Twin 250s, One outboard stronger then the other?

**chevy355mark** · 09-22-2019, 10:09 AM

I have a set of 1998 Mercury 250EFI outboards, xxl and cxxl. they are rigged on a 23' center console 24 degree bottom, closed transom with a bracket, 24p vented 4 blade merc offshore props. They both run great on the garden hose, run same rpm per throttle position on plane. When your getting up on plane the starboard engine seems like it always takes off ahead of the port outboard. I have tried throttling up the port engine first and then rolling the starboard engine in and it still shoots 1000rpms over the the port engine and then they build rpms equally but around 1000rpms apart(with the starboard engine ahead). sorry if that sounds confusing but i want to be clear, the starboard engine is just plain doing the work getting on plane is what i feel like. If you try to plane the boat on either one of them alone they both struggle and hit the wall at the same rpm, it will do it but not easily of course, in this test one is not stronger or weaker then the other.

so the question is this, is one weak or is the boat so overpowered that one just tends to get after it first? on the props, 2 holes are open and 2 are plugged with smaller holes drilled, i drilled them a size at a time until i was happy with performance. would the vent holes be sensitive enough to cause this if i drilled one just slightly larger by mistake?

both outboards compression check at around 97psi give or take 2-3 psi across the board, cold. lift pumps are rebuilt, fuel system all checks out, vapor separators cleaned last year, all sensors ohmed out ok, tps sensors set on the rich side, all new bleed hoses, throttle bodies are clean. reed valves were ok last time i had the throttle bodies off but this may have developed since then or i may have never noticed and its always been this way. I maintain these things meticulously and im running out of ideas, theirs no more maintenance or general wear items i can think to replace, clean, or rebuild.

I will admit though, the outboard(port) in question always sneezes at idle, no matter how much i kick the tps sensor up richer it still sneezes. i had assumed my 40:1 oil mix may be the cause of a lean idle due to the fuel viscosity being thicker. i must be wrong, somethings going on and im out of ideas, i think a broken reed would give me alot of other running problems besides just a lean sneeze at idle. kicking the rpms off idle knocks the sneeze out and its smooth running from their.

sorry to make this so long winded but i really wanted to include all important details regarding the general condition of the outboards.

**mrichartz** · 09-22-2019, 10:18 AM

Sneezing.. than I would say check the reeds again first.
Don't have to be broken to cause a problem.

Did it start doing this? The 1000 rpm

**chevy355mark** · 09-22-2019, 10:22 AM

can i throw a leak down tester on a cylinder at bottom dead and listen for air blowing out of the intake or should i disassemble and inspect? crank seal possibly?

also wanted to add that all spark plugs just as oily brown as the other, no lean cylinders, no water consumtion.

**mrichartz** · 09-22-2019, 10:25 AM

The 1000 rpm did it start doing this or ?

**chevy355mark** · 09-22-2019, 10:25 AM

I often get carried away and just start throwing parts on since its only good maintenance on a 20 year old outboard anyway. In light of that, what would a good replacement reed valve be anyway, stay with oem? are they fiberglass or metal on these?

**chevy355mark** · 09-22-2019, 10:25 AM

i dont understand your question, could you clarify?

**mrichartz** · 09-22-2019, 10:27 AM

Oh you said one motor would rev 1000 rpm higher did it just start doing this recently?

**chevy355mark** · 09-22-2019, 10:30 AM

I can say this, i have recently noticed this. it may have been this way for years. I have recently done alot of work to the boat and have added a considerable amount of weight with new stringers, fishbox, and i added a weight bag to the bow to help even out the center point of the boat.

**chevy355mark** · 09-22-2019, 10:31 AM

i believe it has always sneezed at idle, Im starting to get very picky where i used to accept anything. guess im getting old.

**mrichartz** · 09-22-2019, 10:39 AM

I would inspect and make sure reeds are all closing all the way first.
Then if rpms are still off. Props are matched, check timing?

**chevy355mark** · 09-22-2019, 10:53 AM

I have been lazy about checking the timing, that’s a good idea. Is there a base timing or any other adjustments there or the ecm just does what it does based on sensor inputs?

**mrichartz** · 09-22-2019, 11:11 AM

Best money to spend buy a manual for those motors...You have a Max spark adjustment screw and a primary pickup screw on throttle linkage on the starboard side of motor..

**Chaz** · 09-22-2019, 11:18 AM

It needs the reed cages done and a set of Carson reeds installed. Makes a night and day difference in idle quality on up thru the power range.

The 1000 RPM diff ... my vote is one of the prop's has a spun hub ..

**gotboostedvr6** · 09-22-2019, 11:24 AM

The reeds may be bent slightly or slightly cracked.

3 liter mercs are notorious for this. When the reed does break off you might have to rebuild so it's best to keep on top of this.