rough tick over. any ideas

[il corvo]

I've got a 40hp twin carb 2 stroke 40hp mariner recoil start.

the engine starts fine and shoves my avon along very well, it's just at tick over and very low revs it seems lumpy. i'm aware they are not as smooth as a 4 stroke and it never stalls but just feels, well, lumpy.

before i go talking to a marine engineer is was wondering if there was anything i can check at home.

i'm not much of a mechanic (basic oil changes on motorbikes and such is about it really) so the simpler the better. perhaps the idle is too low? or perhaps the carbs are a little gummed up with old 2 stroke oil. if so does anyone know of a carb cleaning additive i can try to clear it?

like i say, this is before i start paying mucho moolah on someone to strip, clean and set up the carbs.

for what its worth, when not using the boat i always run the engine after disconnecting the fuel line to run down any fuel in the engine and carbs to stop the gummed up thing.

any pointers will be much appreciated. it could just be me looking for issues that don't exist but i like peace of mind.

[Fenlander]

Well I'd check the plugs were fine first... if more than a year old replace to eliminate them.

Then I'd get access to a manual and set the carbs up for idle mixture and tickover speed. Sometimes just a 1/4 of a turn up on the tickover screw will enable a speed that evens the idle loads.

Only once those are done would I start to look for further trouble.

[il corvo]

agreed, plugs and fuel filter are getting done regardless as i don't know how old they are. like i say its just at tick over so it's defo a set up issue and nothing terminal. i just want to smooth it out or i'm just not used to the engines character yet.

the bloke who owed it before me is by trade a marine engineer so should know what he was doing but that said when i changed the leg oil i came out black as the ace of spades (no metal particles though which is good) so i wonder weather he serviced it as often as he could have.

[Fenlander]

I'm fanatical about over the top maintenance of my outboards but in some cases marine engineers... like car mechanics... know just how long stuff can be left as long as it runs!

[uncle al]

It may be that the carbs want to be sycronised,the only way I can explain is, to either take of the air box and look down the throuts of the carbs, are the throttle butterflys both in the same position. Or if you are standing at the back of your OBM if you look on the port side at the carbs,you will see a lonk rod between the two carbs, and on the top carb you will see a screw (it a left hand thread if I remember correctly)loosen the screw and start the OBM up and let it tick over in nutetral, with the engine running it will pull both throttle butterfly valves closed, then very carefully re-tighten the screw.Its easier to do than write how to do it.Please dont put it in gear when the engine is running if the prop hits you ,I promise it will hurt, and obveusly you have to have muffs on with a good water supply, and the engine will run faster with muffs , than if the boat was sitting on the water. Make sure the idle air screws are both adjusted from lightly seated the same amount about one and a half out from lightly seated.Wrong sparkplugs fitted or the gaps are not set correctly.

[il corvo]

cheers. i'll have a look but as it happens, i text the previous owner earlier to see what service schedule he used and mentioned it to him, he has all the gadgets needed to do a full carb balance so he's gonna have a look for me and get it running spot on throughout the full rev range so with a bit of luck it will be ticketyboo in no time.

i appreciate everyones input though. big cheers to all.

[9D280]

Before you do anmything- how long have you had the engine? I ask because if it is new to you, the exhaust back pressure might be set for a slighty different transom.

The extreme version of this is people who tweak the idle speed set screw wit hthe engine on cooling muffs and wonder why it stalls when o nthe baot i nthe water - the exhaust on muffs has negligible back pressure, and so the idle is set waaay too lean to push the exhaust past thre water when afloat.

Back to your boat- if it's been adjusted in a tank or on a differnet boat there may be a difference in the submergence of the prop / exhaust.


+1 for fenlander's comments in the first reply.

[il corvo]

i've not touched any thin as yet. the rough tick over is while the engine is in the water and clears when some revs are applied. I understand the back pressure aspect of using muffs so i don't use that as an indicator (it's like taking the end can off my aprilia v twin, it runs like shite).

but as i said in my last post, the previous owner has all the gubbins to get it running spot on so i'll leave it in his capable hands.

[uncle al]

what sort of gubbins has he got? Just an idle question.

[il corvo]

Hes by trade a marine engineer with full work shop who also was an engineer for the rnli. The only reason he didnt set it up was purely no time to him self hence him selling it. I spose when your hobby turns into a job...

I found all this out whilst having beer with him the other night.

[Poly]

Originally Posted by il corvo

Hes by trade a marine engineer with full work shop who also was an engineer for the rnli. The only reason he didnt set it up was purely no time to him self hence him selling it. I spose when your hobby turns into a job...

I found all this out whilst having beer with him the other night.

a mechanic didn't make some supposedly simple ajustments and get maximum value for the engine before selling?

[il corvo]

Bearing in mind how little i paid im not overly bothered. Either way, its getting sorted for free so no harm no fowl and as hes only 10 miles away not a problem.