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Ben,
Have you found a solution to the problem yet? I've been experiencing the same thing - description and attempted solutions follow:
I've been experiencing a front engine anomaly since buying the aircraft two years ago. N462DA is a 1968 C337C which started out life as a turbo-charged model, and which was subsequently converted to normally aspirated engines, about 10 years ago. So its fuel system is that of a turbo-charged model, although its engines are normally aspirated Continental IO360s, both factory re-mans. The front engine has about 1.5 years and 140 hours since factory re-man; the rear one has about 1150 hours since factory reman.
I usually climb at 25in/2500RPM to 6,000-8,000 feet, then set 65% power @ 50 degrees rich of peak. After 30 - 45 mins the fuel flow to the front engine starts oscillating, dropping slightly below 9.6 GPH and then right back up again, too slightly to notice. After a few minutes (maybe 5 mins) of gradually increasing amplitude, the anomaly becomes noticeable as a slight power drop, then recovery of power. This gets more and more noticeable, as the fuel flow drops to around 6 GPH. Application of the low boost pump for 30 -60 secs cures the problem, and the engine runs fine for another 30-45 mins, after which the anomaly recurs. During the anomaly EGTs rise and CHTs descend. An EDM 760 with fuel flow option is installed, and I've graphed the anomaly.
We checked the fuel tank vent system, and it's fine, as are all the tank cap vents. We removed the vapor return line from the engine driven fuel pump and blew through it, and heard bubbling in both the left main and left aux tanks, so it appears the vapor return line off the front engine driven fuel pump is un-blocked. I've switched fuel tanks from left main to left aux to X-feed during the anomaly, and none of this helped, so it appears it's not a problem with the fuel system upstream from the fuel selector valve. The anomaly has occurred while feeding both from the left main and from the left aux tanks. Opening the cowl flaps doesn't help. I insulated the fuel supply line from approx. 2 feet behind the firewall all the way up to the engine-driven fuel pump.
The front engine driven fuel pump was removed and sent to Continental, who bench checked it. The 600 RPM setting was fine, but the 1600 and 2600 RPM settings were approx. 10 - 20% low. Continental set these flow settings to midrange, and the fuel pump was re-installed on the aircraft front engine.
A few flights back I tried climbing at full power, then setting 65% power and leaning to 50 degrees rich of peak, and the anomaly occurred almost immediately, and also occurred on the rear engine for the first time since I've been flying the aircraft. Application of low boost pump to both engines cured the problem, albeit temporarily - it re-occurred in the front engine within 30 mins. The next flight I climbed at 25/2500, set 65% power and leaned to 50 degrees ROP, and the anomaly occurred in its usual pattern: in the front engine (after a 30 min. delay), and did not occur at all in the rear engine. Is it possible the problem is vapor lock somewhere, and climbing at full power heated up both engines more than the usual 25/2500 climb power did, and this exacerbated the problem in the froth engine, and caused it to occur in the rear engine also? Continental's position has always been that the problem couldn't be the fuel pump because this isn't the way that this fuel pump would fail - it has no altitude compensation - it just turns its little vanes, has the fuel/vapor separator on top, a jet (that wouldn't go bad with such regularity), and a fuel return line down its side to its lower housing.
The problem persists. I've installed GAMIjectors and now run approx. 20 degrees LOP (that's as lean as I can get without running rough), and the problem still occurs regularly. Let me know how your efforts are going.
Thanks!
Paul462
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