Yanmar 2gm20f not going over 2k rpms

Jun 11, 2019
21
Hunter 30 Port Canaveral
I just recently bought a 79 hunter 30, first sailboat ever so I'm still learning. When I first got the boat the motorwould give lots of black smoke and notgo over 1200rpms. Since then I have had the prop and bottom cleaned, prop and shaft were pretty bad but bottomlooked clean. Also I have done fuel filters, added biocide and a stablizer to the fuel. I have done oil, oil filter, air filter, cleaned raw water strainer, allnow hoses, new mixing elbow, and new belts. It now runs good , no blacksmoke but won't go past 2000rmps. I'm getting 5.5 knots at full throttle at the2000rmps, no smoke at all. Any helpwould be appreciated
 
May 17, 2004
5,032
Beneteau Oceanis 37 Havre de Grace
I presume it goes above 2000 in neutral?

Do you know if it is the original prop, or what its size/pitch is? Being over-propped could hold the RPMs low.
 
Jun 11, 2019
21
Hunter 30 Port Canaveral
Actually no it goes about the same in neutral... And I have no idea on the prop, something I'll have to look at
 
May 17, 2004
5,032
Beneteau Oceanis 37 Havre de Grace
Try checking it with an optical tachometer, to confirm the one in the boat is reading accurately?
 
Jun 11, 2019
21
Hunter 30 Port Canaveral
I will have to go get a optical tachometer to check it. Where do you measure the rpms at on the motor?
 
May 17, 2004
5,032
Beneteau Oceanis 37 Havre de Grace
Crank shaft pulley should work. The other pulleys will spin at different rates depending on their size, but the crank shaft is the engine speed, and is generally pretty accessible.
 
Jan 19, 2010
1,169
Catalina 34 Casco Bay
If your RPMs are correctly reading at 2K, then it would appear a fuel delivery issue is at hand. Several possibilities: fuel pump, fuel tank vent, tank shut off valve and lastly injectors...
 
Jun 11, 2019
21
Hunter 30 Port Canaveral
Ok... I have a few things to go on now, I will check the the rpms with the optical tach. I'll look into the shut off valve and the vent hose , also check the prop size and see what I get from that
 
Nov 22, 2011
1,192
Ericson 26-2 San Pedro, CA
If your RPMs are correctly reading at 2K, then it would appear a fuel delivery issue is at hand. Several possibilities: fuel pump, fuel tank vent, tank shut off valve and lastly injectors...
Don't forget the governor.
 
Nov 22, 2011
1,192
Ericson 26-2 San Pedro, CA
Actually no it goes about the same in neutral... And I have no idea on the prop, something I'll have to look at
Clearly you have a problem that has nothing to do with the prop then. Get that solved first and then if there should also happen to be an issue with the drive train you can address it at that time. But deal with one thing at a time. Before you do anything else you need to figure out why it can't go above 2K even in neutral.
 
Jun 11, 2019
21
Hunter 30 Port Canaveral
Ok, glad you pointed that out, didn't put it together about it being in nuetral and the prop not spinning:banghead:..lol. I'll look into fuel delivery and throttle cable and governor
 
Jan 27, 2008
3,045
ODay 35 Beaufort, NC
Do you have a cylindrical electric fuel pump? Those have a filter in the bottom that can clog, and check for any leaks in your fuel lines or connections. Check the fuel pick up tube in the tank for any clogs. Fuel could be old and full of crud. Try running off a small portable fuel tank with fresh fuel.
 
Jun 1, 2007
265
O'Day 322 Mt.Sinai
Verify the rpm number is correct before you do anything. You may be getting more than 2K now. In neutral, that's your no-load WOT (Wide open throttle) RPM. If you're underway, you should also check the RPM at WOT. The RPM should be pretty much the same if your prop is sized correctly.

So, for now, find another way to measure RPM to validate your tach. I was able to borrow a friends electronic tach to verify the real RPM as opposed to my tach. It's like a timing light, for which you add a small piece of reflective tape to the crankshaft pulley and read the rpm.