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Schebler Carbureter
- Thread starter Byron7
- Start date
I went through a similar drill (pun very accidental) on my RV-9A and was dissuaded from attempting it. I traded in my old carb for the new one.
I did the Mooney mod on mine and it worked great. The kit included a new larger main jet and the restrictor for the air bleed. Personally I wouldn't do it without the restrictor as that is what gives the extra fuel through the economizer cct at WOT. Sorry but I don't know the restrictor id.
I did this on my carb a few years back. There was a post some 15 years ago on how to do this. Basically you put a board on the drill press and drill down through it with a diameter just big enough to fit the nozzle (so it holds the nozzle and aligns it with the bit). Then just drill slowly, with the next size bit and use lots of oil.
The Story: I did an exchange instead of rebuilding the one I had. The new one was at peak EGT with the mixture at full rich. Sent it back and it flow tested within spec, but still had the same problem. So I called again and told them I wanted it drilled out. I first got a "No No, don't don't drill it out... let me let you talk to the tech". Then I spoke to the tech who said "don't do it... there are steps and stuff an you will ruin the nozzle". Then I told him I was experimental and he said, "Oh, no problem and I'll drill it out for you for $100". A new nozzle was less than $100, so I did it myself with the above procedure and it worked fine. I have some photos of the process I can find if you need, but it was dirt simple.
The Story: I did an exchange instead of rebuilding the one I had. The new one was at peak EGT with the mixture at full rich. Sent it back and it flow tested within spec, but still had the same problem. So I called again and told them I wanted it drilled out. I first got a "No No, don't don't drill it out... let me let you talk to the tech". Then I spoke to the tech who said "don't do it... there are steps and stuff an you will ruin the nozzle". Then I told him I was experimental and he said, "Oh, no problem and I'll drill it out for you for $100". A new nozzle was less than $100, so I did it myself with the above procedure and it worked fine. I have some photos of the process I can find if you need, but it was dirt simple.
Aircraft Spruce sells a kit to do this. It includes a couple other parts besides the new jet, like a different economizer valve (whatever that is). I think if you just drill out the jet you end up with an opening that isn’t ideal. It needs to be a certain shape. I sent mine to that shop in southern Michigan that does this kind of work. It cost about the same as the kit from Spruce and the turn around time was quick. It came back to me with a new 10-4164 data plate on it.
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I have done a bunch of mods to old Holley 750's & 850's. We used to remove the air bleed restrictors, then drill and tap them. Later models came from the factory this way. Would then take brass set screws (hex key type) and drill those and install. Easy to make changes. have no idea if that would work for marvels.
Can't help on size recommendations as that is not easy to guess at. I can say however that mods to air bleeds have more significant impacts than jets and impact how the system adapts across the flow range, so be carefull there. Halving the area of the restrictor sounds WAY off, unless looking for a significant increase in flow, including mid range flow. We typically tweaked high airflow fuel volume with the jets and only messed with the air bleeds for mid range or low end flow, as full flow was mostly the jet and all the tapering down to the low end was done via the bleeds. My marvel carbs didn't have econmizer circuits, so don't understand how they work. Be sure yours has one because not all do.
There is A LOT to how a carb deals with matching airflow to fuel flow and would avoid taking stabs at orifice sizes unless you enjoy experimentation and have a good feedback process for testing and adapting, including AFR or EGT. Don't forget that fuel height in the bowl is the third leg on that flow matching stool (Fuel height, air bleed volume, jet size) and likely different across carb models. The bowl height was adjustable on the holleys for this purpose. All three of these interact together and therefore should expect changes across the flow range. Changing one often requires changes to others when trying to tweak just one portion of the range.
No way I would change both the bleed and the jet unless I had good data about my setup.
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