Galloping QD 24

[stanley]

I’m visiting a relative and trying to help him diagnose a problem with his QD 24 before I leave.It starts fine,idles good,but about the time the high speed carb circuit kicks in,it starts to “gallop/surge.” Spark,compression and carb settings seem fine.I only have experience with two line pressure tank systems.Is it possible the fuel pump is malfunctioning?

[Mumbles]

It’s quite possible it could be starving for fuel but missing under load is a sign of a bad spark plug to. It could be a few things.

[stanley]

I don’t think it’s an ignition problem.It’s like you get to a certain RPM and then start rythmically twisting the throttle from fast to slow.As long as you let it run tit will do it.It seems to be fuel delivery,just wondering if anyone has seen a failing fuel pump cause this.

[fleetwin]

Well, the fuel pump could cause an issue like this, but not all that likely….Try pumping the fuel line primer bulb while the engine is misbehaving..Suspect a fuel delivery issue if pumping the fuel bulb helps reduce the issue. The other possible fuel pump issue might be a leaky fuel pump diaphragm that is flooding one cylinder. But, this would usually show up at all speeds, and the engine wouldn’t idle very well at all..
Does the engine idle OK, how does it run with at WOT?

[stanley]

Pumping fuel bulb doesn’t seem to help.The engine will idle ok,but not quite as low as I would expect.It will run at wot,but starts surging immediately and so extremely that you’re forced to drop back down to start range.I think I’ll follow your leaky diaphragm suggestiion and put a new pump on.

[fleetwin]

Please don’t replace the pump, unless you can prove it is NG….New pumps are very expensive, and I really don’t think a leaky pump is the issue. Is the mag plate jumping around causing erratic igntion advance? Perhaps something is dragging on the flywheel magnets causing the mag plate to move around.
Are you sure this thing is running consistently on two cylinders? You may have ignition issues causing one cylinder to fire intermittently which will certainly raise and lower RPM. It’s kind of hard for me to judge without actually hearing the engine misbehave…Have you tried replacing the plugs? Just cuz plugs are new, doesn’t mean they are any good…
Anyway you can post a video?

• This reply was modified 4 years, 5 months ago by fleetwin. Reason: addition

[stanley]

It’s a weird,violent oscillating,up and down throttling and I’d like to get it fixed for him.We decided he’d borrow a friends spare and I’ll haul this one back home with me where I can pull the carb and flywheel for a good check before we go the new fuel pump route.Thanks for talking me out of that.

• This reply was modified 4 years, 5 months ago by stanley.

[fleetwin]

Had another thought…Perhaps the prop hub is spun causing the engine to race up and down under load….Or, perhaps, a partially sheared pin….

[Author]

Posts