Perkins 4-108 RPMs

[dancamp009]

wow, so much help in so little time.
Here are some answers to your comments/questions.
1) I was in the water when I changed my fuel injection pump. My 12" 3-blade prop is the same prop as before the F.I.P. change. I hand cleaned the prop last month before going out motoring. When my prop had barnicle growth, I couldn't even get over 1600 rpm's and 3.5 knots. With the prop clean, I'm back to my normal 2000 max (if I'm lucky) state at 5.7 knots in the calm.
2) my prop shaft is not over heating or even getting hot. In fact, i did have to tighten up the stuffing box for the first time in my life the other day, because I was getting too much water drip. But after tightening it up, and then backing it down a little, it still runs cool and with minimum water intake. I also have a grease fitting that I pump with grease when the boat is stopped. I will try diving soon and check to see if I can move the prop shaft by hand. I forget whether I could do it easily when I had the boat out last year, but I imagine if I couldn't move the prop by hand then, I would have been concerned enough to look into it.
3) the transmission is the same, the old Hurtz or whatever the main one was back then, with one gear forward and one backward. The tranny runs cool and the fluid is like new. I think it's a bright red (from what I remember). I checked it last month.
4) My exhaust system is working fine. I removed it two years ago and cleaned it out with muriatic acid.. Put all new hoses on, and it works great. No over heating problems since then.
5) I keep my bottom clean. Though I haven't pulled the boat this year, I did last year and had the same problem after repainting the hull.
More comments:
It feels like it just isn't getting enough fuel, or the fuel supply stops at a certain point.
I imagine the F.I.P. is properly timed, or I doubt I could get the high RPM's at idle. Is this correct?
Still wondering how large of pre-lift pump fuel lines I can add without creating more problems.
The only thing I didn't change was the fuel exhaust line that goes back to the tank. And somewhere I read recently that a clogged discharge line could interferr with the max RPM. I'll change or clean that line next.
I keep new fuel filters always installed and my fuel tank is very, very clean.
Thanks for the info that the gonvernor on the F.I.P. can't be out of adjustment if I'm getting 3600 rpm's at idle.
And thanks for all the help too.

[Bermuda Forte]

I read that careful attention must be taken to get the timing right when re-installing a fuel injection pump otherwise you might get trouble starting and poor performance. This concerned me as I contemplated FIP replacement last month to address a leak at the throttle linkage. My 25 year old Perkins 4-108 has always started with no preheat and runs like a champ pushing my 20,000 lb B-40. Fortunately my local yard replaced the o-ring and I am off and running with no leak. Good luck!

[SkiprJohn]

Sounds like you've got your bases covered. You should be able to spin the prop shaft from inside the boat with the gear in neutral rather than have to dive.

Possibly out of time. Did you have it professionally replaced? Just one little degree off can effect peformance.

Possible clogged fuel return from the injectors as you suggest. That will definitely hinder performance.

Darn! This is not an easy troubleshoot.

Love those 4-108s.

[Captain Jeffry]

Originally Posted by Cheechako

I did a little research several days ago. Evidently the Perkins is continuous duty rated at something like 2800 rpm and Recreational rated at 3000 rpm.... If I'm remembering right... Evidently it produces about 42 hp in this range? and that is the sweet spot in the power curve. It will do higher rpm, but most of the info I found says it's not what's recommmended for fuel economy and longevity. Sounds like a different philosophy than what Yanmar mechanics recommend, which is usually running near peak rpm.... That has always been my rub with Yanmar. I contend they are selling you an engine rated at peak HP at peak RPM and claiming it's a lighter engine, realistically, maybe it's just an over rated engine...

Nice! Thats a great take!

[DeepFrz]

If you aren't seeing black smoke at full throttle then you aren't over propped and you don't have excessive friction in your driveline. It sounds like you just aren't getting the fuel that you need for it to work harder. That extra few knots will probably double your fuel consumption. Somewhere you have a fuel constriction, either inside or outside your F.I.P. If you are getting black smoke, then it is probably something like fouling of prop, hull, possibly a line wrapped around your prop. That's my opinion anyway.

[dancamp009]

Originally Posted by DeepFrz

If you aren't seeing black smoke at full throttle then you aren't over propped and you don't have excessive friction in your driveline. It sounds like you just aren't getting the fuel that you need for it to work harder. That extra few knots will probably double your fuel consumption. Somewhere you have a fuel constriction, either inside or outside your F.I.P. If you are getting black smoke, then it is probably something like fouling of prop, hull, possibly a line wrapped around your prop. That's my opinion anyway.

Thanks for the input. I'm not getting black smoke, and since it's only a 12.5" prop on a 40' boat, I guess I may have a fuel constriction some place considering I max out at about 2100 rpm's if I'm lucky. I just would like to get more rpm's and was hoping it was a solvable problem.. Unless a small prop with a big boat is unable to get a lot of rpm????
who knows? . Thanks for your input.

[Captain Jeffry]

Sounds to me like you are over propped! Take your prop off and have it checked for proper pitch.