Too much carb???

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Too much carb???

Post  GT300TD on September 19th 2012, 1:33 pm

My '64 Galaxie 460/c6 has about 250 miles on it now, about 1/2 of them road miles and a lot of 5 to 10 minute run time in the garage. Checked the plugs and they are sooty black. According to a chart the heat range for the plugs (Autolite 45 gapped at .036) was too cold.
Went to O'Reillys and talked to the asst mgr (he has a '69 427 Camaro w/750 Holley). He said my carb may be too big, I wanted to see what you guys think.
466 cu in, 11 to 1, Crower 22207 cam (.540/.541 lift, 244/251 @ .050 duration), D0VE-C's 2.25 int 1.72 ex, intake bowls blended to match Weiand Stealth 8021 CJ intake, exhaust bowls thermactor removed, 2" Crites headers. Holley 850 cfm (0-80531 w/78 and 82 jet ) vac secondaries/vac advance (pluged, am running old Mallory Unilite cent adv distributor. 1" 4 hole tapered spacer. Is my carb too big, should I go with a 750 cfm or should I rejet the primaries to 70 or 72, get some Autolite 46's and try it? Any suggestions appreciated.
Thanks. Smile





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Re: Too much carb???

Post  kim on September 19th 2012, 1:49 pm

If anything the carb is a little small, Ideally about 970 cfm for a 466 and a 6000 RPM red line.

With the carb being smaller, its pulling a shit load of fuel because of very high manifold vacume. Simply need to tune the thing.

Adjust the idle mixture screws with a vacume gauge and tach, getting the idle set for maximum vacume reading, with the tach reading the speed you want.

Be sure the floats are set right, so it doesnt dump fuel over the vent lines, and or feed excess fuel.

Be sure the power valve isn't blown out, and the thing is bypassing jet area even at idle. Power valve only opens when manifold vacum drops below 6.5" for most stock valves. Some of us have changed the valve to compensate combinations for big cams etc... but out the box they all came with 6.5 power valves.


Lastly, avoid most advice from anyone that drives a chevy. They can fudge up a ford in a heart beat.

Once you have the idle set, then you can start playing with things like the time when the secondaries open, as its a vacume secondary carb, its almost impossible to go too big, the secondaries wont open until the engine can actually use the air flow. Holley does offur a spring kit to allow that opening at differnt levels of manifold vacume, but out the box should be damn close for that carb on that engine, again, a real high manifold vacume...

I would definatly jet down the primaries and the secondaries with that small a carb on that engine. With 78s in the primaries now, I think your guess of 72's is a pretty good start for a check. Best investment is a holley jet kit if your going to race/tune this thing. One size does not fit all, the closer you get to perfect, the more you have to chase the weather. I would not be surprised at all if you don't end up down at some 68's or so primary and some 74s on the secondaries for that carb on that engine.

Start with one thing at a time. Ensure floats and throttle blades are set right. Secondaries should be open just a crack to allow idle air. That will prevent the primaries from being twisted well up through the transition slot to get it to idle.

Before you go adjusting plugs, tune the carb. Cant tell what heat range an engine wants till your air/fuel is right. Heat range for the plug is simply an effort to maximise self cleaning, and minimise plug wear and in extreme cases a preignition/detonation source. Heat range has nothing to do with spark energy.

Once the fuel load is cleaned up, the proper plug should be able to sustain WOT and not have the ground strap change color past the elbow of the ground strap.

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Re: Too much carb???

Post  4604X4 on September 19th 2012, 5:37 pm

Make sure you didin't blow out the Power Valve. Older Holleys did not have the "blow out prevention". A backfire could have blew the PV then you'd be running very rich.
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Re: Too much carb???

Post  GT300TD on September 19th 2012, 5:54 pm

The carburetor is only about 1 year old, no major backfires. Guess it's worth checking though, thanks.

Good info kim, thanks.






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Re: Too much carb???

Post  IDT-572 on September 19th 2012, 5:57 pm

You can flood and foul plugs on a 477 with a 600 Holley and you can lean and burn a piston with a 1250.

It's all about the tune and air fuel ratio.

Kim is correct. On a mild 10:1 472 hydraulic cam, I have ran faster as carb size went up.

One of the best running engines I had was running a stealth dual plane 8012 two inch spacer, and a 8896 1050 Domi. Great street manners too boot.

It will run great with the 850 just get to tuning it.
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Re: Too much carb???

Post  richter69 on September 19th 2012, 6:42 pm

we run a 950 on the 306........ lol yep just gotta bust out the screwdrivers.
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Re: Too much carb???

Post  GT300TD on September 19th 2012, 6:56 pm

IDT-572 wrote:You can flood and foul plugs on a 477 with a 600 Holley and you can lean and burn a piston with a 1250.

It's all about the tune and air fuel ratio.

Kim is correct. On a mild 10:1 472 hydraulic cam, I have ran faster as carb size went up.

One of the best running engines I had was running a stealth dual plane 8012 two inch spacer, and a 8896 1050 Domi. Great street manners too boot.

It will run great with the 850 just get to tuning it.

Thanks Blake, I'll get some new plugs and break out the box of jets...and the screwdrivers, richter scratch Smile




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Re: Too much carb???

Post  rmcomprandy on September 20th 2012, 12:32 am

The air flow size at Wide Open Throttle of a carburetor has little to do with how it is tuned for the rich/lean conditions present during part throttle running.

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Re: Too much carb???

