rochester carb no wot rpm caused

hello gentlemen, 1987 merc 5.7 260 not reaching wot with a simple 4 blade 16 pitch aluminium prop, i only got 4100 wot rpm with little gas in the tank and me alone on the boat. yesterday i went back to trouble shoot and notice when i hit the throttle, gas is only spraying from one of the nosel on the carb the other one barely spray, so maybe the carb is part of my probleb, last problem from day one the engine vibrate when in nutral and i rev it up, i took of the drive and run it with my garden hose and it still vibrate when i give it gas, kind sound rough, the engine idel fine no miss not shaking. i pull all plug wires while the engine is runing one at a time they all make a difrent so i think all the cilinders are firing, i hope.
as you see i have more than one problem here, gas/vibration your thought guys, after searching this forum i came up with a simular problem a guy was havin where don s work on his boat and found one cilinder was not firing after readjust the valve he got compression in that cilinder, leading me to think maybe i did not adjust my valves right. what i did was loose them one at a time antil it start ratle, i then tighten back just as it gets quiet i gave them another half turn tight.

your thought guys. thanks

Re: rochester carb no wot rpm caused

Sounds like your first order of business is a carb rebuild.

Re: rochester carb no wot rpm caused

thanks it seems that way,

Re: rochester carb no wot rpm caused

Personally, I'd worry more about the vibration.

I rebuild my carb about every 4-5 years.
If you have never done it - it's way overdue.

Re: rochester carb no wot rpm caused

yea man that vibration is just not right, in nutral when i hit the throttle it sopose to go vrum vrum instaed of that sweet sound it vibrate a rough sound,

Re: rochester carb no wot rpm caused

I just commissioned my older Mercruiser with a fresh engine. After rebuilding the Rochester 2bbl, by the book, the boat would bog out when I got it on plane.

A little open water diagnosis showed the float bowl was not filling properly.

I bent the float tab a little to allow a higher float height, and the problem solved itself. The boat would fly to max RPM and hold it, no problem.

Oh, the vibration at slower speeds disappeared. It's amazing how well they run when they get fuel .

Re: rochester carb no wot rpm caused

update again today i go back over the valves just to make sure i did not overtight them , they were all good i just loose them antil rattle, then i tight just as it stop i give it another half turn.

now back to mr carb i realy need a new one, i put the boat in nutral and rev it wot real quick i kno thats not good for the enging but gess what the secondary did not open one bit, is it only sopose to open with load. last it smell kinda rich, when i turn one of the screw all the way in to adjust it a little , with the screw bottom out the carb did not make a diffrent it was still running the same lol the starboard did make a change tho, well i hope it is the carb that killing my wot. opps about the vibration it only vibrates from about 1000 rpm to 1200 then it dissapear on the higher rpm s oh well . thank everyone for helping me out this boat will be going on the market soon just want to get it running right.

Re: rochester carb no wot rpm caused

Vacuum controlled secondaries rarely open with no load on the engine.

Re: rochester carb no wot rpm caused

It sounds like the carb is in die or need of an overhaul and it may be causing the vibration but you might take a look at the harmonic balancer on the engine just to be safe because if it is a basic balancer the enertia ring may have slipped .

Re: rochester carb no wot rpm caused

tom i thinkyou might be right on the balancer i will take it of and give it a look over, and yea the carb is a mess. i am going to buy a kit and try to do it myself . maybe lol

Re: rochester carb no wot rpm caused

Rather than take the balancer off, bring #1 cylinder up to top dead center and see if the timing pointer is pointing to 0 degrees. You can do this by taking the balancer bolt out and make sure the woodruf ket that holds the balancer from slipping on the crank is pointed straight up !

Re: rochester carb no wot rpm caused

If you can bottom out the mixture screws and it stays running...time to throw a kit in there.