I've got a 1987 Mercury 135 outboard (B117465) that is running rough and smoking, any ideas?

Question

I've tried just about everything to sort out a problem which sounds to me like it is misfiring (a sort of intermittent dull popping sound), smoking excessively and running quite rough. It runs great at high speed, but cuts in an out at low to mid range rpms.

So far...I've changed the stator, trigger, and had the "distributer" boxes tested (they're fine) spark plugs and even tried replacing some of the coils. I've replaced the fuel pump diaphragm, fuel filters, primer bulb, fuel lines and pulled cleaned and replaced the carbs (checked the reed valves at the same time). The timing has also been checked and adjusted and the compression checked (125 in all six). Changed the thermostats, impeller and pressure relief valve.

The spark plugs are very oily when I pull them out and the cylinder heads appear wet with oil...I'm about to start trying to fix it with a sledge hammer if I can't find the answer soon!!

I hope somebody can suggest something else!

Dave

Thanks for your replies.

I checked the jets I'm running a .50 on the main and a .094 on the top ones. I suspect they are factory issue, but it did get me thinking that it may be running rich at the lower end.

Nigel, for some reason the web links you gave me won't come up. The websites seem to be down; I'll try again later. I'm assuming this question is probably answered there.

helipilot - when I took apart the carbs, I did check and adjust the float bowls (and checked the pins), although I have to admit I probably wasn't meticulous. From memory the adjustment I made were mostly raising the floats to where the mechanic had told me they should be (i.e more fuel in the float)

Sorry forgot to mention that the fuel is only a week old or so. I use the boat regularly, mostly diving and spearfishing. I just avoid going slow (which creates some exciting docking adventures!)

Thanks Phillip - I'm running BU8H spark plugs, surface gap type - only about a month old. In the last few months I have been pulling the plugs recently and while they are always covered with wet oil it doesn't seem to make a difference to the running of the motor when they're clean.

I also forget to mention previously that the idle jets are a .060. So the carbs are jetted (fixed) to .060, .094 and .50

Interesting update...The replies about the float bowls and overfueling jets got me thinking and in the true spirit of "shade tree mechanics" I tried pinching the fuel line shut and waited. Lo and behold, the engine smoothed out remarkably, idle revs came up significantly to over 2000 (from normally sitting at 1000 or so) before starting to lean out. I released the fuel line and the rough running condition returned.

So, my learned collegues...does that point to overloaded float bowls? Surely it means the problems are related to it running rich... Or am I on the wrong track...still, it gives me some hope and a reason to keep the sledge hammer in the shed for a little while yet.

Thanks Philip,

I appreciate your response, I suspect the local marine supplier will be able to provide me with a replacement in the standard type sparkplug. I will try it, (I'll try anything at this point) but I've got to say the boat is rarely run at low - med speeds. Apart from leaving the boatramp and on the flusher, its full throttle to wherever I've got tagged as a dive site (weather permitting). The condition is most noticible when backed off to 8 knots or so after a hard run (30 knots) for 20 minutes or so. I appreciate your advice and I'll try changing the plugs if it helps and saves the powerhead.

Thanks

Answer 1

The oil injection pump probably needs adjusted. Mercury does sell an electric replacement for that Black Max, that takes about an hour to replace. I don't think she deserves the sledge quite yet. Hang in there.............Are you by any chance running a pre mix in your gas tank ???

Answer 2

hi there
all the answers are possible solutions. I have been in the marine business for over 26 years and worked on thousands of engines and run into situations like this quite often. One answer I didn't see was about spark plugs.
Basically Mercs as well as other manufacturers will use two types of plugs, a standard electrode type and a surface gap plug.
In my experience for general running,trolling,skiing etc. the standard electrode type (that you can set the gap on) works very well.
If you do like alot of sustained high speed running such as long runs to a fishing hole where you will be running in the upper RPM ranges for a lengthy amount of time the surface gap plugs cant be beat.(These are plugs without electrodes that have smooth surface across the firing end of the plug).
The surface gap plugs seem to load up(build heavy deposits of gas and oil) in idle and midrange usage to where the standard type plugs don't.
Hope the info helps you out.

I totally understand what you are saying. What I am saying is the way you seem to use your boat you need the standard type plug to properly burn off the gas/oil that is on the plug. I really see this alot and would bet this will cure your trouble.(DON"T ASK JUST DO IT) LOL just kiddin with ya but 4 real I think this will solve your trouble.
Just read your update and will say this I am not saying that it could not be the jet situation but any outboard will pick up RPM's when leaned out on the fuel mixture. Also this new found evidence could lean the motor to much and cost you a powerhead. Why not try the easy solution and the less costly one and try the plugs before you rejet and end up with alot of pain and misery. I am talking from experience not hit and miss guess work.

Answer 3

135 Mercury Outboard

Answer 4

Are the carbs a float type?. I had an earlier Merc 140 Six that had constant problems with the float bowls overfilling due to leakage in the float assembly needle valve. The motor would run like crap at low speed but given enough RPM it would draw enough fuel to keep the float bowls at the right level...slow it down and same thing..
How old is the fuel?
Also i'd check the diaphrams very very carefully. (hold to a bright light)


Addendum: ...If you pinch off the lines and cut the fuel flow and the motor returns to normal running before fuel starvation kicks in....then yup....absolutly...Float bowl overflow..
New needle and seats are required.
Good work....will keep checking in

Answer 5

don't know much about boats. My brother in law always had the same problem. Haul butt at high speeds but idling and slow speeds it's rough and even dies. He's a mechanic and fixed it with "jets" because it was pushing or not pushing enough fuel....think pushing to much. Not even sure you have "jets" but thought any advice would help rather it's right or not.......could beat a sledge hammer. Good luck

Answer 6

http://www.outboardrepairs.com/2_stroke_outboards/#mercury_2_stroke

http://www.marine-engines.net/mercruiser_marine_engines/#mercruiser_30
http://www.marine-engines.net/topics/000651.html?motors

Answer 7

maybe your fuel mix is too rich.