Misfire at low rpm - GT40

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   Hi all,

Got the boat back in the water last week. All good except for a misfire at low rpms.  Here's the story:

- Boat winterised last year (drained then antifreezed) and was running 100% at the time
- 3/4 tank of fuel treated and engine ran for 15 mins or so
- Cylinders fogged through spark plug holes

I changed the leads two years ago along with the cap/rotor, plugs changed last year but  not this year.  I'm hoping that its just a fouled or cracked plug.  I had a quick look at the ceramic on each plug and checked leads were on properly but couldn't see any cracks.  However, I was in a hurry and didn't get much time to do any real troubleshooting or remove them for a proper look.

If not a plug or lead what else should I look at. The misfire seems to disappear at high rpm after a bit of vibration coming out of the hole.  Does this sound like a vacuum leak, coil or anything else?

About 150hrs on remanufactured long block and all was well before putting it away for the winter.

Needless to say, being in Ireland I'm an ocean away form PCM parts but i'm heading down on Thursday for a closer look.

I'd appreciate y'alls thoughts before I hit the road.

D.

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2001SAN Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: June-26-2012 Location: Ireland Status: Offline Points: 246	Post Options Post Reply Quote 2001SAN Report Post Thanks(0) Quote Reply Topic: Misfire at low rpm - GT40 Posted: April-11-2017 at 4:56pm
	Hi all, Got the boat back in the water last week. All good except for a misfire at low rpms. Here's the story: - Boat winterised last year (drained then antifreezed) and was running 100% at the time - 3/4 tank of fuel treated and engine ran for 15 mins or so - Cylinders fogged through spark plug holes I changed the leads two years ago along with the cap/rotor, plugs changed last year but not this year. I'm hoping that its just a fouled or cracked plug. I had a quick look at the ceramic on each plug and checked leads were on properly but couldn't see any cracks. However, I was in a hurry and didn't get much time to do any real troubleshooting or remove them for a proper look. If not a plug or lead what else should I look at. The misfire seems to disappear at high rpm after a bit of vibration coming out of the hole. Does this sound like a vacuum leak, coil or anything else? About 150hrs on remanufactured long block and all was well before putting it away for the winter. Needless to say, being in Ireland I'm an ocean away form PCM parts but i'm heading down on Thursday for a closer look. I'd appreciate y'alls thoughts before I hit the road. D.
	A little knowledge is a dangerous thing... 2001 Super Air Nautique 1989 Fairline Corniche 31 www.bannrivercruises.co.uk

tryathlete Members Profile Send Private Message Find Members Posts Add to Buddy List Platinum Member Joined: April-19-2013 Location: Lake Villa, IL Status: Offline Points: 1797	Post Options Post Reply Quote tryathlete Report Post Thanks(0) Quote Reply Posted: April-11-2017 at 5:09pm
	I'd pull the plugs and see if they're oil fouled from fogging. I'd also check each plug for solid connection at the plug end. Lots of folks suffer the first start of the season with aged fuel and moisture issues and so forth

Gary S Members Profile Send Private Message Find Members Posts Add to Buddy List Grand Poobah Joined: November-30-2006 Location: Illinois Status: Offline Points: 14096	Post Options Post Reply Quote Gary S Report Post Thanks(0) Quote Reply Posted: April-11-2017 at 6:10pm
	I had this problem last year. I took a IR thermometer and found a plug that was cooler. Changed it and was good to go,must have damaged it when I removed it to fog.
	69 Mustang HM SS 95 Nautique Super Sport

2001SAN Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: June-26-2012 Location: Ireland Status: Offline Points: 246	Post Options Post Reply Quote 2001SAN Report Post Thanks(0) Quote Reply Posted: April-18-2017 at 7:43am
	Bad plug - easy fix and back in business! D.
	A little knowledge is a dangerous thing... 2001 Super Air Nautique 1989 Fairline Corniche 31 www.bannrivercruises.co.uk

Bri892001 Members Profile Send Private Message Find Members Posts Add to Buddy List Grand Poobah Joined: September-27-2008 Location: Boston MA Status: Offline Points: 4947	Post Options Post Reply Quote Bri892001 Report Post Thanks(0) Quote Reply Posted: June-21-2017 at 6:54pm
	Just got around to updating the GT-40 Spreadsheet with this fix: http://www.correctcraftfan.com/forum/forum_posts.asp?TID=25584&PID=531736

Misfire at low rpm - GT40

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