JRracing

iTrader: (35)

That sounds like a good idea. After all, it goes lean at WOT and a little lean at cruising speed. So would I need to up the primary jets a little also.

According to the tech sheet on BG.com it has a pink pump cam in it. Shawn, it this what yours came with?

Shawn MacAnanny

iTrader: (12)

Yup mine have two pink cams, i'll be changing them to a different one though to see if i can get away with a slight stumble i have. I've not tuned this carb for more than a half hour or so though, but a few hours on the holley 750 i had.

JRracing

iTrader: (35)

My problem is it starts out a little rich and then goes lean all the way up to wot. I dunno if it needs bigger jets or a bigger cam or bigger squirters.

Shawn MacAnanny

iTrader: (12)

I'd turn the idle screws in a 1/4 turn and up your primary jets about 3 sizes and try that. Dont touch the secondaries yet.

xpndbl3

iTrader: (23)

i wouldn't touch the idle screws 1/4 turn at all if it is already at 13.5:1 at idle, perhaps a 1/16 turn or 1/8 turn but just a little bit will go a long way to reach 14:1 at idle. Next 13.5 cruise is fine, up the secondaries first. See what this does to your WOT a/f ratio. 3 jet sizes on the primaries would be a big jump up. I'd say maybe up 1 more to get it perfect, but for now this would be a clean running engine and wouldn't foul out plugs with a light throttle cruise in that a/f ratio.

JRracing

iTrader: (35)

The float bowls were a bit low so I raised the front to the middle line and the rear to the top line. I also turned the 4 idle screws out 1/4 turn and it idles around11.7 to 12.5:1. It also runs much more rich at cruising speed. 11.9 to 12.5:1. But were I open the secondarys it goes lean again to around 15 to 16:1.

xpndbl3

iTrader: (23)

float bowls are too high, you want them at the middle line or slightly lower. Secondly you'll foul out spark plugs idling that rich, turn them back in to where they were. Follow my advice and report back what happens. You made the idle circuit too rich so it would make the cruise circuit slightly richer as well. You didn't touch the secondary circuit like I stated so it wouldn't make much of a change.

JRracing

iTrader: (35)

I agree, I will turn the idle curuit back down to get it in the 13.5 range. I will also lower the rear float bowl to match the front. My buddie has a set of jets I am going to pick up tomorrow. He said to take the power valve out of the front and raise it 4 jet sizes and raise the rear 4 sizes and see how it responds. I think I may leave it in the front and raise the rear jets a few sizes to see what it does at wot. It seams to be pretty good running on the primaries. It's the secondaries that need to be richened up according to the wideband.


I have been told by several people to do away with the PV's. They say they are not needed in a drag car. Is this true? What do they actually do? They say with big cams, it makes it harder to dial in with the power valves opening and closing.

JRracing

iTrader: (35)

It looks like the bowls have orange rubber gaskets in between the bowl/ main body. Are those the reusable kind?

Also, I learned a little bit about over tightening the float screws. I tore up the gaskets and it started leaking. Of course Napa did not have a set of those gaskets so I had to impervize. I got a blank sheet of paper gasket material and used a 7/16" socket as a template and cut it with an exacto knife. Worked fine after cutting a few to get the right size.

xpndbl3

iTrader: (23)

blue ones are reusable. Also don't ditch the power valve unless it will never see the street. If you want to take it to a cruise night then leave it in place. The power valve is approximately 8-10 jet sizes that you will have to go up to account for it missing. Really follow my advice and see how the wideband reacts. Any speed shop should have the float bowl and metering block gaskets in the blue reusable kind in stock. Report back how it changes and I'll give you more advice.

toxic99

You don't HAVE to do away with the power valve but most racers do. Like I stated on first page, to get the correct size PV just check your maifold vacuum at idle. Divide by 2 and walla. In other words if, ya have 5 inches of vacuum, get a 2.5 PV. Like xpn says, if you do away with it, you will have to go up 8-10 jet sizes in front. I usually start with 6 then work my way up if needed.



For a discription of what a PV does

camrs73

get rid of the secondary power valve, get jet extensions and a notched float for the secondaries and jet up to compensate. a secondary power valve in a drag car is badddd because when you launch all the fuel gets pushed away from the secondary power valve and you go lean. same concept why jet extensions are made expect there's no power valve extension.

hang on to the primary power valve if you're going to be seeing any street driving at all. if you ditched it and jetted up accordingly, you'd be running pig rich everywhere except WOT. its ok without a power valve on the secondary side because the secondaries are usually only open during high load "spirited" driving or WOT when you need the extra fuel

xpndbl3

iTrader: (23)

he doesn't have a secondary power valve in that type of carb.

JRracing

iTrader: (35)

Thanks toxxic. Good info. I have about 10" of vacum at idle with it in gear and holding the brakes. So I need a 5 pv. It has a 6.5 in the primary now. 75 jet in front and a 90 in the rear. 31 squirter and pink cam. I'm not sure what pump is in there.

