I have a 770 SA carb and was doing some tuning today.I had the front fuel bowl off and was changing the jets.On the street avenger carb(not sure about others)there was this black flat spring type thing that was on the two top dowls of the metering block.Kinda hard to explain but anyway I happend to catch it on my finger and I flicked it off and I could not find it.What is this peice used for and is it ok if I just run without it.Your help is apreciated.

Its a baffle for the vent of the float bowl. It should be ok but its used for fuel slosh when braking of accelerating hard.

Dave

thank you

Depending on how hard you take off, it will throw fuel out the vent pipe without it. I ran without and bogged out when it flooded my carb.

How'd the tuning go? What AFR's you get?

I will have to get ahold of Holley and see if i can get that part.

As far as tuning I got 13.5 at Idle (I could get leaner but made no difference in driving) at part throttle cruise its rich at 12.9-13.2 but with my biger cam it works good.As soon as I start to push on the throttle it goes to 13.4 then richer to 12.7 then at full throttle it goes pretty quick to 12.6-5 then when it gets to around 5000 rpm it goes down to about 11.5.

Over all the car runs good.The plugs are very clean.Today Im going to drop the initial back to 18 from 20 and limit the vacuum can back to 10*.I did get allittle detonation in 5th at 60 at 2000 rpm under load.Not bad though.

So the thing is that after all the tuning I have the stock 72's up front and had to go down to 74 on the rear from stock 75 and switch to a 9.5 power valve.I could do some idle restricter tuning but Im kinda burnt on tuning right now.Im sure you can understand.Its alot of work and thinking.

I will have to get ahold of Holley and see if i can get that part.

As far as tuning I got 13.5 at Idle (I could get leaner but made no difference in driving) at part throttle cruise its rich at 12.9-13.2 but with my biger cam it works good.As soon as I start to push on the throttle it goes to 13.4 then richer to 12.7 then at full throttle it goes pretty quick to 12.6-5 then when it gets to around 5000 rpm it goes down to about 11.5.

Over all the car runs good.The plugs are very clean.Today Im going to drop the initial back to 18 from 20 and limit the vacuum can back to 10*.I did get allittle detonation in 5th at 60 at 2000 rpm under load.Not bad though.

So the thing is that after all the tuning I have the stock 72's up front and had to go down to 74 on the rear from stock 75 and switch to a 9.5 power valve.I could do some idle restricter tuning but Im kinda burnt on tuning right now.Im sure you can understand.Its alot of work and thinking.


It's time consuming and a headache to get it totally tuned and it isn't for everyone but the end results are well worth it. My high 10's engine getting 18mpg with a carb and no overdrive is proof of what can happen with tuning. I also tuned a '88 4X4 with a stock engine & 670SV getting 19mpg. That's as good or better than a new Z71.
70-72 is a good main jet size for the 770. I don't care for rear power valves and always plug them but it isn't a must.
In order to really get it tuned, you will need to drill and tap the main body air bleeds, idle restrictions, and power valve channel restriction. I use 8-32 for idle air bleeds/power valve channel restrictions and 6-32 for high speed air bleeds and idle restrictions. Brass set screws make good bleeds/restrictors. You will also need numbered drill bits. Available at any hobby store. .026" is a good main air bleed size that will give you a flat AFR at WOT. You enlarge the idle air bleed until the engine surges/misfires at cruise, then decrease it just enough to make the engine happy. Use the #drill bits to measure everything and write it down. Do one change at a time and keep detailed notes of the effects.

Here's a couple pics to get familiar with the circuits.

Detailed passages (http://i18.tinypic.com/61klx5s.jpg)
Tapped Air Bleeds (http://www.innovatemotorsports.com/forums/attachment.php?attachmentid=1599&d=1170885942) Tapped Air Bleeds #2 (http://www.innovatemotorsports.com/forums/attachment.php?attachmentid=2231&d=1189328607)
Air Bleeds (http://images.chevyhiperformance.com/techarticles/p158299_image_large.jpg) #1= Idle Air Bleed #2= Main Air Bleed
PVCR and IFR (http://www.innovatemotorsports.com/forums/attachment.php?attachmentid=2197&d=1187944682)

The 770 doesnt have a rear power valve in it.When you start talking about everything else its like a different language.I wish I had someone close to me to help.

The 770 Avenger uses 72/75 jets, no rear PV as mentioned. I'm surprised your leaning out the secondaries. Eveybody that I've dealt with had to jet the secondaries up big time. I'm at 84's...had to to get good top-end pull. I was running 72's, dropped to 70's during the summer, and now back to 72 for the cooler temps. Car seems to buck at bit more when taking off from a standstill with the leaner jets.

