When to Decarb 2 st

[dreemon]

I've had a look at the search for decarbing but would like to know what symptoms the engine will tell you that it needs doing,

last time i did it about 1 1/2 yrs ago, spraying into the carbs did'nt kill it, and didn't get the clouds of smoke when I re-started it and , didn't run any diff,

but since then I find that my top speed is slower, will decarbing give the engine more get up and go? used to get just over 60 klm's now 57 klm's,

any comments welcome.

[FNQCairns]

Excluding other possibilities not in your question, head down to your local auto parts store and buy a cheap compression tester, test the engine cold and without throttle, a fuss-less test is fine.

Look for a cylinder/s that is down on the others and use a whole can down this one/s, engine MUST be hot and it does take a good while. Quick application just do not cut it.

What happens is under normal use carbon finds it's way into the ring lands, is compressed mechanically as it slowly builds up insulating the main heat conducting pathway from the piston through the rings, the piston then runs overheated and by very nature over expanded within the bore, the rings are stuck in the lands and cannot conform to the cylinder as designed, there is more to it - regardless the result is an engine completely fubar in comparison to the way it should be operating.

Interested to hear your compression results, one can basically assume stuck rings to broken rings to spit rings all from nothing more than a lack of preventative maintenance that resulted a buildup of carbon in the ring lands.

cheers fnq

[dreemon]

OK, bought the comp tester fr. stuper cheap $29.95, pulled both plugs and pulled the engine rope 4 times (no throttle. cold engine) and got 140 psi on top cyl, same for bottom 4 pulls and got 139 psi. the plugs were the same tan colour, should I do the decarb anyway? and what are common symptoms of a carboned engine, this motor is a 1991, 2 cyl 40 hp mariner thanks for your help

[FNQCairns]

Sounds real good just like mine, log those numbers for future reference and test every 3, 6 months so you can pounce on any potential problem before it becomes a full on one.

If your rings were being impacted by carbon there would be more variability between the cyls, a decarbon is not strictly needed by those numbers but the more distance between the day they do want to stick the better! I consider it within the same vein of reasoning as a water pump.

How old are your plugs? poor/worn gaps will drop WOT rpm a little.

Easy things are the best to tackle without a further confiming symptom, prop in good condition still?, wadded any weight/wind resistance lately?

For it's age it reads really well, you must be well proped and the thermostat must still be operating properly or still be present if they have one. She sounds like a keeper!

cheers fnq

[Roughasguts]

Hmmm only 3 or so Kmh per hour.

Could be lower octane fuel your now using, or stale fuel.
Higher ambiant temps causing the engine to get less fuel air charge = less power.
Your carrying more weight or trimmed slightly differant.
Have you a ding in the prop ?

I would try some PULP in the motor that stuff has higher octane, but has better cleaners in the fuel, so maybe try that just for the one tank.

[tunaticer]

It could also be a tad of wear on your prop causing the loss of speed. The fine leading edge is a pretty important factor to a prop's performance as is the light cupping on the trailing edge for that little extra push.

Jack.

[Kleyny]

from what you are saying the motor seems ok.
How old is the engine?
I am guessing you are getting your top end readings from a gps.
the speed difference could be from a range of things as others have stated.
heres a few that i would also consider.
stale fuel, wave height.

neil

[ozscott]

yammy reckon every year for decarb in my manual for a 2 stroke

[dreemon]

FNQCairns, thanks for the thumbs up ! now having the comp tester will check 3 x a year, the plugs gapped at 30 thou and maybe 10 hrs on em, and yes the t'stat is about a year old (had to break some bolts to do it ) the prop is a yammy 13 p s/steel and perfect condition, no cup on it, but I have a 14 p alloy with cup and only slighty more top speed on GPS, as for added kilos, not much to boat ( the wife flashes me her beer gut sometimes ) but test WOT on my own leaving the trim at one from last hole,

R a Guts, I use 20 L fuel within 4 weeks, same servo and allways valvoline oil, but might try some better petrol to see what that does.


Tunaticer, had a great laugh at your comment on " how to cook Grinner"


Honkeydory, I only test top speed on flat water as it gets really flighty and is a tiller . it's a 91 40hp 2 carb mariner


Thanks for all your advise !! cheers

[FNQCairns]

FNQCairns, thanks for the thumbs up ! now having the comp tester will check 3 x a year, the plugs gapped at 30 thou and maybe 10 hrs on em, and yes the t'stat is about a year old (had to break some bolts to do it )the prop is a yammy 13 p s/steel and perfect condition, no cup on it, but I have a 14 p alloy with cup and only slighty more top speed on GPS, as for added kilos, not much to boat ( the wife flashes me her beer gut sometimes ) but test WOT on my own leaving the trim at one from last hole,

R a Guts, I use 20 L fuel within 4 weeks, same servo and allways valvoline oil, but might try some better petrol to see what that does.


Tunaticer, had a great laugh at your comment on " how to cook Grinner"


Honkeydory, I only test top speed on flat water as it gets really flighty and is a tiller . it's a 91 40hp 2 carb mariner


Thanks for all your advise !! cheers

Just a thought on this above, if you are running a no cup 13p well and typical, and also run a cupped 14 (probably the no cup on the 13 is why you are not turning a 12 atm) the 14 may be working to trash your good compression numbers with each use, sounds like an engine killer. I dunno the diameter of each although the 14 would need to be a lot smaller than the 13 to offset any lugging as your minor increase in overall speed attests to. In absence of rpm readings the 14 might be best left as an emergency backup.

9 times out of 10 the operator is the killer of 2 stroke outboards.

cheers fnq

[dreemon]

The 14 is in the boats storage as a spare and I want to take the cup off for solo trips, both are yammy props about 11.1/4, also have a 12 p that i would like to try but has a large chip to be repaired first and would use for the kids tube. also a small diameter s/steel quicksilver cupped 13p that I will try next time out, but have the 13 s/s yam prop on 98 %, used to have a 12p alloy before I wrecked it, but it would make it rev its head off too easy, i don't have a tacho but maybee one day,

[ozscott]

mate - doubt its overall imiportance to your problem, but I run the Merc (Quicksilver) oil designed for the Optimax. It might be argued that I should run Yammy, but the Opti oil I am assured is very good and does not smoke as much and cleans at it goes. I must admit recently when I did the de-carb, the piston tops were pretty good before I started, so perhaps the good oil is doing its job. I never have plug fouling issues, nor problems with prolonged idling and she smokes less than it did before I started with that oil...then again 100:1 at idle is pretty light injection.

Cheers