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2GM20F doesn't reach full rpms

8K views 18 replies 10 participants last post by  CarbonSink62 
#1 ·
Hi folks, looking for tips where to start, I have no experience with diesels. The motor starts easily, and runs smooth. It will rev to 3600-3700 under no load. Under load it only gets to 2600-2700 rpms. At full throttle it does put out some visible black smoke, barely noticeable, it doesn't seem excessive to me. The fuel in the filter is amber color, looks clean. I have paperwork from max prop recommending the prop size based on a 2.6 to 1 gear ratio. The trans model number is RM2P, can anyone confirm the gear ratio?
One last thing, could anyone post a picture of the governor, and how to lube it?
Any and all help will be appreciated, thanks in advance......


Rich
 
#4 ·
Thanks for the reply, but a fouled bottom can be eliminated. It was just painted in Apr. and is used regularly. The bottom is clean as a whistle......

Does the fact that it will reach 3600 rpms with no load eliminate a partially blocked exhaust elbow as a cause? I would like to eliminate all other possible causes before pulling the prop........

Thanks
 
#9 · (Edited)
Thanks for the reply, but a fouled bottom can be eliminated. It was just painted in Apr. and is used regularly. The bottom is clean as a whistle......

Thanks
If you are in salt/brackish water, just because the bottom is clean as a whistle, doesn't mean the running gear is clean. In salt water, it can build up with barnacles very quickly.

Also, has it been this way ever since you have had the boat or is it just more recent?
If recent, I'l look at the running gear being fouled. If since you have had the boat, I'd look at it being over-propped.
 
#5 · (Edited)
Its a clogged exhaust elbow….VERY common on Yanmar engines. There are 2 solutions.

1. Replace it.

2. Remove it and soak it for a week in distilled vinegar.

I did the second. Worked for a while. Never was completely fixed…but mine was more drastic. Wouldn't rev over 1000 RPM under load. Black smoke. In neutral revved just fine.

Some people say they remove it and soak it once a season as part of the yearly maintenance.

eBay has some stainless steel and aluminum ones for 1/2-1/3 the price of the Yanmar exhaust elbows and those state they won't clog after a few years. I'm very tempted to try one of those instead.
 
#6 ·
Its takes 'work' to move a boat; therefore as a first attempt, your current propeller pitch probably should be lessened by approx. 2° ... the next time you haul your boat. This pitch change should be in accordance with the MaxProp instruction/owners manual.
3600 rpm at max. (unloaded) rpm 'times' .8 (3600 X .8) would give you a max. cruising rpm of aprox 2800 rpm - so youre probably close to optimum, but not there yet.

That you have black smoke at wide open throttle (WOT) means that your engine is slightly 'lugging' (not reaching FULL rpm, which means not reaching full horsepower output). Lugging is the development of extreme combustion chamber pressures which puts excessive pressure on the piston rod wrist pins, crankshaft bearings, etc. and the output result is inability to reach maximum horsepower/Kw output plus 'black smoke'.
That you have a MaxProp ... life is easy for you: Next time you normally haul your boat, decrease the propeller pitch by 2° and see if that propeller pitch decrease results in higher operating (@ full load) rpm thus HP output.
MaxProp (PYI) has some very good videos on how to 'correctly' change the pitch.
 
#10 ·
Ok, some background, I just bought the boat. Without survey. I know, I know.......:hammer But with the condition of everything else, I assumed that risk.
Maybe I'm confused. I was under the impression that I should be able to reach max rpm, or at least close, under load. From what I read that would be 3400-3600. RichH seems to think 2800 would be optimal. I will dive down and check the prop for fouling first, then pull the elbow and clean it. Before that though, I would like to hear what rpms others with 2GM20 Yanmar's are seeing. What rpms do you normally cruise at, and what is the max rpm under full power? Thanks again for all responses.

3600 rpm at max. (unloaded) rpm 'times' .8 (3600 X .8) would give you a max. cruising rpm of aprox 2800 rpm - so youre probably close to optimum, but not there yet.
 
#7 ·
It sure sounds like a classic over-propped condition. Is it a new or changed situation?
 
#11 ·
From what I understand, max RPM is 3600. A good rule of thumb for cruising (with any diesel) is 80% of maximum which would be 2880.
Now if you get your engine to max out at, say 3400, then cruise at 2700.

If you have only been getting 2700 since you had the bottom cleaned, I'm betting on over-propping, assuming the engine is running at its best.

My Universal runs at best, 3100 with freshly cleaned/painted bottom. I normally cruise at about 2500. As my running gear starts to load up and I drop a couple hundred RPM's, I will also drop my cruising RPM accordingly.
 
#12 ·
There's several factors that will influence your engine's HP output and its operating rpm.

