carb jetting

« **on:** April 13, 2008, 09:13:28 PM »

i have a stock 74 455 olds and i just changed the carb from a quadra junk 2 a holley 750. was wondering if there is a general starting point on jetting the carb. not real good at this so i thought i would ask dont want to take it to a shop.

any help would be great thanks

« **Reply #1 on:** April 13, 2008, 09:16:31 PM »

Quote from: bigmike on April 13, 2008, 09:13:28 PM

i have a stock 74 455 olds and i just changed the carb from a quadra junk 2 a holley 750. was wondering if there is a general starting point on jetting the carb. not real good at this so i thought i would ask dont want to take it to a shop. any help would be great thanks

A good start would be to back the idle air screws two turns out.

« **Reply #2 on:** April 13, 2008, 10:03:20 PM »

If the Holley is new run the jetting as it sits in the box and do a plug reading.

« **Reply #3 on:** April 13, 2008, 10:33:46 PM »

Quote from: bigmike on April 13, 2008, 09:13:28 PM

i have a stock 74 455 olds and i just changed the carb from a quadra junk 2 a holley 750. was wondering if there is a general starting point on jetting the carb. not real good at this so i thought i would ask dont want to take it to a shop. any help would be great thanks

What cam and duration? Intake manifold? 750 a vac or DP (what's the list number)?

« **Reply #4 on:** April 14, 2008, 11:50:26 AM »

the holley 750 came off my cousins chevelle with a 396 so it was jetted for that. the 455 is completely stock manifold and everything. im goin to double check to see what size jets are in it now so i can let u guyz know later. i took the boat out sat. n it ran rich n wouldnt start unless i gave it full throttle and would not idle. the carb had a rebuild done to it before i put in my boat so i think its a good carb.

« **Reply #5 on:** April 15, 2008, 08:00:00 AM »

Quote from: bigmike on April 14, 2008, 11:50:26 AM

the holley 750 came off my cousins chevelle with a 396 so it was jetted for that. the 455 is completely stock manifold and everything. im goin to double check to see what size jets are in it now so i can let u guyz know later. i took the boat out sat. n it ran rich n wouldnt start unless i gave it full throttle and would not idle. the carb had a rebuild done to it before i put in my boat so i think its a good carb.

That sounds like a blown power valve. (or they mis aligned the gasket so it leaks - it's easy to do)

Put a set of 65 power valves in it, if it is a 3310 carb (vac secondaries) start off with 71 or 72 primary jets, and if you are lucky and can change the secondaries, 76 or 77. Depending on your impellor, either a purple or plain secondary spring.

If it is a 4779 (DP) use 74-75 primaries instead.

I always found that on a properly tuned single carb app, the idle screws are best about a turn to 1.5 turns out.

« **Reply #6 on:** April 15, 2008, 04:34:21 PM »

it is a 3310 and it has new power vavles also. the primaries r 69 n the secondaries r 76. i'll change the jets n make sure the gasket is on correctly. its a start thanks.

« **Reply #7 on:** April 16, 2008, 08:19:43 PM »

will this carb work with a stock manifold

« **Reply #8 on:** April 16, 2008, 08:26:57 PM »

As long as the stock manifold physically accepts it, yes. If the manifold is set up for a quadrajet, then you will need an adapter to make it work.

« **Reply #9 on:** April 17, 2008, 08:15:56 PM »

i bought the spacer plate so it bolts up.

« **Reply #10 on:** April 23, 2008, 04:20:01 PM »

still having problems carb still not rite. any help out there?

« **Reply #11 on:** April 23, 2008, 05:40:17 PM »

Quote from: bigmike on April 23, 2008, 04:20:01 PM

still having problems carb still not rite. any help out there?

Meaning what? Did you check to see if the PV's are leaking? Pull the carb off of the manifold, set it back down on top of the carb studs so there is a gap. Take your coil wire and put it in the garage. crank the engine over so the carb is getting gas (if you have an electric pump, just turn it on instead) . Look under the carb - any gas dripping? There shouldn't be. Figure out where it is coming from and at least one of your problems will be solved.

« **Reply #12 on:** April 23, 2008, 09:48:06 PM »

Quote from: bigmike on April 23, 2008, 04:20:01 PM

still having problems carb still not rite. any help out there?

There is a lot of help around here but we need some information.

Did you run the boat this season before changing carbs?

What is the engine doing?

Why do you think it is the carb?

Have you checked Timing?

With the engine running, look down the carb, can you see fuel (is it "wet")?

I know it is a lot of questions, but they will help us help you.

« **Reply #13 on:** May 02, 2008, 06:57:22 PM »

well it turns out that the base plate on the carb was warped. put a spare 700 weber on it and it fired right up and seems to run good. it sux i wanted to use it but i'll have to wait to get a new base plate.

« **Reply #14 on:** May 02, 2008, 10:52:28 PM »

Quote from: bigmike on April 16, 2008, 08:19:43 PM

will this carb work with a stock manifold

you will need an adapter q jet to square bore. mr gasket sells one.pep boys ,auto zone, kragens.

be sure and spray out the air bleeds with carb or brake cleaner...before you start tunning.
if it has a secondary metering block you can change secondary jets..if not i *think* the spacer is drilled to 76 's and the primaries will be 73's ootb..3310-1 ?
jetting should be close out of the box..
fuel flow jetted for cfm..air bleeds calibrated for the fuel circuits.

if you can hang an egt in the exhaust.. it would be easier to tune..

auto meter sells one for around a hundred. auto zone pep kragen again.
900 deg at idle 1250 at wot will get you in the ball park.. then you can start drilling the air bleeds.
but for stock 455 should be good ootb..jmho...

bigmike

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