Engine only running 3000 RPM at WOT

[Shamfarlango]

the specs:

I have twin 3.8L OMC outdrives (1985) with 800 Series stringer outdrives.
Both are running the same props (15x15.5 Aluminium 3 Blade).

Last season, it was recommended that i get my timing checked..so I did.
I also had the points and idle configured.

The throttle has full play but when i lay the two sticks down, one tops out at 3000 RPM (Other is about 3900).
I suspect I am incorrectly sized from a prop perspective, but I would like to get the engines working in sync before doing any prop purchases. As it sits right now, there are brand new props on the boat and they are identical.

So...
If the timing is right, idle, points are all configured...what are my next steps that could be limiting the potential of my engine?

 

[Lyle29464]

Re: Engine only running 3000 RPM at WOT

I would start with a compression test. That also lets you take a good look at the plugs. They should all look the same.

 

[Howard Sterndrive]

Re: Engine only running 3000 RPM at WOT

fuel supply line from the tank, anti siphon valve(s) at the tank, poor fuel, dirty flame arrester plates, worn out engines, faulty esa switches/module or shift cable adjustment, engine compartment too well sealed, exhaust leaks blowing back into air intakes, waterlogged hull, dirty hull, trim angle

when your timing was checked - was the advance mechanism in each distributor checked? - that would be near the top of my list

 

[Shamfarlango]

Re: Engine only running 3000 RPM at WOT

Thanks guys, now i can move forward on some more investigation

Howard, my mechanic said i had an exhaust leak on that engine (unsure where). Any advice on how I can rule it out?

 

[Shamfarlango]

Re: Engine only running 3000 RPM at WOT

fuel supply lines are new
anti siphon valve(s)- both engines T in to the same fitting at the tank. not sure where this valve lives, but if one engine is fine. I assume that its not a valve issue if only one engine has the issu
Fuel is was filled and treated at end of season 2011 and issue existed last year with fresh fuel
dirty flame arrester plates - this is possible, I will need to do some reading on how to clean this.
worn out engine - YIKES
faulty esa switches/module - No idea what either of these are.
shift cable adjustment - Not sure what specifically I am looking for here.
engine compartment too well sealed - not sure this is likely with only one engine having issues
exhaust leaks blowing back into air intakes - In hindsite, I'm just going to make an inquiry with mechanic on how to fix (weld/replace).
waterlogged hull dirty hull, trim angle - again, this is one of two engines and the hull is sound so I would hopefully rule these 3 out

 

[JSMoore]

Re: Engine only running 3000 RPM at WOT

When running at WOT, do the engines sound obviously out of sync? A 900 RPM difference between the two would be "painful" to listen to. If they sound like they're close, I'd look at the tach. My 1987 model OEM tach was hundreds of RPMs off when checked against a Sun Pro dwell-tach. If that's your case, you could be getting a false tach reading.

If they do sound obviously out of sync, Howard Sterndrive has already suggested a check of the timing and advance mechanism. I would also check the points and dwell setting. If your timing and/or dwell setting is off, it can make a huge difference in the RPM.

Another place to look is the fuel system for the ailing engine. A bit of dirt or corrosion stuck in a jet, or the needle & seat, can restrict fuel flow.

Lyle29464 also suggested a compression test and a look at the plugs.

Good luck in finding your problem!
JSMoore

 

[Shamfarlango]

Re: Engine only running 3000 RPM at WOT

They do sound drastically different, i've been throttling the starboard drive back to match the 3000 RPM one.
Points were just done on both engines this week. I have to ask about the advanced mechanism as I'm not the sure whether it was looked at or not.

I borrowed a compression tester last night, so I will check that out today. As well I'm going to see if I can find this exhaust leak and whether its at the gasket or a crack today.

Hoping I can baby step through all the easiest stuff first to rule them out. Then deal with the big stuff.

Really appreciate the quick responses. I will let you know how I make out.

 

[Howard Sterndrive]

Re: Engine only running 3000 RPM at WOT

I guess the easiest way to look for that exh leak is open the hatches while someone else drives and listen and look. Probably on the engine that is turning slower. Engines need clean air - if they suck theri own exhaust they can't make power. Also, that soot will quickly coat the flame arrester plates. Brake clean will clean the flame arresters - or soap and water. let them dry fully before reinstalling.

Lyle's first response about compression testing is always a good start.

 

[Shamfarlango]

Re: Engine only running 3000 RPM at WOT

Weather is not being very cooperative, but I have found the exhaust leak.

It's actually on the engine that is running better. Still on the to be fixed list. I have the compression tester on board now, just waiting for the weather to clean up so I can get that completed.

I will pick up some brake clean and clean both of the flame arrestors as well - Thanks for the details Howard