Universal Diesels require unusually long glow plug times 30-60 seconds. This is per the manual.
I found it hard to believe when I saw my boat's previous owner holding the button for >30 seconds. I thought there was trouble with the engine. Heck, after all my buddies with Yanmar engines sometimes do not even use glow plug. When they do, it's just a few seconds.
At any rate, my trustworthy 1982 Universal Diesel 5411 (2 cylinder, 11 hp), fires right up after 30 seconds of glow plug. Less than that--especially if it's cold outside--and I am rolling the dice.
There are other possible issues too. A long power run through undersized wire to the glow plugs can mean only 11 V or so at the glow plugs. For me, I see 11-ish volts at the plugs but it's not a big deal because my plugs are rated to run on something like 10.6 V (convenient). I think after a few seconds of warming up, they are supposed to draw 7 A of current.
My manual also says that sometime you need to keep the plugs energized while cranking the engine. I have never done this. However, if I don't crank the engine immediately after the preheat (say if I wait more than 5 seconds or so), then I notice it's a little harder to start.
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