performance issue
 

Hi everyone i took the seacraft 20 sf out today the sea conditions were 2-3 chop. I could not get the engine to turn more than 4000 rpm. I took it out last week in more or less the same sea conditions and was cruising 4200 no problem with a WOT of 5200. I cant figure out why such a decrease in performance. The only thing I've done different from last time is i added 20 gallons of fuel with 4 OZ of marine STA-BIL and one more person. Could the fuel filter be clogged? Thanks in advance for any advice! Mercury 135 HP 1988.

Start by checking spark, compression, and linkage (make sure it's not binding on something) before ripping apart the fuel system. Sounds like your only running on 5 cylinders. Is the motor running rough at WOT? surging?

If you really suspect fuel system problems, the easiest way to check is run it on a 3 gal can of pre-mix fuel. If it runs fine then you know its up stream from your connection. If it still runs bad, then more digging, like BIG suggested.

I posted on THT - I think its down a cylinder about the right RPM

tautog did you get your SC a short time ago? Did it maybe sit for a long time?

Thanks guys for the quick responses the thing i forgot to mention was i moved the battery under the console and moved the shift and throttle cables a little bit. I also suspected it not running on all cylinders. Ill check spark today and linkage. I just purchased the boat recently it was my second time out. The previous owner had $1200 worth of work done just before i purchased that included new fuel lines carb cleaning, water pump, and thermostats. It also has new fuel tank done this year.

performance update
 

Hi guys the motor is getting spark on all six cylinders.
The compression test with all plugs removed and a cold motor were
#2 85
#4 85
#6 85

#1 85
#3 80
#5 85
I then took boat out in the bay and tested compression in one cylinder with all plugs still in the compression was between 95 and 100 on cylinder #2. I can't understand why I was getting 5200 rpm last weekend and then such a huge drop in rpm's. I'm sure the compression was the same last week. Could a failing fuel pump be the cause? Again Thanks everyone for you're help.

Did it sputter or have trouble starting or just fail to reach rpm?
You mentioned moving the throttle cables - could they been moved or adjusted enough restrict full range of their movement, say not open enough or be able to "floor it" on the gas? If your butterflies are restricting air flow you wont reach your rpms.

Hi Mcgilli thanks for responding to my thread. What I meant by moving the linkage cables was I slid some of the slack under the floor to make room for more storage. I opened the cowling when I was out on the wAter today and adjust the throttle a little bit I was able to get another 200 rpm's. The engine cranks up no problem no sputtering at all. Also when I moved the battery I swapped the engines power cables with the ones that were going the battery to the console when the battery was located in the stern. The cables from the motor were not long enough to reach console anyway both cables seem to be the same gauge. I just figured I threw that out there also. Does anyone know if there is any adjust on the shifter handle itself? It seems I have to push shifter handle pretty far forward before I get any response from motor. I'll probably pick up a service manual to find out how to adjust linkage. Thanks again!!

Do yourself a favor and buy the Mercury Factory Service Manual, not one of the after market manuals. I've had both and there's no comparison. You can probably find one on ebay. You'll need your engine's serial number in order to get the right manual.

The throttle cable adjustment is at the motor end of the cable. To check if it's a throttle adjustment problem, remove the cowl, and then remove the front air box cover. With the air box cover off, you'll be able to see the carb butterflies. With the engine off, push your throttle all the way forward. If the butterflies aren't wide open, that's your problem. It could be a simple matter of adjusting the throttle linkage, or the throttle cam could have slipped out of adjustment.

The manual will tell you how to fix either problem. Or you can check it out and post questions here.
Dave