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Honda 40 2007 Starts and runs smooth for 2 seconds then rough.

T

tropicsilk

New member
I just purchased a boat with a Honda 40. The boat sat for several years. It would overheat so I replaced the water pump and impeller. Overheating is solved. When I follow the pre-start procedure turning the key on and pushing the throttle to full and back, the engine starts and seems perfect for maybe 2 seconds then runs rough. Once warmed up it will take some thottle but only runs on the top cylinder. I accidentally started it with #2 and #3 spark plugs unplugged it didn't have that smooth moment upon start but ran the same without the plug wires attached. My thought is electrical but out of ideas. I swapped coils with another motor I have but still the same rough running. Running out of ideas. Please help.
Much Aloha from Lahaina Maui.
 
Hi,
I take it from the start procedure this is a carbureted model but I don't know for sure.

Since it sat for so long, I think the first thing I would want to know is what the compression readings are. They don't need to be "like new" but should be well clear of 100psi and....most importantly.... fairly close across all three cylinders.

If it is carbureted, I would suspect them for fuel starvation more than electrical. Although, a quick spark test would tell you a lot.

You can get a cheap spark tester from Harbor Freight OR use an old school incandescent electrical test probe connected to engine ground to see if you can get a spark to jump to the tip. Using the old grounded spark plug method doesn't usually work very well on these Hondas.

If you can verify spark and compression checks ok then I would fog the intake with propane gas to verify a fuel starvation issue with those two cylinders.

Just my opinion.

Good luck.

PS...don't leave the plug wires off the plugs. You might damage the coils.
 
You were correct it is a carbureted model. I did perform a comp test. All cylinders within 5lbs at 1@180 2@185 3@185psi. I was pleased with those readings. Just very perplexed as to why it runs so well for a moment time after time. I'll get a spark tester next. Thanks so for the tip on the coils too.
Aloha!
 
Aloha,

Yes, although I don't have service information for that engine, I would agree that those are excellent compression numbers.

I am curious as to what spark plugs are in it and what condition they're in. Should be NGK
DR7EA
Again, I have no SI but hopefully you do and can verify proper gap.
I still think checking spark is a good idea but just wanted to add that, in my experience, including the advice I received from the very knowledgeable guru hondadude who used to help folks here, the Honda engines really "like" NGK sparks as opposed to any other brand....like Denso for instance.
And I wouldn't be above trying a new set on an engine that has a history of overheat and is exhibiting loss of combustion symptoms.
Just sayin'.

Good luck and please keep us posted.
 
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