After replacing the pump diaphragm due to a small hole in it the engine suffered from a flat spot and hesitation at about 4000rpm. Turning the pump in about 1/4 turn seemed to remove that problem but now the engine surges at around 8000 rpm and refuses to rev any higher. 17x12, cut down 17x12 (about 1/8 off the tips) and even an 18x11 carbon prop that revs to 8600 on other engines still peak at around 8000 and surge. The revs we can live with but not the surging.

The main needle doesn't appear to make any difference, turning in or out just slows the engine so that seems to about right. It's currently around 1 and 3/4 turns out, the same as the other engines.

Is this a consequence of fitting a new diaphragm and if so does anyone know the cure? Does it have to be "run in" to soften it up a bit? Is there a trick to installing it that we don't know about or is there another fault turned up at exactly the same time? The pump has been stripped and cleaned carefully again but no improvement.

The first comp is getting very close so panic is setting in!

Anyone come across this and found the solution?

Mel

You mentioned you replaced the diaphram, I have done this in the past and I could not get it to run right either at the top end. So I went through and changed the pump valves and the springs which fixed my problem. It did not make any sense but all I can think is that with a faulty diaphram buy this stage those other compents might be worn as well.

I run 3 DZ160 & one DZ170 great engines heaps of power but as you would understand they do require more maintenance, the price paid for performance. Do those white plastic valves & springs and see how it goes.

Also how much work have this engine done ?

Hi Mel,

You could have a couple of issues, you may be sucking air from the pump and its not an issue untill your revs go up, or there could be an air leak else where, I doubt if changing the pump valves and springs will help, but I have been known to be wrong sometimes. Or there is something as simple as a pin hole in your fuel line somewhere.

Pumps can fail at any time, and having a new engine is no gaurantee of it not happening.
Run the engine and check for air bubbles in the pump to carb line, if there is and there are no bubbles going into the pump from the tank, replace the pump assembly, no if's and no buts, replace the assembly.
Or you can fit two pieces of fuel line to the pump and have each one going into seperate containers with some fuel in them and spin the motor over and watch for bubbles, if there is bubbles replace the pump assembly, you may have to prime the system if the pump is dry or weak.

If the pump is good then also check for air bubbles from the carb to the black injector.

You can test the pump valves to see if they are working, you need a syringe and a piece of fuel tube for this, attach the syringe to one side of the pump and pull on the syringe and push on the syringe, you should have a lot of resistance one way and none the other, then do the other side of the pump and you should have the same thing but opposite, if there is no resistance at all then look at replaceing the valves and springs, but there still can be a leak issue here also. I actually have a assembly that is no good but I cannot find the problem with it.

Other wise you could have a air leak else where and its a case of finding it.

I feel you could do better with your prop choices, try running a 16.5 x 12W APC which should drop your RPM's down also, try for about 8300, your 17 x 12 is more suited to the 170, I only run the 17 x 12 and 17 x 13 on our modified 160's and stock 170's, but, I don't know how the enviroment affects your engines over there, you may be right in your selection.

Will Crossman
DOWN UNDER

Surging is a sign of a lean condition. There are many reasons your engine could have a lean condition. The first of which could be a leak at the diaphragm you just installed. Next it could be as simple as the new diaphragm needs a little more pressure on it to keep is closed longer. Hence making more fuel run through the engine rather than bypassing the fuel to the front of the pump.

I'm not going to go into a detail explanation on how the pump works here but you could just need to dial the pump screw IN (CW) until the surging stops. Or it could be more of a problem like discussed above. You might be surprised what 1/8th to 1/4 turn on the pump Richer (IN or CW) will do to a surging condition.

I have never seen the little valves and springs inside the pump fail. I have seen debris get in there and stop them from closing but this produces a full rich condition not a surging lean one. Not that it can't happen I just have never seen it. The pumps when they fail start to leak in the main body or the shaft of the pump. The piston that is driven by the pushrods or the pump body gets worn either at the top where the tappets are being moved by the pushrods, or the little piston inside the pump body becomes excessively worn.

And yes these problems can cause lean conditions. They can also cause rich conditions too just depends on what happened.

My first step would be to dial the pump in a bit and see if you can get it to stop surging at the midrange...this should also increase your rpms as its providing more fuel....No promises on the rpm increase but I have seen it happen.


Troy Newman
Team YS