VW engine trouble

[LAWise]

My 1834cc VW engine will not maintain takeoff power for more than one minute. After about a minute the rpm drops from 3500 to about 2800 and runs rough. If I throttle back to 1500rpm for a short time I can again achieve full power for a little bit then the drop in power occurs again. What could be the problem?

[Dana]

First place I'd look would be fuel system: Insufficient fuel pressure (whether vertical head for gravity feed or fuel pump pressure). Blocked fuel filter, carburetor inlet, tank vent, kinked fuel line, float needle, float level. A distant second thing to check would be an overheating magneto.

Is this a new problem? If so, what changed since it ran right?

[sr2500]

I had a loss of power once that was due to one of the intake manifolds not seated properly.
Jerry Folkerts

[LAWise]

Before my first flight I really never did hold it at full power for very long. On the first flight after about a minute in the air I started to loose power and thank God I was able to get back to the runway. So, I really never experienced the problem until the first flight. I have readjusted the carb thinking it was running too lean because of the high EGT temps. But, the problem still is there. I am running it up on the ground now. I will have to recheck my flow rate on the gravity feed system. Thanks for your pointers.

[1600vw]

You did do a fuel flow test with the airplane in a nose high attitude and a low nose attitude? The FAA has published standards on how do to these test.

Tony

[LAWise]

Thanks, that is what I am going to do tomorrow.

[rwanttaja]

My initial thought was a venting issue. Make sure you run the fuel flow test with the fuel caps and everything on.

Ron Wanttaja

[LAWise]

Okay, thanks for that I will leave the fuel cap on.

[LAWise]

Well, it looks like the flow rate is not my problem. I was looking for at least 6 or 7 gph of flow and I have 15gph. I just drained 2 gallons in 8 minutes.

[Aaron Novak]

Well, it looks like the flow rate is not my problem. I was looking for at least 6 or 7 gph of flow and I have 15gph. I just drained 2 gallons in 8 minutes.

Rough running at WOT after a few minutes combined with high EGT would lead me to ask a couple questions.

1-What is the CHT doing ( I.E. poor cooling causing an issue by getting into knock, but this usually reduces EGT )
2-Verify the valve lash, lash going tight will cause those symptoms
3-Any evidence of exhaust valve sticking? It does not take much to hang things up and cause missfires.

Being that your issue is follows a thermal change not an RPM change you can probably rule out things like valve spring surge. Ignition might be an issue to look at as has been mentioned. I assume you have verified spark timing and its not retarded?

[Dana]

Well, it looks like the flow rate is not my problem. I was looking for at least 6 or 7 gph of flow and I have 15gph. I just drained 2 gallons in 8 minutes.

Did you disconnect the fuel from the carburetor to do the test? If so, do you have a drain plug in your carburetor float bowl? If so, redo the fuel flow test through the bowl drain, the blockage could be in the carburetor. It could be the float needle, and some carburetors have an inlet screen that could be clogged.

[LAWise]

The CHT is up to about 340 degrees when I start to loose power. Excuse my ignorance but when you say "valve lash" do you mean the valve clearance? I just adjusted the valves to .006" on all valves when the engine is cold. As far as "valve sticking" I do not know how to test for that. What would I have to do to keep the valves from sticking if that is what is happening?

[Aaron Novak]

The CHT is up to about 340 degrees when I start to loose power. Excuse my ignorance but when you say "valve lash" do you mean the valve clearance? I just adjusted the valves to .006" on all valves when the engine is cold. As far as "valve sticking" I do not know how to test for that. What would I have to do to keep the valves from sticking if that is what is happening?

Hello,
Yes lash is sometimes called valve clearance. CHT seems reasonable. Valve sticking is evidenced by scoring marking on the valve stems when viewed through the exhaust port. I think some basic history on the engine would be good as there can be a multitude of things causing trouble here. High crankcase pressure caused by a blocked breather system can cause oil consumption that will raise EGT and cause the engine to run rough and lose power, incorrectly installed piston rings can do the same. Without a complete history its just shooting in the dark here.
-Aaron

[LAWise]

The history of the engine is that I bought the engine as a new kit from Great Planes. It only has about 26 hours on it. I built the engine. I do have a background in mechanics, however, that does not mean I know it all. I know I still have a lot to learn. The engine has a Compufire electronic ignition and a Aerocarb carburetor that has no float system. No starter (I have to hand prop it). It usually starts on the second or third blade.

[Aaron Novak]

The history of the engine is that I bought the engine as a new kit from Great Planes. It only has about 26 hours on it. I built the engine. I do have a background in mechanics, however, that does not mean I know it all. I know I still have a lot to learn. The engine has a Compufire electronic ignition and a Aerocarb carburetor that has no float system. No starter (I have to hand prop it). It usually starts on the second or third blade.

Ok so did you verify all the clearances and such when you built it, or did you just assemble the parts? I am thinking especially valve to guide clearance and piston ring end gaps at this point.
What was the measured compression ratio of the engine?
Is the ignition timing verified to be correct at full RPM?
Is the Cam Timing verified to be correct?
Aerocarbs are pretty simple and provided there is fuel flow running at WOT should be no issue.
What fuel are you running?
What does your breather system consist of?

-Aaron

[Eric Brown]

How about your exhaust system ? Is the system built for this engine ? A restricted exhaust flow will cause a rich mixture and reduce power output.

