HottestZ28

Motor specs
350 stock piston
Trickflow 62cc 202 aluminum heads
Comp 280h with 1.6 full roller rockers
Rpm air gap intake
Hooker long tubes
Summit hei

Trans
th350 2800 stall
Manual vb

Rear
Gm 10 bolt
3.73 posi

I currently have a holley 4160 750 vac secondary carb
I have idle issues as I assume it might be to large of a carb or inadequate timing
I have little to no bottem end and gobs of top end. If I do a calculated cfm it tells me to go with a 612 one size up 650. I want to upgrade my carb to a Holley hp and want to know if a 650 would be a better choice
My idle issue is that I have to jack my rpm out of gear to about 1400 to get a 700rpm in gear. I also jetted up from a 70 to a 74 and put the lightest spring in the secondaries. The squirter was also upgraded to a 35. I really want to get a non choke/ manual secondary carb. I also have been looking into a msd box and dizzy for better timing adjustment and rev limiter

Confuzed1

iTrader: (4)

It would probably help if you posted where you have your initial timing set at with vac advance hose removed and plugged. You shouldn't need any of those other parts to make it run better. Did you look at the sticky on top of the carb board? You should be able to dial in a 750 to work with your setup...

You need to be real specific as to what the exact issues are, and where everything is currently set so others can chime in...

tzim350HO

750 is too big for that setup. Unless you are running a massive lift cam and high compression all its going to do is cause a rich condition. idling will not be as good and will bog at low rpm.

tzim350HO

I actually run the 600cfm holley hp double pumper on my car and it works great. im running a .550 lift cam on semi dish pistons with 64cc heads.

HottestZ28

I tried originally to set base timing as close to 18 as possible. I am not running vac advance and I couldn't dial in the idle with that much timing so right now I am set at 14. When I back the timing down it runs worse but idle drops a lot less. I did see how to tune on a Holley but this is more of a idle issue more then a performance issue. It has alot of top end it ran 13.20 at 107 with no spinning. Very soft launch and very little bottem end( was open rear with radial t/a). With this small of a cam and lower stall I want there to be more bottem end as its a street machine and not a drag only set up.

tzim350HO

Timing is never gonna help! You will have upper rpm performance because the motor is spinning faster just powering over the initial heavy fuel! Pull some plugs id bet they are sooty!

HottestZ28

I do believe its too much carb for the setup. I just did t want to spend 500 and no be completely happy. I bought the 4160 knowing very little about carbs. I now learned that I bought the opposite carb for my needs

Confuzed1

iTrader: (4)

Kinda funny how the original Q-jet on the 305's flow 780 CFM's....750 is not too big.
I'll admit, the Q-jets are a spreadbore carb, but you should be able to lean it out plenty for a 750 to run just fine. Would a 650 do better?...yes, maybe a little on daily street driving because you'll have better air velocity through the smaller primary brrls at idle. Do a search, and you'll find plenty of guys running a 750 carb with a 350 cube engine.

Why are you trying to dial in 18 degrees initial? No vacuum advance, it's gonna suck gas on the street anyway. Straight mechanical dizzys are fine for the strip, but not for street use.

sofakingdom

iTrader: (1)

750 is perfectly fine.

Ignore the comments from whoever up there about "timing". Spark timing is ABSOLUTELY CRITICAL.

And I'm not talking about, put it where some "book" "says" your "88Camaro" or whatever, is "supposed to" be. Believe it or don't, optimum spark timing is not determined either by (a) the year the sheet metal wrapping the motor was stamped in, or (b) the wrinkles in said sheet metal.

Your timing curve, for that cam and those heads, should look like:

"Static" timing (what you set by twiddling the dist body) 16 - 18° BTDC; centrifugal (RPM based) advance beginning just off idle but high enough that it won't cause "hunting" idle (1200 RPM or so) and increasing to about 35° @ 2800 RPM (16 - 20° of "centrifugal" advance); and 12 - 15° of vacuum advance on top of all that, with the vac adv dropping out at around 10 - 14" of vacuum. You won't find an off-the-shelf dist like that, you'll HAVE TO build it yourself. (not, "have" your "mechanic" build it for you)

A man sec carb will outrun and otherwise outperform a vac sec carb in the hands of a knowledgeable driver, EVERY TIME. I don't know how competent your driver is, but if they're good, give em a man sec carb, then teach em how to drive it if they don't already know.

