912 run rough on climb or heavy load

Most likely a fuel supply issue as you get the problem only on the highest fuel demand. It is also highly unlikely to be an intake leak as that is more noticeable at lower throttle settings (higher vacuum), at high power settings you don't have much vacuum at all, so any intake manifold leak won't affect much at all at that power setting.

I would make sure you are getting at least 35 liters per hour at the carb inlets, just disconnect both and put them into a measuring container and time how long it takes to fill the desired amount with just the boost pump (as you don't want to be running the engine while doing this) and do the math from there to work out the flow rate. Don't be fooled by good fuel pressure as you can have good pressure and still have poor maximum flow. I know you aren't testing the manual pump with this method but if the boost pump puts enough out it should override the manual pump.

One place to look for a restriction is right at the carb inlets between the banjo and the float needle. Others have been driven nuts trying to find the restriction right there as all other tests won't show up in this region including the one that I just mentioned.

Hi Murray

"Most likely a fuel supply issue as you get the problem only on the highest fuel demand. It is also highly unlikely to be an intake leak as that is more noticeable at lower throttle settings (higher vacuum), at high power settings you don't have much vacuum at all, so any intake manifold leak won't affect much at all at that power setting."

You make an excellent point. I think a check of fuel filter(s) for blockage ,may be the step before doing a fuel flow check.

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Thanks again guys I will take all of this into consideration.  While I did not pump off a gallon of gas with the electric pump I did pump a cup which only took a second so I really feel that fuel flow to the carbs is not the issue.. my fuel pressure never drops either.  

Food for thought but here is my game plan for when I can get to the plane next. I want to definitively isolate fuel or ignition.  Since there is redundant ignition systems I would have had to have one fail prior and then now the other fall in the same manner.  The one simple test I did not do was to run on only 1 ignition before it breaks down as I fell it would be extremely unlikely that they would operate the exact same.  In other words I assume maybe one would break down as sob as I powered up to 5000. Then pending that outcome I intend to temporarily install a primer so I can give it a bump as soon as it breaks down t to see if it will clear or maybe get worse.  

"While I did not pump off a gallon of gas with the electric pump I did pump a cup which only took a second so I really feel that fuel flow to the carbs is not the issue.. my fuel pressure never drops either. "

Have you checked the float bowls for contamination ?

While you have them off - check for fuel flow. This will eliminate potential restriction/blockage, between carb fuel inlet & float needle valve.

I am surprised your fuel pressure does not fluctuate between engine low & high power - this may indicate a flow restriction.

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Yes I did check for contamination.  Had carbs apart twice. Pulled banjo bolt and needle and checked for any blockage there as well.  Pulled main circuit and evening was very clean.. I even replaced the diaphrams as I had an older set. Not sure what you meant about fuel pressure.. mine had never fluctuates really during power changes. Might just be the smoothing in my guage though.  But does not drop when issue arises.