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Discussion Starter · #1 ·
I have a Shelby Cobra replica with a resonably upgraded 1969 429 under the hood. It has bigger valves, a mild cam, roller lifters etc and a Holley 750 vacuum secondaries carb.
It runs well with no hesitations or flat spots but it is very smokey and sooty when starting and very black plugs. This suggests it is running rich but looking at the various forums my #72 mains seem about right.
I would appreciate peoples thoughts on going down in jet size and/or any other reasons why I seem to be running so rich.
Other details : 6.5 power valve, electirc fuel pump, idles screws 1 1/4 turns set using vacuum, 10 degrees advance idle and about 35 degrees total advance . Oh and the fuel I use here in the UK is 99 octane.
Thanks
 

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I have a Shelby Cobra replica with a resonably upgraded 1969 429 under the hood. It has bigger valves, a mild cam, roller lifters etc and a Holley 750 vacuum secondaries carb.
It runs well with no hesitations or flat spots but it is very smokey and sooty when starting and very black plugs. This suggests it is running rich but looking at the various forums my #72 mains seem about right.
I would appreciate peoples thoughts on going down in jet size and/or any other reasons why I seem to be running so rich.
Other details : 6.5 power valve, electirc fuel pump, idles screws 1 1/4 turns set using vacuum, 10 degrees advance idle and about 35 degrees total advance . Oh and the fuel I use here in the UK is 99 octane.
Thanks
Hello, I have a question, but first. It sure sounds very rich. Are you sure it's set at 1-1/4 turns ?
How is your PCV working ?

I have a 427 Cobra and I'm building a 460 stroker. What oil pan are you using ?
I've had two 428FE's with a Canton with kick outs for the Cobra. It's only 6'' or 6.5'' tall, but I'm having trouble finding one with the short height. A few company's are from 7.5" to 9".
 

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Discussion Starter · #3 ·
Hello, I have a question, but first. It sure sounds very rich. Are you sure it's set at 1-1/4 turns ?
How is your PCV working ?

I have a 427 Cobra and I'm building a 460 stroker. What oil pan are you using ?
I've had two 428FE's with a Canton with kick outs for the Cobra. It's only 6'' or 6.5'' tall, but I'm having trouble finding one with the short height. A few company's are from 7.5" to 9".
Hi,
The idle mixture is definitiely 1 1/4 turns. I have played with it quite a lot. The PCV is just off the rocker cover going straight to the large vacuum port on the back of the carb. What problems can the PCV cause ?

The sump is a problem in a cobra. I ended up modifying the standard sump by making it slightly shallower (about 8") but made up the volume by adding an extension at the back of the deeper section plus a baffle. It seems to work ok but I do run the oil about 1/2" high on the dip stick.
 

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Based on the description occurs at cold start I would check the choke setting. The pull off mechanism on the universal holleys sucks. We've always set the choke thermostat at a minimum.

The as delivered jetting on these carbs is pretty spot on for most mild applications with 72 jets in front and the equivalent of 76's in the metering plate secondary side.

Edit.
Additionally the above statement applies to at or near sea level say up to 1000 to 1500' DA.


The idle and transition are pretty rich due to the rather large idle feed restrictor in the front metering block.

Make sure that the running warm float level is set correctly.








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As MP said, the factory setting is usually pretty close. But I have found in 50 years of playing with Carbs, it's only a very good starting point, if you know what you're doing. Your cam and engine vacuum controls the PV. The engine timing, eng combo, altitude and humidity of where you live is the big factor for jetting. Live at sea level in LA is different then sea level in Seattle due to humidity. Just as 5000' alt in on the east coast (high humidity) is different then Ely, Nv, 6200' alt and 10% humidity. The Silver State Classic is good for this one. Computer engines really have an advantage here. Start at 6200' and drop down to 1700'. Easy way to burn a piston if to lean. Try drag racing at Bandimere in Colo. as compared to SIR, in Wash.
At 4200' with 10% humidity, where I live, I play with them to try and get that just right, (never really have) setting. But I have jets from 58-90, PVs from 2.5-8.5 and all the acc pump different cams, and one of the timing light types with the timing adjustment. If you're not sure what to do. Check the float setting and choke and leave everything else alone.
 

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Discussion Starter · #7 ·
Thanks for the responses. I dont have any way of measuring fuel pump pressure at the moment but will see if my friend might have something I can rig up.
It has a manual choke which functions just fine and it needs very little choke to start.
I notice the black smoke when I start from cold and my friend said it puffs black when I boot it. Plus my plugs are all black.
 

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I have an 850 Holley on my 460, 70 jet size, 4.5PV and 12 degs timing. And I'm at 4200 alt and 10% humidity. I'd drop yours down to about the same thing and go to one heat range hotter on the plugs along with 2 degs more timing.
What brand/model of electric fuel pump do you have? Some will put out 14 PSI (which will lift a float) and need a regulator and some are 6 max and good to go from the factory. I would also get a fuel line for the carb that you can put a gauge in permanently.
 

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IMO... for street driven vehicles...
Holley carbs are usually "ok" out of the box... they control the 'fuel leak' rate under certain conditions better than ...all the other times. Create a log book to record environmentals and spend an hour to find that close-enough tune that seems to work... good enough. Repeat tomorrow or when the air temp, pressure, or elevation changes. And again when anything significant is changed... like cam, intake, heads, exhaust.

Or... save a few hundred (thousand ?) sweaty hours underhood tweeking that inexpensive fuel leak device and get a fuel injection system... like Edelbrock, Fast, FiTech, Holley, MSD, etc. Spend ~$1500 and about 4-6 hours installing the throttle body, fuel pump, O2S, and a few wires. Then, just simply drive it.

My 460 went from 12-13 mpg to 16 hwy. Changing plugs from every 6k to 24k miles. And I'm not washing the cylinder walls or diluting the oil any more.

Again, just an opinion for street vehicles.
 

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Discussion Starter · #10 ·
Thanks for the thoughts and info. I'm going to pick up some 70 jets and see what they do.
I also have a 4.5 power jet in my kit which I might try for the fun of it, although I'm still a little confused how that works. Is it that it opens once vacuum drops to 4.5 therefore making it open later than a 6.5?
I know fuel injection is the way to go for a normal car but this is a Cobra and just a toy so a well tuned Holley will do me fine and tinkering is part of the fun !
 

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The 4.5 or any number is the vacuum in inches that it will open and stay open as long as the vacuum stays at or below that number. A 10.5 or even an 8.5 will open and close with a wild cam that only produces low vacuum numbers. I've seen 7-8 on really radical cams and that's why they'll run rich because of this PV being to high.
If you can get a Vacuum gauge and install it, use a hose long enough to put it where you can see it and drive around. Very informative. That was the old, gas mileage gauge, vehicles had before all this electronic stuff.
 
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