erb64

Had the engine rebuilt three years ago and very happy except it does not like to idle. It's been an issue since day one and the builder told me to get too 400 hp a 650 double pump was the only way to go. It does run well but stumbles at times at lights.
A good buddy , who is a mechanic, keeps telling me the carb is too big and to go smaller. It will improve performance and idle better.
You guys have forgotten more than I know, any input is appreciated.

64 roadster, winters manifold, solid lifters and a medium cam, roller everything . Loves to run but does not like to sit. thanks guys, any ideas are appreciated.
Ed

Nowhere Man

Yes bigger is not always better. The stock L79 and alike 327 used a 575 cfm carb.

Frankie the Fink

As noted above....many old schoolers want to run huge carbs and don't care about streetability and want bragging rights. It can make cars pigs for daily driving. A dual plane manifold (I am told) lets you run a slightly bigger carb because the motor only 'sees' 1/2 the carb at a time (supposedly). Others can confirm I guess.

I've never gone above a 650cfm on small block Fords or Chevys.

Bluestripe67

Do know the specs on the cam, it could be a player, along with the carb. Initial timing and centrifugal advance also could come into the picture. Dennis

MiguelsC2

50cfm difference isn't going to matter. What does matter is jetting,timing,power valve etc.. If these thing aren't correct? It will run like crap no matter the carb size.

Doesn't sound like you are getting very good advice from your "mechanics".

If set up properly,that 650 dp will run fine on almost any V8 configuration.NASCARs got 800+hp with a 600cfm restrictor plate.

climbabout

Ed - I have a 64 327/365hp L-76 that is stock except for a holley 650cfm double pumper 4150/4777-c instead of the stock 4150/2818 600cfm. It runs beautifully btw. I can tell you from personal experience with this setup that timing and proper vac advance are imperative to getting a good idle. Your vacuum advance can needs to be the proper one for the amount of vacuum you have at idle. When I bought my car it would not idle consistently - it was all over the place. Turns out the previous owner had a vac can from an Impala I believe that did not pull in fully below about 14-16" of vacuum. The L76 only produces about 9-10" at idle. So the vac advance was only partially engaged(at idle) giving a very sloppy idle. Once I put the right vac advance can in - a B28 - problem solved.
Hope this helps.
Tim

erb64

Thanks guys. Looks like I have some studying up to do
Ed

65silververt

Study
1. http://www.corvette-restoration.com/...arb_Tuning.pdf

2. http://www.corvette-restoration.com/...ine_Timing.pdf

3. http://www.corvette-restoration.com/...stallation.pdf

4. http://www.corvette-restoration.com/...Valve_Lash.pdf


and if you really do need a new carburetor and want one that has great tuning versatility and will allow you to use the stock small block hard line, filter, and flex hose, this is the way to go. I love these carburetors and will only use quickfuel from now on.

http://www.summitracing.com/parts/qft-bd-1957/overview/

midyearvette

bigger is not better when it comes to carbs.....a 650 d/p should be ideal for your application, check the usual suspects, power valve and timing etc...good luck...

alexandervdr

I would agree with those comments that a 600-650 should work. I actually have an edelbrock 600 on a 365hp/solidlifter/agressivecam and I can have it idle very nicely at 800rpm. Pick up is not a problem.
Now, I have tuned the engine (cam, lifters) down to the reality of traffic conditions where I live (traffic jams) and what I want to do with the car (near-daily driver and cruising). I actually consider installing a 390cfm from holley, they have a good carb selector tool online http://www.holley.com/applications/C...bSelection.asp. For a 327/ 300hp engine that does not go over 5500, that selector invariably picks the 390cfm. Other carb simulators confirm that. Will it work? Don't know yet, I am now doing the body off

midyearvette

i never agree with that formula or carb selector from holley, but thats just me
usually i go one step bigger than the selector
there are times one can make a smaller carb work ok by jetting up and playing around a lot with mixtures and say it works out. but unless you try a bigger carb from the box, one really never knows if better or worse than the formula or a smaller carb.
the "formula" is sometimes ok.....but never always correct in the real world....jmo.....

alexandervdr

agree, that this is not a 100% guarantee.

There is one field where I do have substantial experience, and that is in model airplane engines (don't laugh, these are near 1000 times smaller than the sbc ;-)) Bigger bore carb consistently meant higher top end output (obviously to the limit of what the cubic inches times rpm could 'suck'), but consistently worse (or at least higher rpm) idle and worse transition through mid range. Seems to be in line with physics theory too.

65silververt

Im confused. You had the car running well with the original components, but then you tuned down the engine with a cam and lifter change and now you are looking to change out the 600 cfm carb to a 390cfm unit? You say aggressive cam and then that you tuned it down with new lifters and a cam change. Im assuming you changed out the original solid lifter cam. The original components should work very well if set up correctly, even in traffic.

alexandervdr

exactly, the 365hp engine is a high rpm tuning, gets the 365hp only at 6200rpm. Idle is specced at 800 rpm while the 250/300hp have max hp at 4400-5000 and idle around 480. So I tuned it down, I don't care about max power, much more about practicality. The more that max torque does not vary a lot between all those engines (350 for the 250hp versus 360 for the 365hp) , but the rpm where you get it is way different (2800 versus 4000). Low rpm torque feels much better for the driving style I want. It's all a matter of preference, not about best/worse...just different choices made by different people.

cv67

Tuning issue sounds like to me
Maybe the idle circuits need to be played with some
Most get screwed up and frustrated playing with jets power valves etc with these issues and overlook the idle circuit.
650 is a great all around sized carb for sbc

midyearvette

....good post....

KC66

If I remember correctly, if you are having idle problems and low vac reading possibly from large cam, there is a screw that you can turn. Its on the bottom side of carb on the secondary side I think. You can open the blades a little more to allow more air in while idleing. Not sure if this is the case here, but as others stated timing ect.... to achieve best vacuum will help it idle better.

KC

Westlotorn

I always try to match the carb to the engine, if your cam is all done at 5500RPM you only need a carb good to 5500 RPM, if your cam will pull to 7500RPM you will need much more carb to match. There are web sites that help match Cubic inches, RPM and CFM needed.
They are pretty accurate. Staying at the smaller size gives better throttle response in general. We built a 460 Ford for a Jet boat, the jet pump held RPM to a max of 5100 RPM.
We ran a 650 on this 460 because of this RPM limitation and it ran perfect. Great idle, great response. Pulled really hard right to max RPM of 5100. We might have needed a 750 cfm to make this combo spin 6000 RPM but the pump limited our RPM so there was no gain.
Although double pumpers give great throttle response they sure seem to eat a lot more fuel if that matters to you.

midyearvette

when youre talkin marine applications, all bets are off
my 454 bb i figured the rpm max at 6000 with the holley specs and a 650 d/p did not work without some main jet and power valve changes, once i thought it was good, put on a 750 d/p and made quite a difference and ran a whole lot better....
this was not my first rodeo with the holley chart, i used to use it a lot and if i still do, if the numbers are close to being on the fence, i always go one bigger with perfect results......jmo.....

Westlotorn

You might be right about the boat twist, the jet revs to 2000 with nearly no load and then it is loaded hard up to max rpm.
Pg 1 of the Carter AFB tuning manual attached gives the chart I have used for years. It clearly says if you are running 3.90 or higher gears you should use the next larger size carb.

http://www.moparsunlimited.net/image...rafbtuning.pdf

I tried to copy just the chart and paste it but my working with pdf files is weak.