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Old 11-06-2019, 05:20   #1
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2GM20 blowback thru intake

with other life issues, I have neglected started my port 2GM20 (about 1,000 hours on it) for quite some time, was running fine when last run. seemed to get fuel at the injectors but when I removed the intake silencer, it blows back seriously thru the intake. Spraying WD40 does not get it to fire. I fresh charge on battery, turns over nicely. I pulled the valve cover thinking maybe a valve was stuck open but they all seem to be operating correctly. Water cooling hose is disconnected so not water in the intake issue. What am I missing? It is the blowback that has me concerned.
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Old 11-06-2019, 10:05   #2
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Re: 2GM20 blowback thru intake

Check the valve clearances with a feeler gauge.
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Old 11-06-2019, 11:25   #3
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Re: 2GM20 blowback thru intake

if valve lash is correct then might have a cracked valve. Though Odds are your valves just need adjustment.

also check also check that you discharge elbow is not coked-up. The discharge elbow can get restricted enough to cause back pressure. So check that before pulling the head.
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Old 11-06-2019, 12:09   #4
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Re: 2GM20 blowback thru intake

My Universal 5411 did that once. It was actually backfiring through the intake. I pulled the valve cover off and found the nuts holding the rocker shaft had worked loose. The rockers were still going up and down but they weren't actually opening the valves. I tightened the nuts (and ran lock nuts down over them to make sure they didn't loosen again) and the engine then ran fine.
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Old 12-06-2019, 05:21   #5
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Re: 2GM20 blowback thru intake

Valve may be stuck in open position if you haven't started for a long time. As previous posters said but look that valve stem is up when spring not compressed. Let us know what you find. Unlikely to be anything serious given the low hours. Good luck with it & other life issues keeping you away from boat
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