is 600 cfm carb on a 400 SBC to small?

[justin4260]

so the PO told me the engine was a 350, so i bought a 600 cfm classic holley. well the truck is hesitating really bad. I've tried tuning it for more vacuum but the vacuum doesn't change no matter what i did. i tried adjusting my timing. i didn't have a timing light so i went old school and just loosened it and turned it until the rpms raised and stopped when they started to lower again. then i lowered my idle down. i have to have my idle high in park that way it doesn't die in gear. well i found out i actually have a 400 small block from a g or b body 70-73. so I'm thinking maybe its hesitating so much because it doesn't have enough air? any thoughts?

[1972RedNeck]

No, too small of carb would help the low end drivability. It sounds like you just need some tuning.

[cdowns]

you can run a two barell with about 250cfm with great results if you get it to someone who knows how to tune it

[swamp rat]

How is it hesitating? from a stop and trying to take off? There is an actuating rocker arm that hooks to the accelerator pump there are 3 holes for the linkage, most likely the linkage rod is in the middle hole, try moving the rod to the hole closest to the pivot point, this will give the carb more fuel when you press the pedal

[67 chevelle]

Get it timed properly , and go from there , many are of the opinion "run as much timing as you can" , but too much timing robs horsepower more than too little timing , causes pinging , and can cause the simptoms you are describing .

A 600 cfm holley carb on a 400 small block should run great from idle to about 5000 rpm , and good up to 6000 , but horsepower numbers will be better with a slightly lager carb , maybe 700 cfm .

Check for vacume leaks also , they can cause stalling

[JJLT1]

is that a new carb? what year truck??
?? hesitating off idle? mid range? top end?
was the truck warmed up? does the choke work?


600cfm is fine on a mild 400 as said.

get a timing light, check the timing curve,
and the advance, vacuum and mechanical..
setting it by ear, is usually too much initial timing..

if the idle mixture screws have no effect?
could be plugged up, a blown power valve or vacuum leak or
the throttle blades are open to far, >the high idle.

[justin4260]

the carb is brand new only been on for a few weeks and only run on the truck 3 times. its a 1970 truck but the engine is a 70-73 400 from a b or g body. it idles a little high to keep from dying in gear. it doesn't hesitate in idle only in gear. what it does is if you slowly give it gas its fine but if you punch it it kind of bogs down for like a second or 2 then jumps forward.
i bought a timing light from harbor frieght but its a stupid cheap one and the battery hook ups aren't long enough. so i guess ill spend the $100 and get the digital timing light from orielys? the carbs definitely not plugged up and i checked for vacuum leeks couldn't find any. the secondarys are open ed a bit more to let in more air so i can lower the idle screw down.

[JJLT1]

new carb, did you set the float levels??
check the slack in the accelerator pump linkage??
are you running points, how long since a tuneup? wires, plugs good??

unless you have a big cam/low vacuum? close the secondarys and tune the primarys..
the primary throttle idle and transfer slots need to be set.. gota get the mixture screws working..

heres some holley tuning, 2 pages..
http://forums.holley.com/blog.php?12...ogcategoryid=1

faq's/videos
https://www.holley.com/support/faq/
.
.

[Clavo106]

Just my .2cents it 600cf carb should run pretty good.. but u said it dies when you throw it in gear..mmm.. I had the same problem with a 60s cadi coupe./ oh jesus/ a friend wanted me to install a 750cf elbrok carb on the cadi. it did the same thing.me being New to EB and there crapy carbs .I check everything vacume .timing. spark plugs/ mess around with there ajustment fuel air screws .ect. it just idel really bad . and when u threw it in drive it died. WHAT I should have done at my young age is stop courseing and started opening up that carb. needless to say It kot on Fire..to much fuel. not a good day.. I finally told him to try and find me the original carb. . he did and I replaced ran perfectly. no problems. good luck..
Posted via Mobile Device

[Orange Crate]

Just some quick down and dirty math. Your 400 ci would need 200 ci of air per cycle (720 degrees) so at 4000 rpm that would be 800000in³= 462.9630ft³ per minute. If your engine ran at 100% Volumetric Efficiency (which it is not). So lets say you are at 75% VE at 4000 rpm that would equal about 346 cubic feet per minute. If you have stock exhaust manifolds and cam a 600 cfm carb would be a good choice, also take into account your altitude (back to VE calculation again). High velocity at the throttle valve equals good low end torque (high VE...what again?) making the 400 really shine at low rpm.

[67 cst swb]

Before you do ANYTHING that cost you more than $15.00 or more than 15 minutes of your time... Buy this!!!
http://www.summitracing.com/parts/hpb-hp473



You will not regret it. Its the best thing that you can do for a Holley... is educate the owner/tuner.