But your cruise #'s arent affected by jets as you found out. Its the idle circuit and you cant really lean them out unless you do the wire thing. If I remember, my 770 cruised at about 13.7. Very nice.

I would verify that your afr meter is reading correct. Again, secondaries for me and other was very lean.

The 770 Avenger uses 72/75 jets, no rear PV as mentioned. I'm surprised your leaning out the secondaries. Eveybody that I've dealt with had to jet the secondaries up big time. I'm at 84's...had to to get good top-end pull. I was running 72's, dropped to 70's during the summer, and now back to 72 for the cooler temps. Car seems to buck at bit more when taking off from a standstill with the leaner jets.

But your cruise #'s arent affected by jets as you found out. Its the idle circuit and you cant really lean them out unless you do the wire thing. If I remember, my 770 cruised at about 13.7. Very nice.

I would verify that your afr meter is reading correct. Again, secondaries for me and other was very lean.

Ya I was surprised at the secondaries also.I had 84s to start and was pulling WOT at 9 afr. then I went to 81s and got 11 then 79 got about 11.5 and 77s got 12 then the last was 74 which I have 12.5 for about 3 seconds then it gets abit richer to 11.7 at about 5000.s

The next time out I will do a clean air test then do it again to verify.

The last Avenger carb I did was an 870 and it had dual power valves. After tuning many carbs, it gets confusing.


I looked over my notes and the 770 I tuned ended up with 70/78 main jets, .026" main air bleeds front & rear, .076" front idle air bleeds, .036" rear idle air bleeds, .032" front & rear idle feed restrictions, .063" power valve channel restrictions, and 6.5 power valve. The squirter was a 35 with blue cam.
The combo was a 10.5:1 383cid, Comp XR282HR hydraulic roller, Edelbrock 200cc E-Tec heads, Performer RPM Air-Gap intake. In a '71 Nova with 3.90 gears, 27" tires, th350. It idles at 13.5, cruises 15-16, and WOT 12.5-12.8. He's claiming high teens gas milage at 55-60mph.
Of course, those numbers can only be a reference because every engine is different. What works in one won't be close in another.

Fever68 - great info.

Did you use a w/b O2 to do the tuning? I'm surprised the motor would tolerate 15-16 afr. So you basically reduced jet size and compensated with larger PVCR's?

I was thinking about getting one of those aftermarket metering blocks (quickfuel has a nice one, has adjustable idle IFR's, PVCR's, and some otehr stuff). That beats playing with wire. I like the brass set screws for the bleeds.

that would be great if the metering block was ajustable.I will have to check into that.

Something like this:
Metering blocks are a simple bolt-on component that can upgrade the performance of your existing carburetor. “Universal performance” carburetor calibrations are developed to work on virtually every engine, including stock engines without any performance upgrades. A primary component in the “Universal performance” calibration is the metering block. Upgrading the metering block to a more aggressive calibration is the key to unlocking the true potential of a performance engine.
Billet metering blocks offer the highest quality metering performance available in a metering block. Precision CNC machined from aluminum billet, porosity causing cross well bleeding is virtually eliminated. All billet metering blocks have screw-in idle feed restrictions. Standard calibration metering blocks have race breed emulsion calibration pre-drilled directly into the metering block. Adjustable metering blocks have screw-in emulsion bleeds that can be altered for ultimate fuel curve tailoring


http://www.quickfueltechnology.com/carb_parts/images/metering_34_5/hex0.jpg

Gasoline
Street Use
Professional Series
Adjustable Metering
Changeable Emulsion
Five Stage Emulsion
Changeable Power Valve Restrictions



$72.18

Fever68 - great info.

Did you use a w/b O2 to do the tuning? I'm surprised the motor would tolerate 15-16 afr. So you basically reduced jet size and compensated with larger PVCR's?

I was thinking about getting one of those aftermarket metering blocks (quickfuel has a nice one, has adjustable idle IFR's, PVCR's, and some otehr stuff). That beats playing with wire. I like the brass set screws for the bleeds.

I use an LM-1 and LMA-3 from Innovate Motorsports. I've been carb tuning on the side and thinking about getting into it big time. I do reduce the jets and compensate with larger PVCR's. I set the idle and cruise where I want it with main jet, idle feed restrictor, and idle air bleeds, then adjust the WOT AFR with primary PVCR and secondary main jet. 15-16 is kinda low. I had a regular Performer RPM intake that I could tune to 18:1. The cooler manifold is good for performance but fuel performance is better with a heated manifold.

