Engine maxing out at 2500RPM in FWD - 3000RPM+ in Reverse

[MFC]

I have so many darn questions today . . .

To start with one of the more perplexing: I just tied the boat hard to the dock and ran the engine at full throttle fwd and rev. In FWD the RPM's max out at 2500 (something which is down from a couple years ago at 2800+) but in Rev it hit 3000RPM+. I have a single lever throttle / shift control and I am wondering if perhaps the culprit. Any thoughts on why this might be happening?

TIA
Matt

[Paul D.]

I don't have any answers but I wondered also it may be good to know what sort of prop you have.

[tartansailor]

That's perfectly normal because of aerodynamic considerations.
You need to do some studying.

[bottomscraper]

I'm not sure what gearbox you have but I do know that many of the Hurth boxes have different gear ratios for forward and reverse.

[Brian2]

What is the max rating of your engine? My old engine, a Universal M18, was rated at 3200, but I only got 2400 in forward due to the prop being overpitched. I never tried to max out in reverse.

[MFC]

Hi Brian -

Max is 3000RPM. Used to run WOT at 2800-3000. Either got some bad fuel or maybe a wave when adding a jug when bound for Bahamas . . . appeared to be a slow drop off then to about 2500 (with corresponding loss of power). A few filter changes were pretty knarly (confirming water) and had white smoke (steam I think) for a while. Now the filters come back clean, the smoke is gone but the RPM's are down to 2300. I am working on the assumption that it is a fuel restriction issue somewhere (if anyone disagrees please say so). I am considering making a temporary tank and running some fuel with SeaFoam or another injector cleaner (I have something called Diesel Kleen) through the system to see if the RPM's return. Thoughts on / experience with this? My tank has essentially no access so really cleaning it is at best very difficult.

Thanks
Matt

[Keith]

I had a similar problem, it would only rev up to 1800. It turned out to be the fuel shutoff was not opening all the way. The cable had slipped. I manipulated by hand to diagnose then adjusted and tightened the cable clamp. I hope your problem is this simple.

Good luck,

Keith

[MFC]

Somehow it escaped me that I can still get it running @ 3000RPM in neutral. I take this to mean that it must be a prop / shaft or tranny issue. Does that make sense? Anything I am missing?
TIA
Matt

[MFC]

Just an update. Pumped tank empty (tank was basically clean), ran 50% Seafoam through the primary filter (and another dose in the tank). No immediate response (well the RPM gauge fluctuated like it might be broken) and then took off on a trip for a few days. Engine performed flawlessly at full RPM over 4 days . . .. Another victory for the bloody knucklers . .

[Zeida]

Matt: My 1982 CD 33 "Bandolera" has a Universal #30 - #5424 engine. It is also rated for 30000 rpms, BUT like you described, with my single lever (fwd. & rev) I can only get it to run up to 25000 rpms. Like you, my fuel tank is an aberration, extremely hard to access and/or clean. I would LOVE to pump it clean of all fuel - how did you do it, using what? I keep my filters clean, and change them relatively often. Still I would love to have a clean tank for once before I fill up again; what is that 50% Seafoam you refer to that you ran through the primary filter and exactly what dose in the tank? Maybe I can also get my engine to FINALLY run to its assigned 30000 rpms…that would be something!
Thanks for any explanation you can give me.

[MFC]

Hi Zeida -
First the easy stuff: I took out the fuel gauge and used that hole to pump out the tank. I pumped it out with a $14 battery operated kerosene heater "siphon pump" which I bought at Lowe's (they had stowed it for the winter so you may have to ask customer service). I don't know that I got the tank "clean of all fuel", but I got enough out to assess the condition of the bottom of the tank (there weer black spots -- corrosion or dirt I am not certain -- but there was no accumulated crud as I feared). I am not sure if you have followed, but on my trip to the Bahamas, I took a wave on board while adding fuel and may have taken some sea water in the tank. Either because of that, or because of some bad fuel along the way on that trip, I ran into a problem which expressed itself as a) some smoke at high RPM, lowered RPM and lowered power. A couple of my filter changes showed black crystalized crud on the filters (which I understand to be indicative of water in the fuel). After a number of clean filter changes which resulted in clean exchanges (old filters were not obviously dirty) and reduction of smoke (but continuing low RPM and low power -- I decided to use an injector cleaner. The brand I used is called Seafoam which is commonly sold in autoparts stores. It can be used in a number of ways but one suggestion on the can is to place it directly in the primary filter as a 50% fuel / 50% Sseafoam mix. I did that. I can't swear that the Seafoam was "the" thing that worked, but I got great performance from the engine (2800-2900 from the engine in many circumstances -- and good boat speed under power). Like I said, I am not sure what was ultimately responsible, I sort of subscribe to the theory that sometimes our engines just demand a certain level of tribute in knuckle-blood before they act properly! Still, I would say Seafoam is worth a look.
Good luck!
-Matt

[Dick Kobayashi]

This may or may not have any relevance. A few years ago I had a new head and exhaust elbow in stalled on my 1 GM. I had poor running conditions but the enginge would get up to 3000=3000 rpm. After the installation the engine ran very smoothly - no black smoke, etc but maxes out at 2700-2800 rpm. No change in prop. Effect on propulsion: none. So I leave it alone. btw the engine will turn 3300 rpm with no load. Unsolved mystery!