200 rpm
 

Hello guys,

I have a question for you, tell me what do you think or if it has happened to you :

I have two 525sc clones on my boat, and when I am running wot, i have 1 at 5280 rpm, and the other one at 5080...
I have sent the prop to be labbed, and the same thing happens..
I have changed all the spark plugs, and nothing new....
I have checked the carburator/accelerator cable, and it is ok...

What can it be...? compressor....? timing...?

I want to know if someone had the same problem before?

Thank you.

Try a different tach

Swap props and run the boat in reverse (which will be forward)....or swap sender wires on the tachs to see if the problem is the tach.

x2 check the tachs first. Does it pull to one side?

Did you look down the carb bores and see if they were actually WIDE OPEN ? Throttles plates vertical ? One may be not fully opened. :drink:

Hello, the tach are digital floscan, I haven't swap them, I will do it, to see.
And yes the carbs look exactly the same when wide open....

In fact the problem is begining after 2000rpm : the port engine begins to loose power and throttles must be seperated to keep engine rpm's equal to the starboard engine, and, when wot there is still these 200 rpm........:eek:

Is the boost equal? My first thought was a slipping belt. If you have an IR gun see if the upper pulley is getting hot on the one dowm on rpm.

could be a stretched throttle cable.

A friend of mine had a similar problem with a pair of 1997 carbed HP500 Mercs. Turned out to be fuel water separator filters one clogged before the other, changed filters problem went away.

Do NOT do this if you have TRS or speedmaster drives, or you will burn up the transmissions.

Just swap the wires at the back of the tachs and verify the tachs are correct.

Also, when you are out there running the boat, synch the engines "by ear" at say 3500-3800rpm. You will be able to hear them in synch. They kinda "hum". Take note of the tach readings. Also, take note of the boost gauges when they sound in synch.

Checking the timing would be a good idea too. Not necessarily for only this problem, but timing is crucial on blower motors.

Check the timing for sure. A few degrees could cause that 200rpm difference. Make sure both engines are advancing at the rate and have the same total advance. They should both be at 35* total advance at 4300rpms. The stock merc balancers should have a mark on them for 35* also. Timing at idle should be 13*.

When running at 3800 both (on tach) I can ear that it's ok for the tach.
But I have to check the timing ( i will do it this saturday and tell you after)
Cables and fuel separator seems to be good point too, if i can i will change both before saturday.

Thank you guys, I will let you know what happen saturday !!!

Ok, so after checking cables and timing, we have made a sea trial, and still nothing different...
after talking with a mechanics, he told me to check the blower boost, but the gauges are not ok.....

So I have put a smaller top pulley only on the engine that wasn't running 5200, and made a sea trial yesterday....
Everything is ok now, both engines are running 5200 rpm when wot.

So my question is:

what was the problem?

Blower need to rebuild ?
valve problem?

what else...?

I would put the same pulleys back on each motor. Then swap the blowers. Really shouldnt take more than 1/2 hour to that. If the other motor is low on rpm, than the blower needs a rebuild. Which sounds like it could be the problem.

However, it still can be anything from the engines having different camshafts, to valvetrain issues. So i'd verify the blower situation first. Good Luck and keep us posted

It is common for belts to slip as they age on a 10 rib setup. Did you try a new belt as I suggested earlier?

I have ordered a pair, and I am waiting for it now...