what size should a stock 273 running a holley 600 vac sec carb jets be?
the jets in there are stamped '66' it runs really rich. is it just a case of trial and error?
273 jet size?
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Dave-R
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To give you an idea Joe. I use #66 jets in the centre carb of my 440 six pack (works like a primary on a 4bbl) so I would agree the jets are a touch on the rich side for your engine.
As steve says. Disable the secondary side and wire it shut so it will not open as the engine sucks.
Use a vacuum gauge on the port at the base of the carb (manifold vacuum) to set your idle mixture so that is is only just rich enough to give you max vacuum. Then make sure your power valve is rated 2-inches of vacuum lower than this reading.
Then keep fitting smaller jets on the primary side and drive around until the plug colour is a nice tan. This can take some time and experimenting.
A rolling road will help (expensive) with a gas analiser up the pipe. This would be a LOT quicker.
The DIY version is an oxygen sensor in the exhaust near the engine. You can buy gauges that tell you how rich or lean you are as you drive.
Or if on a budget pull a sensor out of any scrapped modern car (all computer controled cars have them) and wire it up to a volt meter that measures 0-1volt.
You want to measure around 0.85 volts on the sensor to get an ideal 13:1 mixture.
Once the primaries are right you allow the secondaries to do their thing. Tune those for the same 13:1 mixture or just rich enough to give max MPH on the strip.
Too many people bolt on a bigger carb thinking it will make their car faster and more powerful. They don't realise that there is a lot of tuning required to bet the best out of any carb but when it is right you will notice a BIG difference to the way the car drives.
As steve says. Disable the secondary side and wire it shut so it will not open as the engine sucks.
Use a vacuum gauge on the port at the base of the carb (manifold vacuum) to set your idle mixture so that is is only just rich enough to give you max vacuum. Then make sure your power valve is rated 2-inches of vacuum lower than this reading.
Then keep fitting smaller jets on the primary side and drive around until the plug colour is a nice tan. This can take some time and experimenting.
A rolling road will help (expensive) with a gas analiser up the pipe. This would be a LOT quicker.
The DIY version is an oxygen sensor in the exhaust near the engine. You can buy gauges that tell you how rich or lean you are as you drive.
Or if on a budget pull a sensor out of any scrapped modern car (all computer controled cars have them) and wire it up to a volt meter that measures 0-1volt.
You want to measure around 0.85 volts on the sensor to get an ideal 13:1 mixture.
Once the primaries are right you allow the secondaries to do their thing. Tune those for the same 13:1 mixture or just rich enough to give max MPH on the strip.
Too many people bolt on a bigger carb thinking it will make their car faster and more powerful. They don't realise that there is a lot of tuning required to bet the best out of any carb but when it is right you will notice a BIG difference to the way the car drives.