I have not had time to get the car out this week except Monday when I jetted it up. But it runs around 13.5:1 at cruising speed and when I go wot it hits a lean spot then levels out to 12 to 13.5:1. I need to get the laptop hooked up to log the entire fuel curve. Hopefully I can do this next week. Stay tuned guys. Thanks for all your help and advice.

forcd ind

iTrader: (7)

i didnt read thru everything, sometimes these posts get long, lol-as some said, the carb is a little on the small side, but would have a good signal on the street though
using a p/v in the front even on a race car can help keep the plugs clean on the return road, and in the lanes-you can increase the pvcr also, but better off not if your not fam. with doing it-dealing with .060+ # drills, small changes
i had to go to green pump cams on the LS motors, did some testing with 3 diff carbs, all didnt respond good to the pink cams-the green cams are a little more agressive than the pink, but dont have the duration, but hopefully your into the boosters by then-i moved my p/v closer to my idle vacume, as i had a mild dip into cruise, as the transion to the boosters needed to get fatter
you want your squirters just big enough to do the job-to big and you can drain your pump well to early-your getting pretty big on the jets for a 650, might want to think about dropping the hi sp a/b's down to around .026-.029, should fatten up the circuit, might allow dropping back the jetting-i run a procharger, so some of my tuning is diff., but some of the basic stuff i have found the LS motors want-shame, i just sold an 850 BG
that would have flowed all the fuel you needed, lol
sometimes you have to go back to square one and make one change at a time
if you get to far out-too much info can be really confusing, lol

JRracing

iTrader: (35)

Sounds like you know a lot about carbs. To clear things up, my carb is a mighty demon 750GC. I have been talking with a lot of old school guys about there setup's and going from what they say about a race setup you want to take out the power valve and square up the jets. But not good for street driving.
So I decided to see what it was about. I started with 82frt and 83Rr. It was a little lean at wot so I went to 84frt and 85Rr. I don't have 4 of each jet. It is at 12.5:1 at wot but is very rich at cruising speed around 2500 rpm's like 10:1 to 11:1 and stumbles a little on take off. I got the idle set at 14:1. So I see the importance of the PV. But I'm about to start hitting the track a lot in two weeks. So I was giving this a try.

Today I lowered the frt to 82 and raised the Rr to 87. It leaned out the bottom a little bit like 11:1 to 12:1 and wot is still about the same. Kinda seams like I'm on the right track but that is not squaring up the jets.

I would like to make it work on the PV so the streetability is better. When lowering or raising the PV what does it do?

The last setup on the PV was a 6.5PV with 75frt and 90Rr and it drove fine. Very responsive no hesitation at all. Idled around 13.5 to 14:1, cruising around 2500 rpm's it was around 13.5:1 and wot was about 12.5 to 13:1 with a slight lean spot (14:1)around 4000 rpm's then quickly going to 13:1.


I'm enjoying messin around with different setups to see what it does. I can always go back to square one if I get way out of tune. The box says it came with a 6.5pv 75frt and 83 Rr jets and pink cam with 31 squirters.

The carb was bought for a stock 346ci motor with future plans on going bigger ci later. I also called BG tech and a guy there said it is capable of flowing 840cfm. he said it was plenty big enough for a 383ci.

JRracing

iTrader: (35)

I did get my laptop fixed so I can hook it up and try to log runs. I don't know **** about doing that so that will be fun also!

The timing starts out at 11@0 rpm's then straight to 22 @ 1000 rpm's then staight to 28 by 3000 rpm's. It seams to run fine at that point. Thanks for the advice xpndbl3. You are also right about the pv deal. I just had to try it lol.

xpndbl3

iTrader: (23)

Sounds great then when you hit the track try upping the timing, I've seen 32 total timing out of these motors with carbs on top. So keep going up 1 degree at a time and shifting the same, tire pressure the same, etc and watch the mph to go up to base your motors hp off of. Mine won't be out and about until mid april at the soonest but I have a wideband and carb'd motor just the same.

JRracing

iTrader: (35)

Oh so you are running a carb'd lsx setup also. What size carb?

What does a larger or smaller pv do in relation to fueling the carb?

I'm thinking about putting a 5.5pv and going back to 75/85 jetting. Another old school drag guy told me he ran a pv on his old dragster and it actually ran better by leaning it out on the big end or I think that is how he put it.

xpndbl3

iTrader: (23)

power valve is just open fuel flow below that vacuum reading, so if there's a slight tip in leanness during the vacuum drop then a higher power valve would help smooth out that curve. It's just metered fuel to match up the front jets to the rear jets size wise to make it square jetting when the front jet + power valve is open. Thus why usually it's a 8-10 jet spread front to back along with a front power valve. This allows for leaner cruising AFR on only the front jets, but when the vacuum drops the power valve takes over. Stop talking to "old school" drag guys who never street drove them or fouled out plugs. Your setup before messing with this when the cruise, idle and WOT levels were fine IMO was a better idea. Then you started messing with stuff back and forth. Your motor makes max hp at a certain AFR, and leaner than that won't help anything. It takes tuning to find that level, with a wideband no longer do you have to pull plugs. Get it to 12.8-13.0 at WOT and play with timing first to find max mph at the track. Then you can work on other tuning issues as well.


I'm running a bigger carb than you but I don't have any problems with tuning them myself to make up for the part throttle driveability issues that you would run into. At the track the bigger carb will usually be faster than a smaller one.