[Aaron Novak]

Ok so did you verify all the clearances and such when you built it, or did you just assemble the parts? I am thinking especially valve to guide clearance and piston ring end gaps at this point.
What was the measured compression ratio of the engine?
Is the ignition timing verified to be correct at full RPM?
Is the Cam Timing verified to be correct?
Aerocarbs are pretty simple and provided there is fuel flow running at WOT should be no issue.
What fuel are you running?
What does your breather system consist of?

-Aaron

I was talking this over with my dad and he brought up vapor lock isues that were common on the vw engines. How is your gascolator and fuel system isolated from engine heat?

[Dana]

Going back to the original post: Full power for awhile, then it fades. Full power recovers after low power for awhile. This sounds like a fuel delivery problem; the engine uses up the available fuel and power fades, then at the lower power setting the engine draws less fuel so it recovers, etc. Many of the suggestions don't explain the recovery after a short period of low power running. Anything temperature related would be on a longer time cycle.

[Byron J. Covey]

Going back to the original post: Full power for awhile, then it fades. Full power recovers after low power for awhile. This sounds like a fuel delivery problem; the engine uses up the available fuel and power fades, then at the lower power setting the engine draws less fuel so it recovers, etc. Many of the suggestions don't explain the recovery after a short period of low power running. Anything temperature related would be on a longer time cycle.

Ron mentioned venting problems is a post above. Note that it is possible to have a venting system that feeds fuel just fine at low airspeeds, but not well, or not at all, at higher speeds. Some of the old "blast tube in the fuel cap" type, when put on backwards, would run just fine until there was a little airspeed.


BJC

[Dana]

Good point. My T-Craft had one of those caps with the forward facing tube on the wing tank. Even if it was pointed forward, if the locking tabs inside the cap weren't tight the low pressure above the wing would draw more air past the leaking gasket than the tube could provide, and it wouldn't feed. Took quite awhile to figure out what the problem was.

[LAWise]

It looks as though the valve adjustment has solved the problem. I had just adjusted the valves before I took it to the airport to fly it for the first time, so I guess I thought it was good to go. When I just checked the valves they were a very tight .006 so I opened them up to a loose .007 and ran it up to WOT for over two minuets and I did not loose RPMs. Now I have to wait for warmer weather to try to fly it again since my plane is an open cockpit Teenie Two. I really thank you for all your help.

[Dana]

It doesn't seem like opening the valve lash from .006 to .007 would make any major difference. But you may have accidentally "fixed" it when you disconnected the hoses for the fuel flow test.

I would suggest considerably more than a two minute WOT engine run before you fly it again.

[Aaron Novak]

It doesn't seem like opening the valve lash from .006 to .007 would make any major difference. But you may have accidentally "fixed" it when you disconnected the hoses for the fuel flow test.

I would suggest considerably more than a two minute WOT engine run before you fly it again.


I agree,
There is also a temperature difference now which could be causing other fuel issues. You aren't running much head pressure on your fuel system, so it is very sensitive to vapor lock. Also your CHT was probably lower due to the colder weather, and your ignition system component temperature was probably lower. I am still thinking its thermally induced so please re-test in warm weather.

[sr2500]

One other item to check. I used a Great Plains gascolator. After a year, I changed the filter and replaced the gasket with the one that came with the filter. I had a similar problem and when I investigated, I found the large flat gasket that came with the filter was incorrect. It was partially blocking the output port of the gascolator and restricting fuel flow. The engine would begin to stumble after takeoff at high power. Throttling back would correct the problem. I swapped it out with an o-ring I found at a local tractor company. I haven't had a problem since.
Best,
Jerry Folkerts

[LAWise]

I do intend to run the engine at WOT before I fly again, however, the engine build instructions I got with the engine said not to run it at full power more than five minuets. It seems to me that a static run-up at full power for five minuets would be more harmful to the engine than if I were climbing out at 80 MPH for five minuets. The ram air in flight would be better for it I think.

[Dana]

Just keep an eye on the temperatures...

[rwanttaja]

I do intend to run the engine at WOT before I fly again, however, the engine build instructions I got with the engine said not to run it at full power more than five minuets. It seems to me that a static run-up at full power for five minuets would be more harmful to the engine than if I were climbing out at 80 MPH for five minuets. The ram air in flight would be better for it I think.
Perfectly true, but it's better for the engine, not the aircraft. An unexpected loss of power on the ground results in a furrowed brow, an unexpected loss of power in flight results in furrowed ground.

The engine build instructions allows a five-minute WOT static run, so I'd do that. I've read too many accident reports where the pilot thought that an engine problem could best be checked in-flight.....

Ron Wanttaja

[Rons Pup]

I had loss of power on two occasions. The first time the engine just slow down. The second time the engine stopped and I made an off-site landing. In both cases the weather was very hot and I discovered that my fuel line was very close to my exhaust pipe. I assumed I had a vapor lock and installed a fuel pump. Have not flown it sense but I hope it will work

[1600vw]

After a good warm up my VW started to run rough. Lower the throttle and it would clear up, then throttle back into her and she again would start running rough. This was after I had the engine bored and stroked. Bigger jugs, crank, rods and pistons. I found the oil level was a little to high. When it warmed up it started to foam. Lowering the oil level a little helped in my case. This could be your issue to. You may be using a little to much oil.

Just a thought.

Tony