Need Assistance

O-320 150 HP, fixed pith prop, RV-9A, MA-4SPA carb. After replacing the old carb float with the new blue epoxy float we have encountered a problem that we cannot explain - advice - info please. Normal smooth running during takeoff and climb. After leveling off and reducing power slowly, around 2150 - 2200 rpm the rpm abruptly drops approx 100 rpm. A little unsettling. Runs smooth before and after. Float replacement was done in conjunction with annual condition insp in which the normal items were done. Comp ck - good, mag retime, plug clean and regap. Leaning at altitude is normal except mixture knoob is not quite out as far as before float change. Any ideas anyone?

Sounds like the throttle cable may be slipping in adel clamps, I think I'd uncowl it and have someone pull the throttle the same as you do in flight and watch the whole works moving under the cowl. Check for binding.
Just my 2 cents....

ditto

I agree with 170driver.

Also, if the throttle cable was somehow interfering with and cracking open the carb heat, don't think you'd get a 100rpm drop, nor would it be that sudden, but I'd look at that also. Carb heat leans the engine so might explain difference in mixture setting at cruise you mention?

Only other thought is that if you're at WOT and then come back to get this effect, perhaps your carb's enrichment feature, if it has one, has an issue? Gil or Mel will have to chime in on that one.

Check to verify that your carb is securely mounted.

I had something similar to what you describe happen when my carb was slightly loose.

I was able to rock the carb side to side slightly. Once I tightened everything and replaced the little tabbed lock washers, the problem disappeared, never to reappear.

Also, disconnect the throttle cable and actuate the throttle arm by hand. Look for a "catch" or any looseness in the shaft bore. Sometimes the throttle plate will wear a notch in the body and hop the valve when it goes by.

RPM Varaiation

Kyle Boatright said:

Check to verify that your carb is securely mounted.

I had something similar to what you describe happen when my carb was slightly loose.

I was able to rock the carb side to side slightly. Once I tightened everything and replaced the little tabbed lock washers, the problem disappeared, never to reappear.

Click to expand...

Kyle probably has the answer here. I have a friend who is an A&P with Inspection Authorization who has found several carburators with loose screws holding the float bowl to the upper portion of the carb. These are NOT the four nuts that hold the carb to the intake/sump.

Let us know what you find.

Throttle action is smooth, and both throttle and mixture have full travel available. Blue epoxy float was just installed per E-1000 instructions. New base gaskets, new bowl gasket, new tab locks for bowl screws and bowl screws as well as carb mount are tight and secure.