Hot Rod Forum banner

Choosing the right carb

2K views 25 replies 9 participants last post by  C10-400sbc 
#1 ·
I am building a 1985 chevy C10 with a swapped 400sbc. The engine has a comp cams 292h cam but other than that there is only slight modifications. I currently have an older Holley 4 barrel but it tends to hiccup pretty bad on acceleration so I'm looking to upgrade. I am open to pretty much any brand, it is my daily driver/ occasional street racer. Any suggestions as to what carb I should get? Cfm? I was leaning towards an edelbrock performer 700cfm but have heard that hollers are a better option. Thanks
 
#2 ·
A rebuilt 1967-1971 Rochester Q-jet from AwfulZone is a good choice for a part timer. Of course you will need a 1967-1971 Q-jet intake manifold, divorced choke kit or a electric choke conversion kit. Use a stock 1967-1971 Q-jet cast iron intake manifold.

That carburetor has air valve controlled secondary throttle bores that will make the carburetor up to 750 CFM on demand.
 
#4 ·
QuickFuel Slayer 750cfm vacuum secondary.

What's the List # from the front of the choke tower on your current Holley??
If it turns out to be big enough it may just need a rebuild kit and some tuning to make a great carb.

Hiccup at the start of sudden acceleration is usually an accelerator pump problem, could just need fresh gaskets and diaphragm or could be wrong pump cam or pump nozzle size for your engine combination.
 
#13 ·
QuickFuel Slayer 750cfm vacuum secondary.
Great Carb for a Great Price :thumbup:
If your buying a new carb C10 you can not go wrong with that Slayer. Almost 1/2 the price of a Holley, looks better, and way more user friendly. Last one I bolted on (beefed up 351c Mach 1). I took it out of the box, bolted it down, adjusted NOTHING, and it was absolutely perfect. The car had way more power, was way more responsive, and it stayed perfect from idle to WOT until I sold the car....

PS
Your current carb is running way to rich


The Modified 850 i took off my 351c (was also running crazy rich)


The Slayer 750 I put on
 
#7 ·
Looks like it could be a 750 on there now that just needs a cleaning and re-gasketing, and then proper tuning to match the engine.

Impossible to tell for sure the size from just that picture.
WHAT IS THE NUMBER ON THE DRIVER'S SIDE FRONT WALL OF THE CHOKE TOWER??
That way we can figure out exactly what it is.
 
#11 ·
That looks like a date code you posted and there is a number that should be something like 1850- and the number after the dash should be something like a 1 or 2 etc. An 1850 and 80457 are 600cfm carbs and a 3310 or 80508 are 750cfm carbs and 80670 is a holley 670 street avenger and a 80770 is a 770 street avenger. I can't tell cause the site is not showing the picture right and your current holley can be made to run your setup just fine as I have had that cam before among others and have no problems tuning my holley carbs to run well but it may take more then just changing jets and doing your timing etc as other factors will come into effect such as your ignition system and what tranny your using along with the rear gear ratio.

First thing is to get your carb settings back to stock from out of box and get it running good enough to idle and get your timing set correctly before doing any carb tuning and after your timing is verified and your ignition has been checked and is in good order then you can then go to proceed to tune and adjust the carb and then you will have to go from there but its more then just turning some screws and changing out a few jets as you need to know how a carb operates and how to tune each circuit at a time and how to do advance tuning such as changing the idle fuel curve calibration cause with big cams like that with the big amount of overlap it will take a lot of timing to get the carb and engine to run right and also most likely a richer idle fuel curve depending on what exactly your carb is.

Out of the box my holley 650cfm double pumper was to lean on the idle circuit for my 350 with a lunati voodoo hydraulic roller cam 280/292 231/239 535/550 and my jetting was excellent but I had to change the idle fuel curve which is done by changing out the idle feed restrictors but I must warn don't do anything with that type of stuff till you know the basics of carb tuning and are really educated on how each circuit must transfer to the next smoothly without having any bogs or pauses or backfires etc.

Below is a link that is a start to give some basics but check out amazon and there are some good books on there with some great info on how to advance tune a holley carb and feel free to ask questions as many folks on here are really great on giving some great advice and have helped me grow in knowledge over the years and I can now pretty much tune with out having to go back to my books and I know what to look for and trouble shoot without having to take forever like I used too.
 
#14 ·
I think I'm probably gonna go with the fuel slayer. I'm not a huge fan of holleys anyway. Do you guys have any suggestions on cfm wise? The cfm calculator says I should be running about 650 with my 400 cu but that doesn't factor in the heavy hitting 292h comp cam. I'm thinking probably about 750. Thanks for your help, I really appreciate everyone that took the time to help me out. And sorry for the late response, I've been out in a cabin with no internet
 
#23 ·
Engines with less than 10:1 compression ratio can run on 93 octane pump gas but need 13 to 16 degrees initial timing advance. If your engine has higher than 10:1 compression ratio, you can run the engine on 10 to 12 degrees initial advance. The factory engineers recommended a conservative 6 to 10 degrees initial advance in the shop manual for high compression pre 1970 engines because they did not know what octane gasoline the new car owners were going to use. The factory engineers played it safe in order to prevent warranty problems caused by burned Pistons from car owners using cheap 87 octane gasoline.

In 1965, my dad borrowed my 1961 Chevrolet Impala 348 for a 60 mile trip and burned a piston when used cheap low octane gasoline.
 
#25 · (Edited)
That is a good policy, whether your engine needs 93 octane or not.

My 1962 Chevrolet Bel Air 327/300 +.030" OS has 9.6:1 actual calculated static compression ratio and can run on 87 octane pump gas. In 1962, the Chevrolet "front office" rounded off the actual compression ratio to 10:1 and the owners manuals suggested premium gasoline, which at that time was 103 octane at some of the major stations, such as Gulf Crest at the purple pump. That was part of the GM warranty "cover their ***" policy.

According to the previous owner of my '62 Chevrolet, he used .018" crushed shim head gaskets when the motor was blueprinted in 1986. Those gaskets bumped the static compression ratio up from the actual 9.4:1 to 9.6:1 . It is equipped with a Chevrolet L79 camshaft which needs at least 10.5:1 compression ratio and 3.70:1 gears for best performance. I am pleased with the performance with the 3.36:1 Positraction and a 1967 Rochester Q-jet.

Today, I use 93 octane EXXON/Mobil gasoline with the initial timing advance set at 12 degrees as indicated on a 8" harmonic damper.
 
#26 ·
I don't know a whole lot about all the compression ratios and timing stuff but I know how to install pretty much anything you give me because my family owns a mechanic shop and I have always been working on them down there and I go to school for mechanics. Right now I'm only 16 and all my paychecks go to my truck, this was my dads old truck and I'm trying to honor him by doing what he always wanted to do and build this thing. I'm trying to get into the rodding world because I love everything about it and this truck is like a member of the family to me. I really do appreciate taking the time to help me out and doing the things that you do, because getting really into this stuff can be pretty intimidating and you guys really do help a lot. I can't thank you enough.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top