Edelbrock carbs, lean bog when running on the transition slots. (Throttle position just off idle)
Posted: Wed Apr 18, 2018 10:46 am
Recently I have been messing about with my 500 Performer carb, I have modified the primary boosters so that they are now annular boosters similar to the ones fitted to the AVS2 carbs. I had to richen up the calibration of the carb in order to obtain air fuel ratios for all the modes of operation that were similar to the ratios prior to the booster mods. This seems odd because annular boosters are supposed to have a better 'gain' than straight leg boosters but I reckon that Edelbrock had the same issue with booster gain because they supply the new AVS2 carb with a much richer out-of-the-box setup compared to the Performer or Thunder series carb. The idle fuel screws had to be wound out to 2+3/4 turns which I now think is a good indication of a slight issue that I found after doing the mods. Researching on the web I found that other people have also suffered the same problem that I have which is a slight lean bog whilst the carb transitions from the transition slots to the primary boosters. I could actually watch this happen if I slowly revved the engine whilst in neutral on my drive, I could see the air fuel ratio lean off as the revs climbed to 2000 RPM, the AFR was around 16.5:1, above 2000 RPM the mixture richened back up to around 15-15.5:1 which is the ratio that my engine is happy with whilst cruising under a light load.
I was unable to find a solution to the problem by researching on the web but some people were trying things that I also thought were worth trying. The first thing I tried was drilling the idle jet out from 30 thou to 33 thou, this seemed to make a small difference but I was reluctant to go too far with this because it is a one way street, it is not easy (but not impossible) to make the jet size smaller. I also thought that there was a limit to how much effect this mod would have due to the fact that the economizer is also a restriction in the fuel circuit.
I then turned my attention to the air bleed at the top of the booster, not the brass tube one but the one that is just a hole in the aluminium body. The idle circuit actually has two air bleeds, one being a small brass jet in the side of the booster. I measured the diameter of the hole at the top of the booster, with all of the 500 boosters that I have the hole is 66.5 thou, I also have some 600 boosters and the hole is 59 thou. I then made some tiny brass jets that were slightly tapered so that I could push them into the air bleed without them falling out the other side of the hole! The jets were proud of the booster by about 30 thou, there is only around 60-80 thou of space between the booster and the underside of the top of the carb. The hole in the jets was 40 thou.
I was very surprised by the huge difference that the jets made, without adjusting the two fuel screws the engine was now idling very rich, the AFR was around 11:1, before the jets were fitted the AFR at idle was 13.2:1. I had to turn the fuel screws in one full turn (1+3/4) in order to lean off the idle ratio. I could not get the engine to idle on the same 13.2:1 AFR that it was running on prior to the fitting of the brass air jets, it is happy on 12.5:1 but went rough if the AFR was any leaner, I guess this is down to the fuel being supplied at idle now being a bit more blobby as it now has less air being bled into it. I then slowly revved the engine to 2000 RPM and was pleased to see that the AFR never went leaner than 12.5-13:1. I think that I need to try some jets with a 50 thou hole but my first set of tests indicate that I am on the right path.
I think that the fuel screw setting are a good indicator of how well the idle and slow speed circuit is running, if the screws are getting on for being 3 turns out then I think that this is an indicator that you might run into a lean issue whilst running on the transition slots, especially as the slots start to get to the end of their range. The good thing about modifying the air jets is that you can always revert back to standard which is nice! I know that other people have suffered the same sort of issues that I had especially when using a carb that is borderline too big for the engine that it is fitted to, I had this issue when running a 600 carb on my engine, making small air jets is not too hard if you are handy with tools so it is worth a try!
I was unable to find a solution to the problem by researching on the web but some people were trying things that I also thought were worth trying. The first thing I tried was drilling the idle jet out from 30 thou to 33 thou, this seemed to make a small difference but I was reluctant to go too far with this because it is a one way street, it is not easy (but not impossible) to make the jet size smaller. I also thought that there was a limit to how much effect this mod would have due to the fact that the economizer is also a restriction in the fuel circuit.
I then turned my attention to the air bleed at the top of the booster, not the brass tube one but the one that is just a hole in the aluminium body. The idle circuit actually has two air bleeds, one being a small brass jet in the side of the booster. I measured the diameter of the hole at the top of the booster, with all of the 500 boosters that I have the hole is 66.5 thou, I also have some 600 boosters and the hole is 59 thou. I then made some tiny brass jets that were slightly tapered so that I could push them into the air bleed without them falling out the other side of the hole! The jets were proud of the booster by about 30 thou, there is only around 60-80 thou of space between the booster and the underside of the top of the carb. The hole in the jets was 40 thou.
I was very surprised by the huge difference that the jets made, without adjusting the two fuel screws the engine was now idling very rich, the AFR was around 11:1, before the jets were fitted the AFR at idle was 13.2:1. I had to turn the fuel screws in one full turn (1+3/4) in order to lean off the idle ratio. I could not get the engine to idle on the same 13.2:1 AFR that it was running on prior to the fitting of the brass air jets, it is happy on 12.5:1 but went rough if the AFR was any leaner, I guess this is down to the fuel being supplied at idle now being a bit more blobby as it now has less air being bled into it. I then slowly revved the engine to 2000 RPM and was pleased to see that the AFR never went leaner than 12.5-13:1. I think that I need to try some jets with a 50 thou hole but my first set of tests indicate that I am on the right path.
I think that the fuel screw setting are a good indicator of how well the idle and slow speed circuit is running, if the screws are getting on for being 3 turns out then I think that this is an indicator that you might run into a lean issue whilst running on the transition slots, especially as the slots start to get to the end of their range. The good thing about modifying the air jets is that you can always revert back to standard which is nice! I know that other people have suffered the same sort of issues that I had especially when using a carb that is borderline too big for the engine that it is fitted to, I had this issue when running a 600 carb on my engine, making small air jets is not too hard if you are handy with tools so it is worth a try!
