Yamaha 8hp 2 stroke not running idle

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Yamaha 8hp 2 stroke not running idle

Post by juan » Sep 16th, '20, 14:28

Hi TSPers,

I'm trying to teach myself outboard maintenance.
I have recently replaced the impeller and gearbox oil myself. All was working ok until this last weekend when the engine didn't want to run in idle anymore.
I changed sparkplugs and check fuel intake and it all looks to be fine. I took this short video to illustrate the problem.
Any advice on what to try next?
I want to solve by myself and avoid giving up and taking it to a mechanic if possible.
thanks

Juan
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Re: Yamaha 8hp 2 stroke not running idle

Post by Davidjohn » Sep 16th, '20, 17:03

Hi Juan
The first thing that comes to mind is that your fuel tank may be too low for the engine vacuum to maintain adequate fuel supply to the carburettor, can you try it with the tank elevated, perhaps on a chair something similar. Of course the fuel tank vent is open as well? :) It all seems to start and initially run OK. I am assuming was running OK prior to the job?
Regards
David
Last edited by Davidjohn on Sep 16th, '20, 17:43, edited 1 time in total.
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Re: Yamaha 8hp 2 stroke not running idle

Post by Davidjohn » Sep 16th, '20, 17:08

Another separate thing noticed straight off is that the tell-tale water is not running as freely as it should - a common issue with those yamaha 2-strokes, the final water chamber is a dead-end and becomes blocked with salty crud. it's an easy enough job for anyone good at spanners, I'll dig up pictures if I can find them
David
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Re: Yamaha 8hp 2 stroke not running idle

Post by MoodyBlue » Sep 16th, '20, 17:18

First port of call for the idling is fuel supply. As previously suggested ensure the air vent on the tank is not clogged, and try raising the tank.

As to water telltale the attached pic is of an 8hp Yammy with powerhead removed. Look carefully at the picture along the right hand side and you can see the small outlet hole for the water. If you run a small drill 5 - 10 mm into the hole from below it usually solves the issue. Plan B of course is to lift off the power head as I did in the picture and clean the exit gallery of built up crustys. Removing the powerhead is easy, but you will need a new base gasket.
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Water Telltale.jpg
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Re: Yamaha 8hp 2 stroke not running idle

Post by Slammin » Sep 16th, '20, 17:27

Since you haven't changed anything and it was idling fine previously then:

Did you set the spark plug gap? If not do so. Then test again. If no change:

Check the carburetor float bowl and clean. If you're wanting to learn more then servicing the carby would be a great lesson. Lots of vids online. I suspect your low range jet may be blocked. It may have a drain screw which flushing (hand pumping the pressure bulb with drain open) may dislodge. If not then clean the carby. Video everything as you go, it's not that hard.

If that doesn't help then come back to the brains trust.

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Re: Yamaha 8hp 2 stroke not running idle

Post by Davidjohn » Sep 16th, '20, 17:40

My old 6hp Yamaha before a good clean out:
DSCN0371.JPG
The tell-tale hole is somewhere under all that crud - I took the time to clean out all the rest of the water jacket while I was at it.
DSCN0373.JPG
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Re: Yamaha 8hp 2 stroke not running idle

Post by granty » Sep 16th, '20, 19:02

Hi , Noticed your tell tale water stream, it must be partially blocked,this is a common problem,all i do with mine is use a cable tie and push it up as far as will go ,twist it a few times and that usually clears the blockage,works for me every time.

Cheers David.
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Re: Yamaha 8hp 2 stroke not running idle

Post by juan » Sep 17th, '20, 10:25

Thank you all for your replies.

1- tank level. I put it on a chair, but the problem started when the outboard was on the boat, tank is at the same level there, so I don't think that's the issue.

2- tank vent was indeed closed (from when I transported the tank back home) but it was also open on the boat when the problem started. Having said that, I opened it, tried again and outboard could run in idle. So I am not convinced the problem has been solved.

