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Old 25-11-2020, 12:49   #1
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Yanmar SB12 won't fire - injectors work?

Looking for suggestions. My 1 cylinder Yanmar will not fire. So far I've tried fresh diesel and bled all lines. I also removed the single injector and reconnected it upside down and the spray pattern looks good. What else can I look at? Could it be compression related?

Video of injectors spraying:
https://youtu.be/KiCapgB5jZY
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Old 25-11-2020, 13:30   #2
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Re: Yanmar SB12 won't fire - injectors work?

After reading a few other related threads, I'm going to assume it's compression. It's located in fresh water and has alwasy started the first time every time - except the last time. What do I look at for compression? I read that I can try to spray started fluid in the air intake? What else?
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Old 25-11-2020, 14:06   #3
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Re: Yanmar SB12 won't fire - injectors work?

have you checked the tappet settings? also blocked exhaust elbow is very common.
If you on a marina you can try hair dryer in air intake. Do you advance the throttle to at least 1/2 way?
You can spray a bit of WD40 or similar in the air intake too. Before I recoed our single yanmar I used to have to have it on full throttle & spray crc into air intake or squirt a little engine oil in air intake. Dont use the little engine oil in air intake trick though as you can bend the pushrod if you use too much.
If the problem happened suddenly its unlikely to be compression as usually it gets harder & harder to start over time.
Whats the air temp there?
Forgot to add I think I've got a service manual for that engine PM me if you want a copy ( free )
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Old 25-11-2020, 15:34   #4
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Re: Yanmar SB12 won't fire - injectors work?

Thanks for the reply Compass790. A few questions:

Tappet settings?

WD40 or is starter fluid better?

"spray crc into air intake" What is "crc"?

The temperature here is 60 degrees. I can try the hair dryer trick too. Like I said it has always started within 3 seconds of the key turn. I hadn't used it in about 8 weeks and wouldn't fire this last time.
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Old 25-11-2020, 15:49   #5
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Re: Yanmar SB12 won't fire - injectors work?

Quote:
Originally Posted by rockin411 View Post
Thanks for the reply Compass790. A few questions:

Tappet settings?

WD40 or is starter fluid better?

"spray crc into air intake" What is "crc"?

The temperature here is 60 degrees. I can try the hair dryer trick too. Like I said it has always started within 3 seconds of the key turn. I hadn't used it in about 8 weeks and wouldn't fire this last time.
Tappet settings = valve lash, should be 8 thou.
Starter fluid is sort of better, just use a short squirt initially and a squirt or two after the engine starts to fire. If you don't have starter fluid, try the WD40.

CRC is a range of spray can products. CRC 5-56 is similar to WD40.

Have you tried starting it using the decompression lever?
Is the battery spinning the engine over as rapidly as it did in the past?
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Old 25-11-2020, 15:55   #6
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Re: Yanmar SB12 won't fire - injectors work?

Advance the throttle to full power.

Decompress the engine, spin it over with the starter motor for about 5 seconds before moving the decompression lever back to normal while keeping the starter motor engaged.

Report back
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Old 25-11-2020, 18:11   #7
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Re: Yanmar SB12 won't fire - injectors work?

Challenge accepted - I will report back on Friday with my results

What does holding the decompression lever do to fix the problem?
If is starts with starter fluid, what could be the culprit?

I'm just trying to understand how it went from working to not. Stuck rings?
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Old 25-11-2020, 18:19   #8
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Re: Yanmar SB12 won't fire - injectors work?

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Originally Posted by rockin411 View Post
Challenge accepted - I will report back on Friday with my results

What does holding the decompression lever do to fix the problem?
.........
It won't 'fix' the problem but it will help diagnose the problem.

Spinning it up decompressed allows for a faster rotational speed at the starting point (ie when you reset the lever to normal). This effectively increases the 'starting compression'. The energy contained in the momentum of the spinning flywheel helps the starter motor (ie battery) if is weak so it can also point to a weak battery or battery wiring.

If it starts as described, then try starting in the same manner but with the throttle less than half power. If it fails, it may point to a worn regulator needle in the injection pump but let's not get ahead of ourselves ATM.
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Old 25-11-2020, 18:27   #9
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Re: Yanmar SB12 won't fire - injectors work?

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Originally Posted by rockin411 View Post
......
If is starts with starter fluid, what could be the culprit?

.......
Low compression.
Lack of fuel.

The SB12 has a old design injection pump and as the regulator needle wears, it reduces the fuel at all power settings below full throttle.

Your video suggest plenty of fuel but I assume the throttle was fully open when you shot it. If so, have you tried starting with WOT (wide open throttle)?
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Old 25-11-2020, 19:29   #10
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Re: Yanmar SB12 won't fire - injectors work?

