Engine Prob, Carbs?

[Rapson]

I have a 1987 Yamaha 140Hp 2 Stroke with a intermitent engine prob.

Starting out from the ramp go to full throttle with no probs. Get to fishing grounds and shut down for about an hour. Start the engine and get to about 1500rpm when engine starts to struggle, if i continue to push throttle it dies. Does sound like one cylinder isnt firing. Continue along for a couple of minutes and missing cylinder comes good and can push to full throttle no probs. drop to idle and then to full throttle no probs.

Bit of history. Bought boat about 4 months ago. Fuel filter was full of crap so replaced fuel filter, fittings, filter on engine and removed and drained fuel tank. There was probs with electrics so engine had to be stopped by flicking on the choke, probs have been fixed now. New spark pligs installed.

Any tips on where to start trouble shooting would be appreciated.

[bigjimg]

Sticking choke possibly.Jim

[testlab]

Sounds like fuel starvation to me. Perhaps some gunk obstructing the inlet needle valve on one carby... so it takes a bit of time for fuel pressure to build up enough to force sufficient fuel past it and fill the bowl up enough to make sure the main jet gets a decent drink.

Do those carbs have the fine screen filter where the fuel line enters the carby? (in the carb itself)

With fuel that dirty in its past it may be wise to pull the carbys apart and clean them out anyway - especially if water may have found its way up there.

Then again I could be completely wrong.
.

[Rapson]

testlab I think you could be on the money. There was heaps of water in the old fuel filter bowl, but the little one on the engine was clean. I will givethe carbys a good clean this arvo an see how it goes.

[Rapson]

Pulled the fuel pump, filter and carbies apart this arvo and gave them all a good clean with a carbie cleaner and some compressed air. Once reinstalled the engine started first go every time, which was not the case before. Wont know for sure until I do a test run in the water but it is looking good to have fixed my problem.
Thanks for the input guys.

[rogerb]

We had problems with the carbs on 2 stroke Yams gumming up after being left in storage for some months, even though we ran the fuel out before putting it to bed.

[Rapson]

We had problems with the carbs on 2 stroke Yams gumming up after being left in storage for some months, even though we ran the fuel out before putting it to bed.
I think that may have had something to do with my problem as I think the boat had been sitting for some time before I bought it.

[Spaniard_King]

How Did you go Dave???

[Rapson]

Headed out today and not much difference. The engine was running a lot better so cleaning the carbs and fuel pump did do something but didnt fix the problem. Gave the plugs a bit of a scrub before to so I might replace them as well.

To top it off conditions and fishing were rubbish!

Oh well, still beats work!

[dreemon]

Try tilting the motor all the way up (with air box off) and primer bulb till really hard and check to see if fuel dribbles out of a carb, if it does, that could be crud in the needle and seat, best to do it when it's playing up,

I was having that same sounding prob and added anouther fine filter , if your fuel lines to the carbs are really old it might part of the prob, good luck

Also what did you find in the carb bowls ?

[Rapson]

Thanks for that dreemon, I will give that a go.

Didnt find anything in the bowls and all the needles looked clean and moved fine. I drained the carb bowls when this first happened so there was nothing in them when I dismantled the carbs, the smaller filter on the engine had a tiny bit of muck in it which could have been from when I installed the new water/bowl filter.

The fule lines look original and could be part of the problem so I might change them over.

[FNQCairns]

Consulting the crystal ball,

At home on the muffs on a dark night (on the water even) start the engine up with the cowl off (look for light shows) and run at a fast idle 1200 odd, have a spray bottle of water to mist all engine parts till wet.

Sorta kinda sounds like you may have a fine crack/whatever somewhere that allows moisture laden air to cause a problem when on salt water because of all that salt air dragged past when running to go fishing, if you where boating on freshwater probably wouldn't be an issue.

What you doing-doing full throttle from the ramp!.... looking to upgrade early to that flash new 4st:)

Anyway it's a guess..

[Noelm]

OK, you need to start right at the very basics, get a remote tank and hose, and fresh fuel and run the motor like that, it will eliminate any tank/hose/primer problem, when you removed the plugs to 'wire brush them" did they all look the same? REPLACE them anyway, they are not that dear, while they are out, do a compression test (even though it is probably not compression related), do not pull things to bits and throw parts at it just yet, that is a sure way to empty your wallet and get nowhere, do the basics. Next step is to buy a GENUINE manual, not a cheap no name or CD copy, downl load version, and if the carbs need doing, buy new kits for them and do them as per the manual, magic cleaning sprays and all that sort of stuff is not the way to go.

[Jarrah Jack]

And if all those things don't work then I would be looking at the black box. They can do some strange things at times when they are on the way out as mine on the 115 yammi did.

[mustang5]

Your not adding too much pre-mix are you???? Fouling spark plugs?? Similar things have happened to my yammie when the oil wasnt mixed in the tank properly and it was sucking up too much oil for a little bit. Another possibility is the fuel diafraghm. They sometimes leak which could be flooding it at low rpm.

Do you smell fuel at all??

With another yammy I had (Funny thing is I have only ever owned yammies), I found the problem with the carbs gumming up only to be in the mid to high rev range. Not so much idle and starting. It ended up being a particle blocking the jet in the top carb. You have checked this but from my experiences the V4's idle and start the same on one carb. I could be leading you astray but Im only talking from my own account on things.


Just a thought.

Hope it works out :)

[Rapson]

Update. I have dismantled the carbys and cleaned, new spark plugs and a new inline filter. Took her for a test run with a seperate fuel tank and line and there is still no change.

Gets to about 1500rpm and if I push the throttle any further she dies. Let her go for about a minute and then slowly push the throttle it sounds like a cylinder isnt firing and then kicks in and she runs fine. Slow down to idle and then can push the throttle flat out with out any dramas.

Should I now be looking at electrical?

[Jarrah Jack]

At the risk of upsetting Noelm again I would be getting a manual from readmanuals.com for $8. It should give you the tests to preform to check all the electrics. They are not as good as the genuine but you can get them straight away and cheaply. Then if you think the genuine would be worthwhile it hasn't cost you much.

[Gon Fishun]

G'day. Have you rekitted the fuel pump or just cleaned it? Is it getting sufficient vacumn. ? Just a thought, is there any chance of corrosion internally and water getting at a spark plug.

[D river]

Mate I've got a 130hp yamy 2003 2stroke and had the exact same problem your describing, It turned out to be a sh*ty old fuel line that in its day was the best around (grey 1 I think evinrude or something). It had a lining inside it and this became brittle, and little pieces were breaking off and some how found there way through the filter to the carby's. I pissed it off and cleaned cerby's myself and complete new fuel lines only to have the same trouble but this time it stuck proper n i limped home from the reef. Took it to yamaha and $600 later its perfect. They did everything I did again but this time replaced alot of the carby components. Motor is running like a dream again.

[Rapson]

Update, Problem fixed.... dodgy fuel pump.

After much troubleshooting and swearing and cursing I found a tiny hairline crack in the fuel pump housing, $40 for one from the wreckers and she is as good as new, atleast ran fine in the river today.

Thanks for the input guys.