3- before attempting to dismantle and cleaning the carburetor (seems like a lot of small parts there), someone suggested playing with the choke knob. I see there are two knobs I can play with, can someone explain what are each one of them?
https://photos.app.goo.gl/j2B4TJeW4BcaSA83A

4- regarding the telltale. I've also noticed it's weak and tried cleaning with a wire. It improves for a little while and then back to being weak. I suppose the best thing to do is open the whole thing and giving it a good clean, I bet it's very dirty inside, like those photos sent by some of you.
Is this the kit I need to buy to seal it off after cleaning? https://www.outboardspares.com.au/osls6g1
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Re: Yamaha 8hp 2 stroke not running idle

Post by Davidjohn » Sep 17th, '20, 11:36

Hi Juan,
The kit you've shown is not for here. Here's a diagram from the Yamaha parts catalogue - I think all Yamaha 6 & 8hp 2-strokes are similar from around those years but don't hold me to that, please check your model and year etc.
This one covers parts for '03 models 6CMH (6M82), 6DMH (6M92) AND 8CMH (6N02) The diagram shows the gasket you need plus the others if you take off the other cover plates to clean. i.e. #s 19, 22 & 25. you may like to replace/check the thermostat while you have the cylinder head cover off for cleaning.
Yamaha crankcase diagram.pdf
(1.6 MiB) Downloaded 4 times
Cheers
David
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Re: Yamaha 8hp 2 stroke not running idle

Post by Davidjohn » Sep 17th, '20, 11:56

Hi Juan, A few more thoughts: You say your motor started playing up when it was on the boat prior to working on it so we assume it was going OK up to that point, something has changed to cause that. Simple things first, it fires up, so spark plugs seem sufficient to spark and fire. It does seem more of a fuel issue. Before starting to change existing carb. settings, check first for any obvious deterioration/hardness,splitting/leaky joints under pressure/vacuum in the fuel line including under the cover, leading to the carburettor. it only needs a small amount of air getting into the fuel line to create an air lock, it does sound like you're "running out of fuel" on the video. You could try to elevate the fuel tank higher than the motor so the motor is gravity fed, does it run differently now? If so, go back and check your fuel delivery system for leaks/blockages again. The fuel line connectors wear with constant movement and the rubber seals in them can fail to seal sufficiently. Is your fuel fresh and mixed to the correct ratio? (really fresh) it's hard troubleshooting with stale fuel.
Good luck and I'm sure it's nothing serious.
Cheers
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Re: Yamaha 8hp 2 stroke not running idle

Post by Davidjohn » Sep 17th, '20, 12:04

....and here's the parts catalogue I have, invaluable for finding what you're looking at before you pull it apart!
Yamaha catalogue.pdf
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Re: Yamaha 8hp 2 stroke not running idle

Post by Davidjohn » Sep 17th, '20, 12:13

Juan, I'm going out on a limb here. I've just watched the video again, there appears to be a great deal of fuel line hanging off the fuel line to motor connection, try supporting the line whilst pushing the connector in firmly when it's running - does that fix the issue? air could be getting sucked in at that point.
David
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Re: Yamaha 8hp 2 stroke not running idle

Post by juan » Sep 17th, '20, 12:19

Thanks David,
I'll have a look. Fuel is not fresh. I haven't really cleaned the tank well for a long time. Last saturday I drove to Batemans Bay and filled the tank with 13 litres, then added 130ml of oil (I do 100:1). All was going well, in fact I had to motor for 3 hours to Moruya as no wind, and the problem started on arrival trying to get to the jetty. Luckily light wind and no tide at the time.

Maybe I'll get rid of it all, clean the tank well and check all stuff there.

Regarding the other stuff (opening engine and cleaning), I guess the gaskets I will need are:
https://www.chsmith.com.au/Products/Sie ... 13-A1.html
https://www.chsmith.com.au/Products/Sie ... 14-A0.html

seems to me that this kit includes all?
https://www.chsmith.com.au/Products/Sie ... 00-00.html
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Re: Yamaha 8hp 2 stroke not running idle

Post by Davidjohn » Sep 17th, '20, 12:35

Looks like your on the right track with the gaskets.
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Re: Yamaha 8hp 2 stroke not running idle

Post by MoodyBlue » Sep 17th, '20, 14:03

If you do pull the power head then a "Top Tip" is to add a reasonable dose of grease inside the shaft tube to item 15. These are prone to seizing up due the the heat from the exhaust in the leg chamber.

Unless you are stripping the power head you only need a base gasket not the whole set.
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Re: Yamaha 8hp 2 stroke not running idle

Post by juan » Sep 17th, '20, 14:13

MoodyBlue wrote:
Sep 16th, '20, 17:18
As to water telltale the attached pic is of an 8hp Yammy with powerhead removed. Look carefully at the picture along the right hand side and you can see the small outlet hole for the water. If you run a small drill 5 - 10 mm into the hole from below it usually solves the issue. Plan B of course is to lift off the power head as I did in the picture and clean the exit gallery of built up crustys. Removing the powerhead is easy, but you will need a new base gasket.
So if I unscrew those 6 screws I am able to access the base and clean the bits I need? As easy as that? I'm slightly reluctant to mess around there.
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Re: Yamaha 8hp 2 stroke not running idle

