Please advise !

[chuckberry]

I have twin 1987 YAMAHA 200hp 200ETXH engines that have idling issues. When running engines on trailer they run fine in neutral and in gear. But when the boat is in the water (still on trailer) it idles rough and stalls when put in gear. Money spent on new plugs, fuel pumps, and filters has not cured the problem. My fellow garage "boat mechanics" think it may be a vacuum issue but have no idea how to diagnose the problem. All help and input will be greatly appreciated.

[CatCam]

I would connect your motors to another fuel tank and test it. May be a vent problem with your boat's fuel tank since BOTH engines are having trouble.

[chuckberry]

CatCam, That was our mission today. We took it to the ramp and ran each engine off of a fresh 6 gal tank. Same symptoms. I will add that the original problem started with the starboard engine last year but with twins it didn't become a problem until they both started acting up. I'm thinking some part on both engines failed within a year of each other. (??)

[rodbolt17]

I am thinking, test spark, test compression go fix the fuel system.
easy peasy.

[chuckberry]

Not so easy peasy. Checked spark and comp OK. Like I said, it runs fine in the driveway, put it in the water, idles rough then stalls in gear.

[rodbolt17]

now we know the cylinders can seal.
we also know it can spark all 6 .
can the spark be con*****ed?????
that's ign timing.

that pretty much leaves fuel.
if the CORRECT air to fuel ratio cannot be delivered to this sealed cyl and ignited at the proper TIME, it simply runs poorly to not at all.

like I said.
easy peasy.

I started doing this type work at about age 8,maybe a tad earier.
my dad had a wrecking yard since I was about 5 till about 13.
Ill be 53 in November.
I have done most of this at least once and have stayed in a holiday inn occasionally.
yesterday I started on a V6 4.3L Volvo with freshwater cooling about 10 AM.
by 6PM I had changed both heads,both drive belts and the seawater pump and seatrialed it.
had two trips for parts and two rain delays.
work was done with the boat in the water.

most jobs are simple.
testing the cylinders ability to mechanically seal is paramount.
then test spark,can it be generated and con*****ed?
then fix the fuel system.
the fuel system consist of everything from the liquid in the tank to the reed cage.

[chuckberry]

Maybe if someone could tell me what could cause both engines to run rough when the boat is in the water and run smooth when in the driveway, I could get to the heart of the matter. Anybody been there, done that?

[99yam40]

when was the last time the carbs were cleaned.

Motor needs more fuel to create power under a load.
putting in the water creates more back pressure and needs more power to turn the prop in the water.

Pilot passage/jets probably need some attention along with the rest

Just my opinion

[robert graham]

Most motors will run apparently well in neutral....it's when they're put under load(like in gear) that problems become very evident....you may have some plugged carb jets or injectors/filters...whichever...

[rodbolt17]

water in the fuel,old fuel, contaminated fuel.
or it simply sat and both engines carbs gummed up.
happens all the time.
this aint nuttin new unda da sun folks.

it aint rocket science.
but based on what your posting the carbs,fuel lines and fuel pumps need attention.

the reason it seems fine on the trailer is lack of load or back pressure.

same as why some idiots can get 4200 RPM from a Z200 reving in N.
by then its ONLY running on TWO cylinders, yet they will do it.

[chuckberry]

Ok, now I think it's getting narrowed down. We sprayed the carbs twice with carb cleaner and air hosed them. We didn't take them off or mess with any adjustments. We replaced all the fuel pumps after seeing a small crack in a diaphram, we ran them off a clean 6gal can of fuel with a new line and primer.
The engine needs more fuel for more energy to push the exhaust through the water clear of the engine. The back pressure of the water in the exhaust ports acts like a potato in a car's exhaust, can't breathe, suffocates.
So, what senses the exhaust being under water (back pressure) and what or how (vacumm?) does it increase the fuel to the carbs?
This is the problem (I believe). How do i diagnose and fix this?????

[boscoe99]

You clean the carburetors.

Fully disassembled and put into a heated ultrasonic cleaner for a period of time. Passage ways are rodded out if necessary. Verification that all passage ways are open is performed.

You then reassemble the carburetors. Finally, you set them up per the manufacturer's instructions.

A motor running fine out of the water means absolutely positively nothing. Well, actually it means it runs fine out of the water but who gives a rats ass how it runs out of the water? It is when it is in the water that matters.

Rodbolts alternator works fine when it is on a tester. It does not work well in his truck. Must be something wrong with the alternator.

[99yam40]

We sprayed the carbs twice with carb cleaner and air hosed them. We didn't take them off or mess with any adjustments.

You cannot clean a carb without taking it apart