Need a Bigger Carb

goodnigh

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My mechanic took the car back to his place to install a new bumper

and grill. First time he has driven the car since delivering it to

me after installing the new engine, about 6 months ago.

He had the car up to about 100 mph and when he gave it more gas

the car would accelerate but not "launch" as it should. The engine

is a newly rebuilt 351C 4V quench with a 0.58 lift cam. A factory

cam for the Cleveland would be around 0.43 lift. Another issue is

the smell of gas when I "accelerate" in city traffic. Seems like the

engine is getting more gas than air when it needs it. The carb is a

Holley 670 with vacuum secondaries and all the adjustments are

properly set. We used the same carb for five years before the old

engine cracked a block and got a new one from Don of OMS, with a

cam. The timing is set at 14 and we get 14 in. vacuum at 750 RPM

idle.

mike

 

marks73

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Need a 780 carb set at about 12 degrees on initial timing. With the set up you have at .580 lift cam with closed chamber heads, The engine will really live between 4000-7000. Not so good for 25-45mph city driving though :s.

 

goodnigh

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Need a 780 carb set at about 12 degrees on initial timing. With the set up you have at .580 lift cam with closed chamber heads, The engine will really live between 4000-7000. Not so good for 25-45mph city driving though :s.
It's a toploader, from stoplight to freeway you only need one gear,

2nd. The other three gears are Ford options.

mike

 

MeZapU

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Any engine with vacuum secondaries isn't going to really "launch" at speed when you mat the throttle.

 
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Wolverine

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Born an I-6, spent the teenage, 20 and 30 years as a 302, but at 40 will reach full potential as a 351C.
What gear are you running?

What is the diameter of your rear tires?

What torqure converter do you have?

What gear was he in when and what was the rpm at 100 MPH?

Are you happy with your acceleration below 100 MPH?

How often are you going to accelerate at 100MPH?

What do you want your top end speed to be?

If you believe the carb is throttling to much gas change the secondary jets to something smaller.

 

hyena429

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My mechanic took the car back to his place to install a new bumper

and grill. First time he has driven the car since delivering it to

me after installing the new engine, about 6 months ago.

He had the car up to about 100 mph and when he gave it more gas

the car would accelerate but not "launch" as it should. The engine

is a newly rebuilt 351C 4V quench with a 0.58 lift cam. A factory

cam for the Cleveland would be around 0.43 lift. Another issue is

the smell of gas when I "accelerate" in city traffic. Seems like the

engine is getting more gas than air when it needs it. The carb is a

Holley 670 with vacuum secondaries and all the adjustments are

properly set. We used the same carb for five years before the old

engine cracked a block and got a new one from Don of OMS, with a

cam. The timing is set at 14 and we get 14 in. vacuum at 750 RPM

idle.

mike
Smell of gas...Could very well be because of too small of a carb...Some folks dont realize when you put a carb that is too small for a car..It delivers too much gas and not enough air...So it will flood out...Same if you put a carb that is too big for your car...You lean out...Alot think you flood out when you put a carb too big..No sir...Its the other way around...lol..Cam lets your heads take in more air..and if your carb too small...It cant take in enough air for what it needs and will gas out.....I got a 5.30 lift...And i need atleast a 750 holley to run right or bigger....factory carb was already a 716 cfm spread bore...I think your carb is a little small for that build....And if i get your numbers right there...a 5.80 lift? wow.....Thats getting close too the 500 hp mark man...My cousins trans am is 504 hp and needs a 850 cfm to run......a 600 lift she hitting up too 500 or 550ish...670cfm is waaaaay too small for her..lol..Btw what is the cams duration? 2.80ish? or over 300?.....Im sure most will recomend you step up too a bigger carb.

 
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marks73

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My Car
1973 Mach 1 Q code 4 speed
1973 Chevelle 454 SS
2005 Magnum RT
2008 Colorado
My mechanic took the car back to his place to install a new bumper

and grill. First time he has driven the car since delivering it to

me after installing the new engine, about 6 months ago.

He had the car up to about 100 mph and when he gave it more gas

the car would accelerate but not "launch" as it should. The engine

is a newly rebuilt 351C 4V quench with a 0.58 lift cam. A factory

cam for the Cleveland would be around 0.43 lift. Another issue is

the smell of gas when I "accelerate" in city traffic. Seems like the

engine is getting more gas than air when it needs it. The carb is a

Holley 670 with vacuum secondaries and all the adjustments are

properly set. We used the same carb for five years before the old

engine cracked a block and got a new one from Don of OMS, with a

cam. The timing is set at 14 and we get 14 in. vacuum at 750 RPM

idle.

mike
Smell of gas...Could very well be because of too small of a carb...Some folks dont realize when you put a carb that is too small for a car..It delivers too much gas and not enough air...So it will flood out...Same if you put a carb that is too big for your car...You lean out...Alot think you flood out when you put a carb too big..No sir...Its the other way around...lol..Cam lets your heads take in more air..and if your carb too small...It cant take in enough air for what it needs and will gas out.....I got a 5.30 lift...And i need atleast a 750 holley to run right or bigger....factory carb was already a 716 cfm spread bore...I think your carb is a little small for that build....And if i get your numbers right there...a 5.80 lift? wow.....Thats getting close too the 500 hp mark man...My cousins trans am is 504 hp and needs a 850 cfm to run......a 600 lift she hitting up too 500 or 550ish...670cfm is waaaaay too small for her..lol..Btw what is the cams duration? 2.80ish? or over 300?.....Im sure most will recomend you step up too a bigger carb.
Exactly! however if you get over the.280 duration you'll run into loss of vacuum issues for power brakes. but that can be resolved as well. That was why I only went 571/284 on my 454 with initial of 10 deg. Brakes still work fine.