Follow the "Holley tuning" sticky on the top of this forum. I didn't exactly "write" that; Apeiron actually composed it out of a handful of repetitive posts I had written; but he did a fine job of capturing the essence and presenting it faithfully, for which I am thankful for not having to write it so many times since then. Follow it to the letter, in order, AFTER getting the ignition curve set as I just described. After you do that, feel free to come back and ask questions, especially if something doesn't behave as predicted.

tzim350HO

Im not going to get into a pissing match on here with you but its never going to matter where you set timing if you are getting too much fuel to start with! 750cfm on a small lift 350 is too much, sure you can trim them back but its not worth the trouble. Theres a little more involved than just changing some jets and calling it a day!

tzim350HO

It's not that it's bad to use a 750 VS or double pumper on a 350...it depends on how the motor was built and it's intended use. If you've got a bone stock 350 with 275hp...sure you can use a 750, but, the carb is not being used for it's full potential. It'll work, but a 600 or 650 will be able to do the job just as good, if not better then the 750. If you have a 350 or 383 that is used for street and strip, and it's pumpin out a respectable 350hp+, then, also depending on how it was built, a 650, 700, or 750 will do the job just fine. The main factors are cam, compression, and intended use. Remember another thing, sometimes people get caught up in "bigger is better"...you put a big carb on a motor that isn't put to par with the carb's potential, you'll end up drowning/choking the motor with too much fuel. I hope I helped make sense of this...good luck

HottestZ28

I wanted 18 initial because with the curve in full advance I would be close to 33-34. I just can't seem to have a solid idle without setti the idle up close to 1400 to get a 750 in gear idle. I read that it can be due to throttle blades being open to much and pulling off the Venturis, timing which I got bushings, weights and springs but I have a china made summit brand HEI and feel I'd be better off with a small cap MSD and an ign box. I basically want to get away from a choke and vac sec... So felt this would be the time to get a comparison on the 2 cfms

tzim350HO

Yeah if they are open too far it will cause problems and never idle right! I hope i helped! I think a 600cfm would be perfect but thats just my own opinion! I have that on my race car. Im running a .040 over 350 with 64cc heads and a .550 lift cam across the board with a 600 cfm double pumper (that expensive *** carburetor you hate to buy). I had it dynoed at 425hp at the flywheel. Runs great and never misses a beat!

HottestZ28

Just did a search for 600 hp and I can't see spending $200 more to get 50 less cfm

cuisinartvette

I had that exact combo yrs ago and it ran way better with a well sorted out 650 double pumper night and day. Instead of pulling when you stabbed it the tires just started smoking no joke.

The 3310 I tried....with 74s if you havent fouled plugs you will thats just way too rich. 70s tops...I think 68s would do it. Ck your throttle blade openings again make sure your idle bleed screws have a good seal with those gaskets they can back out and suck air making things finicky.

Main jets dont really come into play at idle so much. Go smaller so when your cruising about 40 wait til you get a lean surge then go back up 2 sizes.
Sofa pretty much covered all the rest (actually all of it lol).

What size torque converter are you running? I had a cheapie 2800 and needed a better quality and possibly larger stall. That will drag the motor down at idle also.

InfernalVortex

iTrader: (4)

Ive followed a LOT of your advice, especially the holley tuning sticky, and my car runs absolutely fantastic when its warm thanks to you. I suggest the OP read it over and take it to heart. It made a night and day difference to my car.

But that's just carb tuning, Im still in the dark on setting up distributors.

How do you adjust where the vacuum advance falls out?

