mercury inline 6 90hp dies at idle

[LCCRUISER]

My 1986 Merc 90 Horsepower engine is acting up. It ran great through 1/2 of yesterday until we got into heavy chop and bumpy water. It runs good anything over about 1000 RPM. At idle she dies and I have to rev her up hard and slam shift to get her to go from a start. It acts like it is running only on 4, but no trouble with extra throttle.

COMPRESSION IS 135-140 ON ALL 6. Switchboxes (powerpacks) are new this week as is the stator. Running like a top until half way through yesterday's fishing trip. Ran bad again today. Fresh fuel and 2 different tanks.

Once I get past that initial hesitation, it runs great at all RPMs above about 1K. I ran it on the muffs and pulled back the throttle to the point she was almost dying and squirted a bit of carb cleaner in each carb without any improvement. Enrichening has no affect, other than lowering RPM.

Whether cold or hot, I have to go to full throttle to get it to start. As soon as she screams to life, I can throttle back and it will run about 1500 and then go down to 1000 or so and keep it alive.

 

[latin monkey]

Re: mercury inline 6 90hp dies at idle

have the carbs been taken apart and cleaned?

 

[blue90HP50]

Re: mercury inline 6 90hp dies at idle

LC: Try this. Submerge lower unit in water. Start engine and get it to idle at 1000 as before. With cowling wrap off, back out idle set screw rapidly a couple more turns then back in to "set-point". Do this on all carbs. Must do each separately and quickly. This may dislodge any trash in needle valve. Then, stop engine. Turn idle set screw in to light seat, then back out 2.5 turns(CCW) plus 1/8 th turn more. Do this on all carbs. Start engine to see if it will idle. If so, good, try hole shot. If not, check timing and link and sync(instructions are on this forum). If the above fails, then look at rebuilding carbs and linc and sinc. I'm not an expert, just my suggestions, gained from experience with my 82. Good Luck
Blue

 

[LCCRUISER]

Re: mercury inline 6 90hp dies at idle

have the carbs been taken apart and cleaned?

Not yet. I did a SeaFoam treatment on it yesterday without improvement.

I'm on vacation and not equipped to do much at the moment, although I have my DVA meter with me and a few simple tools. I'm not about to tackle a carb rebuild under the current vacation scenario. (I don't think the spouse or the people who own the cottage would want carb parts and carb cleaner all over the kitchen table.)

I have had the boat 2 weeks and the motor has been grief since I got it. Ran fine during a test run before purchase. The I lost spark on 5 cylinders. I replaced stator and packs and it ran great for the first 1/2 day of our trip.

I really have to enrichen for minutes on end for it to get enough fuel to start. Once its running and I have jumped it into gear, the thing flies on my lightweight 17' Sea Nymph. Yesterday it wouldn't start for over an hour trying it off and on and I had to use the kicker all the way in from Nantucket Sound. Fortunately it was good weather with moderate winds following us in. Naturally, I was able to get it started about 1/2 mile before the launch ramp. So, we took it for a joy ride to get some life into the batteries and I had it on charge overnight to get them maxed out for tomorrow's adventure.


Today we went kayaking and my stress level has greatly diminished. The fishing has been slow, so maybe there will be more kayaking on Friday. We leave here Saturday and the wife has offered to put a For Sale sign on it before then

 

[LCCRUISER]

Re: mercury inline 6 90hp dies at idle

LC: Try this. Submerge lower unit in water. Start engine and get it to idle at 1000 as before. With cowling wrap off, back out idle set screw rapidly a couple more turns then back in to "set-point". Do this on all carbs. Must do each separately and quickly. This may dislodge any trash in needle valve. Then, stop engine. Turn idle set screw in to light seat, then back out 2.5 turns(CCW) plus 1/8 th turn more. Do this on all carbs. Start engine to see if it will idle. If so, good, try hole shot. If not, check timing and link and sync(instructions are on this forum). If the above fails, then look at rebuilding carbs and linc and sinc. I'm not an expert, just my suggestions, gained from experience with my 82. Good Luck
Blue

Couldn't I just remove the idle screw and jet and blast some carb cleaner through it instead?

I'm also not sure where the screw is, any guesses?

 

[daveswaves]

Re: mercury inline 6 90hp dies at idle

Most of the replies are for fuel starvation, it sounds like you have the opposite since adding fuel in the carb throat lowers your rpm. My guess would be a bad float or needle valve, flooding out at low speed. At high speed it will drink everything you can give it. Floats can become fuel logged or cracked and quite simply.....don,t float.
Probably not what you want to do on the holiday but removing the fuel bowl covers after squeezing the primer bulb to fill the bowl will show you the culprit.
Sometimes after trailering a boat with the engine tilted up the floats can jamb in the fuel bowl if the floats are worn.

 

[LCCRUISER]

Re: mercury inline 6 90hp dies at idle

I considered the possibility of flooding at idle. I know that hitting the enrichener at low rpm causes it to die. On the other hand, I need to enrichen like crazy to get it to start. I think it is the top carb at fault because I pulled the plug cap off #2 and it didn't change a thing when it was "running on 4". But pulling the cap on #3 made it die immediately. Short of pulling the bowls, how about a sharp rap on them in the even it is just stuck?

 

[daveswaves]

Re: mercury inline 6 90hp dies at idle

You can tap on the fuel bowl with the handle of a screwdriver and see if it helps. If it does then the float was stuck, if it doesn't it does not mean anything, the float could still be stuck or you could have a bad needle/seat or the float could be cracked. If you suspect the top carb that is the easiest one to remove the bowl cover to have a look.
You always need lots of fuel to get an inline 6 started, that is normal and is characteristic of the inline 6.

 

[LCCRUISER]

Re: mercury inline 6 90hp dies at idle

Weatherman was wrong today. Supposed to be 10 knots E & NE. It was SE 10-20 knots all day off the sound with 2-4 foot seas going out and coming back. I'm impressed by the ability of the SeaNymph to handle rough water. She rides high, but not dry.

Motor was a little less cantankerous getting going this morning and only stalled 6-8 times trying to shift from fast neutral to forward. Never quit running with a fair amount of throttle even when bogging into the back of a swell or hammering over the tops.

I now concur that flooding at low RPM is much more likely than fuel starvation. I will be odering a carb kit and do the top carb first.