adjusting carbs

[ron_a218]

I have a 90hp Merc three cylinder that makes an unusual noise at low forward speed. It kind of rattles. The noise stops when rpms are increased or its shifted into neutral. I've retimed it and checked carb synchronization. When I tried to adjust the carbs according to the service manual, the needle on the top carb can be turned until it seats with out bogging the motor. Manual says turn in till it bogs then back out quarter turn. The lower two carbs respond normally. Is this a problem with the top carb or normal? Is this rattling noise at lowest speed something to be concerned with? Thanks Ron A

 

[Backlash]

Re: adjusting carbs

Have you determined that the top cylinder is making power? Sounds like you may be running on two cylinders. If you need advice on how to check it let us know.<br />Backlash

 

[ron_a218]

Re: adjusting carbs

I think its running on three because it idles fine and runs great with any amount of throttle. It just makes this noise at the lowest forward speed. This may be normal for two strokes but it seems like an odd noise to me. I've checked most of the ignition out and have recently replaced the stator. A motor wouldn't run that good on just two cylinders, would it?

 

[Backlash]

Re: adjusting carbs

Ron,<br />What prompted my thought is that you can completely close the air/fuel mixture screw in the top carb without any change in engine performance. This could indicate that the cylinder the carb feeds is not making power. <br />Could you be a little more specific on the "unusual noise" it makes at forward low speed?<br />Also what year Merc do you have?<br />Backlash

 

[ron_a218]

Re: adjusting carbs

Its a '95 model Merc 90hp 2-stroke. The unusual noise is hard to describe. Its kind of the noise one would make if it were idling too low and trying to die. It sort of comes and goes, a kind of rattle like it running rough. It never dies and with the least amount of throttle will go away. It only makes this noise in forward gear and the motor vibrates or shakes a little when it happens. Any of this make sense?

 

[dilligafda]

Re: adjusting carbs

The noise you describe sounds like clutch rattle caused by the idle set too low. Check your idle speed and sync the carbs and linkage. Don't be overly concerned about the #1 carb not responding at idle. I have worked on several of these and found that the engine has a unique idle quality in that it has enough fuel in the recirculation system to support the top cylinder, so adjustment doesn't make a difference. Set the mixture screw at the recomended factory setting and perform a link and sync. Then set the idle at the prescribed RPM. If you still have the rattling noise, you may need to look into the gearcase. Hope this helps!

 

[aeronutt29]

Re: adjusting carbs

This may be a long shot, but.....<br />Is it possible that your top carburetor has a blocked idle passage? That would make it work as if the screw is all the way in regardless of were you actually put the screw. The engine will usually idle very nicely without one cylinder firing because it requires no power at all to spin an unloaded engine. The carburetor idle circuit is completely unused at high speed which would explain why it runs great at high power settings. It can't hurt to pull that carb off and give it a real good cleaning including blowing it out with compressed air after soaking in solvent overnight.

 

[MurdockJr]

Re: adjusting carbs

Do you have a stainless or aluminum prop. If stainless these motors will have what seems to be a rattle only in gear at idle speed, must be the weight difference in the prop. Hope this helps

 

[ron_a218]

Re: adjusting carbs

Thanks for all the help. It has a stainless prop and I don't have an aluminum one to try. I never thought about the gear case. It sounds like it might be something I could live with as long as I'm confident I'm getting adequate fuel to each cylinder. I'll try to reset the carbs and get a service tach and set idle. I've been just guessing. thanks, Ron A