Stay away from new and improved billet metering blocks. They are hard to tune. I'm having a hard time getting my Proform to act right. I've done had to block emulsion holes and modify everything. If it was a Holley, I'd been finished days ago. It is very easy to drill and tap a standard metering block and throttle body. The taps are readily available at the local auto parts store or hardware.

Wow..sounds good. Wish I could somehow get you to tune my 770 to my 454, but I dont see me shipping my car to you :(

Sounds like I'm wasting a lotta fuel in the 13 afr range.

Maybe you coukd strike a deal with UPS or Fedex for discounted shipping rates.

A set of numbered drill bits

http://img.inkfrog.com/pix/whataworld2003/60pc_drill_bits-2.jpg

and a few brass set screws (http://www.mcmaster.com/param/asp/PSearch2.asp?reqTyp=parametric&act=psearch&FAM=set&FT_104=4054&FT_125=499&FT_137=60708&FT_147=2074&FT_518=122652&FT_1381=55706&FT_2816=123451&FT_6432=255558&desc=Brass+Set+Screws&dscIDs=43813&sesnextrep=467894559858945&ScreenWidth=1280&McMMainWidth=774)in 6-32 X 3/16, 8-32 X 3/16, and 10-32 X 3/16 would get you tuned with better gas milage, drivability, and performance.

Fever, let me ask you something.

When tuning the carb, my guess is you start with the IFR's to lean the cruise afr, then adjust the main for power, and bleeds to adjust when the mains start right? Sounds like we dont even need a powervalve if you can delay the onset of the jets far enough.

do you just need one tap for the brass set screw? Ok let me get this you drill out the idle feed resrictor with the bit size for the brass set screw then tap the hole then drill it out with the number drill bit for the size hole you want.If you happen to go to big you just pull out that set screw and install another one and drill it to the smaller size that worked? Ok can you put up pics of all the parts I need and more detail on how to do this and tell me like you would a 5 year old so I can get it:)

ALSO TELL ME WHAT ALL THE ABREVITIONS ARE THANKS

Wow..sounds good. Wish I could somehow get you to tune my 770 to my 454, but I dont see me shipping my car to you :(

Sounds like I'm wasting a lotta fuel in the 13 afr range.

Maybe you coukd strike a deal with UPS or Fedex for discounted shipping


rates.
Hey 69 I had the exact same thought but you beat me to it.:yes:Well maybe if we keep this post up we can learn how to do it.:beers: Just about everything I have read about it is so confusing or maybe I just need someone to draw a picture.:)

Fever, let me ask you something.

When tuning the carb, my guess is you start with the IFR's to lean the cruise afr, then adjust the main for power, and bleeds to adjust when the mains start right? Sounds like we dont even need a powervalve if you can delay the onset of the jets far enough.

Close, The size of the IAB does effect the mains. I open the IAB's until the engine surges or misfires at cruise. I then decrease their size until the engine cruises smoothly. Sometimes I have to play with IFR to IAB relationship and find something that it likes. The transition circuit runs dry usually around 2600-3000 rpms and that's where the mains should start to pull over. Too big of an IAB and too small of IFR, it will stumble before the mains come in. That's where you must find the right combo. You need the PV because you actually cruise in the main circuit without and OD transmission. The MAB is the correction for main jets. Without it, the carb would get richer as rpms came up.

do you just need one tap for the brass set screw? Ok let me get this you drill out the idle feed resrictor with the bit size for the brass set screw then tap the hole then drill it out with the number drill bit for the size hole you want.If you happen to go to big you just pull out that set screw and install another one and drill it to the smaller size that worked? Ok can you put up pics of all the parts I need and more detail on how to do this and tell me like you would a 5 year old so I can get it:)

ALSO TELL ME WHAT ALL THE ABREVITIONS ARE THANKS

You need 3 taps. 6-32, 8-32, and 10-32. You drill the hole the size that goes with the tap. Then tap it and blow out the cavity with carb cleaner. The numbered drill bits actually are sized in thousanths. Each number crosses over to a size in thousanths. That is what you go by for sizing the orifices. Once the carb is tapped, changing set screws is a breeze. If you go too big, get another and start over drilling it smaller.

