Engines not running at full throttle

serpico1

I recently had to have one of my shift cables replaced and then adjusted as the shift levers were not aligned after the replacement. Now, after the mechanic completed that task, it seems my boat will not reach full power.

When I throttle up to go on plane, my engines are only reaching about 3300 rpm. (full throttle should be about 5400) if I throttle back to about the 3/4 position, the rpms stay the same, there is no change in rpms from 3/4 throttle to full throttle and the engines do not sound any different either. I also noticed that according to the sync meter, the port side engine is running faster then the starboard engine, from the 3/4 thru the full throttle position, the engines are in sync up until the 3/4 position.

I am assuming this is a simple cable adjustment issue, at least I hope it is. Prior to the shift cable being replaced. I did not notice this problem.
How do I correct this, do I adjust the throttle cables at the helm in the throttle control box or back at the engine?

Michael T

I'd take it back to the dealer to set-up properly. IMO they did not conduct the repair properly. from what you have stated it seems to me as if they have misaligned/misadjusted your throttles/cables.

[Deleted User]

If this was my boat I would firstly disconnected the cable to the throttle body, then open the throttle manually and see what RPMs you can get. If it revs to 5K, then the problem is in the cable adjustment. If you paid good money for the work, as mentioned above, take it back and get them to do it right.

Andy

MarkB

Do not rev your engines without them being under load.  You might get away with 1000 or 1500 rpm, but no ways 5000 rpm is going to be good for it.

Michael T

@MarkB, I agree. You'd probably be getting some serious valve float at 5K. I don't like to rev even at 2K during a test. If I do it's only for a few seconds.

[Deleted User]

I've heard people before say revving an engine to redline without a load will result in damage and a potential catastrophic mechanical failure. In my opinion there is no risk to free revving a modern marine engine (in my case a 2006 EFI 5.0L GM V8) without load for five seconds or less with the following caveats:

1) The engine should be warmed up first at idle
2) Advance the throttle slowly in 500 RPM increments, because...
3) ...free revving results in a reaching higher RPMs faster, increase RPMs slowly to avoid over revving the engine, where you CAN do damage
4) Reduce throttle slowly in 500 rpm increments when backing off the throttle

Opinions on free revving I've read on the internet are all over the map, but what I take away from reading various forums and postings by credible experienced mechanics and automotive engineers is that its not harmful without over revving and modern engines can take it.

This is not something I would do routinely without a good reason, but I believe a lot of opinions about this cross over into the "old wives tale" territory, similar to a lot of old timers saying never put a battery directly on a concrete floor - modern plastic battery cases have no problems with that, unlike maybe the old Bakelite battery cases Grandpa had in his Model A.

Bottom line, I have no fear that revving my engine briefly to 5k RPMs will do any damage. But that's just me. Everyone else, if you think it's likely to do harm, by all means don't do it!

Andy

serpico1

I agree, I think they screwed something up when they replaced the shift cable.  But, I am almost afraid to let them touch it again, who knows what they will mess up next.  After replacing the shift cable, they didn't align it right and my shift control was off.  So, I had the mechanic fix it and the idiot, after removing the control box to make the adjustments, disconnected the neutral safety switch wires, as there is no play in the wires, as soon as you pull the box, they will disconnect.  I guess he didn't know that and put the control box back and then the engine wouldn't start.  I had him look at it again and he said it was an electrical issue and would have to take a more detailed look at it. well, I looked at it after he left and saw the wires weren't connected.  took my about an hour to find the female connections.  but I got it done myself and got the engine started.  So, do I really want him to mess with the control box again?

Michael T

@serpico1- I think you have answered your own question.

serpico1

Well, yesterday, I checked the throttle cables and they both seem like they are good.  I had a friend push the throttle lever forward as I watched the cable move on the engines.  When the throttles were in WOT position, the throttle on the engine was also in WOT position.  So, the engines, not being able to get above, 3300 rpm, doesn't appear to be a throttle cable issue.

I did notice though, that when we had the boat out last week, the engines were pulling gas primarily from the port side tank, at least according to the gas gauge.  we went out with  just under a half tank in each side and when we returned, the starboard gas gauge didn't appear to have moved, but the port side was reading near empty.

I can isolate the tanks to see if that makes a difference.  If not, maybe I will change out the fuel filters.  But, usually the engine bogs down when they are clogged

LaRea

"... the engines were pulling primarily from the port side tank ..."

Don't you have a crossover valve in the fuel line between the two tanks?  That valve should be closed during normal operation, so the port engine draws ONLY from the port tank, and stbd from stbd.  

The only time you should open the crossover valve is if you are intentionally running both engines from one tank or the other.

serpico1

The crossover valve was open as was the port and starboard tank valves, so that the engines could draw from both tanks at the same time, instead of individually.  but, it seems that the engines drew mostly if not totally from the port tank.  So, there may be a problem with the starboard pickup line. But, I am pretty sure that's how the valves have been all last season and this season.  Never had a problem before.  But, this weekend, I am going to run it with the crossover valve shut.  so each tank can only draw from its respective tank and see if there is an improvement.

Michael T

@serpico1. Good idea. In the middle of the summer I noticed that my crossover lever had been placed in the open position. I must have snagged it on my jeans when leaving the engine compartment. It did not make a difference to my acceleration or gas mileage. I did return it to the closed position when I noticed it as that's where I leave it. IMO your idea of forcing each engine to draw from its own tank is a good trouble shooting idea.

MarkB

Does the cross over valve sit inbetween at the bottom of the tanks? Basically, if you have a line a the bottom of the tanks connecting them together, even if the engines are drawing from only one tank, then that line acts as a balancing line between the two tanks and the level should stay the same no matter what. But if the cross over is on the pick up lines, then I agree, something could be wrong with the one pickup line.

bella-vita

Crossover is on pick up line mine was open too in beginning of season no problems but I have since closed it still fine

LaRea

If you have plenty of fuel in both tanks, the crossover valve shouldn't affect how the engines run.  However, I can think of two reasons to keep the crossover valve closed.  

(1) With the valve open, you could accidentally drain one tank down to empty.  Then the engines might ingest whatever sediment is at the bottom of the empty tank.  

(2) With the valve open, you lose the ability to know whether both engines are burning the same amount of fuel.  Not a huge deal, but it's a bit of engine health data that you'd be throwing away (unless you have another way to measure fuel flow to each engine).  

Just saying, standard practice is to leave the valve closed.