Try the old tri power. I had it on my old GTO. The car was fast for its time but trying to keep them tuned together was a nightmare. I'll take the 650 dule anytime.

 

Krystal, look forward to seeing your results.

 

i am all ears!!!!!! i have 408 sbc with 10.6 cr,AFR 195 heads all CNC , roller 1.6 rockers .air gap rpm manifold and Lunati .541 ,.541 lift (with 1.6 ratio included )231/241 duration @ .050.and 112LC. msd/HEI with 36 mechanical advance and 54 degrees total with vacuum advance at 3600 rpm . 3100 ibs 71 corvette,700R4 3.06 1st and 3.55 gears. stomp on gas and car literally stops from dead stop stumble is not the word. need to bring rpm to 2400 to take off (2600 stall converter) i have try everything with this new style Holley aluminum 750 cfm, vacuum secondary.went from 82 jets to 77 in the primary,vacuum set idle jets ,set accelerator pump from .015 to .030 nothing -----nothing ------nothing ---help please

 

This may help. Watch these videos in order-

 

I was setting accelerator pump at(.015) without holding the throttle to wot! -------DUH!!!!! first attempt at changing cam never did this before. will check squirter tomorrow and report ,

 

as the saying goes, a little knowledge is a dangerous thing.

 

These threads are like a train wreck....you know it's going to be bad...but you just have to keep looking! They fall in the line with what oil should I use, solid lifter or hydraulic lifter, oval port or rectangular port BBC heads, should I use blue interior lights or white ones..??

That "sense of power" you get when you mash the pedal...IS power. It's not imagined....the engine is accelerating because it's getting more air. If comparing spread bore to equal bores...then yes...you'll need to mash the pedal less with equal bores to get the same area. Not sure how that relates to anything?

A small primary spread bore will need throttle plates opened more even at idle to get enough air even to idle. It takes X area to feed whatever it is. If you've got a big cam/heads/intake or even low compression with a weaker signal...it will need a larger opening. One reason spread bores don't work as well there because they have to be opened too much just to idle.

GM installed 2 BBl's on a lot of 396"-425" range engines. Had great throttle response...but made no power. They were cheap and worked great for grandma's station wagon. Is that what you're building??

When I first put EFI on my car I used a 4500 Dominator style throttle body (2000 CFM) with 1 to 1 throttle linkage. It opened all four barrels at once just like the 660 center squirter carb pictured above. The throttle response was incredible off idle...to the point when I let new people drive the car they had a difficult time at first because it was so responsive. Definitely would think it was a relatively "horrendous" velocity going through it...especially with a big cam and huge intake. I later swapped to a progressive version to make it easier to drive. You can't say it's just because it was EFI...it's still airflow and the EFI just adds fuel to match it. Obviously the engine could take the airflow right off idle with no issue.

If you want to be super super safe.....put a 650 vacuum carb on it. But don't be surprised if it's lazy once you're moving and want real power unless you do serious tuning. If you never get over 4000 RPM...then not even sure why you're building a hot rod motor...might as well build it stock and enjoy the slowness.

JIM

 

Bob,,,this should get you fixed up. The pump should shoot fuel as soon as that arm moves a hair.


JIM

 

i took my time with a vacuum gauge and set those needle valves perfect and my idle vacuum is over 20 inches at 750 rpm so that's good. the car drives really smooth but from idle speed to 2000 rpm car falls flat on it`s face when I power brake to 2400 the car lifts that front end (held in place with VB&P tie bar) and melt those huge Nitto drags radials so the engine roars above 2400rpm to 6000 rpm that leaves the accelerator pump, squirters, plastic cams as the problem. after that, i guess i could perhaps re-jet with the use of my A/F gauge while driving.the car pins you in the seat all the way to 140mph were i let off. carbs are not my expertise so i figure i made a mistake buying a 750 cfm instead of 650 cfm. but now i guess i can live with it. you guys get pretty tough with a newbie.thats not my style i got all shot up trying to save other people the least you can do is steer a old veteran the right direction.

 

I think your timing curve needs work first. What's your initial advance? You can retard total timing because you have aluminium heads and that's a recommendation from AFR. They recommend 32-34 total in by 3000 rpm. See if that improves anything before chasing your tail trying to tune the carb. Are you using ported or manifold vacuum? Try each and see how it works.

 

i am using manifold vacuum. i should mention i have a vacuum adjustable can on the HEI dizzy. i also use the HEI as a trigger by removing the modular and using the MSD 6A to fire.as it stands now i have one heavy spring and one medium spring in mechanical advance of HEI.the light springs were no good even more stumble. i have 22 degrees with vacuum and mechanical at idle. 18 mechanical and 4 from vacuum advance.

