Help 351C guys - Carb

[Wyflyer]

Correct. I should of stated that I thought the 750 was too much carb. I do think it is too much. However I like double pumpers and I would start with a 650. Now as far as that rocket science stuff goes? Dont you have to figure runner size and flow capacities before you can come up with a total possible cfm? Explain in a little more detail please Nails.

Nails, thanks for posting that. I was sorta trying to say that in this thread http://classicbroncos.com/forums/showpost.php?p=1171873&postcount=13 but I didnt' recall the exact formula so I blew it off.
Another thing that I don't hear much about is read the spark plugs for color to determine lean/rich mixture.

 

[Johnny]

I just think there is a little bit more to your equation.

There can be, but your trying to make it more complicated than it needs to be...


Oh... and my name is Johnny..... Nails is the name of my Bronco.... that I stoled off the Island of misfits...

 

[bax]

So Johnny, what you are saying is the formula used is just kind of a to get you close. Tune from there if you will. I was really hoping for an argument.

 

[Johnny]

Yeah, thats pretty much what I'm saying... and I'm to tired to argue Bax.....

 

[bax]

No your not!

 

[Johnny]

Yes I am...

 

[bax]

What are you doing, trying to start an argument!!!

 

[Wyflyer]

You keep it up, just keep it up and he'll sic his lab on you..

 

[bax]

You keep it up, just keep it up and he'll sic his lab on you..

I posted pictures of my lab with stuff on it's head in off topic. He's a cool dog
http://classicbroncos.com/forums/showthread.php?t=127031

 

[feitctaj]

this thread had such potential as a very informative subject, instead it has become a dribbling contest

 

[bax]

this thread had such potential as a very informative subject, instead it has become a dribbling contest

Sorry Man, I was trying to get the info out of him but he wouldn't play.

 

[feitctaj]

thats OK, we can corner him later

 

[bmc69]

this thread had such potential as a very informative subject, instead it has become a dribbling contest

LOL...never saw a thread with Bax or Johnny in it that didn't sorta meander off somewhere in the ether. Now I, on the other hand, try to keep things strictly on topic and avoid being silly...whenever I can.;D

 

[feitctaj]

LOL...never saw a thread with Bax or Johnny in it that didn't sorta meander off somewhere in the ether. Now I, on the other hand, try to keep things strictly on topic and avoid being silly...whenever I can.;D

would you like to see a fight between me and Bax? That is, You are the filler between us LOL?

 

[Johnny]

this thread had such potential as a very informative subject, instead it has become a dribbling contest

Bax did it...

 

[Madgyver]

A single Holley 750 DP is not too much carb for a free flowing 351C4V (single box plenum intake and SuperComp headers) You could run dual 660 center squirters on it...

You want better gas mileage but you'll sacrifice performance,
Try the Holley 1850 carb rated at 600cfm. I ran it on an 82 bronco with a 351C swap. I did it for a customer and he was very happy with the combo....

I forgot what jets I ran but it did a good baseline.. And after doing your basic sparkplug read tuning, I got it to where the motor was happy throughout its RPM range..

 

[broncnaz]

I agree with Nails. And with his formula. while there may be a formula for volumetric efficency for given port size CID, ect. basically hipo engines such as a 4V 351C will only be at 80% max and more than likely even lower especailly at low RPM. As everyone knows the 351C is not a real good low rpm engine.
a 600 cfm vac secondary carb will be much easier to tune than a 750 and produce better vacuum signals to boot. But a 670 TA might be a very good choice. While I realize a 4V 351 can use more CFM in reality I dont think Kirk is spinning his above 6K much.
I'm sure the Cleveland guys will correct me if I wrong but if Kirk has 4V heads shouldnt there be a #4 cast into the corners of the heads and should be visible? No 4 should be standard 2V heads.

 

[bax]

I agree with Nails. And with his formula. while there may be a formula for volumetric efficency for given port size CID, ect. basically hipo engines such as a 4V 351C will only be at 80% max and more than likely even lower especailly at low RPM. As everyone knows the 351C is not a real good low rpm engine.
a 600 cfm vac secondary carb will be much easier to tune than a 750 and produce better vacuum signals to boot. But a 670 TA might be a very good choice. While I realize a 4V 351 can use more CFM in reality I dont think Kirk is spinning his above 6K much.
I'm sure the Cleveland guys will correct me if I wrong but if Kirk has 4V heads shouldnt there be a #4 cast into the corners of the heads and should be visible? No 4 should be standard 2V heads.

Correct, 4v should be visible in the top corner of the head. However Kirk said he had 4v heads. I would think he knows what he has. My point with the Aussie heads recommendation was to give him option that will give better performance to a motor that is turning mostly all of its rpm's under 5k. True I am not really helping kirk solve his immediate problem. How can I tune my carb and make my truck run better? I do think kirk has too much carb. I do think extending the bowl vents would help with his hill climbs. I do think the backfiring issue is a lean situation. He may have a float set too low or a regulator set to low or a fuel delivery problem. he tells us he has tan colored spark plugs not black wet ones. Even with all that cfm carb it is still possible to close the fuel flow from it to a to lean situation. I feel that if he spends time tuning that carb He will have good results. if kirk has another smaller carb laying around the house he could give it a try or maybe one of his friends has one he can try. But I would not recommend buying another carb just hopping that would help. Advancing the camshaft a few degrees may help with low rpm vacuum but that is getting into the motor to do that. Checking out the fuel delivery system and adjusting the carb would be my first plan of attack. Aussie heads would be on my save up list.

 

[CDHUNTER]

What Bax says makes sense.... But in my opinion... your running to much carb... best way to figure what size carb to go with, is to find the volumetric efficiency of said engine...

To do this, you take the engine size (cubic inches) times maximum rpm, divided by 3456 equals cfm...

example... 351ci x 6000rpm = 210,600 divided by 3456 = 609cfm

Since NO engine achieves 100 percent volumetric efficiency... a 600 cfm carburetor should be the correct size for this engine...

I've used this equation on every engine I've ever built.... and it hasn't let me down yet...... yeah saying "I'm running a 600cfm" doesn't sound as cool as "I'm running a 750 duel line, double pumper" but the engine will preform better when it's not drowning in a sea of gas....

Here is the explanation on the equation. The cubic inches is for how much airfuel mixture the engine can hold. Multiply it times the max rpm and it'll give you the amount of mixture used per minute but since it is a four stroke engine you only fill it up every other revolution so you divide by two. This will tell you the max amount of air the engine can use but it is in cubic inches per minute instead of cubic feet per minute. Dividing by 1728 converts it to cubic feet per minute. For simplicity everyone combines dividing the 1728 and the 2 into 3456.

By the way some very good engines can exceed 100% slightly due to exhaust creating a vacuum and pulling in excess air. Boosted vehicles also don't follow this formula.

 

[GrillMaster]

I'm back!

Well I took the small filter off and ran it with out it... made a big difference! I ordered an off set base for a larger 14x3" air filter that should allow it to breathe better. Hope it all fits under my hood. I did get a backfire that sounded like it came from the carb (burped?). Not sure why it did this but the previous owner said it had happened before and shot fire out of the carb. Would like to avoid this is possible

How do you extend the bowl vents? Is that the copper tube mod?

I was told they were the 4V heads but will double check tonight