IFR= idle fuel restriction, the jet that controls fuel to the idle port and transfer slot
MAB= main well air bleed The air bleed jets at the top of the carby , the inner ones of the 4 that are on each side.
IAB= the outer air bleeds at the top of the carby, they air leak into the idle and transfer circuit.
E-bleeds= the air bleeds visible in the metering block behind the bowl to metering block gasket they emulsify the fuel hence e-bleeds.
T-slot= the slot cut in the base plate near the butterfly valve closed position, Fuel for low power operation comes from here.
Idle port= the exit point of the idle fuel, its in the throttle bore below the butterfly.
Butterfly angle hole= any holes drilled in the butterfly's.
Main jet= the jet at the base of the metering block facing into the float bowl.
Power valve= the vacuum operated valve on the body side of the metering block.
PVCR= power valve channel restriction. Its the holes in the metering block that are visible when you remove the power valve. They control the additional fuel added to the mix when the valve is opened by vacuum reduction in the manifold.
Kill bleed= the little hole at bump in the casting of the dogleg channel in the metering block. Its drilled into the main well. It helps with the starting control of the booster.
Booster= the venturi that is located inside the body venturi, its a venturi within a venturi which makes its vacuum strength boosted.
Dump tube= the tube leading out to the booster venturi, they come in different sizes.
WOT= wide open throttle, the fun stuff.

Here's a chart for what each number is equivalent to.

gauge inches mm
80 0.014 0.343
79 0.015 0.368
78 0.016 0.406
77 0.018 0.457
76 0.020 0.508
75 0.021 0.533
74 0.023 0.572
73 0.024 0.610
72 0.025 0.635
71 0.026 0.660
70 0.028 0.711
69 0.029 0.742
68 0.031 0.787
67 0.032 0.813
66 0.033 0.838
65 0.035 0.889
64 0.036 0.914
63 0.037 0.940
62 0.038 0.965
61 0.039 0.991
60 0.040 1.016
59 0.041 1.041
58 0.042 1.067
57 0.043 1.092
56 0.046 1.181
55 0.052 1.321
54 0.055 1.397
gauge inches mm
53 0.059 1.511
52 0.064 1.613
51 0.067 1.702
50 0.070 1.778
49 0.073 1.854
48 0.076 1.930
47 0.079 1.994
46 0.081 2.057
45 0.082 2.083
44 0.086 2.184
43 0.089 2.261
42 0.094 2.375
41 0.096 2.438
40 0.098 2.489
39 0.099 2.527
38 0.101 2.578
37 0.104 2.642
36 0.106 2.705
35 0.110 2.794
34 0.111 2.819
33 0.113 2.870
32 0.116 2.946
31 0.120 3.048
30 0.129 3.264
29 0.136 3.454
28 0.141 3.569
27 0.144 3.658
gauge inches mm
26 0.147 3.734
25 0.149 3.797
24 0.152 3.861
23 0.154 3.912
22 0.157 3.988
21 0.159 4.039
20 0.161 4.089
19 0.166 4.216
18 0.169 4.305
17 0.173 4.394
16 0.177 4.496
15 0.180 4.572
14 0.182 4.623
13 0.185 4.699
12 0.189 4.801
11 0.191 4.851
10 0.194 4.915
9 0.196 4.978
8 0.199 5.055
7 0.201 5.105
6 0.204 5.182
5 0.206 5.220
4 0.209 5.309
3 0.213 5.410
2 0.221 5.613
1 0.228 5.791

Awesome :thumbsup: wow I apreciate the help.That helps alot in understanding what everything does and what its called.I keep reading post with all these abrev. and didnt know what everyone was talking about.Do you mind if I send you a PM when I get all the tools I need to perform surgery on my carb so you can kinda walk me through it??
Thanks
Kevin

Were did you get the drill bits?Taps and set screws?

Don't mind at all, PM me and I'll send my number. Maybe I could help you understand by talking to you with the carb in front of you.


Here's a pic (http://www.innovatemotorsports.com/forums/attachment.php?attachmentid=1600&d=1170885959) with a set screw installed for the IFR's (Idle Feed Restrictions)

And here's a pic (http://www.innovatemotorsports.com/forums/attachment.php?attachmentid=1367&d=1163134746) of what your block should look like.

The taps I get from Napa, The numbered drill bits I bought off Ebay, and the best place I've found for the set screws is McMaster-Carr (http://www.mcmaster.com/nav/products/products.aspx?sesnextrep=467894559858945) Click on the link and type it in the search box to pull them up. They also sell the taps and numbered drill bits and they are better than the ones on Ebay. You will need pin vices to hold the small drill bits. Also available at McMaster or Ebay.

