cv67

750 platinum seried D.P.

350
big heads
Large solid roller etc

Pulls between 10-11 in of vacuum around 1000 rpm

Some say use a 4.5 PV some say if its 10 or over use a 6.5


Not as snappy as it could be and running rich at idle.

Primary jet is 74
Secondary is 80 with P.V. blocked off.

Squirters are stock
Has 50cc pump up front which Im taking off and putting the 30 back on if only I could find the pink cam for it.

Car ran Ok but seems starving up top and rich down low. Floats, timing are set, did the best I could with the mxi screws

Anyone have some baseline suggestions? Keep thinking maybe I have the wrong power valve...possibly too much jet? Id guess Id need a 74 with this mabe not.

Rule of thumb on P.V sizing is confusing most say the 1/2 if vacuum rule is bs. if its over 10 in vacuum runa 6.5 if less go 4.5

Im about to throw this thing through the window...

dannyman

Have you tried the manufacturer's site?

http://www.holley.com/TechService/FA...ory=Carburetor

palmbeachvette76

Aww, don't ruin a window

Power valve rule is half of the vacuum. 10-11 means a 5.5 would be the appropriate valve to use, never heard if over 10 run a 6.5 only, a stock engine with a holley would need a 8.5 or 9.5 so go the 1/2 guideline and remove the plug and try 5.5 and even a 4.5 if your gauge is like mine and bounces around a bit.

Could you pull the carb number off the carb? I have a 770 Street avenger and had to increase the primary and secondary jets. I pulled the rears to the front and bigger jets in the rear, switched the power valve to a 4.5 and increased the squirters in mine.

The 50cc isn't going to squirt more fuel, it only gives you mor available fuel to discharge, so changing that back to a 30cc is fine but it won't change the amount of fuel going in the carb, the 50cc is needed for big squirters. I think i have #32 squirters in mine that came with #25 and from the sounds of it your engine is more radical than mine.

Idle mixture screws, easiest way to get in the general ballpark is 1 1/2 turns out and while the engine is running screw one side in until it starts to die down a bit then back out til it recovers, then rotate out until it dies down and try to reach the middle of each of these adjustments. So you'll need to count the amount of turns you make in each direction. You can also use your vacuum gauge with your goal being to reach max vacuum but some radical cams make this more difficult than listening to how the engine reacts, then set the other side to the same number of turns out (according to Holley, sometimes each side will want a slightly different adjustment from the other.) When you think you close give it a few quick revs and let the idle settle back down and see how shes sounds, then test drive.

cv67

4779-7
3678 stamped underneath that.

I keep hearing about transfer slots uncovered too much leading to a rich condition what do you make of this?


Primary





Secondary

MotorHead

That's he classic Holley 750 DP. First make both pri and sec trasfer slots the same, make them so they are square that's close enough. All four idle mixture screws should be turned the same amount.

Next buy an LM-1 wideband so you can tune it properly

palmbeachvette76

It appears the secondary throttle plates have been opened up a bit. The transfer slot is the very small vertical slit, which I see on the primary but not on the secondary, but the black circle appears to be below the throttle plate which is why I'm thinking it's been adjusted, granted I could be wrong, just going by what I can see in the pic.

I do see that the primary plates have been drilled out, I haven't ever done this, but have heard many people who do but most of the time their trying to get a better idle transition on an automatic car and many people don't approve of the drilling techinque when a secondary throttle plate adjustment can me made instead since once you drill it's permanent.

If you still have the carb off, the adjustment screw for the secondary throttle plates in on the fuel inlet side (passenger) and is a very small screw in the base plate where the throttle shaft has a small arm on the outside of the carb close to the rear pump arm. If it's been adjusted you'll probably see some burrs on the screw since they are normally tight and you could try adjusting it but make sure you keep count of the position it is currently in and count the number of turns and increments you move the screw so you can get back to where you are now if the changes don't work, mark it with a sharpie too, if you want.

Grumpy 427

Power valve is worth close to 10 jet sizes. Connect the vac gauge and see what it read in gear with your foot on the brake. Drop your power valve 2 sizes from what the vac gauge reads. If you have 74 with PV in the front put in 84 jets in the rear without the PV.

My rule has been you jet for MPH on the top end, and the squirter is for the 60 foot on the launch. Go out and give it squirter increase till it blows black out the back when you hammer it then go down 2 sizes. 30 cc accelerator pumps will be fine .

cv67

its a stick at 900-1000 rpm its about 10 in
When it had an auto in it the primary plates were drilled.

cv67

TRansfer slots are the vertical looking ones not the small round holes?

