50hp Perkins won't rev past 1900rpm

[Martinini]

Had the same problem with mine. Check for a one way ball valve ( and here is the good part) inside the Fuel In line to the fuel pump. the 5 mechanics I had working on mine never found it and it was semi stopped up and would choke off the fuel supply and kill the engine at higher demand rates, hence around 1800 rpm! I started at the tank replacing all the fuel lines and found that stinker by accident. Cleaned it out with WD 40 and have had NO problems since.

[SkiprJohn]

Originally Posted by Hoohaa

Just hooked up to a mooring at the zoo on Sydney Harbour. Cold beer, good food & 6.4 knots at 1400 rpm.

Sent from my GT-I9505 using Cruisers Sailing Forum mobile app

I think you are overpropped but that's another subject.

[westwinds]

Originally Posted by Hoohaa

When I bought my boat, over 2 years ago now. The engine had a reving problem. It would not rev over 1900rpm, we flattened the batteries one day and had to leave the motor running for a couple of hours to get everything fully recharged. At the same time we got a mechanic to call in and have a look at the reving problem. He came on board after the motor had been running for a couple of hours and when he revved it up, presto. She rev
ed over 2200 rpm. The problem had fixed itself..
Well, it’s doing it again.
The engine will only rev to 1900rpm while in neutral (cable connected and disconnected) and only to 1100rpm when under load.
The only difference this time is that there may have been a trigger for it. The last time I filled up, I forgot to put the fuel back on when I motored away and promptly sucked air into the fuel system...

There is reving problem in neutral so not propeller I think. Put an electric fuel pump on it and see if fuel flow is adequate, like more than a pint a minute. You could also pull injectors, crank engine and observe spray pattern. The best thing to do is pull injection pump and injectors and have them bench tested at a repair shop.

[Irish rambler]

If the engine has an intermittent revving problem I would first check that the fuel tank is clean and all the fuel lines are cleaned out and joints checked for leaks, (both fuel and air) before going any further.
Check the gauze & non return ball valve at the lift pump before buying anything.
I once repaired an engine with a tiny air leak at an over tightened fuel pipe olive, the guy was too enthusiastic and had squashed the olive and deformed it allowing a tiny seepage of air into the fuel line, not enough to stop the motor but enough to stop it revving fully out.
I hope this helps.

[Hoohaa]

Update. My Perkins is also a Volvo Penta D2-55.

We are still having rev problems with occasional overheating thrown in, just to keep us on our toes.

After talking to a bloke from Volvo at the Sydney boat show recently, who sugested that my rev problems could be my "Exhaust Elbow Riser". Apparently they can become blocked & corroded where the water inlet mixes with the exhaust fumes and should be replaced about every 5 years.

This sounded feasable to me and could also be the cause of the over heating.
The over heating doesn't always happen. It ocures if I push the motor hard for a bit and the last time it accured was after we motored for about 1.5 hours anchored for lunch and then motored for about 20 min but not pushing it. The one common thread with the overheating seems to be after we have been running the motor for a while. ie it doesn't over heat straight away. It lulls you into a false sense of security (tricky situation) and strikes when you least want it too.

This brings me to my latest project.. Fixing the Exhaust mixer elbow (i will call it that for now as apparently it is called a few names).
Here are some pics, please feel free to post your ideas theories and opinions.
Before

Heat exchanger.jpg Views: 155 Size: 82.1 KB ID: 106991" style="margin: 2px" />

[SkiprJohn]

Originally Posted by westwinds

There is reving problem in neutral so not propeller I think. Put an electric fuel pump on it and see if fuel flow is adequate, like more than a pint a minute. You could also pull injectors, crank engine and observe spray pattern. The best thing to do is pull injection pump and injectors and have them bench tested at a repair shop.

Hooha mentioned going 6.4 knots when his engine is well below peak hp and fuel efficiency performance rpm. That's why I mentioned that he might be overpropped. Like I said that would be another subject.