Numbered drill bits on Ebay (http://cgi.ebay.com/60pc-hss-numbered-drill-bit-set-new-retail-40-00-qualit_W0QQitemZ270168466577QQihZ017QQcategoryZ503 83QQrdZ1QQssPageNameZWD1VQQcmdZViewItem)

Pin Vices (http://cgi.ebay.com/PRECISION-PIN-CHUCK-SETS-EXPAND-YOUR-DRILL-CHUCK-USES_W0QQitemZ150102892426QQihZ005QQcategoryZ2594Q QssPageNameZWDVWQQrdZ1QQcmdZViewItem)- I use these in a drill real carefully as to not break the bit

6-32 Tap & Bit (http://cgi.ebay.com/6-32-Tap-Drill-Combo-36-Drill-Hobbies-Crafts_W0QQitemZ160168520546QQihZ006QQcategoryZ295 28QQssPageNameZWDVWQQrdZ1QQcmdZViewItem)

8-32 Tap & Bit (http://cgi.ebay.com/8-32-Tap-Drill-Combo-Hobbies-Crafts_W0QQitemZ160158957342QQihZ006QQcategoryZ295 28QQrdZ1QQssPageNameZWD1VQQcmdZViewItem)

10-32 Tap & Bit (http://cgi.ebay.com/10-32-Tap-Drill-Combo-21-Drill-Bit-Hobbies-Crafts_W0QQitemZ160160842059QQihZ006QQcategoryZ295 28QQrdZ1QQssPageNameZWD1VQQcmdZViewItem)

One more question which type of set screw do you use.I see a few different types.

One more question which type of set screw do you use.I see a few different types.

Brass cup point 3/16 long. They use allen wreches. Also, on the taps, get tapered/strarting taps. You only want to tap deep enough for for the set screw to be flush. When putting them in, just snug them. The tapered thread tightens them and if you try to tighten too much, it will crack the boss. Don't ask me how I know, lol. I needed a Proform center section anyway.

http://www.mcmaster.com/param/images/set/socket_cup_120x80.gif


I also forgot to mention that you may need to change pump cams and squirters. Get the assorted pack of cams (http://cgi.ebay.com/ebaymotors/HOLLEY-PERFORMANCE-CARBURETOR-ACCELERATOR-PUMP-CAM-KIT_W0QQcmdZViewItemQQcategoryZ33551QQihZ006QQitem Z160168288185QQrdZ1QQsspagenameZWDVW)and of 31 (http://cgi.ebay.com/ebaymotors/HOLLEY-CARB-DISCHARGE-NOZZLES-ACC-PUMP-SQUIRTERS-31_W0QQcmdZViewItemQQcategoryZ33551QQihZ006QQitemZ 160136485418QQrdZ1QQsspagenameZWD1V) & 35 (http://cgi.ebay.com/ebaymotors/HOLLEY-CARB-DISCHARGE-NOZZLES-ACC-PUMP-SQUIRTERS-35_W0QQcmdZViewItemQQcategoryZ33551QQihZ006QQitemZ 160131612982QQrdZ1QQsspagenameZWD1V) squirters. They come two in a pack so if needed, there will be a couple extras to drill to size. I know this tuning stuff adds up but it's money well spent. The gas milage will pay for it in short time.

http://i6.ebayimg.com/01/i/08/09/40/c3_2.JPG

http://i13.ebayimg.com/05/i/07/9f/5a/09_2.JPG

If Im getting three different sizes of drill and taps (6-8-10 32) then I guess Im getting three different size of brass set screw.how many of each would I need.

Ray,

Why not do a nice "O2 carb tuning 101" write-up here? We have an ignition write-up, be nice to have a carb one.

If Im getting three different sizes of drill and taps (6-8-10 32) then I guess Im getting three different size of brass set screw.how many of each would I need.


I think they come in 50 packs for under 10.00 a box. That's way more than enough but it will leave you extra to work on a few more carbs. Once you learn by doing yours, all your friends will want you tune theirs.

Ray,

Why not do a nice "O2 carb tuning 101" write-up here? We have an ignition write-up, be nice to have a carb one.

I don't know. Maybe I can walk Kevin through his tuning and the mods may sticky it. Although all carb/engine combos are different, the procedure of finding the right combo of bleeds, restrictions, jets, etc. is the same.

Right now Im getting all the tools and parts together.:thumbsup:

I think they come in 50 packs for under 10.00 a box. That's way more than enough but it will leave you extra to work on a few more carbs. Once you learn by doing yours, all your friends will want you tune theirs.

I take that as a yes to needing three different sizes?

I take that as a yes to needing three different sizes?


Yes,

6-32 X 3/16
8-32 X 3/16
10-32 X 3/16

Ok so far I have the drill bits (harbor freight 10.00) the taps and I have ordered the sqirters and pumpcams and all of the brass set screws.I still need to find the pin vise.I will check a few hobbey shops in the area first.