How do you know when to go up a carb size?

[CobraSix]

With all the debates about carb size, especially on teh Oz250 heads, it got me wondering. How do you know when your carb is too small as you make some more modifications?

Slade

 

[addo]

Watch the dyno curve. That's assuming your carb and ignition are dialled in.

Also, bench-testing the (your own) head for flow numbers will give you induction CFM out of an engine calculating program like EA or DD2K.

 

[MustangSix]

If you can pull 2-3" of vacuum at WOT, usually your carb is too small.

However, be careful that excessive vacuum is not caused by a restrictive manifold and ports.

 

[addo]

So true! I imagine you could get some freaky combinations of turbulence that gave a phantom "shelf" on your power curve.

 

[Anonymous]

Mine pulls 0" of vacuum at WOT...is that a good thing??

Thanks,

Doug

 

[addo]

Means it's not undercarbed!

 

[Anonymous]

I was at 3-5" WOT with the 61 jets, dropped to 2-3" with the 62's, this at high rpm. It will now drop to 0' when floored from low rpm's, wouldn't do that before.

The Bosh platinums look good at 600+ miles, still a tiny bit leaner on the front 3 cylinders.

Wondering now if I need the 2300 500 cfm. The cam is stock, the head has Mike's SI valves and 1.6 roller rockers, and of course the Pacemaker dual exhaust. Compression is 9.4.1, DS2 ignition.

It's a 200 mile trip for the dyno, I don't want to do it twice due to having the wrong carb.

 

[Mustangaroo]

cfmustang....."still a tiny bit leaner on the front 3 cylinders"

I'd fatten up the jets to about 65-66 to start and see if your lean condition in the front three cyls clears up, I was running 68 jets in my Aussie 2V before I switched to the 4V intake and carb. I had it dynoed with the Holley 500 and it was running 12:1 A/R with 68 jets, I think you are running too small of jets and that's why it's lean in the front three cyl's. You'll also get greater performance at or near 12:1 A/R instead of 15-16:1 A/R and you won't burn any valves

 

[wsa111]

Don, you stated the front 3 cylinders run lean. first do you have the pcv valve at the front of the intake manifold? if you do move the vacuum to the pcv to the fitting on the carburetor & move the pcv valve to the valve cover.
Second, i find it rare to richen up a holley carb. Do you have 7# of fuel pressure to the carb, i bet you don't.
third once you have the correct fuel pressure to the carburetor you should be able to run 59 jets, an x factor is to put 1 size richer jet in the front jet to richen up the front cylinders.
Fourth, make sure your float level is set properly???
The next choice is to put a 500 cfm on and give it a try. Don't be afraid to de-jet a holley carb, they are always rich unless you run an engine with over 400 cubic inches. Let us know your findings or results. William

 

[Anonymous]

alrighty, i have a question that involves just a stock 250...and a carb off a 200. i have no idea what the carb is, but the floats look like there nitrophyl or similar and they say holley...did they come with holleys? anyways, I just finished boring it out, so now its a good 2-1/2" diameter or so, way bigger than stock, i cant bore out my YFA that much...but i had to fill in some areas with JB weld, but i never hit any vitals so I'm good, i have to figgure out jetting still...but with a 1bbl carb, is it possible to get good low and top end power or will i lose all my low end with this? is this too much airflow for the carb/engine?

 

[Anonymous]

Don, you have the Oz head right? It´s lean in the front because of the thottle plate config. The Aussie intake manifold was not designed to use a carburetor with throttle blades perpendicular to the crank. As they open, they will favor the front or back cylinders. The only good solution IMO would be to use a bigger Weber style carb.

 

[Anonymous]

Don, you have the Oz head right? It´s lean in the front because of the thottle plate config. The Aussie intake manifold was not designed to use a carburetor with throttle blades perpendicular to the crank. As they open, they will favor the front or back cylinders. The only good solution IMO would be to use a bigger Weber style carb.

I agree but as usual am puzzled since I understood the original Stromberg used the same orientation. Maybe I confused progressive and synchronous.

Thank you for pointing out what should have been an obvious solution as I did have the H/W 5200 mounted at one point. This solution does present a problem as I have been contemplating moving to a TBI/Megasquirt and the Weber's are pricey. Does anyone know the orientation of the Ford TBI unit's that fit the 2300 platform?

 

[Anonymous]

P warrior, I was thinking about the same as you were, I have a 250, mild porting on exhaust and a carb that came off a '77 fairmont, carter carb, it idles well starts good but I haven't been able to take it on the road yet to really test it, so far it seems to be good enough for it. It was a great deal for no cash outlay.

 

[Anonymous]

Ii finished boring it out and need to figure out a good size for the main jet hole...its pretty pathetic cause if u co too big theres no going back (without some unique sort of jetting, since the jet is just a hole...pretty pathetic so i think i'm gonna get some different size drill bits and start small and work my way up....my bored out YFA is on the car now...it was perfect when it was running rich, i put in a slightly larger than stock size jet in today, since i need to save gas for awhile, but i cant go WOT or it will stumble and shake without acceleration, which is fine for now.

 

[mustang6]

placid warrior, when you bore these carbs out, how do you increase the size of the throttle plate butterfly to cover the bigger throat? If you don't bore them out where the throttle plate butterfly is, then how are you really increasing airflow at all?

 

[Anonymous]

because in the centerish part of the carb is the venturi, it gets narrower than the rest of the throat of the carb....take a carb and look down it, its not one solid diameter the whole way through...i dont have a picture so thats the best i can explain it, but increasing the sizze of the venturi greatly increases airflow, but i am not very knowlegable on why parts of the carb are built the way they are, so the venturi may be there to create the power curve, rather than having only high or low end power from it, but i'm not sure, untill i try out the carb that i just bored out (which will most likely need to run very rich for low and high end power...since thats the way my previous bored out carb seems to work) i wont have any more first hand knowlege, even then i may not learn much..I wont be working on the carb again till next weekend or so, but i will be sure to post results, i may have screwed something upm but maybe not...we'll see...but the throttle plate and butterfly valve stay the same size.

 

[xctasy]

Rule 1 of carby selection

Smallest carb venturi area for resonable performance:-
Cubic Inch displacement divided by 115

Best carb for performance and economy compromise:-
Cubic Inch displacement divided by 70


Best carb for maximum performance:-
Cubic Inch displacement divided by 34 to 57. Use the smaller number if engine is able to rev above 6000 rpm, use larger if engine is not likely to go above 6000 rpm

If you have a 250 cube engine, you need 2.17 sq inches of venturi area. One 1.675 " venturi is fine.


Rule 2

In order for gas to be pulled into the engine, a pressure drop from the venturi to the throttle bore must be present. That is, the venturi must be a size smaller than the throttle blade. A factor of 1.35 creates a great signal, while on some carbs, like race spec 32/36's, can go down to a minimum of 1.06 on the primary side. Ideal difference is about 1.16 to 1.10.


For the best performance, a 1.84 inch throttle blade is ideal.

In practice, a 1.75" hole in a log head is ample. If the existing carb has a 1.75" throttle, you can ream away to about 1.59" without ruining the carb. Past 1.65", your carb will stop metering fuel.

When you take the swanky hips of a carb, it ruins the gas/air metering, and drilling jets and fiddling with idle circuits always results in poorer results unless you have a colortune, a gas analyser or wide angle oxygen sensor to read the mixtures.

Best of Colonial Fortune to ya, old stick!