What gear are you running?

What is the diameter of your rear tires?

What torqure converter do you have?

What gear was he in when and what was the rpm at 100 MPH?

Are you happy with your acceleration below 100 MPH?

How often are you going to accelerate at 100MPH?

What do you want your top end speed to be?

If you believe the carb is throttling to much gas change the secondary jets to something smaller.
::shrug:: That would only make it worse.

 
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guys72Mach1

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I am running a 750 with mechanical secondaries that is jetted way up and nozzled way up with the 50cc secondary acc pump.

Our engine specs are pretty close, and because I am jetted so far up, I probably could have ran an 800cfm with mechanical secondaries.

My car pulls hard at any gear, although because of the gears(2.75), is a little slow, just off the line. At 25-45, in first gear, if you romp on it, it launches hard and pulls all the way up through the gears to 120.

On the freeway at 80mph, I'm taching about 2800rpm. If I put my foot to the floor it takes off instantly and pulls hard up thru 120mph. Kinda scary actually how hard it pulls at that speed and make me want to get some higher gears before I get myself into trouble.

Anyway, sounds liek you need to dump that vac secondary and step up to a 750, maybe even 800 mechanical secondaries.

If you want to take a ride to Dublin, I'll take you for a little ride and you can decide for your self if like the response of the mechanical secondary. I work in San Leandro and driv ethe Mach pretty regularly, if you wanted to meet up there.

If you decide to go with the Holley, I'd be happy to give you some jet, nozzle and pump settings to get you real close. Or you could bring it over to me and I'll set it up for you?

 

hyena429

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Smell of gas...Could very well be because of too small of a carb...Some folks dont realize when you put a carb that is too small for a car..It delivers too much gas and not enough air...So it will flood out...Same if you put a carb that is too big for your car...You lean out...Alot think you flood out when you put a carb too big..No sir...Its the other way around...lol..Cam lets your heads take in more air..and if your carb too small...It cant take in enough air for what it needs and will gas out.....I got a 5.30 lift...And i need atleast a 750 holley to run right or bigger....factory carb was already a 716 cfm spread bore...I think your carb is a little small for that build....And if i get your numbers right there...a 5.80 lift? wow.....Thats getting close too the 500 hp mark man...My cousins trans am is 504 hp and needs a 850 cfm to run......a 600 lift she hitting up too 500 or 550ish...670cfm is waaaaay too small for her..lol..Btw what is the cams duration? 2.80ish? or over 300?.....Im sure most will recomend you step up too a bigger carb.

Exactly! however if you get over the.280 duration you'll run into loss of vacuum issues for power brakes. but that can be resolved as well. That was why I only went 571/284 on my 454 with initial of 10 deg. Brakes still work fine.

Its sure does...Thats why i went with a 280 duration my self...And good to hear you didnt need it with a bit more duration than im going....I didnt want to install a brake booster..lol....My cam is custom ground 5.30 lift with 280 duration...Im expecting 425hp i hope out of my 4v.

And ya...Good thing about having a cousin who is a engineer to explain carbs..lol...I use to think bigger carbs flooded a motor when i was a teen...Come to find out its the other way around..lol...A carb too small will flood a motor...Motor will get all the gas it needs from the little carb..Just not enough air...So its pulling and begging for air and just getting gas.....Same with putting a carb that is too big...Its begging and pulling for the right mix...And getting mostly air.



I am running a 750 with mechanical secondaries that is jetted way up and nozzled way up with the 50cc secondary acc pump.

Our engine specs are pretty close, and because I am jetted so far up, I probably could have ran an 800cfm with mechanical secondaries.

My car pulls hard at any gear, although because of the gears(2.75), is a little slow, just off the line. At 25-45, in first gear, if you romp on it, it launches hard and pulls all the way up through the gears to 120.

On the freeway at 80mph, I'm taching about 2800rpm. If I put my foot to the floor it takes off instantly and pulls hard up thru 120mph. Kinda scary actually how hard it pulls at that speed and make me want to get some higher gears before I get myself into trouble.

Anyway, sounds liek you need to dump that vac secondary and step up to a 750, maybe even 800 mechanical secondaries.

If you want to take a ride to Dublin, I'll take you for a little ride and you can decide for your self if like the response of the mechanical secondary. I work in San Leandro and driv ethe Mach pretty regularly, if you wanted to meet up there.

If you decide to go with the Holley, I'd be happy to give you some jet, nozzle and pump settings to get you real close. Or you could bring it over to me and I'll set it up for you?
{thumbs up} I agree

 
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will e

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If one carb is good, three must be better:

1024.jpg


But seriously. Generally a smaller vacuum secondary is the best choice for street use but your cam and setup might want a little more CFM.

 

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