Wotties given you good advice. Another thought struck me, check that air intake is not obstructed. I had a case that a rag got sucked into air intake of a Bukh 20 which prevented it from getting enough air to start.
When was the exhaust elbow last checked/cleaned?
60o isnt cold enough to make it hard to start normally if it has good compression & no air in system.
Leaving for 2 months can sometimes cause a longer start as injection pump leaks back slowly but your injector appears to be spraying ok.
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Old 25-11-2020, 21:39   #11
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Re: Yanmar SB12 won't fire - injectors work?

Sorry rockin 411 I take that back saying that your injector is ok ITS NOT OK.
Didnt look closely enough before on phone. On laptop it shows up better.


UNLess the wind is blowing the spray



Spray is crooked & too wide, needs fixing. Not sure if thats your no start problem as could be other things but probably needs a new nozzle or at the least a cleaning. There are you tube vids on that if you are mechanically minded but if not take it to a diesel injection shop.
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Old 25-11-2020, 22:30   #12
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Re: Yanmar SB12 won't fire - injectors work?

Be extremely careful spraying any combustible material (WD40, starter fluid etc.) into the air intake. It can ignite before the piston gets to the top of it's stroke with devasting results.
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Old 25-11-2020, 22:54   #13
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Re: Yanmar SB12 won't fire - injectors work?

Quote:
Originally Posted by rockin411 View Post
Looking for suggestions. My 1 cylinder Yanmar will not fire. So far I've tried fresh diesel and bled all lines. I also removed the single injector and reconnected it upside down and the spray pattern looks good. What else can I look at? Could it be compression related?

Video of injectors spraying:
https://youtu.be/KiCapgB5jZY
Yes, but WHAT is it spraying? It could be lemon juice.
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Old 26-11-2020, 16:47   #14
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Re: Yanmar SB12 won't fire - injectors work?

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Originally Posted by Compass790 View Post
have you checked the tappet settings? also blocked exhaust elbow is very common.
If you on a marina you can try hair dryer in air intake. Do you advance the throttle to at least 1/2 way?
You can spray a bit of WD40 or similar in the air intake too. Before I recoed our single yanmar I used to have to have it on full throttle & spray crc into air intake or squirt a little engine oil in air intake. Dont use the little engine oil in air intake trick though as you can bend the pushrod if you use too much.
If the problem happened suddenly its unlikely to be compression as usually it gets harder & harder to start over time.
Whats the air temp there?
Forgot to add I think I've got a service manual for that engine PM me if you want a copy ( free )
Hi All, CaptVR here
Not totally correct, If it happened suddenly, it has a good chance of being a compression issue. These small engines, 1 -2 cylinders, a lot of the times a chunk of carbon breaks loose from head or piston and gets stuck under the exhaust valve. I've seen this a number of times over the years. Four and six cylinder engines will normally run fine and within a short period of time, at a good running speed, they will dislodge the obstruction. The small engine will just stop or be very hard to start, single cylinders won't. For what ever reason, the maintenance of decarbonizing is not thought of for these small engines, there the ones that really need it. Most of these small diesels only have 16:1 compression ratio new, it takes a minimum of 14/15:1 for a diesel to run, so it does not take much to kill ignition temps.
If it was running fine, and starting good without a long loping start. Your fuel is being delivered on time, can't be anything but compression. You only need compression and fuel to make a diesel run. Pop the head, clean, clean valve seats and check valve for proper rise and fall. Put it back together and off you go. Iron main back in service. Hope this gives you some insight.....
Capt. Vince Rakstis, Ret. MS St.Petersburg, Fl.
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Old 26-11-2020, 17:05   #15
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Re: Yanmar SB12 won't fire - injectors work?

Quote:
Originally Posted by CF32907 View Post
Hi All, CaptVR here
Not totally correct, If it happened suddenly, it has a good chance of being a compression issue. These small engines, 1 -2 cylinders, a lot of the times a chunk of carbon breaks loose from head or piston and gets stuck under the exhaust valve. I've seen this a number of times over the years. Four and six cylinder engines will normally run fine and within a short period of time, at a good running speed, they will dislodge the obstruction. The small engine will just stop or be very hard to start, single cylinders won't. For what ever reason, the maintenance of decarbonizing is not thought of for these small engines, there the ones that really need it. Most of these small diesels only have 16:1 compression ratio new, it takes a minimum of 14/15:1 for a diesel to run, so it does not take much to kill ignition temps.
If it was running fine, and starting good without a long loping start. Your fuel is being delivered on time, can't be anything but compression. You only need compression and fuel to make a diesel run. Pop the head, clean, clean valve seats and check valve for proper rise and fall. Put it back together and off you go. Iron main back in service. Hope this gives you some insight.....
Capt. Vince Rakstis, Ret. MS St.Petersburg, Fl.

Not totally correct, lol,
Not looking for an argument but his SB12 has a compression ratio of 20.9/1. & the SB8 is about 23/1
Decarbonisation is part of the scheduled maintenance in the service manual.
Very true that compression is vital.
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