwards. The power valves open when the vacuum drops low enough to relieve the spring pressure. The #45 valve will open at 4.5in of vacuum or less and so on for all of the other number power valves.


edit: go HERE (http://www.holleytv.com/featured_landing.php?reset=true), click on CARB TUNING, then click All 4 and on the right side, click the 1st video called Holley carburetor power valve. Watch that video.
Das rite. No obstruction, just too large of an opening in the throttle bores. Pulling 1.5-2.0" vac Hg now.

George said it right. Y'all are just hearing him wrong and saying the same thing.

Look at it this way. Go get a PV, look at the spring side. That end of the PV is in the bowl and exposed to fuel. Engine down, there is no vac to move the PV diaphragm. Look right inside the spring, see that little tapered piece? You may have to move the windings of the spring to see it. That is the seat and with no vac to pull it against the PV body fuel is open to the PVCR's. But nothing flows because there is no air movement. Engine starts and vac pulls on the diaphragm and overcomes spring pressure and pulls the little tapered seat into the PV body, sealing fuel from the PVCR's. Floor it under load and vac drops, opening the passage.


IMO

George said it right. Y'all are just hearing him wrong and saying the same thing.

Look at it this way. Go get a PV, look at the spring side. That end of the PV is in the bowl and exposed to fuel. Engine down, there is no vac to move the PV diaphragm. Look right inside the spring, see that little tapered piece? You may have to move the windings of the spring to see it. That is the seat and with no vac to pull it against the PV body fuel is open to the PVCR's. But nothing flows because there is no air movement. Engine starts and vac pulls on the diaphragm and overcomes spring pressure and pulls the little tapered seat into the PV body, sealing fuel from the PVCR's. Floor it under load and vac drops, opening the passage.


IMO

Amen to that^.Sounds like a few folks need a lesson in power valve 101 class.Gotta love the spin doctors though.:D

870 Avengers are just rich, period. Nothing to do with PV's. They cruise in the ~12:1 AFR range, way to rich for a street car IMO. I would try and lean out the IFR's a bit, maybe take a jet size or 2 out of the primaries. I was able to run 76's in the primaries vs. the 78's....that alone will clean up your plugs.

I liked the way the 870 ran on my car, but the secondaries seem to come in too late no matter what spring I tried, even took the check ball out. I replaced it with a 3310 and it has much sharper throttle response, and secondaries come it excellent. The 3310/770 Avengers work as good or better than DP's. I have a 800 DP on the shelf, and I like the way the 3310/770 runs a bit better. Cleaner too!

George said it right. Y'all are just hearing him wrong and saying the same thing.

Look at it this way. Go get a PV, look at the spring side. That end of the PV is in the bowl and exposed to fuel. Engine down, there is no vac to move the PV diaphragm. Look right inside the spring, see that little tapered piece? You may have to move the windings of the spring to see it. That is the seat and with no vac to pull it against the PV body fuel is open to the PVCR's. But nothing flows because there is no air movement. Engine starts and vac pulls on the diaphragm and overcomes spring pressure and pulls the little tapered seat into the PV body, sealing fuel from the PVCR's. Floor it under load and vac drops, opening the passage.


IMO

OK, I think I am see what each is saying , hard to envision but with air movement, the fuel is pulled out of an open pv that was already open to start with motor off.

I do believe I am headed in the right direction on carb selections when checking vac at WOT. With the 850 WOT vac was at 5", 750 pulled about 2". Comments? I am looking at the pressure diff between atmosphere(14.7 or depending where you are) and 2'' vac Vs. 5". I am understanding that 2" is better but 5" makes me think , you would have more differential . Maybe go back and find a mamometer and strap it to the outside door skin.

I do believe I am headed in the right direction on carb selections when checking vac at WOT. With the 850 WOT vac was at 5", 750 pulled about 2". Comments?

.


The 850 has a problem, as discussed in your thread a few days back. And correct on your PV summary, open to begin with until eng starts.