1995 Mercury 90 OB : Puzzled by slight miss at idle ?

[reccrew]

I'm hoping some members may offer their thoughts on a minor issue. Everything seems right on my 1995 Mercury 90 OB and on 11/10/12, I gave it a through de-carb with the recommended 16 oz seafoam method & the plugs also came out like new.
I then sea trialed it for another 2 hours and it ran very well with one minor exception:
After running for at least 5 minutes under load in fixed RPM ranges from 3400 to WOT of 5500 RPM, when I eased back down to idle speed, I have a slight miss. The motor will jump to 1100 RPM down to 900 RPM with a slight shutter and a (pfffft) sound. What's strange to me, is this lasts for just less than 1 minute and the outboard settles back in at 1000 RPMs. I repeated this same thing 6 times so its consistent behavior. In my search for perfection, I'd like to solve this issue & appreciate your thoughts & suggestions.
Thanks,
reccrew

 

[Don S]

Re: 1995 Mercury 90 OB : Puzzled by slight miss at idle ?

Moving to Mercury OB forum, you do not have a Mercruiser IO.

 

[Dave1027]

Re: 1995 Mercury 90 OB : Puzzled by slight miss at idle ?

I would start with a refurb of all three carbs paying very close attention to float level. Probably should rebuild fuel pump too just in case you are getting a leak there. Then sync and link the motor.

 

[CharlieB]

Re: 1995 Mercury 90 OB : Puzzled by slight miss at idle ?

Fuel treatment alone is not a guaranteed method of cleaning the carbs. Accumulated mineral or crub restrict fuel flow in the small passages of the idle jet and slightly 'lean' the idle mixture. You might get lucky and while running at idle try turning each idle mixture screw in 1/4 turn then back out that same 1/4 turn, sometimes this will dislodge the mineral accumulation and restore fuel flow. Repeat on each carb. If this fails to cure the lean idle mis-fire (that Pfft) then a thorough carb cleaning is recommended.

 

[reccrew]

Re: 1995 Mercury 90 OB : Puzzled by slight miss at idle ?

Charlie B & Dave 1027,
Thanks for the info; being a bit of a perfectionist I believe I'll refurb all carbs, rebuild the fuel pump & sync & link. I suspected the leaning out especially on cylinder #1 & appreciate the suggestion on the procedure for the idle mix screws; I think I'm going to just go ahead and do the oultined repairs. The motor is running very good now and it's not a very costly fix: hoping it will last a few more years. I really appreciate the knowledge and advice of you guys!
Regards,
reccrew

 

[reccrew]

Re: 1995 Mercury 90 OB : Puzzled by slight miss at idle ?

Thank you Sir for moving to correct forum...should have kept my glasses on when posting.

 

[Texasmark]

Re: 1995 Mercury 90 OB : Puzzled by slight miss at idle ?

You are experiencing part of what I experienced recently and solved....look under my recent post herein of a 90 sneezing. Even though my carbs had no varnish, with carb cleaner and compressed air, I dislodged a wad of what appeared to be oily fine grain sand plugging the #3 high speed jet (the other part of my problem) and something similar in the top (low speed area) of #1.

On the high speed, after spraying with cleaner, I put the air hose into the vertical cast cylinder in the bottom of the bowl which is connected to the high speed jet and shooting a blast of 100+# air dislodged this blob which I was unable to detect with a visual observation prior to the treatment.

On the low speed, I took the top covers off, exposing the low speed "circuits" and had removed the low speed adjustment screw for observation of the seating surface. I doused good with carb cleaner and shot with more air. The crud in #1 was easily seen when I removed the top cover.

In reassembling everything, I followed the procedure in the manual to the word, setting the carb butterfly gang and then in setting the linkage adjustments. I had earlier reinstalled the low speed screws, seated lightly and backed out 1 1/4 turn, all being the same adjustment. As I later found out in the water, that was ok and no further tweaking was necessary.

My low speed timing, set with the idle adjust screw had a listed of range of from 2 to 8 BTDC and on muffs, after the engine warmed up, I found that 4 degrees was what gave me the best combination of idle and desired idle rpms. Remember that on that engine the book says that the idle screw adjusts the timing to obtain the desired idle, not the carb low speed jets, although they play a part in it.

Last it says that your goal when finished is that, after adequately warmed up....which mine has always liked, the check for the correct low speed idle adj. is that when you goose it the engine will take throttle while in F and in the water. I think CharlieB told me that when he needs to do that he just leaves the boat on the trailer on the ramp which makes it a lot safer than hanging over the transom in rolling waves.

Last comment, the manual says that to get the operational performance necessary to get on plane (adequate fuel supplied through the low speed adj. screws) and not bog the engine, when in N at idle it may be a little rough. Mine used to be, but after this, it was very smooth.

HTH,
Mark

 

[reccrew]

Re: 1995 Mercury 90 OB : Puzzled by slight miss at idle ?

Mark,
Thanks for sharing your expereince & the great information. Today, I went over the boat & pulled off & replaced the main fuel line to the OB as well as the fuel line from the built-in tank to the fitting on the splashwell; with modern EPA type hoses. I'm pretty sure, they were both still 1995 originals; no telling what they may have expelled into the carbs with our modern fuel? Hopefully, over the next couple of months I'll get in to the carbs and will rebuild the fuel pump too; hope to have a favorable report.
Regards,
reccrew

 

[Texasmark]

Re: 1995 Mercury 90 OB : Puzzled by slight miss at idle ?

Mark,
Thanks for sharing your expereince & the great information. Today, I went over the boat & pulled off & replaced the main fuel line to the OB as well as the fuel line from the built-in tank to the fitting on the splashwell; with modern EPA type hoses. I'm pretty sure, they were both still 1995 originals; no telling what they may have expelled into the carbs with our modern fuel? Hopefully, over the next couple of months I'll get in to the carbs and will rebuild the fuel pump too; hope to have a favorable report.
Regards,
reccrew

I recently had my 10 year old Quicksilver gray fuel line generate a bunch of tan crud that completely clogged my fuel system up and including the filter. Thinking back now, maybe this is the stuff that found it's way into my carbs and caused me the trauma that I experienced recently. Filters can only filter what is larger than their pores. Smaller things leak through. I think Chris hit it in his comments on this post. I found a couple of problems, but I feel that the blasting with carb cleaner and compressed air cleaned out other subtle problems.

HTH,
Mark