View Full Version : Hunting and lack of power

04-13-2009, 01:02 AM
"Hi all, I own a Volvo AD31B 130hp with DP drive (A2 props), originally the boat was in Bundeburg and would cruise at 16 knots @ 3600 rpm using about 9 lts/hr, top speed about 21 knots at 3900 rpm. Now it resides in Darwin. My problem is the best I can achieve is 3700 rpm at which stage the engine hunts by up to 200 rpm and top speed is about 14 knots, fuel economy is down to about 20 lts/hr. I have reached comparable performance (fuel economy not included) at one stage on a fresh water dam after changing out fuel lines and filters, but the problem returned 1st trip out in the harbor. I have since had the top off the fuel tank and thoroughly cleaned the tank, the Turbo reconditioned, the injectors have just been reconditioned and the lift pump is fine. I have fitted the boat out extensively but don't believe the overall weight is greatly different than when it was used in Bundy.
I will be grateful for any suggestions of what to try next."

04-13-2009, 09:55 AM
"When you fitted the boat out was the prop serviced at all? What where the major changes? Did some of the changes move weight in the boat forward or aft? Is there any smoke at any time at any RPM especially at the top end? When you say the lift pump is fine, was it replaced and have you tried by-passing it with an electric fuel pump to see if it helps at all. This is an easy and inexpensive test to perform. Make sure you use a proper demand fuel pump with the correct output. Do not by pass any filters. What type of primary filtration do you have? ie; racor. If it is a racor like the 500 series has it been fully service like new check valve and seat etc;. I have seen them plug up inside or have a stuck check valve. Since the performance improved after changing a filter I would suspect the fuel system. I had one customer that plumbed his racor backwards, meaning the out was in and the in was out, the only reason it ran at all was the check valve which is an aluminum ball was stuck open. The engine ran poorly from Brazil to St Maarten and final quit after the check valve finally worked loose and stopped the fuel flow. I would service the entire fuel filter and put an electric fuel pump by passing the mechanic lift pump and see what happens before I started digging into more expensive solutions. If that works then replace the lift pump. Was the fuel tank cleaned before or after the problem started? I had another customer who plumbed the fuel return line to the suction on the tank and vice versa. On this tank the return took a 90 degree bend at the bottom of the tank and as he was using it for the suction line it was sucking up algae in the tank. The tank had been cleaned but some hard reach places (behind baffles) still had algae hiding behind them and when they became dislodged it would plug the fuel line. The normal fuel pick up is suspended about an inch from the bottom of the tank making it harder for debris on the bottom to be sucked up. I hope this helps."