So some should show but no more than 35 thou?

Wonder if this is part of my rich idle issue did the same thing on my old motor too havent been able to figure itout. paid 2 shops who lightened my wallet and car ran the same. last guy drilled the primary blades and gave it back to me.

Carb has less than 1000 miles on it if you can believe that.

Something simple Im missing here to get this thing right.

snoopykissedlucy

Are you using a fuel pressure regulator?

Alot of my Holley carb weirdness was fixed by adding a fuel pressure regulator between the mechanical pump and carb.

cv67

No regulator, Carter 172 mechanical. Stays steady at about 6 lbs.
Heres what I just did...
Stuck a new 4.5 PV in and
Using the screw opposite side the throttle arm (under the choke, rests on a brown/red plastic piece) I adjusted it almost al the way out so theres somewhere between .025-.030 of that slot showing. Prior to that almost the whole slot was exposed, cant tell it from the pics. This could have made it go rich?


Wondering it I should throw it back together and try it along with a new power valve. If the 50cc pump wont hurt anything should I? Im missing the pink cam and the shorter screws for the 30cc pump. Saved that stuff for years now it fell out of the bottom of the box lol. Guessing if its rich now I may need to bounce down a size in jets?

Also..last shop that worked on it eliminated the rear power valve but jetting was left stock at 80. If its blocked off shouldnt I jump up a few sizes.....or leave it.....or stick another power valve in there?

Worked great stock.

cv67

Never mind that was the fast idle from the choke.

Timsride

Also..last shop that worked on it eliminated the rear power valve but jetting was left stock at 80. If its blocked off shouldnt I jump up a few sizes.....or leave it.....or stick another power valve in there?

Worked great stock.[/QUOTE]

Put a new power valve in the secondary side as it will lean out on wide open throttle. When You plug the PV You need to increase approximately 8 jett sizes to make up for it. Also make sure Your not using the two stage PV. Put it back together and try it with the changes You have done with the throttle plates. Too many changes with out testing will put You in a tailspin.

midyearvette

Put a new power valve in the secondary side as it will lean out on wide open throttle. When You plug the PV You need to increase approximately 8 jett sizes to make up for it. Also make sure Your not using the two stage PV. Put it back together and try it with the changes You have done with the throttle plates. Too many changes with out testing will put You in a tailspin.[/QUOTE]

.....re place the rear pv if that was stock out of the box....

cv67

Spoke with the tech at Holley and also Steve B from Westech performance and they both said it came stock with 80s in secondary and the blockoff so I went 82 on it.
Went 72 on the primary 6.5 PV put the 30cc pump back up front, night and day differnce. Can get plenty more out of it just need more tiem with it.

Seems the rumor of sizing these PVs to 1/2 the vacuum is BS. Both those guys said go roughly 1.5-2" less than vacuum reading. Been doing the 1/2 method for yrs they must be doing something right cause Im not. Running pig rich before the adjusments.

Grumpy 427

Congrats glad you got it working.

coconut vette

what would be the advantage if any in blocking off the PV and increasing jet size? why did the shop do that to his carb? Just wondering??? Glad to hear that its working good now.

cv67

After calling Holley tech and giving him the list# it came that way...thoguht it was done. See its been a number of yrs since I drove it. Some will stick a plug and go up a number of jet sizes but since it came like this figured it was probably close just went up 2 jet sizes as the motor is pretty highly modded. Well see.
Little green behind the ears with this but picking up quick, guess in a primarily race situation the PV isnt needed so they would block them up and go up a few jet sizes as a cruise situation never took place someone correct me if Im wrong.

Jets are cheap enough and easy to change can be switched back easily if need be.
Surprising the amount of jet a motor doesnt need

chevymans 77

To eliminate one part that can give you issues.

To have a PV in the secondaries would suggest that one will cruise fast enough that the secondaries will be open with the vacuum high enough that the PV will stay closed (a scenario that doesn't happen very often) so the AFR will stay in the 14:1 range. Than when the load on the motor is increased and the AFR needs to be at 12.5:1 the PV is timed so that it opens and draws the AFR down. With the realization that the chance of cruising on the secondaries is slim the removal of the PV in the secondary side is a feasible option.

A carb with a very small primary would support the possibility of cruising on the secondaries and would benefit from a PV on the secondaries.

Neal

MotorHead

I have a 825 Drag Race Demon ( flows 975cfm ) has jet extensions etc and works fine on the street and at the the track. The secondary PV is plugged and the jets are 8 sizes bigger.

Putting a PV in the secondaries can really complicate a device that not many really know how tune properly in the first place.