Re: suzuki dt 9.9c 87
Hi there. I've also got a DT9.9C, also about 1988. I don't know how relevant this is to your problem, but anyway. I had removed the oil pump and was running premix for a while. I was concerned as every now and then there would be a small air-bubble in the oil feed line from the pump to the "Y" joint where it connects to the fuel line. After removing the pump, I plugged the hole with an aluminium plate and gasket. However, this was not enough, as the shaft driving the oil pump would rattle intermittently - and as you say, it sounds like it's coming from the top of the leg / bottom of the motor region. There's not enough room to slide it out with the engine cowling in place. Talking to a local Suzuki mechanic, having this shaft rattling loose long-term isn't a good idea, as it's driven from the crank and if it wears and/or jams, then that's potentially a big problem. He suggested that if the plate could be modified to hold the shaft in position, but of course you also don't want to jam it tight. But the mechanic also assured me that he hadn't seen these pumps (Mikuni) fail. A replacement pump is still available new, cost around US$150.
So if the pump is still in place in your motor, I'd be checking it out carefully.
With pump removed, I set it up with the full tank above, check valve on the outlet in place, and ran it with an electric drill on slow speed connected to the pump drive (I used some heat-shrink tubing as a flexible drive and to connect to the screwdriver type head of the pump shaft). In operation the shaft is driven anti-clockwise, looking from the shaft side of the pump. This showed the same problem, the pump was pulsing properly, but intermittently some air would get in. So I disassembled the pump. Remove two screws on the plate on the end of the pump. There's a small spring-loaded piston that slides up and down in a rotating cylinder driven by a gear from the drive shaft. There's not a lot that can go wrong with these things. The pump "pulses" when the main cylinder rotates, and the small piston is depressed by the raised ridge on the backing plate. But when the piston drops off the ridge as it rotates past where it "pulses", the pump sucks back a little. The check valve in the oil line between pump and fuel line needs to be working - it needs a little pressure to open - that's normal. I checked oil seal, and O ring on the backing plate for leaks under a little pressure, and they seemed fine. My problem was the hose between the pump and the check valve was old and hard, not sealing perfectly, so that when the pump "sucked back" between pulses, a little air must have slipped in at the pump end connector. I couldn't buy this thick-walled hose anywhere. Thinner walled hose is not going to work properly. Replacement hose and new clips were available from Suzuki. I replaced the other hoses as well, then reassembled pump hoses and tank, bench tested again. This time no problem. I've refitted the pump to the motor, running on 50:1 premix to be double sure. The new hose is much clearer, so I can see if air bubbles are in that part of the hose, not just if bubbles appear between the check valve and "Y" joint. I have run the motor for at least an hour in a tank, and no problems - no more air - the oil-injection system is working fine.
I was looking at replacing this motor - auxiliary on my 25' trailer yacht. But the DT9.9C with extra long shaft, sail drive prop with conversion flange for rear-exhaust flow in reverse, electric start, and decent charging circuit would be expensive to replace.
I have also just removed the cylinder head, thermostat cover, exhaust heat-exchanger assembly, and cleaned it out thoroughly. It has had 20+ years mainly salt-water use. Salt/corrosion build-up between barrels and cylinder sleeves was pretty bad. Water channels in the head actually not too bad. It is still running on the same set of plugs it had when I bought it 10 years ago - a new spare set have been sitting in my boat for 10 years, waiting for the day that I couldn't start the motor or it was running rough - that's never happened yet, and the plugs are still good and clean. There was minimal carbon build-up on either the head or piston crowns, bores look almost new - no significant scuffing. The only problems replacing cylinder head is that I couldn't remove the anode in the block, and that's in the way of cleaning the remains of the old head gasket from the surface of the block easily - which you need to do, and the bottom (#7) head bolt isn't accessible with a torque wrench with the cowling in place (no problem as you can get a ring spanner on it, and with torque wrench held at the length of the spanner on another bolt, estimate torque pretty closely. After cleaning it out, the water tell-tail is now working reliably, as well as general peace of mind that the water cooling passages are now all clean and unobstructed.
