My Yanmar 3GMF (1983 Hunter 34) does not run above 2500-2600 RPM"s in neutral or in gear. The manual calls for 3600 wide open. It has run at the same RPM's since I got the boat three years ago. I have changed the fuel filter and have removed the entire mixing elbow to clean but stil the same RPM's. A couple of questions: 1) Is there any special way to clean the elbow? I don't know how "clean" the passages need to be. Upon initial inspection it doesn't appear to be blocked but there is alot of carbon in there on the walls. 2) If it's not the mixing elbow what do I look to next? I have very little smoke at the exhaust. Sometimes when I start the engine after a long layoff it will smoke a little (blueish). The only other time it smokes is when I'm docking in my slip and I need to give it lots of power quickly to maneuver. Thanks for any advice. Joe Mullee
Low RPM's. Not The Filter Or The Mixing Elbow
- Thread starter Joe Mullee
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I think you should remove it.
Joe: I think you should remove the elbow for the inspection. I would have your prop checked at a good prop shop. You can also have an overload problem with the prop if it is pitched to high.
Joe: I think you should remove the elbow for the inspection. I would have your prop checked at a good prop shop. You can also have an overload problem with the prop if it is pitched to high.
Excuse The Obvious
...but one suggestion would be to remove the throttle cable and activate it at the engine manually to see if you are getting a full throw. At least that would be my next step, assuming you think the RPM's are being displayed correctly. Rick D.
...but one suggestion would be to remove the throttle cable and activate it at the engine manually to see if you are getting a full throw. At least that would be my next step, assuming you think the RPM's are being displayed correctly. Rick D.
Second Rick's responce
I have to agree with Rick, remove the exhaust at the engine, 4 bolts and clean out all three sections. You mentioned there is smoke on rapid throtle up, Has the boat sat for any extended period of time? how long? Have you ever serviced the injectors?
I have to agree with Rick, remove the exhaust at the engine, 4 bolts and clean out all three sections. You mentioned there is smoke on rapid throtle up, Has the boat sat for any extended period of time? how long? Have you ever serviced the injectors?
Back Pressure
Back pressure from the exaust manifold. 20years is about the correct time period. They start to decay and clog the line creating a back pressure which in turn prevents the engine from reaching full RPM. Unaddressed the RPMs should continue to decrease. My buddy's Hunter had this problem.
Back pressure from the exaust manifold. 20years is about the correct time period. They start to decay and clog the line creating a back pressure which in turn prevents the engine from reaching full RPM. Unaddressed the RPMs should continue to decrease. My buddy's Hunter had this problem.
Check your prop pitch
If you are not getting the correct RPM it is more likely you prop pitch. Too high means an overloaded engine that cannot reach its designed rpm. Mike Gibson
If you are not getting the correct RPM it is more likely you prop pitch. Too high means an overloaded engine that cannot reach its designed rpm. Mike Gibson
Calibration?
It could just be that the tac is calibrated to that reading if its always been that way. When my tac was replaced i had a 600rpm difference. If the boat cruises at the proper speed when motoring it could just be a calibration issue.
It could just be that the tac is calibrated to that reading if its always been that way. When my tac was replaced i had a 600rpm difference. If the boat cruises at the proper speed when motoring it could just be a calibration issue.
Joe,
With the problem occuring in neutral and in gear identicaly, I don't think it's your prop. Call Mac Boring and get the gaskets for the manifold to the head, and manifold to the ehxaust riser. You can do this job, Drain the coolant out of the manifold, both fresh and raw water. Disconect the throtle cable. Startin at the front, remove the small hose from the raw pump, then the large hose from the circulating pump, NOTE this on is hard to get to, kind of under the raw hose. On the top front of the exchanger is a hose to the thermostat, just loosen the clamp for now. Around the side you'll find 4 bolts and 2 nuts, all 12mm remove them. The manifold will not fall off, the 2 nuts are on studs that will support the manifold. Unbolt the exhaust elbow assy. Should be 4 bolts with 13mm heads, remove the hose from the elbow to the heat exchanger. Set the elbow assy, aside If you have enough room to slide the manifold to stbd and clear the studs it easy, take it off. Other wise you'll need to remove the studs to get the manifold off. I'm betting the passages in these are greater the 50% clogged Pat McCartin
With the problem occuring in neutral and in gear identicaly, I don't think it's your prop. Call Mac Boring and get the gaskets for the manifold to the head, and manifold to the ehxaust riser. You can do this job, Drain the coolant out of the manifold, both fresh and raw water. Disconect the throtle cable. Startin at the front, remove the small hose from the raw pump, then the large hose from the circulating pump, NOTE this on is hard to get to, kind of under the raw hose. On the top front of the exchanger is a hose to the thermostat, just loosen the clamp for now. Around the side you'll find 4 bolts and 2 nuts, all 12mm remove them. The manifold will not fall off, the 2 nuts are on studs that will support the manifold. Unbolt the exhaust elbow assy. Should be 4 bolts with 13mm heads, remove the hose from the elbow to the heat exchanger. Set the elbow assy, aside If you have enough room to slide the manifold to stbd and clear the studs it easy, take it off. Other wise you'll need to remove the studs to get the manifold off. I'm betting the passages in these are greater the 50% clogged Pat McCartin
Follow the suggestions below.
I would change the mixing elbow as a matter of course. It probably needs it anyway. Otherwise, it will bite you soon. If you are overpropped (too much pitch) the engine will be overloaded and smoke, also and you won't hit your rpm's. Another thought: Low rpm's means fuel starvation. If it is constant, as you say, it may be a clog somewhere. You have changed your filters - OK. Now check your fuel pick-up tube in the fuel tank. It may be partially clogged and starving the fuel. Have you also changed your primary fuel filter? That could be a source of fuel starvation, also. Good Luck!
I would change the mixing elbow as a matter of course. It probably needs it anyway. Otherwise, it will bite you soon. If you are overpropped (too much pitch) the engine will be overloaded and smoke, also and you won't hit your rpm's. Another thought: Low rpm's means fuel starvation. If it is constant, as you say, it may be a clog somewhere. You have changed your filters - OK. Now check your fuel pick-up tube in the fuel tank. It may be partially clogged and starving the fuel. Have you also changed your primary fuel filter? That could be a source of fuel starvation, also. Good Luck!
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