 

What PV is in it? With 20" at idle (which is indicative of a relatively mild cam considering the cubes)...and let's say you have a 6.5" one in there...vacuum has to go down a long way before it opens. When you're power braking it, vacuum drops and it will open. You might need an 8.5 or even a 10.5 PV to get things started earlier without counting on pump shot to cover the hole.

Remember...as vacuum drops you can lose some of that vacuum advance as you tip in depending on characteristics of the vacuum can.

I don't remember on the MSD HEI...does it allow you to "lock out" the timing like a regular MSD distributor? It would be a simple way to test things for no $$$. Lock it out at 36* with no vacuum and see how it does. I've often used initial timing well up into the high 20's and beyond.

If nothing else...install the heavy springs on it and crank initial way up to 26-30 or so for a test. Just do the initial whack of the throttle without going above 4000 (to keep from over advancing things) or so to see how it responds to increased initial.

JIM

 

427 Hotrod i am following your lead along with a few others and i thank you for your time like a brother would do. i have Holley 4160 with vacuum secondary, electric choke, aluminium hp with 6.5 PV and 31 Squirters so far i held accelerator at WOT with a Zip tie and adjusted the setting to .005, and set the plastic cam to top screw (maximum opening). today will start and warm up with vacuum from the manifold port,A/F gauge connected.will also check timing with a without vacuum advance

 

The .015" accelerator pump measurement is to be sure it doesn't bottom out. I always adjust it so it has an instant fuel shot with slight movement of the throttle and THEN go WOT to check for bottoming out. That spring on the adjuster screw can have an impact on responsiveness if it compresses before moving the pump arm. Just set it for operation...go WOT and see if you can manually depress the arm on the pump itself slightly more to be sure it doesn't bottom out.

Another thing that can help is to isolate the secondaries by removing the linkage and blocking them from opening and theb dialing in the primaries first....then work on the secondaries for WOT. This helps if things seem way out of whack and you feel the need to get back to basics.

JIM

 

with the accelerator pump adjusted properly , idle at 900 rpm in park ,vacuum to dizzy plugged , timing at 18 degrees initial, vacuum from manifold is 16 inches steady A/F at 12.95 take the throttle and slam open the engine almost stops the stumble is so bad! at that moment A/F is 20.2 then three seconds later engine responds and vacuum goes to 25 inches at 2000rpm A/F at 12.53 at 2500 A/F at 12.40 and at 3000 A/F at 11.98 . power valve is 6.5 and squirter 31 do you change both or one at a time ? thank you in advance

 

What LIST number is the carb?

For the PV on a street car...I like to find a nice stretch of road with hills. I hit them at say 40-50 mph or so and see how it responds to the added load/low vacuum as it goes up the hill without moving the throttle. It should smoothly handle it as the vacuum drops and the PV opens to add fuel/power to get up the hill. if it goes flat and lays down until you mash the pedal more...then I look for an earlier opening PV.

Why did you drop 5 jet sizes on primary?? That's a lot. Can we put it back to it's baseline and start from there?



JIM

 

Do changes one at a time and forget about the pcv for now. I had same problems. I think your squirters are too small. Try a 0.034" and a 0.037" and without changing anything else, see if there is any change, better or worse. I had to go from a smaller squirter that came with the carb up to a 0.040" testing different sizes in between. But it worked! Don't screw around with air bleeds, at least not yet and the same goes with the accelerator pump cams. As you increase the size of the squirter with car in park and secondary side isolated to prevent from opening, give the car a shot and check your exhaust. Keep increasing squirter size until you see smoke coming from exhaust. Now your about one size too big and hopefully your bog has gone or gotten a whole lot better. Oh yeah, car must be at normal operating temps to do this stuff.

Edit: Just saw your post, Jim. I think he needs to address the squirter size first before going further.

 

Don't disagree....I'm just concerned a lot of stuff has been changed and we could end up chasing our tail if we don't have a good baseline.

He's talking accelerator pump tuning it sounds like more than anything. I was steering away from PV concerns for this.

JIM

 

the first thing i did was pull into the driveway and bring to temperature ,yes choke is off. holley carb model 4160 part # 0-80508S .factory jets were 72 i went up in size to 80 and cam down and down because plugs fouled and A/F gauge was crazy rich (i hook up and drove with it A/F constantly running).what is the real number i want at 2000 rpm from A/F gauge? the car is used for "spirited driving" cannot get ticket ------will not explain that

 

Barrels 4
Billet Color None
Booster Straight
Brand Holley
CFM 750
Choke Electric
Circuit 2
Emission Code 1
Finish Shiny
Fuel Gasoline
Fuel Inlet Dual
Material Aluminum
Model 4160
Primary Main Jet 72
Primary Power Valve 6.5
Primary Pump Nozzle Size 31
Product Type Carburetor
Secondaries Vacuum
Supercharged Application No
Throttle Bore 1.688 inch
Part Number 0-80508SA

Neal