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Author Topic:   84' 70hp Yamaha idles rough
Schroeder posted 08-07-2002 03:19 PM ET (US)   Profile for Schroeder   Send Email to Schroeder  
Figured I'd give everyone a whack at this.

I've got an 84' 70hp Yamaha with oil injection that runs a little rough at idle, more so at trolling speeds but then runs pretty well above 3000 rpm all the way to WOT.

I changed out the spark plugs with fresh regaped plugs and it seemed to smooth out a bit but trolling speeds still were rough.

The old plugs were fairly clean but had a slight oil build up on the threads.

My question is...
Does the fact that I'm running on 50:1 oil in the oil injection system have anything to do with my rough running.

I have a 90hp Yamaha manual from that same year and it recommends running 100:1 oil. The oil in the engine is from the previous owner.

Has anyone had a similar experience?

John Schroeder

Bigshot posted 08-07-2002 03:59 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Wait! Engine is oil injected. Is it still connected, do you add to the resevoir? If so you should be running straight gas, no mix. The mix in the manual is for when the injection is disconnected and during break-in. You will ruin an engine double oiling it. I assume I read your post right.
Bigshot posted 08-07-2002 03:59 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
My question is...
Does the fact that I'm running on 50:1 oil in the oil injection system have anything to do with my rough running.

Explain this part please.

Schroeder posted 08-07-2002 05:12 PM ET (US)     Profile for Schroeder  Send Email to Schroeder     
I'm only running the oil injection system. It is functioning, it is connected and it has oil in it. The gas is straight, no mix.

I think you've helped me answer my own question. The 100:1 referred to in the Yamaha manual is only for premix situations.

Thanks for helping me to help myself. The rough idling problem still exists however. I guess I'll be looking for another solution to the rough idling problem.

John Schroeder

skred posted 08-08-2002 09:17 AM ET (US)     Profile for skred  Send Email to skred     
I also have a 1985 70 Yamaha and it idles roughly at times(sounds like it's missing now and then), but as soon as I throttle up, everything smooths out. I just picked up a set of plugs thinking that might be the problem - I won't know till this weekend, however.
Salmon Tub posted 08-08-2002 01:00 PM ET (US)     Profile for Salmon Tub  Send Email to Salmon Tub     
Welcome to my nightmare. Actually it is not that bad. I have a 90 2-stroke Nissan and I have learned that rough is part of the territory.
Offcourse it all depends on what you call rough. Is the idle steady but the engine shakes and vibrates (my idea of rough), or does the idle lope or sound like it occasionally misses (my idea of something wrong)?
If it is vibration and shaking, you will not get rid of that. If it is missing, then you will need to do some work. There is one thing that carbed 2-strokes do, and that is they load up after long periods of idleing. After trolling for a few hours, you may notice they run real rough when getting to speed and then smooth out.
On my auto-mix system, whick is probably carbon copy of all the Japanese auto-mix systems, the oil is injected into the gas at variable rates. At idle the pump lets proportianaly less oil in to give a mix of approx. 120:1. At WOT, the rate is about 50:1.
Make sure that the linkage to the pump is set to the proper marks and you should be fine as far as the oil ratio goes.
where2 posted 08-08-2002 01:05 PM ET (US)     Profile for where2  Send Email to where2     
Poor idling characteristics can be related to dirt plugging the idle jet of the carb. The reason it gets better when you throttle up is because you stop relying on the idle jet and start pulling fuel from the high speed jet which is larger and less likely to plug with dirt.

Disassemble the carbs, and blow all the passageways out with compressed air. (WD-40 before we had a compressor), then reassemble. Alternatively, call your mechanic and ask how much to "rebuild your carbs".

Dirt in the carbs doesn't have to come from the fuel tank, it can form as a result of the fuel drying out in the bottom of the carb, creating varnish. This is a GREAT reason to use the boat more frequently, once you get the problem fixed. "Honey, we need to use the boat so the carbs stay clean". Just don't let anyone figure out that you could keep the varnish from forming by running the engine on a garden hose and earmuffs every few weeks.

Schroeder posted 08-08-2002 02:17 PM ET (US)     Profile for Schroeder  Send Email to Schroeder     
Thanks Guys,
Both suggestions dually noted.

Concerning whether its missing the answer is sometimes. Besides the roughness and vibration it seems to run ok though. I'd prefer to make it through this season before I pull it out of the water for work.

I actually want to repower the darn thing but I will probably be selling it this coming spring. If I repower now I'll get killed on the selling price.

John Schroeder

Schroeder posted 08-08-2002 02:21 PM ET (US)     Profile for Schroeder  Send Email to Schroeder     
Oh one more thing...

Is there anything I can add to the gas to help clean the carbs out?

John Schroeder

Bigshot posted 08-08-2002 02:26 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Techron or Berryman's B-12. Follow directions and do not spill on paint...nasty stuff. run a couple tanks through and see. Works for many.

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