I have a moki 180 with 10 hours running time, just mounted into a new model.
I have a new vp30 perry regulating pump.
I have a fuel tank 10" from pump.
I have the pump 3" from the engine.
I have a new moki glow plug.
I have a 18x10 prop.
I have GX-5 fuel (no nitro, high castor content).
I have the top needle set around 2.5-3.5 turns out for starting.
I have the idle disk set at about 4 degrees lean from the neutral.
I have problems..


The engine starts and idles well. The engine transitions from idle to full throttle immediately. However, if I keep the throttle fully open for a few seconds the rpm drops from ~7500 to ~5000 where it remains for a few seconds then goes back to ~7500 and the high/low RPM cycle repeats. Typically the engine sits at each rpm for around 3 seconds.

I've tried different idle and top needle settings nothing seems to help.
Thought top end was lean, tried richening to no avail.
There are no air bubbles in the fuel.



Any ideas?

Surging is usually an indication of air leaks.
Your backplate pressure tap may be the cause - Did you just install one?

Suggest you start process of elimination by taking the Perry pump out of picture.
Plumb the clunk straight to the carb, and vent the tank to atmosphere. Plug the backplate tap.
Run the engine level, on the ground. If that eliminates the surging, you know where to look.

If not shoot some WD-40 on joints where leaks could occur, while engine is running.
This trick is good to momentarily plugs hairline leaks below carburetors, and around backplates and heads.
If the running engine changes behavior, you have spotted your leak.

Good luck! Let us know what you found.

The backplate pressure tap was not changed, though the crank pressure pipe to the pump was.
I'll eliminate the pump and plug the pressure tap as you suggest and try again, ergo checking if it's pump or other.
I'll report back on progress.

Other ideas welcome.

I went through this with my BUSA Citabria Pro with a 1.60, cowled in and no fun to get to much less see what was going on. After an hour of messing with the set up I pulled the cowl, put the prop back on and started the engine. What greeted me was a nice stream of bubbles comming from the pump to the carb, NONE from the tankt to the pump though. The only possible explanation for this would be a ruptured diaphragm in the pump letting the air through from the crankcaase.

Since I really wanted tto fly the plane that afternoon and it has the tank mounted on the CG I was stuck. In a pinch I pulled the Cline regulator off my UCD 60 a put it on the C-Pro and fired it up. I was so impressed with this set up I trashed the pump and the Cline is still on that plane.

Folks,
There were some issues with the Perry VP30 pumps ... some were generating air bubbles in the line to the carby. I had one of those faulty units and Gary Conley (the manufacturer of Perry pumps) suggested I return the pump to my LHS where it was replaced. Apparently Conley engineering now has an improved testing procedure to ensure this type of fault does not occur in newer units - Good work Gary Conley!!
RTD