Sucking a tank dry means you've definitely got some foreign bodies in your fuel system so going through the whole system and cleaning it up wouldn't be a bad idea.

[Hoohaa]

I am almost certain this is not a fuel issue. We have allready done allot of work on this side of the equation.
Can a restricted exhaust cause these problems? Ie. Reduced revs.

[SkiprJohn]

Originally Posted by Hoohaa

I am almost certain this is not a fuel issue. We have allready done allot of work on this side of the equation.
Can a restricted exhaust cause these problems? Ie. Reduced revs.

Definitely it can cause rpm loss and overheating. Same for air restriction on the intake side can cause low rpm.

[SkiprJohn]

Originally Posted by Andrew Troup

Exhaust/cooling water injection/mixer gooseneck/elbow, notorious for clogging on some engines in particular, (because sudden temp change causes condensation inside, which in turn encourages particulates to build up, notably carbon)

The constriction will cause back pressure and prevent a motor from revving freely.

Here's post 212 from this thread.

[SkiprJohn]

Originally Posted by sailorchic34

Oh a mixing elbow or discharge elbow, where the raw water discharges into the exhaust. The cool raw water cools the exhaust gases. This causes the carbon in the exhaust gas to condense and build up over time. The minerals in the raw water also will drop out due to the hot exhaust gases. The carbon builds up over time, increasing back pressure, until the engine can not operate at full speed. Mine was 1/2 blocked after 800 hours, though still ran fine.

One of the symptoms of a clogged discharge "mixing" elbow would be an engine not able to reach rated RPM's.

It's a maintenance item on my boat that I check every 300 hours or so. I'll take it off and run a hose stream of water through the exhaust gas side, should be no backup at full flow. I use a small dog chain snaked through the elbow, with a sawing action to clean the rocks out. I've gotten pretty big chunks out of it.

Here's post 214

[SkiprJohn]

Originally Posted by edwmama

John tank you for your kind response

I'm only trying to rev the engine while tied up at dock

And in neutral. Supposedly at 1500 cruising RPM is the optimal for gas Economy
And is the sweet spot.

Someone on the web had suggested checking mixing elbow assuming the fuel side is clear, and also the lift pump filter screen

Thanks again
Edwmama
(Edwin)
Cal 46 Lahlia

Here's post 184

[SkiprJohn]

Originally Posted by sailorchic34

Have not read every response, But a clogged mixing elbow will cause the low rev issue for sure.

Here's post 195

[SkiprJohn]

Too large a prop or too much pitch on the right diameter prop can cause overheating. The sweet spot you're looking for is nearly full hp at whatever rpm that is for your engine at rated hull speed for your boat. Every kind of engine is different. Some engines don't develop their rated hp until well over 3000 rpm. Usually a repair manual or operators manual will give you fuel economy over all the rpm range. You want your best economy at about hull speed.

Good luck in sorting this all out. I'm really glad you got your engine running now all you have to do is getting it not to overheat.

[Hoohaa]

Thanks Skip. So what your saying is. Go back and read my own thread. Great advice. I guess when these first appeared I didn't know what this part was. Couldn't see the forest through the trees. I am making a call today to see if I can buy a new elbow. I will post the results.

[SkiprJohn]

Originally Posted by Hoohaa

Thanks Skip. So what your saying is. Go back and read my own thread. Great advice. I guess when these first appeared I didn't know what this part was. Couldn't see the forest through the trees. I am making a call today to see if I can buy a new elbow. I will post the results.

You got it! I didn't realize there was a part identification issue so sorry for the snippiness. Can't you clean out the old one sufficiently?

If you want to check out the prop size that you might need for your engine, you'll need to know your max hp at rated rpm and the gear ratio on your transmission as well as the weight of your boat and the waterline length. Then you can come close.

Do you know the size and pitch of your propeller now?