Also, FYI... you can determine if a Holley is too big or small for a engine by:
If you have to change the original factory supplied jets to a larger jet size by 8 numbers or more (such as 75 to an 83) then your carb is too small.
If you have to change the original factory supplied jets to a smaller jet size by 8 numbers or more (such as 75 to 67) then your carb is too large.

And... Don't be concerned with too small of carburetor if its on a daily driver application. If you read the book above and properly tune everything, you will be pleased.
If you are going to a race track with it, then that's a different story.

[MAC71]

I had a 400 small block in a 71 LWB c10 in high school with a 600 holley that ran 12.9 1/4 mile. 600 cfm on a 400 cid is just about right for the street.

[tranz am]

Just need vacuum gauge to know if its too small. Have a friend watch the gauge as you give it full throttle through all the gears. If at any time it reads more than 1.5 hg the carb is too small. Or there is restriction above the carb base.

[MAC71]

Originally Posted by tranz am

Just need vacuum gauge to know if its too small. Have a friend watch the gauge as you give it full throttle through all the gears. If at any time it reads more than 1.5 hg the carb is too small. Or there is restriction above the carb base.

If it does show vacuum wide open and it is a vacuum secondary carburetor it could be the secondary's are not fully open. In a Holley the stock spring will not let the secondary's open up until you reach a really high rpm somthing around 8000 rpm.
Posted via Mobile Device

[tranz am]

Originally Posted by MAC71

If it does show vacuum wide open and it is a vacuum secondary carburetor it could be the secondary's are not fully open. In a Holley the stock spring will not let the secondary's open up until you reach a really high rpm somthing around 8000 rpm.
Posted via Mobile Device

Any restriction above the carburetor that limits the cfm of what a given engine wasnts will show an increase in vacuum under wide open throttle. You do not want Anything over 1.5 hg. If you see that your engine wants more flow. I run a 750 quick fuel on my .030 over 454 and it doesn't need anymore cfm and I rev it to 6400. Keep the carb as small as you can and you will benifit from increased fuel economy, throttle response, low speed torque, and prevent unwanted fuel pooling. You benifit from the high speed volocity of the incoming air that helps atomize the fuel beter.

[burnin oil]

Originally Posted by tranz am

Any restriction above the carburetor that limits the cfm of what a given engine wasnts will show an increase in vacuum under wide open throttle. You do not want Anything over 1.5 hg. If you see that your engine wants more flow. I run a 750 quick fuel on my .030 over 454 and it doesn't need anymore cfm and I rev it to 6400. Keep the carb as small as you can and you will benifit from increased fuel economy, throttle response, low speed torque, and prevent unwanted fuel pooling. You benifit from the high speed volocity of the incoming air that helps atomize the fuel beter.

This is the best description. My daily 400sb has a 830dp and now runs correctly. Fuel mileage is the same as the 750vs that had 5"s of vacuum at WOT. The 750 had the lightest spring also and then modded to a DP. The rules don't always apply but its best to error the small side for the street. A lot of the 400s came from the factory with a big 2 barrel carb. GM probably got away with that because it had the same power house of a cam as a 307 that did not rev to high.

[justin4260]

lots of great input. I'm getting out of this that 600 cfm is good for just daily driving just needs a good tune, and if I'm racing want more power than consider going bigger.

[67 cst swb]

Originally Posted by justin4260

lots of great input. I'm getting out of this that 600 cfm is good for just daily driving just needs a good tune, and if I'm racing want more power than consider going bigger.

yes, that is my thoughts on the subject.
But, don't forget about the other things mentioned in post #11

[justin4260]

ok so i found out that i have on the passenger side the heads that are prone to cracking and on the driver side i have a corvette head. so thats one of the reasons its running like crap

[frankslagoon]

Originally Posted by justin4260

ok so i found out that i have on the passenger side the heads that are prone to cracking and on the driver side i have a corvette head. so thats one of the reasons its running like crap

I read your other thread. if you got different size valves on each side you never get it to run right.

[gonnuts]

I have a 650 on mine, works great. When I first put it on it was doing the same thing, I finally got a good timing light & didn't have enough timing in it. Advanced timing & fine tuned carb a bit & bog is gone.

[JJLT1]

different size heads is not really a good idea,
but its not going to make your carb idle mixture screws inoperable..

[justin4260]

I'm going to be changing the heads to the same size eventually. is there anywhere I can rent a timing light from? like orielys or auto zone or something?

[JJLT1]

yes some parts stores rent tools..

[justin4260]

either the head cracked or i blew a head gasket...either way next thing I'm doing is new heads and gaskets. any suggestions for some nice but still cheap heads that will be a just for now until i can afford the $1500 heads? and best gaskets for the money