Throttle and RPM Differential

[Bingo Boyz]

Question for twin engine boatowners. The throttles on our 340 DA are not very even. At full throttle (used very rarely) the RPM difference is over 300 rpms. How mis-aligned are the throttles supposed to be if at all? The service reps say that Mercruiser's specs allow for a difference, not sure of the tolerances though. Any thoughts, observations?

 

[jg300da]

I think you'd be hardpressed to find many twin engine boats whose engines are synched with the throttle levers matching exactly. You learn to live with it.

 

[MichaelNJ]

Are you talking about the alignment of the two levers, meaning thatthe levers don't line up when you have both engines running at the same RPM? That's a simple adjustment. When I got my boat, they didn't line up and it bothered me. I had it adjusted. There is and will be some play in the levers, that's acceptable but you should be able to have the handles togeether when the engines are in sync.

 

[osd9]

Are BOTH engines making it into the recommended max RPM range?

Could be simple throtle adjustment, or, possibly a propping issue. Either way, you don't want to lug an engine by being overpropped.

 

[fwebster]

If I read your question correctly, I think you may be looking at this backwards........

The throttle control levers should be set from the idle position with the engines' controls on the engine against the idle stop, not from WOT position. With the cables disconnected at the engine end, the engine is put at idle, then the levers are set even with each other at the idle position and the cable ends or control adjusted so the cables can be reconnected without moving the engine idle control, then and the throttle control lever stops are set. This way the throttle control levers are against a stop and aligned with each other at the same position that calls for the engine to be at idle.

It is normal for there to be some difference in the position of throttle control levers at WOT. But, if the control is set from the idle position then the levers stay pretty much aligned with each other up past normal cruise rpms.

I may be having a mental block, but this was really hard to put into words a reader can follow...."throttle lever" or "throttle control lever" is the part on the dash you grab; "engine lever" or "engine idle control" is the "thingy" (belcrank) on the engine that controls throttle position.........hope it makes sense!

 

[Bingo Boyz]

I am toalking about the throttles at the dash console. Does anyone know the acceptable difference in RPM at WOT? Sea Ray says mine is within spec but it still seems signifigant to me.

 

[osd9]

Bingo:

From a purely mechanical point of view, both Throtle levers at the dash should provide FULL open throtle "position" at the engines when the throttle levers at the dash are pushed fully forward.

That is a straight forward adjustment as per Frank's post above.

Once you are sure of that adjustment, you can then be sure whether or not both engines are achieving full RPM. You must first confirm one, before you can trouble shoot the other. Also, you have to be sure that your dash mounted tachs are accurate....but that is a whole different issue

IMHO, more than a 300 RPM difference between engines, after all of the above adjustments are made and confirmed, would warrant further investigation. It just seems a little too much difference to me.

 

[Bingo Boyz]

I have contacted Mercury to find out the tolerances on the items above. I hope to hear back from them soon.

 

[fwebster]

Do you hear the engine out of sync? You should easily hear a 300 rpm difference.........the harmonic vibration of 300 rpm would drive me nuts.

My guess is that your problem is as much a tach error as an engine difference. There is a resistance adjustment on the back of the tachs for a mechanic to sync them, but that cannot be done unless you are sure the engines are running the same rpm by verifying engine speed with an accurate test tach.

 

[Skip]

This does not relate immediately to your query, but if this is a boat you intend to keep for a while, and a boat that you intend to run long distances or for extended duration cruising, you may want to consider a Glendinning Engine Synchronizer. When I bought my 420 used it had one to synch the Cummins. I had never used a synchronizer before. On my 320, i synched the Mag Horizons by ear, running attitude, synch gauge, and tachometer.
The glendinning is one sweet piece of gear. At about 800 RPMS, you engage the synchronizer, advance the slave throttle to the stop, and accelerate/decelerate the boat using the master throttle lever. With the glendinning synchronizer, the dash mounted engine synch gauge becomes superfluous, because the Glendinning synchs your engines for you.
We ran the boat 7 hours home on the delivery run from the Bay Bridge marina on the Chesapeake to Quantico Marina, and I was on synchronizer the entire way. A real delight.

regards
Skip

 

[marqbloq]

My new 320 has a 260RPM difference and I am following this thread to hear the outcome. Please continue the post Bingo Boyz.

marq

 

[Bingo Boyz]

Once I take the boat to the dealership to get all of my warranty items finished off before it expires, I will address that issue. I will keep you posted.