Engine Top Speed Changed. Ideas Please.

[Clark]

A little background. I recently changed our prop - a folding 2 blade - to a Campbell Sailor 3-blade; 14x8. Goodness, it feels like a different boat (well, I guess it is sorta). 20GM20F by the way. Very smooth and with a lot more bite in reverse but here's the issue.
While taking the new prop out for a test run, I recorded boat speed vs RPM and was pretty happy. I had pushed it to 3400 for a couple of minutes after some steady runs at 2900-3000; generating some extra smoke. Next time out; with the engine thoroughly warm and after running at 2900 for 20 minutes I decided to check to see what max RPM would be. I eased it up to ~3400 for a minute then pushed the throttle to max. Well, it did not get any faster so I thought, "I guess max is 3400 and that is still OK". Just then, the RPM started to slowly drop so I pulled the throttle back to 2800 and it ran with no issues. From that point on however, I cannot get above 3000 RPM. Discussions with some old heads around the docks suggested that I might have clogged filters which made sense in light of the stumbles I experienced the next time out. With that in mind, I replaced the primary and secondary filters (see other thread). Well, she runs beautifully now; starts easily, runs smoothly and doesn't smoke very much but I still cannot get above 3100.
What do you guys think? Remember, this happened fairly suddenly. Could the tach be off? Isn't it controlled by the alternator? Could I have lost compression even if it runs well? Could I have dislodged some crap in the elbow and am now getting an exhaust restriction?

TIA everyone.

Clark

 

[Winkfish]

Clark, when the tach RPM reading dropped, did it sound like the engine RPMs dropped?

 

[Winkfish]

Clark, having read your other post on changing your filters, I wonder if maybe the higher RPMs from before were caused by the "black" fuel, some restriction. Your engine might have been working harder, thus higher RPMs. Now that your filters are clean, the engine might be working at its "unrestricted" lower RPMs. (Did that make sense?)

 

[Steve Dion]

Clark:

You can purchase a Laser Tach for about $30 to check the actual RPM.

Neiko Professional Digital Laser Photo Non-Contact Tachometer - Accurate to 99,999 RPM Measurement.

Is sort of strange that all of this happened at the same time, but Neptune works in mysterious ways.

I think your tach works off of the flywheel, but I am not sure.

1. I assume that the bottom is clean (you just pulled the boat to install the prop)
2. Have you removed the fuel pickup from the tank. Check it out and see if there is a screen on the pickup (sometimes you pick up stuff there).

All of our above assumptions may be turn, but I would work through them one at a time.

 

[Don12364]

My H376 had a little cylinder of gauze round the bottom of the pickup tube inside the tank.
It was intended as a primary screen and this was the first thing to clog when the diesel bug first struck.
Its not there any more!

 

[Clark]

Hmm; It only lasted for several seconds (the drop) but I think the engine RPM was dropping, not just the tach. Based on many others observations about tach accuracy, I'll probably get a laser tach. And no, I did not pull the pickup.

If I do, it would seem that I'd have to do a prolonged bleed to remove the air from an empty pickup . . . ugh.

 

[Winkfish]

If I do, it would seem that I'd have to do a prolonged bleed to remove the air from an empty pickup . . . ugh.

On a bareboat charter we did once, that happened to us. We did not bleed it after replacing the pickup and it worked just fine.

 

[Steve Dion]

Clark:

Another thing you may want to check out is the actual throttle position on the engine. Be sure that it is opening all the way. If your cable housing has slipped you may not be getting the full throttle that it "appears" that you should be getting. This could have happened when you pushed the throttle to the WOT position.

PS: Have you checked your speed with GPS. You should be getting around 6.5kts +/-.

 

[Steve Dion]

I doubt that you will need to bleed it if you shut off the fuel at the tank before you remove the pickup. You appear to be an expert at the bleeding process anyway.

 

[Clark]

Good suggestions Steve. A slipped throttle occurred to me as a possible issue but I forgot to check that yesterday. I'll be back to the boat this weekend so I'll do another speed check with GPS and examine the throttle linkage. Good to know that pulling the pickup shouldn't be a huge deal - assuming I can get it out w/o removing the tank. What material is the pickup tube? I would have to flex it to get it out.

Expert? HAH!