Post by Furstin » Sep 17th, '20, 14:25

You can always try freeing up with a wire to dislodge crust.
I did our yammy 50 powerhead and the only bit with any salt crust was the outlet (always flushef) but it was rock hard! Had to drill nit out

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Re: Yamaha 8hp 2 stroke not running idle

Post by zebedee » Sep 17th, '20, 16:33

juan wrote:
Sep 17th, '20, 10:25
Thank you all for your replies.
...
2- tank vent was indeed closed (from when I transported the tank back home) but it was also open on the boat when the problem started. Having said that, I opened it, tried again and outboard could run in idle. So I am not convinced the problem has been solved.
If the tank vent is closed and the tank is full to the brim, with very little air space, then you don't need to draw very much fuel out before the vacuum drawn by expanding the small amount of remaining air to fill the space left behind becomes sufficient to cause problems.

If the tank vent is closed and the tank is nearly empty, you may never draw enough fuel out to cause enough vacuum to be a problem. Drawing the last one litre of fuel out of a rigid unvented 11 litre tank would only increase the space occupied by the air by 10% and reduce the pressure by 10%.
juan wrote:
Sep 17th, '20, 10:25
4- regarding the telltale. I've also noticed it's weak and tried cleaning with a wire. It improves for a little while and then back to being weak.
At least you know from this that the telltale is weak because it is blocked, not because the water pump is weak.
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Re: Yamaha 8hp 2 stroke not running idle

Post by juan » Sep 17th, '20, 17:10

thanks.

So the situaiton is now that the engine is running on idle ok. I will still replace the fuel and wash the tank, although not sure if waiting to use the fuel in the tank or getting rid of it now (don't know where I can discard it).

Bought the gasket and will open the base when it gets here.
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Re: Yamaha 8hp 2 stroke not running idle

Post by Davidjohn » Sep 17th, '20, 17:45

Our old 100:1 two stroke fuel usually gets relegated to the four-stroke mower and/or the 1988 nissan patrol - it seems to not affect either! Good news that it idles again :thumbup:
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Re: Yamaha 8hp 2 stroke not running idle

Post by juan » Sep 17th, '20, 17:56

good point, I could leave it for my lawn mower (2 stroke in my case).
Spring almost here and it will start to have a lot of use again.
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Re: Yamaha 8hp 2 stroke not running idle

Post by MoodyBlue » Sep 17th, '20, 18:37

juan wrote:
Sep 17th, '20, 14:13
MoodyBlue wrote:
Sep 16th, '20, 17:18
As to water telltale the attached pic is of an 8hp Yammy with powerhead removed. Look carefully at the picture along the right hand side and you can see the small outlet hole for the water. If you run a small drill 5 - 10 mm into the hole from below it usually solves the issue. Plan B of course is to lift off the power head as I did in the picture and clean the exit gallery of built up crustys. Removing the powerhead is easy, but you will need a new base gasket.
So if I unscrew those 6 screws I am able to access the base and clean the bits I need? As easy as that? I'm slightly reluctant to mess around there.
Pretty much, BUT there are a few ancilliary items to disconnect like the gear shift lever which from memory is at the front of the leg, Fuel hose, wiring loom, but the six nuts for the power head are the main ones.

Do not force the power head up. You need to watch the drive shaft stays in the gear box. The Yammy I repaired had the shaft frozen (Corroded) into the long seal holder under the power head, so ended up pulling the whole shaft out which was a world of pain as I then had to dismantle the gearcase to replace the drive shaft end clip (items 37 & 39 in the catalogue).
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Re: Yamaha 8hp 2 stroke not running idle

Post by juan » Sep 17th, '20, 19:41

Thanks.
I'm pretty confident the shaft will be fine because I got it from the other end 2 weeks ago when I changed the impeller. It was all good, and I put some more grease before reassembly.
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Re: Yamaha 8hp 2 stroke not running idle

Post by davem » Sep 18th, '20, 16:49

Furstin wrote:
Sep 17th, '20, 14:25
You can always try freeing up with a wire to dislodge crust.
I did our yammy 50 powerhead and the only bit with any salt crust was the outlet (always flushef) but it was rock hard! Had to drill nit out

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I had an old motor when I got my boat that had these symptoms - poor flow due to a combination of worn impeller and clogged with crust. I gave the motor a run in a tub with a good dose of vinegar in the water which really shook things up internally. I had to repeatedly clear the nozzle but loads of crap came out !

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