Its been way too long since I did any tweaking on the carburetor. The WMB has been running fine but, it can be improved.
Getting together with my buddy Karl we pulled the carb and looked back at what had been done earlier. Currently the truck will not idle down below 900 rpm in gear. The idle mixture screws will not respond to bring the idle down any farther. The joys of late 70's emissions carburetors.
Previously we had drilled, tapped and installed some plugs in the base plate to remove these massive air bypass passages. (Photo 1) Then Karl plugged two idle air bleeds in the main body (Sorry to say I didn't get pictures of this.) and to replace those air bleeds he drilled two holes in the top plate for air bleeds like a pre-emissions carburetor would have had. (Photo 2) These modifications allowed us to get the idle mixture screws to start responding, whereas before they had no affect on the idle. After a looooong test period we know that the air bleeds needed to be further modified to get the idle mixture screws to operate properly.
This leads us to the work we did this week. Basically an idle circuit is like a straw with a hole in the side of it. You adjust the size of the hole in the straw to make idle air to fuel ratio correct. You also have to have the correct size straw to get the engine the correct amount of fuel for the engine to idle at the desired RPM. To accomplish this the factory drills holes in the carburetor body to allow a certain amount of fuel and air to mix and then be introduced under the primary throttle plates. How much of this mixture is allowed into the engine is controlled by the mixture screws. This mixture of air and fuel is then combined with the rest of the air passing the throttle plates to make the idle mixture complete.
Our previous modifications had allowed us to get some response from the mixture screws but there was still to much fuel passing into the engine. In other words the hole in our straw was too small. In order to get the correct amount of fuel we needed to make further changes to the previous modifications.
To start we removed the plugs we had put in the base plate and installed ones with holes drilled in them. Then Karl reduced the size of the air bleed in the top plate by pressing in small brass tubes in the previously drilled holes. (Look closely at Photo 2 and you can see the tubes) once this was done we reassembled the carburetor and bolted it back on the engine. Now the engine would idle all the way down to 550 RPM in gear and 650 out of gear. Finally I got to use the idle speed set screw!
Not only did the idle speed set screw work but, the idle mixture screws now worked as they should. After a test run to get the truck to operating temperature I was able to set the idle speed in gear at 650 and out of gear it was about 775. The A/F ratio was dialed in to 14.7. Most excellent!
Now on to the next step, dialing in the A/F ratio.
When I am referring to the Air/Fuel ratios they are not an exact number but .3 or .4 above and below the number I state. For example above I said it was 14.7 the reality is the readings were between 14.4 and 15.2.
So back to testing the truck. I found the A/F to be fine in the upper speeds but around 40 mph it had gone to 12.5 and at 25 mph it was at 10.5. Way too rich, especially when driving with the choke on. It was turning my quarter panels black. UGGG! The 90 plus degree weather wasn't helping things either.
Since the higher speeds were okay I decided that larger primary metering rods were needed. Smaller jets would have leaned the A/F mixture too but that would have leaned it out every where instead of just at the lower speeds. Karl recommended that we remove the ATP screw cover to allow more adjustment of the metering rod position.
The ATP adjustment is one advantage of the later emission style Q-jet carburetors. The ATP screw is in the front center of the base plate. It allows you to change the height of the power piston and thus which part of the taper on the metering rod is used at steady state cruise. There is about 1/8" of height adjustment provided by the ATP. The screw in the front moves the small lever on the base plate. (In Photo #3 the red arrow indicates the lever and the green arrow is where the screw is.)
This was a factory adjustment back in the day that was made to adjust individual carburetors to pass emissions, then it was blocked off never to be adjusted again.
There is a staked in plug that must be removed to access the adjustment screw. (Photo #4 shows the plug with a hole drilled in it and the 3 staked areas to be removed.) You have to drill a hole in the plug and then grind the 3 stakes off with a Dremel and then twist a small screw into the hole so you can pull the plug out. And there's the ATP screw. (Photo#5)
Now with the ATP turned all the way out, which drops the power piston to it's lowest point, we installed the larger metering rods. (I will have to post the new rod sizes later. I forgot my notes.
) Then the carburetor was reinstalled and I was off on a test run.
Things were much leaner now. So lean in fact there was a stumble upon acceleration. Just at the point you would expect the accelerator pump shot to have been used up. At that point I could see the A/R ratio jump to 20.0 (which may be the upper limit on the guage). At 20 mph A/F ratio was 15.5, and at 40 mph it was 17.0 and under deceleration there was amount slight back firing from the exhaust.
Back to the shop where we turned the APT screw all the way in which raised the power piston. The higher power piston meant that the part of the metering rod inside the metering jet's orifice was smaller thus allowing more fuel to flow through. And back for another test run.
Much better this time. The 20 mph A/F was 12.8 the 40 mph A/F is 13.5 and at 55 mph it is 14.2. The stumble was gone, as was as the back firing. Most excellent!!
We decided to leave things as they are for a while and get a longer term feeling on the carb's performance. I have noticed the engine starts easier when it is hot and heat soaking would be a concern. I have also backed the choke off as it seems starts better with less choke. Of course we've been having lots of 90 plus degree days lately, so I may end up setting it back when the weather gets cold.
Such a long post! Man I'm tired. Not to mention I had to repost some of it as I somehow lost the photos and some of the text at the 3/4 point.
Any questions? Don't hesitate to ask. I'm good at confusing myself as well as others.