carb jetting info
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Joined: Feb 2009
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carb jetting info
Hello everyone out there just got my first baby,a 77 4sp 95% stock it has true duals with the cats off.My question is for better performance should the stock q jet have any jetting changes?Thanks for any help.
Safety Car
Joined: Aug 2000
Posts: 3,711
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From: San Antonio TX
What an engine likes is application specific. Just changing fuel metering -up or down- without data to support it is usually counter-productive and ruins an otherwise functional carburetor.
Thread Starter
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Joined: Feb 2009
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carb jetting
Thanks for the reply,I have worked on a lot of harleys and allways looked at the plugs after exaust changes and jetted the carb to reach the best looking burn on the plugs I guess it would be the same on a larger gas engine
Drifting
Joined: Aug 2008
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Yes, changing the way the carb is set can improve performance - if the carb is stock. Leave the primary side of it alone as it will be very close for all but the highly modifyed engines. The secondary side will make a bigger difference and it is easier. Pull the metering rods out and get the letter designation off them. There are several places on the net that post a chart with all the q-jet rods sizes. Odds are good yours are probably in .057 to .067 range. You can drop down to an .052 or even an .041. Then adjust the secondary air valve opening rate - tighten it all the way, then back it off 1/2 to 3/4 turn. Do this and get your timing curve close to 35 - 38 total (vac advance unhooked) all in by 2,600 rpm (hook vac advnc back up) and you'll have a noticable difference for just a few bucks.
