Yamaha 9.9 (white) running rough when warm

greggron

greggron

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I have a Yamaha 9.9 4 stroke (the older white model). It starts fine and runs fine until warm then runs very rough. It is the wax/solenoid type of auto choke

Sparks are new
oil has been checked and is the right level
carb has been cleaned
pilot tube runs strong and cool
fuel is fresh

Could anyone shed some light on what might be happening and offer solution(s)?

Help gratefully received
 
Hi - Thx for replying

As its a 4 stroke I don't think it will be the mixture. How can I diagnose a possible problem with the autochoke? Any ideas?
 
I imagine Maxi77 meant the air fuel mixture. If the carb is not as clean as you think then once the choke is disengaged the mixture could be weak.

have you checked that it is firing properly on both cylinders ... an ignition fault that only occurs when warm could be the problem.

I think though the first thing you need to do is to check the operation of the choke.

What year and model number are we talking about?
 
I had a similar problem that defied all attempts to fix it until I found (yet another) jet in the carb. It was at the bottom of a hole that was covered by a friction fit 'cap'. Once cleaned, the engine ran as well as it had when new. Find ALL the jets in the carb.
 
my late 80's 9.9 Yamaha long shaft defied all attempts at repair. It would run full speed and die to an idle, then up to full speed, down to an idle. Never died, but always up and down.
The final verdict was the oil sensor unit. It is there to idle the engine down if you lose oil pressure. My Mexican mechanic disabled the sender and it runs great! He had two other similar engines he was going to check as no one could ever find the problems.
 
Jmolan said:
my late 80's 9.9 Yamaha long shaft defied all attempts at repair. It would run full speed and die to an idle, then up to full speed, down to an idle. Never died, but always up and down.
The final verdict was the oil sensor unit. It is there to idle the engine down if you lose oil pressure. My Mexican mechanic disabled the sender and it runs great! He had two other similar engines he was going to check as no one could ever find the problems.
Click to expand...

Mine is the old white 9.9 too and I've got a similar problem that has defied diagnosis! It seems to run ok for a few minutes and then power drops to virtual idle. It keeps running but even on full throttle does little more than putt along - then suddenly it kicks in and your off (as much 'off' as you can be with a 9.9). It then drops in and out of power randomly every few minutes. Really frustrating! I have noticed that when it's in idle mode water seems to gush from under the power head near the pivot mount adjacent to the transom clamps - I can't see where it comes from or why it happens. I got a belt off the HT leads so have replaced these and the coil, new plugs, new fuel, cleaned the carbs thoroughly, played with the mix, checked the thermostat - any ideas please????
 
Not being expert at these things, I took it to Red Line Outboard services in Abingdon (no connection, just local to me). Miraculaously, they found the same problem as I did having cleaned out the carb again. They have now diagnosed a faulty CDI unit. Due to the postal strikes, the new one has not yet been received or fitted. I hope to goodness that will cure the problem - CDI units are EXPENSIVE (£250ish). I did however hear that it could be a blocked exhaust. Is there any likelihood of this?
 
Greggron

Sorry did not respond when you posted model details. I have not identified the year. I thought the letter L would be the year identifier but they did not, it seems, use the letter L :confused:

A faulty ignition module does fit the symptoms ... an intermittent fault cutting out one cylinder. I did say check that it was firing on both cylinders!Hopefully the new module will cure it.

Seems unlikely that the exhaust could be blocked.


Rutland mike

You too could have an ignition fault that is cutting out one cylinder. Could also be the ignition module
 
Mike

Thanks for getting back. The cdi has now been replaced and the motor is running perfectly. I shall be picking it up today.

Thanks for sticking with me.

Greg
 
Are the spark plugs the same size ? as some engines had one shorter than the other and if they were in the wrong way round it would run lumpy.
 
I put 2 new sparks in at the pre-season service but according to spec and identical. Was running fine after they were installed. Thanks for the input though
 
Are the spark plugs the same size ? as some engines had one shorter than the other
Click to expand...
That's pretty unusual I'd have thought. What engines are like that? Must make a note for future reference.
 
Mee Too

Hello!

I am VERY interested in this thread!

I have a 1990 white 9.9 four stroke No. 6G8 L 301952 (1990) F9.9A (I am 95% sure that the L is Long shaft)

It has the (very complicated) old style vacuum choke system, that I sorted all out about 5 years ago. (I have the flow path diagrams and description of workings if anyone wants them - in German but lots of piccys)

In the last year or so I have had very similar problems to those in this thread, in fact pretty much identical to those of Rutland Mike, but without the water gushing out.

it starts perfectly, runs perfectly on choke, and still runs perfectly when the choke cuts out.

Then, after running under load for some time (anything from half an hour to an hour or more - it varies, but usually does it just as I am congratulating myself for solving the problem, yet again :) ) it cuts down the revs suddenly and wont rev up properly.

Then it will frustratingly cut back & forth between normal higher revs and low 'stiffled' revs without touching the throttle. Finally it only runs at lower revs.

It seems to be a problem occuring after it is hotter than warm (but it nevers gets 'hot') after having been run under load for some time. An air leak opening up when warm? under load problem = coil?

I have stripped and fully cleaned the carb countless times (found that 'hidden' jet), changed the inlet manifold-to-carb gaskets and studs & nuts. I have also re-adjusted the float level.

It has also been in to two seperate Yamaha dealers here in Lisbon, both of which ran it in the test tank (with my fuel tank) and both returned it saying there was no problem!

I initially thought it was electrical because of the suddeness of the switching between low & high revs. Also It seemed to flick back up to high revs after I 'wiggled' the CDI/Coil mounting plate when I first had the problem, but this has no effect anymore & maybe this was always just coincidental.

So I also checked all the electrical connections and earths.

Changed the plugs - they are both same spec by the way.

So, I am at a loss, and am using it anyway (9.9 on a 22 footer still enough even when not running properly) but I want to get it going as its not right & with the currents around here I want full power, expecialy with a new crew addition arriving in Jan, whom I want to get on the water asap :)

So I will try oil sensor, not sure where it is though,

Maybe its finally time to bite the bullet and try the CDI?
And the coil, perhaps?
Valve clearences?
Have also heard from Yam mechanic that maybe exhaust could be blocked (could be as motor is permantly in well).

Also a final Q- is the pilot needle jet (mixture screw) on these a valve in the air flow or the fuel? i.e. screwing it in richens or weakens mixture respectively?

Glad to hear you sorted your problem Greggron, is it running perfectly on the boat?

Did you get your problem sorted out RutlandMike

Any more ideas?

Please?!

PS how is the sailing up there at the moment? I miss sailing at home!
 
The vacuum choke system, I believe, has two sensors, one for the cylinder wall, the other for air temperature. When the engine starts running oddly, can you see the little choke button moving? If so then a temporary fix may be to remove the clear dome and use the slidy lever thing to hold the choke in position.
 
Hi It's an electric choke - 2 wires leading to a black cylinder, there is no clear plastic dome. I dont think there is an opportunity to manual overide
 
I am assured by the mechanic that it runs fine now having tested it in a tank. The acid test will come when I'm battling against an outgoing spring tide trying to make way up the small boat channel into Portsmouth harbour!
 
Finally got to try the motor in the boat on Saturday. A bit of clearing the throat after a winter in the garage but now works perfectly. Seems it was the CDI was at fault. Expensive but a good outcome.

Thanks for all your suggestions:)
 
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