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Old 07-08-2019, 18:12   #1
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Yanmar SB12 Rebuild Follow Up

Dee and I just returned from two weeks on our Bristol 29.9 cruising the coast of Maine. I am happy to report that our little Yanmar SB12 is running perfectly. The modifications that I made to the fuel system are also working well. There were no air leaks and no fuel leaks. The engine starts on the first rotation every time. We even had a situation with a low battery (different story) and could just get the engine to barely turn over. One compression cycle and it was running!

I watched the oil use carefully. Though the oil consumption is way down from where it was before the rebuild, the engine is still using some oil. Last year, before the rebuild, it was using about 5 ounces of oil an hour. In the first thirty hours of operation after the rebuild, the engine used a bit less than a pint of oil. This equates to about 0.48 ounces per hour. In the next twenty hours of operation, the engine again used about a pint of oil, which equates to about 0.74 ounces per hour. In that twenty hour period, ten hours were spent trying to keep up with another, faster boat. We ran the engine at a steady 2,850 rpm. Normally we keep the engine at about 2,600 to 2,750 where it is much quieter. At 2,800 rpm and above every bolt and bulkhead on the boat vibrates and resonates. It's quite a cacophony of sound! But, this higher speed use may account for the increased oil use.

Does anyone understand the governor mechanism on this engine? If I put the throttle all the way forward, the engine goes to 3,000 rpm but settles back to 2,850. It then hunts a bit between 2,800 and 2,850. As noted, I don't like to run the motor at it highest possible speed, but I'd like to know what keeps bringing the speed down from 3,000 to 2,850.
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Old 07-08-2019, 19:56   #2
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Re: Yanmar SB12 Rebuild Follow Up

I don't have a copy of the SB12 w/shop manual so don't know if it's the same governor setup as our ysm8.
All I can tell you is the spring tension is important on the linkages. I think on ours the spring has stretched over time & I can only get 3150 rpm in neutral when it should go to 3400. It will go to 3400 if I push on the lever a little bit to help the spring.
Saw where someone said the spring was $200 from Yanmar! so I'm not rushing into buying the genuine article.

Your oil consumption is a lot higher than I would expect. Have you checked the level in the gearbox? In the ysm it's possible for the engine oil to leak through the rear main crankshaft bearing seal into the gearbox.
Otherwise it leaves new liner & valve guides but you aren't gunna be doing that so you'll have to live with it.
Does it smoke noticeably at 2850 revs?
Your oil consumption as you already noticed will be lower at lower revs. It took about 50 hrs for our oil consumption to drop as the rings bedded in the first time I put new rings in an old liner.
You could try 20w-50 diesel rated oil or straight 40 if you want in your next change.
I have seen 25w-70 for old motors here which would be even better in terms of reducing consumption. Will be a bit harder to crank the engine in the cold though.
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Old 07-08-2019, 22:00   #3
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Re: Yanmar SB12 Rebuild Follow Up

Quote:
Originally Posted by Homer Shannon View Post
........
Does anyone understand the governor mechanism on this engine? If I put the throttle all the way forward, the engine goes to 3,000 rpm but settles back to 2,850. It then hunts a bit between 2,800 and 2,850. As noted, I don't like to run the motor at it highest possible speed, but I'd like to know what keeps bringing the speed down from 3,000 to 2,850.
First up, thanks for the feedback of your repairs .

Here is a recent thread on the SB8 governor / fuel injector which is exactly the same as the SB12. Read all the posts but particularly post #10.

http://www.cruisersforum.com/forums/...or-221204.html

It is possible you have a spring issue but also possible it is just some wear that can be adjusted out or even a worn out regulator valve. Surprisingly they aren't very expensive from Yanmar.
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Old 07-08-2019, 22:19   #4
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Re: Yanmar SB12 Rebuild Follow Up

Whoops CORRECTION: Forgot you were using mono 30 oil, in that case 20w-50 or 25w -70 will be no harder to crank when cold.
Mono 40 will be harder though.

Wotnames post about the linkages is a beauty.