And how do you adjust what RPM the centrifugal advance kicks in? All I know is swapping out springs will give a difference advance curve, but I just kept swapping springs until I got full advance around 3000 RPMs and left it alone. No matter what springs I used, the total mechanical advance was always the same, so mine is set now to 18 degrees initial (probably a bit much) and 32 degrees total. But I just recently hooked up vacuum advance and now the car runs tons better all over again. Should have done that years ago. But this distributor stuff is somewhat voodoo to me too, but it makes a huge difference in how well the car runs.

sofakingdom

iTrader: (1)

Agreed... but it's equally impossible to tune a carb, if the spark is at the wrong time. Can't tune that out no matter how much you work on the carb.

Look at it this way: it's ALOT easier to predict what a reasonable timing curve will be for any given motor, than to predict how to set up some random carb for it. THerefore, to reduce the number of carb/timing tuning iterations, it makes the best sense to get ONE of them as close as possible FIRST, then optimize the other, then dial in the first, then tweeeek the other.

Infernal, what you observed with the springs & weights is exactly what it should do; so you're good there. Might want to try bumping it up a degree or 2 at a time and tune for best compromise of mph in the ¼ w/o pinging under any normal driving conditions.

With an adjustable vacuum can...

http://www.summitracing.com/parts/crn-99600-1/overview/

First, adjust its limit (i.e. how much vac adv it gives) such that it almost but not quite pings at very light throttle tip-in during high-speed cruise, like, driving at 70 mph and accelerating to 71 mph; then, adjust the drop-out vacuum to where it doesn't rattle during a heavier acceleration like say 70 to 80 mph. Should get it mighty close.

I'd suggest hooking it up to full manifold vacuum if possible, and use the ported vacuum only if the full vac causes unstable idle.

HottestZ28

I pretty much sold on the 650 and yea I wish I did a larger stall and better name brand but when I was building the motor tried to saw alittle and go with a summit brand converter. When I upgrade it will be a taco 3000 or 3200 was more worried about stall speed around town but with the gear and tire I am over 3000 in third by 55-60

camarito

i had the same problem and found out the wights on my distributor were too heavy and mechanical advance was all in by 1500 rpm even with the heavyest springs so i drilled a bunch of holes in them and it's all good now, idle goes down a little but not much, and full advaced by 2500 rpm

HottestZ28

Never heard of that. I did get a mechanical advance spring and weight kit and didnt fit the pins so I just put the light springs in first and was ok then put the heavy springs in and felt like it was dragging an anchor so I put the light weights back in

camarito

if your advance is bouncing at idle you never get the idle steady, i litthe load on the engine and the advance drops and the engine dies

camarito

also the lack of vacuum advance is the reason it feels like you are dragging an anchor when the mechanical doesn't go in quick, with the vacuum advance you have that extra advance without the mechanicals disturbing the idle

birshking

i have an aem 750 double pumper that was recently rebuilt. im not going carbed for my motor swap, so if interest shoot an offer.

HottestZ28

Update..... Got the 700+ 600 cfm carb directly from holley for far less. Now my problem is no vaccum port for brakes and no room for a 90 due to where edelbrock decided to install a bung under the secondary fuel bowl. I will be needing either a 1 in spacer or find a 1/2 in spacer with vaccum port in it. I will let ya know when I get it running. Porting the intake now. I have a 3 in cowl currently but with a wieand single plan and a 1 in spacer needed a major drop base for a standard 14x3 air cleaner which I don't like
Anyone recall how much taller the rpm air gap is along side. Single plane

camarito

Why don't you just dtill and tap on another location? If you don't have the tap tool just drill bigger and epoxy an adapter, JBWeld is good stuff

HottestZ28

I thought about drilling but don't really like to rig stuff as I am looking to effectively complete my install without modifying a intake when all I my need is a $30 spacer. If good clearance is to much of an issue my last route will be modification.

I do have a full master set of tap and dies so drilling and tapping would be no issue for me

Confuzed1

iTrader: (4)

They sell a low profile 90 degree fitting that works great on that....

EDIT: I used the fitting plus a 1/2" spacer for the brakes, and used the port on back of the carb for the PCV......

sofakingdom

iTrader: (1)

There may also be a port on the back of the carb throttle plate with a plug in it... if so, a 3" or 4" long 1/8" pipe nipple might be all you need.

HottestZ28

Carb has no provisions for any vaccum none on baseplate and metering block. I will have to look into that 1/2 spacer and special 90.