135 Perkins RPM's Issues

Hi everyone. I just recently purchased a 40ft cruiser with a 135hp Perkins. Its and old motor but seems to work fine except I'm having an issue with the rpm's. The guy I bought her from said he used to run her at 1800 rpm but she could go to 2600 rpm flat out. In neutral she'll actually go over the 2600 rpm's but in gear I can't get her to go over 1800. Any ideas what would cause this. Other boats I've had never had this issue. Thanks

Darren

Encountered the same with a Perkins 6.354.The secondary fuel filter on engine was restricted. Changed filter and back to WOT underway.

Is bottom and prop clean? Is boat loaded with more stuff (weight) than when it could make 2600? Prop been changed since then?

Nothing has been changed regarding the boat and its weight and the bottom is a bit dirty but I will try the fuel filter until I can get her hauled out of the water.

In addition to previous suggestions, does the temperature stay around 175 - 180 when you give it prolonged full throttle when in gear? Also, are you confident the previous owner could recently get 2600 RPM? Or was it long ago?

Temp stays on 180. Has for the previous owner, I do believe him because when I questioned him about the issue he said the same thing about the fuel filter and thought it was strange. I've known him for quite some time and he never seemed like the kind of guy to bs anyone especially me, but then you never know someone's true colors till they shine through I was always told.

dardurfr said:

Temp stays on 180. Has for the previous owner, I do believe him because when I questioned him about the issue he said the same thing about the fuel filter and thought it was strange. I've known him for quite some time and he never seemed like the kind of guy to bs anyone especially me, but then you never know someone's true colors till they shine through I was always told.

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Then as suggested by DK and Ski either there is likely a fuel system issue, prop is too big or you're dragging a garden around.

BTW, years ago I beard about a low hour 1800 RPM genset that was converted to marine use. The necessary revamps for higher RPM fuel delivery and governor settings seemed an issue.

Change primary and both secondary elements. Sure sounds like a fuel restriction.
Btw there is a fine strainer on the mechanical fuel pump “under the dome “ as well as in the injection pump.

"Btw there is a fine strainer on the mechanical fuel pump “under the dome “ as well as in the injection pump" Thanks for the info, had no idea going to clean lift pump strainer, injection pump rebuilt recently ,strainer ok.

David Kimmel said:

"Btw there is a fine strainer on the mechanical fuel pump “under the dome “ as well as in the injection pump" Thanks for the info, had no idea going to clean lift pump strainer, injection pump rebuilt recently ,strainer ok.

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I dont recommend going there unless the primary and secondary filter changes don't solve the problem. You can't get parts for the lift pump anymore. You get to buy a new pump.

passed along to me by a diesel guy.....

" if the exhaust is not belching black smoke the engine is not overloaded by prop or bottom condition or towing something. It simply does not get enough fuel to increase power output. The OP never said a word about the boat speed and that is a critical piece of information that is missing. Is it running slower than before or what might be expected?

If it is not the fuel filter(s) then it may be the fuel stop on the governor. It doesn't take much fuel to reach high idle. It might be something as simple as a misadjusted or loose throttle cable "

The previous owner used to run the engine @ 1800rpm's and get 7 knots or a little better. I'm running her at what I'm getting @ full throttle 2050rpm's and just barely making 6.5 knots. He told me she should turn up around 2600rpm's. Haven't had the chance to change the fuel filters but hopefully get aboard this week and do so. I'll report any new findings.


Darren

Id be checking the prop shaft speed its easy to lose 250 rpm in clutch slip

Is your tachometer accurate?

I have analog tachometers at my lower helm and digital on the fly bridge. One of the lower helm tachometers is consistently lower by just over 100 rpms.

"I'm running her at what I'm getting @ full throttle 2050rpm's and just barely making 6.5 knots."


Sounds like a foul prop.


Mr Herrishoff used to suggest viewing the prop as if it was the same size as the boat.


Then a few growths look like garbage cans , and its EZ to see where the speed went.

dardurfr said:

The previous owner used to run the engine @ 1800rpm's and get 7 knots or a little better. I'm running her at what I'm getting @ full throttle 2050rpm's and just barely making 6.5 knots. He told me she should turn up around 2600rpm's. Haven't had the chance to change the fuel filters but hopefully get aboard this week and do so. I'll report any new findings.


Darren

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Have to ask the obvious; Is there a current running where you are boating?

Hi guys. I Finally got the fuel filters changed and although there was a nice bit of gunk in the racor and some in the secondary filter there has been no change in the performance of the engine. Seems like she runs a bit smoother but other than that I'm right back to square one. Next plan of attack is get her out of the water and inspect the bottom and clean and paint it and see if that improves anything. I'll post any new findings
Thanks