Cheers for posting the update too, it's interesting.
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Old 08-08-2019, 09:15   #5
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Re: Yanmar SB12 Rebuild Follow Up

Thanks for the comments on the governor/fuel regulator. I'm guessing that the most simple thing may be the right thing - the spring on the speed control lever. Several years ago I spent half a season unable to get the motor over 2,000 rpm because the spring was in the wrong position. I moved it to the other available location and the problem was fixed. At that time I did not have a tachometer, so motor speeds were a guess.

Perhaps a second spring over the existing spring would add enough pull to keep the engine at full speed, 3,000 rpm. I'll measure it next time I'm on the boat and see if I can find one to fit. It won't be difficult to install it and see if it makes any difference.

At any rate, I still don't want to be motoring around at high rpm - it's just too hard on the ears and nerves. With just a hint of favorable wind, I can hit 6.0 kn at 2,600 rpm.
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Old 08-08-2019, 15:23   #6
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Re: Yanmar SB12 Rebuild Follow Up

Yes I was going to add a 2nd spring too but it wont be a yanmar one.
Yanmar singles can rattle your fillings out alright but you just have to find the sweet spot & then they are not too obnoxious.
Ours is good at 2000rpm ( waay overpropped only gets to 2400 rpm before it starts over-fuelling, long story))
I doubt your oil consumption will get any better now as you sound like you have given it enough of a run in.
Be a lot better for it's longevity & oil consumption running at 2600 than 2850 anyway not to mention your states of mind lol.
Enjoy the season. I'm jealous it's winter here
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Old 08-08-2019, 17:07   #7
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Re: Yanmar SB12 Rebuild Follow Up

Glad to hear that your trusty old Yanmar is going well. We had our YSE8 rebuilt about 2 years ago by a nearby Yanmar workshop and it's been brilliant ever since - like yours it starts first go when you turn the key. I haven't got around to putting back the hand crank, but it's on my to-do list.

As far as the oil goes, the Yanmar shop recommends the Yanmar brand single grade oil - for both the crankcase and the gearbox. I am not on the boat at the moment, but I think from memory, it's about 30 grade. We live in quite a warm climate (Sydney, Australia) with a really mild winter (0 - 20 degrees C) so the oil is perfect for our climate. With the increased oil usage, please bear in mind that the engine has a horizontal piston, these are a wee bit prone to use more oil than a vertical piston - mainly because of the oil splashing about in the crankcase and some gets into the cylinder. This isn't a bad thing, but it does use more oil. I have a BMW K series motorbike (the one with the horizontal 4 cylinder engine) which does exactly the same thing. Even to the extent that if you leave it on the side stand for more than 10 minutes, it blows a bit of blue smoke on start up as a bit of oil runs down into the cylinders. Incidentally, the Yanmar starts easier than the Beemer does - even prior to the rebuild!


Apparently, the YS series engines are designed to run at pretty much maximum revs all day - they just love it. Like your engine, it's hard on the ears and there is a wee bit of vibration. Our boat, a Compass 28 does have the engine box soundproofed, but not the rear of the engine bay where it's a big space going all the way back to the stern of the boat - so the soundproofing on the sides of the engine box etc don't really do much. But then I would much rather sail - the sooner I get the sails up the better!


cheers

Andrew
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Old 08-08-2019, 20:29   #8
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Re: Yanmar SB12 Rebuild Follow Up

@ Hommer, be aware that the YSE, YSB and SB injector pumps are quite different to all other Yanmar injector pumps.

In essence, they close the regulator valve to achieve full flow of diesel into the engine. The spring (s) assist this closing action but if the regulator valve is worn too much, no matter how good the spring is, the regulator valve remains slightly open thus preventing full fuel flow (therefore decreased max rpm under load).

A quick and dirty way to tell if the regulator valve is working properly is to listen for the "squeak" of the injector when turning over the engine by hand (decompressed) with the throttle set to full power. If the squeak is positive and quite noticeable all is good but if it sounds weak and indistinct, you need first to adjust it by the method shown in your operation manual (also available in the link posted upthread). Of course if your are hearing impaired, this test isn't so good

However let's hope it is just the spring tension!
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Old 08-08-2019, 21:59   #9
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Re: Yanmar SB12 Rebuild Follow Up

Homer take Wotnames advice as he knows that pump whereas our YSM8 pump is quite different. My advice is limited to the spring problem only.
He rebuilt his injection pump so he knows the quirks for that pump.
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