Carb Size

Moderator: Moderators

Carb Size

Postby AusRs » Wed Jun 23, 2010 8:31 am

OK
My car is running great however the tuner has told me i need a bigger carby as mine through reading the plugs shows it is lean and he says to upsize the jets any more than they are has no benefits
at the moment i am running a 570 street avenger holley
he has suggested a 750 vac secondary holley
on the dyno with 570 it runs 230 hp he tells me that there is another 20 hp in the carby upgrade

anyone care to comment /give suggestions ?
Last edited by AusRs on Wed Jun 23, 2010 9:28 am, edited 1 time in total.
User avatar
AusRs
 
Posts: 2364
Joined: Mon Jun 26, 2006 5:20 am
Location: Wollongong NSW Australia

1979 Chevrolet Monza Spyder


Re: Card Size

Postby gerbsinmd » Wed Jun 23, 2010 9:18 am

I'm not sure the benefit out weighs the cost. If you were an all out drag racer, then yeah change the carb, but I would bet that your drive ability and throttle response is crisp and instant the way it is now. I think if you like the way the car drives right now and its not running way lean on the street, (check the plugs). I'd leave it alone. You've got 100 more hp than the car came with, and we all want more HP, but is it worth it to mess up the way it cruises 99% of the time for that 1% of the time you take it to the track?
1999 Saturn SL2
2017 Cruze Hatchback
2008 Town & Country - Family transport
2018 Cruze - 35mpg - avg, 49.7mpg - best so far.
2011 Impala - commuter
RIP-2002 Saturn SL2 Commuter car - burned a valve @ 234k miles

1977 Monza Mirage being Restified!!
User avatar
gerbsinmd
 
Posts: 2062
Joined: Mon Jun 26, 2006 2:48 pm
Location: Dunkirk, MD

1977 Chevrolet Monza Mirage

Re: Carb Size

Postby Roccosvega » Wed Jun 23, 2010 10:47 am

I put a 670 street avenger with vacuum secondary on mine but I have a 350 and don't want to over carb the motor.
That's why I like a vacuum secondary carb. The CFM is controlled by engine RPM, never to much.

Robert
User avatar
Roccosvega
 
Posts: 618
Joined: Tue Feb 03, 2009 9:03 pm
Location: Illinois

Re: Carb Size

Postby chevy art » Wed Jun 23, 2010 11:10 pm

hey aus any chance the plugs are too hot of a heat range art
chevy art
 
Posts: 1124
Joined: Fri Feb 26, 2010 11:33 pm

Re: Carb Size

Postby AusRs » Thu Jun 24, 2010 4:29 am

chevy art wrote:hey aus any chance the plugs are too hot of a heat range art


ummmmm did i mention that engine tuning is NOT what i am good at .........i asked the tuner to sort it out he told me that the car is running well BUT could use a bigger carby ........i wish i knew more but i dont :(
i know how to do a basic set up but anything more than that i leave to people that know what they are doing
on the highway now the car runs well ,at 60 mph i can plant my right foot drop the car back to 2nd and it pulls well (3.23 rear end) .........it is crisp on the accelerator peddle and feels nice to drive .........not very economical though with those gears on a highway ..........will just enjoy it for a while then decide what to do
User avatar
AusRs
 
Posts: 2364
Joined: Mon Jun 26, 2006 5:20 am
Location: Wollongong NSW Australia

1979 Chevrolet Monza Spyder

Re: Carb Size

Postby iowagold » Wed Jul 21, 2010 7:53 am

I would not change much untill you check the air fuel ratio!!

I installed a aem guage on the IECO setup and I am glad to have it!!
cost was 200 bucks USD but worth every penny!!
I am running dual 45 dcoe's (1000+ cfm by the wierd weber math)
And I am still working on the perfect jetting... I am getting close!!
nice snap and perfect on the Wide open throttle.

if you do not want to spend the bucks on the guage..

The next time you have the car on the dyno make sure the hook up the afr guage...
then you can look at the afr at partial and full load...

there are so many things that make rich or lean!!

Fuel
spark
exhaust
temp and humidity

just to mention a few things...

A good afr guage will tell you if you are close to melt down!!

Paul in Iowa
See my vega project at:
http://www.poustusa.com
User avatar
iowagold
 
Posts: 37
Joined: Fri Sep 18, 2009 9:16 pm
Location: OTTUMWA, IOWA

1972 Chevrolet Vega GT

Re: Carb Size

Postby starfire383 » Sun Jul 25, 2010 9:54 am

If you want ultimate throttle response and midrange torque, and don't mind losing 2mpg and gassy idle, go for a 650 double pumper.

When it comes to vacuum secondary holleys, I'd choose an edelbrock carb every time. :th:

I recall that engine is a mild 350???? GM woulda put a 780 vacuum Qjet on it. Rule of thumb, subtract 100 cfm when using a true mechanical secondary 4-squirter carb. I'd totally believe your tuner about leaving power on the table with that 570 carb.

good luck
User avatar
starfire383
 
Posts: 952
Joined: Tue Jul 04, 2006 4:06 pm

Re: Carb Size

Postby Astre-mutt » Sat Aug 07, 2010 5:22 pm

The carb you have now should be good for your engine. This carb is rated up to 300hp out of the box. Not sure what your tuner means by "upping the jets from where they are now, won't have any improvements". Getting the car off lean condition will be an improvement. There may be a horse power increase with a bigger carb at the top end, but only by sacrificing the low end performance.

This is part of a quot from one of the websites I use for Holley tuning.

"The factory determines jet sizes using several factors on each carburetor, including the air bleed size, the venturi size, booster venturi size and shape, and the power valve channel restrictions. So if you find that your car will not run properly with a jet that is within 5 sizes one way or the other of the factory recommendation, you have problems somewhere else, like a bad or misadjusted float, bad needle and seat, a vacuum leak, a dirty carburetor, or a plain old worn-out carburetor. "

I would stick with the carb you have now. I would check that there is no vacuum leaks, and that the float level & accelerator pump are adjusted properly. If those are good, and your running the stock jets in the car. Then I would increase the primary jets up by two sizes bigger, and see if that takes care of the lean condition. If it doesn't I would look other places for the problem.
Scott
76 Astre panel express
User avatar
Astre-mutt
 
Posts: 1932
Joined: Tue Jan 08, 2008 6:19 pm
Location: Plainwell, Mich.

1976 Pontiac Astre Safari Wagon

Re: Carb Size

Postby Sirshredalot » Sat Aug 14, 2010 2:47 am

Im a firm believer that an engine cant really be "over-carbed" just "under-tuned"

A 750cfm vacuum secondary is rated at 750cfm of air not fuel...a carb will only flow what is demanded of it.
There are lots of daily driver 850cfm carbs running around(quadrajet). GM used them for YEARS and YEARS.

A smaller carb might seem "snappier" but is just going to be your bottleneck. And fuel economy is not to be a concern either.
Matter of fact, many have seen mileage improvements by switching to a larger carburetor.

I would however say that a 350ci engine is more than capable of using much much more than a 570cfm carb, especially in a light car...even more so with a manual trans.

God bless
-Shred
User avatar
Sirshredalot
 
Posts: 1384
Joined: Wed Aug 23, 2006 3:12 pm
Location: Muncie, Indiana

1980 Chevrolet Monza Spyder


Return to Carb/Intake/EFI Induction Tech

Who is online

Users browsing this forum: No registered users and 4 guests

cron