to block off or not

[jeff5347]

If i run a 357 chevy and a holley 750 vac sec are there any advantages to running a pv blockoff. i run the car on the street but like it to move. Was weondering if putting a block off and upping the jets 10 numbers (72 to 82) and such. Would this hurt the performance or give me a bit more with the right tune.
Jeff

[CarlC]

The PV is a great help for a street car. Adding excessive fuel to the mix will waste fuel, foul plugs, and increase engine wear while reducing overall power. A properly tuned PV will allow for better fuel mileage while still adding needed fuel at higher engine loads. Your carburetor is designed and tuned to use a PV. If it is removed and larger jets installed, your fuel curve could be thrown way off.

Dave Emanuel (sp?) has a pretty good book on how to tune Holley carbs.

[68sixspeed]

Depending on the motor, you may find a nice comprimise in blocking the secondary pv; but I'd definately leave the primary one in place for drivability. I found a big (3mpg) difference in my highway mileage with blocking the secondary one, I think both pv's were just openning at times on the highway and dumping way too much fuel. But that is with a motor with a lot of cam. -Dan

[jeff5347]

The carb is a 3310-1 or 2. it was originally a 4 bbl off a 60s or 70s car i got from my father. It had a metering plate on the secs and i took it off and put a block on it. im sure i have 1/16 of a clue on how to tune the carb aside from the cams, accelorator pumps and air bleeds on the blocks. What i dont know is i live in an area about 1000ft above sea level and dont even know where i should start with jet size. How do i know if the prim and secs are too big or small. what would be a good starting point. Also what are the symptoms of a mismatched pv. Meaning if i have a pv rated for 6.5 hg but the vacuum at idle is say (guessing , i need to check the vacuum) 10hg or 5hg, what are the signs its not the correct one. I feel my car would pull some more ponies by a properly tuned carb. I just go the new chevy hi perf and by using the same enigine they went from a 750, 850, 950 and the 950 gained 20 horse over the 750. Carbs mistify me. I do need that book. Any help greatly appreciated.

[68sixspeed]

Someone else may have to jump in on this one on vac. secondary spring selection since I'm a mech. secondary kind of guy, but on the other stuff--

Basics- (simple version, actually the latest PHR magazine just came with a good article, gets into air bleeds, more detail) Jets are for wide open thottle mixtures. Idle screws and idle air bleeds are for just that, idling. Power valves are for the transition between. It's very important that your power valve is closed at idle and even just off idle. 6.5" pv works for most applications, even as much cam as I run, mine got boggy when I tried a 5.0pv; If you have a lot of idle vacuum and you are getting a stumble at part thottle you may need a higher number power valve, but other than a stock cam I haven't seen too many people running over a 7.5;

Does your carb have interchangeable squirters for the accelerator pump? There are some gains in drivability and launching by playing with this, a larger size dumps the fuel faster.

Getting back to jets-- call Holley, and ask them for a baseline setting for that carb combo, or look in their online catalog-- I'm going to guess that by taking the metering plate out of your carb and going to jets made it basically into a normal vac. secondary model; if you can find that p/n out Holley's catalog will tell you the factory jet settings. Normally you don't need to deviate from these by more than 4pts of jet size for our altitude (I'm at 750ft myself).

If you can borrow or add an air-fuel ratio (oxygen sensor) it is a great tool for carb tuning.

Hope that helps-- Dan

[MonzaRacer]

The general rule of thimb is 2 sizes below your idle vacuum in gear at idle. But for the most part i use 3, 3.5, 4, 4.5 in most carbs.
I have 355 that ihave running a LT4 HOT roller and it idles pretty good but I am using a 84017 ,4011 spreadbore too.
But for the most part I would start with stock 72 jets front (if it had metering block in rear just keep jetting up till it slows down or quits make ing more power.On powervalve I would start with a 4 or 4.5 and go down. IF it opens at idle in gear it will run engine rich. Also dont forget to get either spectre or mr gaskets blow out preventers.
If Iremeber corectly you can go down to 2.5 but after that you will have to block it off and go with more jetting but if your running that much cam you need a better carb than the 3310.
Another thing is use a 31 or 35 accelerator shooter with the extension tubes,, it helps the engine pull over fuel better and makes them work better anyway.
I love my 3310's and I am getting ready to convert one over to 4 hole idle soon. BUT my 800 is now back working great as of today after having to make floats for it.
Oh yeah if it stumbles and wont tune of with accelerator pump tuning then you need to go up with power valve number ,,, try 1/2 sizes at a time.
Good luck and Pm me if you need help.
Lee