Jump to content

carb jet sizes

Recommended Posts

I have a 1982 pick up. Five speed, 720 engine w/70,000miles. I replaced the carb with a Weber 32/36 DGAV 2 years ago. Runs well enough, but I think it could do better. I think I need to adjust the size of some of the jets. Right now it's running   #55 primary idle jet, #50 secondary idle, #145 primary main jet, #135 secondary main, #165 primary air jet and#160 secondary. Those are the numbers stamped into the jets. Are there any changes in jet size that will work better for this carb in this truck? Also, does manual linkage serve any purpose if I'm running an electric choke? I'm kinda new at all this, and I'm a gal, but I'm learning! Thanx  you may not ALL be handsome, but your handy!

  • Like 1
Link to comment
  • Replies 1
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

Unless wildly out in size (unlikely as it has been running) small tweaks in jet sized are not likely to be discernible to the driver. While there is a theoretically perfect fuel ratio (14.7 to 1) that matches correct fuel to correct air needed to burn it, on a graph it would be represented by a rather broad bell curve and it will also run pretty much the same on either side. Say 14 (slightly rich) to 15.5 (slightly lean) Keep in mind that there is almost infinite variables for correct jet sizes and the same carburetor will act differently at a different location with someone else. What works at sea level will not be exactly the same inland because of the altitude difference.    


The old school way to guesstimate the fuel ratio is to 'read' the spark plugs. This is the laborious process of driving along for 15 min to stabilize the plugs and allow them to 'self clean' and quickly shutting the engine off to preserve the plug color and pulling over to remove an inspect a spark plug. Too rich a mixture and the plug will have a dark insulator from carbon that is not completely burned. Too lean and the plug runs hot and burns off the carbon and the insulator is a very white color. You want to see a light tan color. This will confirm that you are close enough.  


Today you can buy a wide band O2 sensor that is installed in the exhaust pipe that reads the oxygen content in the exhaust gasses compared to ambient air. The gauge will accurately display your mixture ratio in real time as you drive. They are about $200 or so and a threaded bung welded in the exhaust down pipe for the sensor is needed. Invaluable for tuning a carburetor and the final arbitrator of fuel mixtures for tit picking. Once set not much for the O2 sensor to do, but if you reading should vary from the norm it is a good diagnostic tool. Too rich could be the carburetor is flooding, too lean possibly a blockage in the jet(s)   



Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Create New...

Important Information

By using this site, you agree to our Terms of Use.