My Franklin 150 hp on take off roll stumbled at about 2000 rpm causing me to abort. Checked electrical and fuel found nothing that would cause that problem. My carburator accelerator pump has a ball valve that keeps gas from flowing back into reservoir. Marvel website says to blow into the pump to check if air passes the valve. My valve lets air pass. The ball is located in a small hollow bolt that appears crushed on the threaded end. I don't know if that is designed to keep the ball in place or if it is damage. I sent an email similar to this one to Marvel. So I have 2 questions for the group. Could the accelerator pump be the problem? can I replace the ball valve without a $600 re-build? AS sells kits but they don't include that valve.
The operation of the accelerator pump is super easy, just open throttle quickly fuel pours out. Yes the ball is swedged in place, all normal. Why do you suspect a carb problem? Franklins are notorious for induction leaks and sticky valves both cause what you describe. I have noted on my 150 Franklin it stumbled and I found several induction leaks. Fixed them problem went away. The 150 uses a smaller carb and is more sensitive to rapid throttle movements than the 165 as well. If I jam the throttle it just burps waits a second then accelerates. Scary to say the least when on short final doing a go around.
Thanks Andrew, Carston, I ruled out carb ice since Colorado 10% humidity and never experienced it on take off but did not apply heat and should have. My run-up after included a static run up to 2100 and it faltered again but I neglected to shut off a mag and should have. I ruled out stuck valve since did a comp check and all better than 70/80 and never had a valve stick closed. And I thought (maybe mistakenly) that induction leaks were a bigger problem and very noticeable at lower power settings. My mag timing showed 27.2 deg BTDC and very close left to right. I still need to check for cross-firing in mag. A web page of Marvel Schebler said that one should blow in the accelerator pump chamber by mouth and no air should leak past the valve. Mine does hence my focus on the carb. I could taxi back to the hangar with no problem at low power. Since this problem is not intermittent I should be able to fix it.
Thinking outside the box?? Carb accelerator pump should cause a stumble upon initial acceleration, not when you get to 2000 RPM. I would look for an obvious intake leak then shift my search to a bad spark plug or 2. Plugs can and will break down under load hence your stumble at higher RPM. Mags would be suspect also with maybe internal spark jump at higher RPM. Hope you find the problem easily.
Since you are an A&P/IA and since all individual parts for the carb are available, you should be able to repair whatever it needs. It won't constitute a legal overhaul, t\but a legal repair can produce the same results at a much lower cost.
Check the cross-over's for leaks.