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Old 11-11-2014, 12:46   #16
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Re: Erratic RPM's - Yam15hp

A lot of things can be going on to affect RPM at 1/4 throttle. I am thinking you have a slight blockage in the main circuit . When the throttle is cracked 1/4 open there is still enough vacuum to draw from the idle circuit.......hence the typical little stumble outboards tend to do when they come off idle. Running a 2 stroke at 1/4 throttle isn't doing it much good either. Keep using the fuel cleaner additive and take it out on some hard runs. I am betting the RPM Hunt will clear up. If not........have the carb thoroughly cleaned.

What is happening at 1/2 and full throttle?
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Old 11-11-2014, 12:47   #17
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Re: Erratic RPM's - Yam15hp

Whoops, A64pilot beat me to it.

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Old 13-11-2014, 13:04   #18
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Re: Erratic RPM's - Yam15hp

Thank you... let me do a bit more investigation and I'll get back to you. I'm guessing that if I get that brass plug out, it can stay out!
Bry
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Old 13-11-2014, 14:01   #19
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Re: Erratic RPM's - Yam15hp

Yea, I left mine out.

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Old 13-11-2014, 14:14   #20
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Erratic RPM's - Yam15hp

The way I get those plugs out is to drill a very small hole in the middle, then tap in a sheet metal screw and pull the plug out.
Be sure though that is what that plug is for and for example it's not the cover for the butterfly shaft or sometinng
Yes, leave it out, it's only function is to keep you from adjusting the idle mixture screw
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Old 14-11-2014, 10:32   #21
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Re: Erratic RPM's - Yam15hp

First, thanks again everyone. Appreciate all the input.

I think we are onto something with the plugs. Here's the one from the top cylinder:
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Old 14-11-2014, 10:38   #22
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Re: Erratic RPM's - Yam15hp

Now, the plug from the lower cylinder. Quite the difference. Could I have a bad plug? It was replace a month ago but has been on the boat for years. Maybe I should swap the plugs and see if the problem moves with the plug?
Thoughts anyone? Bry...
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Old 14-11-2014, 10:45   #23
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Re: Erratic RPM's - Yam15hp

That last plug looks pretty bad.
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Old 15-11-2014, 04:13   #24
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Re: Erratic RPM's - Yam15hp

bcripps... you said:

I hardly ever run the engine above quarter-throttle; as indicated by the little bars on the grip. Never bring the dingy up on plane.

I believe that is a large part of your problem. Outboards need rpm's put to them regardless if they are 2 or 4 strokes. Those plugs show evidence of "wet stacking".....for lack of a better term. Running any outboard continiously at 1/4 throttle is going to do the same thing. You are not getting the engine up to temp at 1/4 throttle. Clean or replace the plugs, and go run the SH-T out of it for a few hours. I'll bet you will be pleasantly surprised how well the engine runs after.
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Old 15-11-2014, 04:29   #25
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Re: Erratic RPM's - Yam15hp

We had our 30HP 2 stroke in storage for a while with the boat on the hard. When we got back to the boat I used the old fuel and had some similar issues which obviously I related to the fuel, but ontop of all that I could not get decent revs even with the throttle full open. I stripped the carb and removed the float with needle and seat to clean out the jet a number of times not realising that under a black rubber insert was another jet. Eventually after scratching my head several times and another strip ... pried at the rubber which came loose ... screwed out that jet and cleaned it. Things got a whole lot better from there. Then I looked at the cam where the throttle cables pull to accelerate the engine and realised the little roller was not reaching anywhere close to the cam indent. So I adjusted until the roller dropped nicely into the indent at full revs. The engine got quieter and performs like a racehorse. Of course I had discarded of all old fuel replenishing it with new. For what this is worth ... it may help! Oh ... just adding to the comment here ... yes .. 2 strokes want to be throttled hard ... do not pamper them ... race your motor every now and again ... you will see the difference!

Just for fun ... took this clip a while ago ... motor rus sweet now after doing what we did above ...
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Old 15-11-2014, 12:22   #26
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Re: Erratic RPM's - Yam15hp

Thanx everyone:

With regards to running the engine too slow: Wouldn't that affect both cylinders? The upper plug looks perfectly fine to me. Not fouled and the insulator is a tan color. It is only the lower plug that is... well, yuk! Also this is a recent problem... I've putted around for years. I'm not trying to be obstinate here, but gas is expensive, and worse, I have to jerry-jug it... a one mile hike! And I'm Old, damn it! The less gas I use the more I like it. If I could find a 4-hp outboard for running back and forth, I would buy it. But still need my 15!

With regards to Old Snipe's post, I tried running at speed but it was still missing at higher revs. I know this did nothing to clean up my plug, too little... too late! But I hope it answers your question; and thank you.

Anyway, I'm going back to the boat in a few minutes and I'm going to rev the living daylights outta the bitch. The engine will be so surprised, it will probably blow up. I'll let you know!

Best everyone...
Bry
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Old 15-11-2014, 17:08   #27
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Re: Erratic RPM's - Yam15hp

Quote:
Originally Posted by bcripps View Post
Thanx everyone:

With regards to running the engine too slow: Wouldn't that affect both cylinders? The upper plug looks perfectly fine to me. Not fouled and the insulator is a tan color. It is only the lower plug that is... well, yuk! Also this is a recent problem... I've putted around for years. I'm not trying to be obstinate here, but gas is expensive, and worse, I have to jerry-jug it... a one mile hike! And I'm Old, damn it! The less gas I use the more I like it. If I could find a 4-hp outboard for running back and forth, I would buy it. But still need my 15!

With regards to Old Snipe's post, I tried running at speed but it was still missing at higher revs. I know this did nothing to clean up my plug, too little... too late! But I hope it answers your question; and thank you.

Anyway, I'm going back to the boat in a few minutes and I'm going to rev the living daylights outta the bitch. The engine will be so surprised, it will probably blow up. I'll let you know!

Best everyone...
Bry
Just a quick question since you mention the plug going tan in colour ... are the plugs the same spec? One gets a 'Hot plug' and a regular. 2 Strokes want the 'Hot type plug' ... I am trying to recall the number on them ... I think B8HS 10 - the issue we had at one time was the 'heat spec' was out. Yamaha said we needed to pay attention to this ... replaced the plugs and that took care of it ... ours were also going 'tan' on those plugs not designed to take the heat.
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Old 15-11-2014, 17:11   #28
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Re: Erratic RPM's - Yam15hp

Check this out .. quite interesting ...

NGK B8HS Spark Plug Standard
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Old 16-11-2014, 11:39   #29
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Re: Erratic RPM's - Yam15hp

Well I can only say that I'm using the NGK plugs that Yamaha recommends for the engine. My only concern is that these plugs were bought many years ago when I was in Florida,(I'm talking 1992, here). But they have been kept dry and look brand new. The fact that one plug looks perfect, after a couple of months, and the other looks terrible, leads me to believe that either that lower plugs is defective or that it is not firing properly due to a fault in the electrical system.

First of all, I propose swapping the plugs to see, as mentioned, does the problem follow the plug? If the lower cylinder still fouls, then which component in the electrical system would most likely be suspect. Is it something I can change out myself?

Thanx everyone, for you help...
Bry
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Old 18-11-2014, 08:37   #30
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Re: Erratic RPM's - Yam15hp

Ok. Been running at speed the last couple of days. The engine certainly sounds better at the higher rpm but is still missing first thing in the morning when it's cold. Maybe a few more days will help!

Any further thoughts on why one plug looks good and the other doesn't?

Many thanks,
Bry
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