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270 hp not reaching high RPMs

J

jayj123

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I have twin 270 hp (remans with ~400 hrs) and one will not come up to top end RPMs. Goes to ~2400, then no more, engine sounds fine and seems to throttle up, just no RPMS. Suspected fuel issues and found water/crud in the fuel water seperator (never used ethanol). Had tank cleaned and have solved the fuel issue but still no RPMs. I ran it at the dock this weekend for ~30 minutes and then checked fuel water seps again. I only got ~ 9oz of fuel out of the affected engine's seperator where I got ~16 oz out of the other seperator. There are no other inline fuel filters.

Does this indicate a fuel pressure problem or what?

Thanks for any suggestions as mechanic is also at a lost.

P.S. bottom was clean when this issue started (end of season last year) and was supposed to get a fresh cleaning again today, so we are addressing that probabilty.
 
"Fuel pressure problem" - possible; what did the pressure gauge indicate?

Does the ignition timing advance per spec?

Are these carb equipped or injected? Finally, a single tank feeding both engines, right?
 
Don't have pressure gauge.

Not sure about timing advance, everything was fine on previous trip before problem.

Two tanks, one per engine with transfer switch. Each engine running off seperate tank.

Carbs.
 
jayj123 said:
I only got ~ 9oz of fuel out of the affected engine's seperator where I got ~16 oz out of the other seperator. There are no other inline fuel filters.
Click to expand...

The above part of your post bothers me. I would suspect that the fuel pump may be sucking in air somewhere in the fuel line to the tank. Can you run that engine off the other tank? You could try starting out with a separator that you fill with gas, run the engine for a while and recheck the filter.
 
I'll agree with the ignition timing suggestion.
No gasoline Marine engine will produce any realistic power if it's not seeing the correct progressive or total spark advance.

Mark off your harmonic balancers up to approx 35* BTDC.
You can use the Mr. Gaskets decals if you know the balancer diameters.

Or.............. you can do the math and mark them off yourself.
Here's how to do the math.

Here's what you'll end up with for a LH Standard Rotation engine.
RH Rev Rotation engine markings will be just oposite of the TDC line.


Now strobe each engine and jot down the advance at various RPM, and up to approx 3,200 RPM.
Here's a short video. This automotive..... but the idea is similar.

Now compare notes between the two engines.
At/near 3.2K RPM, you should be seeing approx 28* of ignition advance.

Example only..... see your OEM specs for the correct curve and advance number.


Also, if you suspect fuel lines/fuel filters, pick up some new fuel line long enough to allow you to swap these with each engine.
It will cost you a little bit, but if you've not upgraded fuel lines, it's not a bad idea to do so.



.
 
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Update: We found some minor water leakage at the Riser/manifold joint and mechanic is suspecting that the risers may be clogged. It has been more than 10 years since these were changed out. (~400 hrs) My question now is: Can an obstructed exhaust cause the loss of top end rpms that I am experiencing? Note: I've been measuring temps throughout the exhaust path for several years and there are no hot spots anywhere. I was expecting to see hot spots in the exhaust system if there was any obstruction. I will probably pull the risers/elbows for inspection since they are pretty old, but I am wondering if this will do anything to solve the top end rpm problem.
 
Do yourself a huge favor and replace them!

No one mentioned a sticking anti-siphon valve at the tank. That will gas starve the engine (bad news) at full throttle while allowing normal low speed running.

Jeff
 
I am definitely pulling them to inspect and or replace. My question is whether some blockage in the exhaust would cause the high end RPM problem? I have two fuel tanks with a crossover switch and have run this engine off the other tank and still have the problem so I'm thinking that rules out the anti-siphon valve.
 
I've heard of a collapsed exhaust hose causing that problem. Rare, though.

Jeff
 

Attachments

  • Collapsed Exhaust Hose.JPG
    Collapsed Exhaust Hose.JPG
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You might pull the spark plugs (look at them closely) and then do a compression check. If you suspect leaking elbows you want to be sure all is well inside the engine before you change them. Sometimes leaks will lead to corrosion causing sticky valves which will give the reduced rpm you experienced. Also be sure to drain the elbow/risers by pulling the little drain plugs on the side before you loosen and remove. Otherwise they will empty some raw water into the exhaust manifold when you take them off. Let's not ask how I know...
 
If the exhaust is restricted, the engine won't breathe and the issue gets worse as RPM increases...at the "too much" point, the RPM won't increase any more.

Jeff's attachment shows a blocked exhaust hose...from what I can see, from overheat due to lack of sufficent raw water flow.

If you ran the "problem" engine using the cross over valve selecting the other tank, I'd agree you have ruled out the anti-syphon valve.

Have you measured the fuel pressure or the timing advance yet?
 
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