Foremost is the infamous 'hull speed' at which the displacement hull boat produces a larger bow wave that it simply cannot 'climb' over. So, what you have to do if first - yourself - construct a graphical 'plot' of engine rpm VERSUS boat speed (speedo or GPS and when in smooth flat and zero current water).

Then look at the engine's specific rpm to horsepower output - a chart in your owners manual, etc.
Then scrutinize exactly where the 'curve' / plot that you constructed begins deviates from a straight line as you measure boat speed vs. engine rpm ... and then notice that the boats bow is now 'rising' when at faster speed - indicating that youre now moving the boat in that transition speed approaching 'hull speed'. Then compare the Yanmar published torque or horsepower output curves VERSUS engine for several rpm values that you recorded versus increasing boat speed.
1. If youre not 'anywhere near' hull speed then you probably have a prop pitch problem, and you probably should do a trial pitch reduction on your MaxProp in order to allow the engine to spin at a higher rpm; hence higher horsepower output.
2. If youre 'close to' or are approaching hull speed, the boat will struggle to go faster and the engine will struggle to produce more output horsepower, and changes in prop pitch are probably not going to make much difference in boat speed ... even if you reduce the propeller pitch to allow a high rpm.

FWIW - cruising rpm is anywhere along that straight portion of the curve (speed vs. rpm), lugging of the engine will occur when your 'plotted curve' starts to deviate from a straight line.

Again, suggest that after scrutiny of the bottom and prop for fouling, etc., simply wait until the next time you haul out, reduce the pitch on your MaxProp by 2° (one MaxProp 'index') as a trial, and then compare the differences in MaxProp settings with respect to engine rpm output vs. boat speed.
It takes 'work' (horsepower) to move a boat, that amount of 'work' is deducted from the max. engine speed at no load (in neutral). For a boat reaching near hull speed, the higher the bow rises, the MORE work it takes and the result is lower (max.) rpm if youre already at 'max' possible engine HP output due to the wrong prop pitch.

For reference: your 'hull speed' is 1.34 X the square root of your waterline length. ... and this is only an approximate value.
 
#13 ·
I'd add one thing to Rich's excellent post - the torque peak RPM is the most fuel efficient running speed for an engine. 80% of max RPM is a piece of conventional wisdom that has little to do with broad reality - it's just safe.

My 3GM was rated at 3600 Max and 3000 Continuous - Continuous is just that, it can run there long term.

The Yanmar manual will have HP and Torque curves charted in it. Look up the RPM at which peak torque occurs and work from there - you want to achieve hull speed at that RPM, maybe with a bit in reserve for headwinds etc.
 
#14 ·
Ok, update... I have learned a lot from this. According to my manual the torque peak for a 2GM20F is 3000 rpms. That also happens to be where it is most fuel efficient. However, the torque and fuel consumption curves are pretty well flat from 2200-3200 rpms. Hull speed, by my calculation, is 6.4 knots. On the water testing shows the boat at 5.3 knots @ 2200 rpms and 5.6 at 2400 rpms, with no visible smoke. I didn't bother to push it to 2600 because it does start to smoke a bit. This leads me to agree with the posters who said it was slightly over propped. In searching the web, it seems that over propped is the normal condition for most boats with this motor. By that I mean that most people run their motor in the 2400-2800 range at normal cruising speed. So, for now, I'm going to leave well enough alone until the next time she is out of the water. At that time I will reduce pitch by 2-4 degrees. From everything I've read, these motors love high rpms. I would be more comfortable if the rpms were 2800 to get that 5.6 knots, and still have a little left in reserve. If anyone thinks I'm on the wrong track, post up, suggestions are appreciated....
 
#15 ·
Sounds like a plan. A prop shop can tell you how much 2 degrees and 4 degrees will raise your RPM so you can choose before you take it off and deliver it to them.
 
#16 ·
I installed the same engine in my UFO 34 a couple of years ago and had the same problem, engine would only rev to about 3,000 rpm. Played with the pitch of the prop and that fixed the problem and it now revs to around 3,800rpm max and i generally cruise at 3,200rpm. I have a Autostream 2 blade self feathering propeller which you can adjust the pitch in the water. Took a couple of dives to get it right.

Ilenart
 
#17 ·
You can still hut 3600rpm in neutral with partial choked mixing elbow.its when you put into gear that the choke becomes a problem.
You'll be surprise how bad the chokage could be. A hole of 1.5" diameter could become finger width!!
 
#19 ·
I had a similar issue on my way back from Casco Bay last week. My 2GM20 wouldn't go past 3,100.

I worried all day and then saw the prop fouled with seaweed when I got back to my mooring.

Does it make sense that fouling the prop would reduce RPMs? It made sense to me.

I need to check before every trip now.

Ken
 
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