1999 90 Merc running rough at idle speed

[ssp]

I have a 1999 90 3cyl merc that was recently serviced. It fires up great and idles smooth. When I bump it into gear, it begins to shutter significantly. Once I give it some more gas, it runs fine. It runs great from there to WOT (5200 rpm without trimming it out too much). I took it back to the shop to check it out and they said everything looks fine. The idle and timing are set to spec. The mechanic suggested that it could be the torque from the 4 blade prop (original trophy 4 ss 19 pitch), but I have never had it happen before. They replaced the fuel filter on the line and put a new primer bulb on when they serviced it. Anyone have any suggestions as to why it runs so rough in gear at idle speed? Thanks in advance for any help.<br /><br />Scott

 

[12Footer]

Re: 1999 90 Merc running rough at idle speed

Greetings, Scott. Welcome to iboats.<br /><br />Go to an auto parts store (NAPA, Discount, PepBoys ect), and pick up a spark tester. They are only about 8 buks,and the gap can be adjusted . The plug wire is unclipped, and clipped to the tester, and the aligator clip is connected to the block.<br />It sounds for all the world like you're down one cylinder. It could also be a fouled plug, or running too lean. Iether way, this can be deduced by the condition of each plug, so you'll know at a glance.<br />If you need help with "reading" the plug's condition, use the little chart they have in all shop manuals, or just ask here.<br />But I highly suspect it's ignition missing on one cylinder. Lettuce no what you find.

 

[ssp]

Re: 1999 90 Merc running rough at idle speed

12Footer, <br /><br />Thanks for the reply. I will do it this weekend. Someone else I was talking with suggested that perhaps the fuel might be slightly constricted or even that the plugs could be the wrong ones. I was wondering if anyone had any thoughts on those theories as well.<br /><br />Thanks again,<br /><br />Scott

 

[12Footer]

Re: 1999 90 Merc running rough at idle speed

If it has the wrong plugs, it can cause major engine damage. What are the plugs? we can check that.

 

[KIWI MERCMAN]

Re: 1999 90 Merc running rough at idle speed

ssp<br />Before you go looking for spark problems,(as you did say that it was ok before it was serviced) first check the idle speed in gear, it sould be around 750rpm. Big props will pull your idle down in gear, and check it with a shop tach not the one in your boat. The one in your boat could be 1 to 200 out, this very common. If it is ok then I would suggest that the tech at the shop has set your idle mixtures wrong.<br />That model has odd ball mixture settings and they need to be set dead right or they will do what you are saying.Top carb only runs about a 1\4 turn where the other carbs are up to 1 1\2 turns out. Also it is very important on the 90 to make sure all the carb butterflys are CLOSED at idle otherwise they run like a piece of s--t.The other thing I would check is the running temp, 90's dont like running cold<br /><br />Safe Boating<br />Kiwi

 

[blazed4x4]

Re: 1999 90 Merc running rough at idle speed

Gotta agree w/ kiwi on this one. Doesnt sound like spark to me. If ya had no spark on one cyl. she wouldnt run up to 5200. Get yourself a manual and run through the linkage adjustments. One more thing, did your shop run your rig on the hose or in the water?

 

[ssp]

Re: 1999 90 Merc running rough at idle speed

All,<br /><br />Thanks for all the good advice. KIWI MERCMAN, I will check the idle speed while in gear. saltydog660, the mechanic said they "tank tested" it, so I am assuming that they tested it in the water.<br /><br />Thanks again,<br /><br />Scott

 

[ssp]

Re: 1999 90 Merc running rough at idle speed

KIWI MERCMAN, <br /><br />I have some questions about your response. I apologize if this is a dumb question and please let me know if this is information that I can get from a shop mannual (I plan to purchase one this weekend). What do you mean by the idle mixture? How do I determine if each cylinder has a 1/4 turn or 1 1/2? I am assuming this is a set screw that is adjusted for each cylinder. Where do I find these adjustments points (screws?)? Thanks again for your help.<br /><br />Scott

 

[bryanredfish]

Re: 1999 90 Merc running rough at idle speed

Get a manual. It has all the information you are lookin for.