Post  ifucan on September 20th 2012, 12:45 am

similar combo here, but w/a 950...78/86 produced a 10.8 afr...I too, will be taking out the screwdriver

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Re: Too much carb???

Post  rmcomprandy on September 20th 2012, 10:27 am

ifucan wrote:similar combo here, but w/a 950...78/86 produced a 10.8 afr...I too, will be taking out the screwdriver

YEP ... 10.8 is almost 3 ratios to rich for part throttle cruising.

However, A real racing engine makes a bit more power with a rich part throttle circuit when going to WOT.

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Re: Too much carb???

Post  ifucan on September 20th 2012, 11:01 am

rmcomprandy wrote:
ifucan wrote:similar combo here, but w/a 950...78/86 produced a 10.8 afr...I too, will be taking out the screwdriver

YEP ... 10.8 is almost 3 ratios to rich for part throttle cruising.

However, A real racing engine makes a bit more power with a rich part throttle circuit when going to WOT.

Randy, those numbers were at WOT from 5k to 6500...My plan is to go to the track and log mph while cutting jet, think there is power left on the table w/the afr that rich? I know they say high 12's low 13's but I also know that everything is just a reference point and every motor likes its own 'sweet spot', my plan is to try to leave t he primary alone because while I know it is rich cruising, it has excellent throttle response and idle characteristics, so I'm gonna focus on cutting secondary jet, I new to carbs but think I will start a trial and error by cutting 4 sizes in the back, any opinions?...thanks guys

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Re: Too much carb???

Post  kim on September 20th 2012, 11:49 am

throttle response, is adjusted by tunning the pump cams, the squirter shot size and the pump shot duration, leaving the thing pig rich to avoid tunning the accelerator pump circuit(s) is a waist of fuel and performance.

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Re: Too much carb???

Post  ifucan on September 20th 2012, 1:55 pm

I had the carb done by a reputable performance carb shop locally and they were supposed to get the fuel curve close Rolling Eyes , while throttle response and driveability increased they left the same jets in it I had before Mad Kim, if that is the case, then I shouldn't worry about changing jets in the primary either right?

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Re: Too much carb???

Post  kim on September 20th 2012, 2:10 pm

Its running rich. You need to change the jets, what requires tunning to tweak depends on where its running rich.

As I said, the first things that have to be done, as the idle circuite affects ALL circuits is set the floats, ensure the power valve is in tact, and then set the idle mixture/rpm properly. From there the next steps are taken.

Once the idle is done, typically we tune for total fuel surface cross section, or WOT, wich is a combination of the jets, and the power valve, as well as all fuels deliverd via idle and intermeadiate circuit.

When WOT fuel delivery is correct, then there are areas to tweek the cruise fuel and of course the throttle responsiveness.

When the carb is "sized" for the engine, throttle response is usually real easy as the system is under so much vacume little accelerator pump tuning is required, as we move into free-er air flowing carbs, BIG carbs for smaller displacement engines, then the acclerator pump is more critical and more difficult to tune. IT takes a little longer for the fuel to flow through the booster, so we have to mechanically add it till its there.

For main jet tunning to expidite the process we usually move in steps of 3 for primary and secondary size jets, ie from a 78 to a 75 and such.

Your tunning WOT< so you tune both sides at the same time, the only reason there is a jet split is because of the power valve and the progressive linkage of the carbs opening the throttle blades. The primary are used for the first 25% of engine operation solely on most cars, so to make it easier to tune the intermeadiate circuite (light load cruise) the power valve is equivilant of about 6 jet sizes surface area. You get by running the smaller surface area on the primaies, and thus less fuel under low load. That way there is less tunning with the air bleeds and the car still runs decent at part throttle.

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Re: Too much carb???

Post  4604X4 on September 20th 2012, 5:24 pm

kim wrote:Its running rich. You need to change the jets, what requires tunning to tweak depends on where its running rich.

As I said, the first things that have to be done, as the idle circuite affects ALL circuits is set the floats, ensure the power valve is in tact, and then set the idle mixture/rpm properly. From there the next steps are taken.

Once the idle is done, typically we tune for total fuel surface cross section, or WOT, wich is a combination of the jets, and the power valve, as well as all fuels deliverd via idle and intermeadiate circuit.

When WOT fuel delivery is correct, then there are areas to tweek the cruise fuel and of course the throttle responsiveness.

When the carb is "sized" for the engine, throttle response is usually real easy as the system is under so much vacume little accelerator pump tuning is required, as we move into free-er air flowing carbs, BIG carbs for smaller displacement engines, then the acclerator pump is more critical and more difficult to tune. IT takes a little longer for the fuel to flow through the booster, so we have to mechanically add it till its there.

For main jet tunning to expidite the process we usually move in steps of 3 for primary and secondary size jets, ie from a 78 to a 75 and such.

Your tunning WOT< so you tune both sides at the same time, the only reason there is a jet split is because of the power valve and the progressive linkage of the carbs opening the throttle blades. The primary are used for the first 25% of engine operation solely on most cars, so to make it easier to tune the intermeadiate circuite (light load cruise) the power valve is equivilant of about 6 jet sizes surface area. You get by running the smaller surface area on the primaies, and thus less fuel under low load. That way there is less tunning with the air bleeds and the car still runs decent at part throttle.

Excellent points/tips. This should saved as a sticky for all of us beginner tuners.
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Re: Too much carb???

Post  kim on September 20th 2012, 6:19 pm

nah, not even a close proximity of a sticky.. there are 3 page write ups I've written on how to.... but it does cover the honest truth, that ya got to adjust the things to optimise them.

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