Q-Jet jet question

Hi,
My FB  350 engine has been rebuilt 30 over with a HO cam. What would be a good jet size to start with for the 69 carb(7029268). The engine is currently running lean and I was thinking of using larger than stock jets. Any suggestions would be appreciated.

Thanks
Sam  

Hi there, I just looked up the carb and its for the 69 350 HO . Do you have the #48 heads? If so, its good for 330hp with the stock primary and secondary metering rods! Excellent combo! Tom  

Hi,
I have 47 heads. The engine  originally was a 350 2 barrel.  

Hi Sam, That carb should flow plenty in stock form, it's for bigger valves(2.11 intake). Are you sure the carb is working correctly? Sticking metering rods could be the problem. The jets should be big enough, as a rule for performance driving or racing, the rods should be about 30 in number selection less than the jet number. Make sure the carb is working properly first,check jet size(number),then change jet size accordingly. For racing, primary jets will vary from .070 to as high as .080. Hope this helps, Tom  

Hi Tom,
The carb was rebuilt by someone that I know, he worked at Rochester Products at the time when Q-Jets were built.  The problem is the engine only smooths out when a hand is put over the top of the carb no matter how much air/fuel adjusting is done.

Sam  

Sam, sounds more like a vacumn leak. When you cover the carb you increase the intake charge, does it blow black smoke out the tailpipe when covered? If it doesn;t , then its drawing air from somewhere else! Mostly a carb or intake gasket. If you can;t locate by a hissing sound, try blowing smoke at these points and see where it gets pulled in, while the carb is covered. All the adjustment to air/fuel in the world won't help with a vacumn leak. Let me know if this helps.  With air/fuel ratios at 14.7 ppm (air)to 1ppm fuel, that,s a big leak, I'm thinking. Tom  

Hi Sam

Q Jets are a great carb for the street, but they take a little effort to get tuned right. Were you  running this carb on your setup before you changed to the HO cam? Usually, if you decrease the idle vacuum (as with a swap to a high performance cam)you will experience trouble in the idle circuit. Changing the main jets will not help!

So:
Do the idle mixture control screws provide adequate control? does the engine run rougher as you open the mixture screw? if not, you will need to enlarge your curb-idle discharge ports. If you are up to this type of tuning then I suggest you get Doug Roe's book "Rochester Carburetors". It is an excellent "how to" and explains Q-Jets thoroughly.  

If not, be prepared to pay lots to have someone do it for you, or switch to another brand. Q Jets are very sophisticated and are application specific. They are set up for the engine at the factory and were not intended to be re-tuned,but they can be. When tuned right they can't be beat on the street, for power or drive-ability. Most guys don't have the patients  because it involves drilling.  
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Good Judgment comes from Experience...Experience comes from Bad Judgment

If the carb was fine before the switch.Chances are your vaccum dropped.I use q-jet almost in every app.You can order a spring kit from edelbrock for the performer carbs.the q-jet is vaccum operated as opposed to a fuel dump like holley.There is a spring under the metering valve.you can change it with the edelbrock spring or pull it out and snip a small amount off.It looks like a spring from an ink pen Ive actually seen one used before.This will allow you to get it the run correctly even with an engine that makes very little vaccum.As for application specific I get alot of free q-jets and if it were true how many performer carbs would edelbrock have to offer?By the way mine 66 326 with a q-jet off a 454 marine engine.