Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 18-11-2014, 11:26   #31
Moderator
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Albany Ga.
Boat: Island Packet 38
Posts: 17,058
Re: Erratic RPM's - Yam15hp

You did either replace with new plugs or at least swap the plugs, right?
If so then obviously one cylinder is running richer than the other, has a bad ignition system (probably coil) or maybe the crank seal is leaking, or maybe the compression is down on one cylinder.
Another possibility is the cylinder with the plug that looks good is actually running lean due to a vacuum leak and the rich looking plug is the good cylinder (unlikely, but possible I guess)

I'd swap coils to eliminate at least that, not much else you can simply swap to see if the fault goes with the swapped part.
__________________

__________________
a64pilot is online now   Reply With Quote
Old 18-11-2014, 11:30   #32
Moderator
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Albany Ga.
Boat: Island Packet 38
Posts: 17,058
Re: Erratic RPM's - Yam15hp

This thing has independent coils? After thinking a little, it may have only one coil and use a "lost spark" ignition?
Wasted spark - Wikipedia, the free encyclopedia

I believe it's applicable to two strokes as well?
__________________

__________________
a64pilot is online now   Reply With Quote
Old 18-11-2014, 16:32   #33
Registered User
 
impi's Avatar

Join Date: Aug 2010
Location: From Cape Town now New Caledonia
Boat: Lagoon 440
Posts: 952
Images: 8
Send a message via Skype™ to impi
Re: Erratic RPM's - Yam15hp

You may well find that a piston ring got stuck ... that happens to 2 strokes when they do not have enough throttle ... they carbon up. With new plugs installed and running the engine at a 1:100 mixture at good throttle under load ... you may find it eases up ... just another thought for you before you start stripping too much
__________________
In our own style and our own time ...
http://cat-impi.blogspot.com
https://www.youtube.com/channel/UCIy...Uhlfkd34f8FrEg
impi is offline   Reply With Quote
Old 21-11-2014, 08:52   #34
Registered User
 
sparrowhawk1's Avatar

Join Date: Jan 2014
Location: Miami Beach Fl
Boat: Colombia Cc 11.8
Posts: 1,252
Re: Erratic RPM's - Yam15hp

If you replace the plugs and the bottom one fouls again. Take it out and ligh it against the side of the engine and see if you get a strong spark if you do most likely it's a stuck ring like mentioned earlier. if you don't then reinstall it and start the engine at night and look to see if there are any sparks jumping anywhere
__________________
sparrowhawk1 is offline   Reply With Quote
Old 21-11-2014, 09:35   #35
Registered User

Join Date: Jul 2011
Posts: 134
Re: Erratic RPM's - Yam15hp

Thanks 64pilot: Yes, I cleaned the bottom plug and swapped it to the upper cylinder which seems to be running fine. I did this a couple of days ago and have not checked the condition of the plugs since. Wanted to give it a few days. Engine, as of this morning, is still missing.

You mentioned swapping the coils. Each plug wire goes to a black cylindrical part mounted on top of the block. TWO Coils? Can I just pull out the plug wires and cross them over?

Thank you impi: I have been running it harder the last few days and using the RedLine fuel system cleaner that is reported to help break up carbon, but who knows. I'll give it a few more days.

sparrowhawk1: You're suggesting that I might have dirty wires, causing a short? That's certainly possible. And running at night sounds like a good way to find out. Thanks. I'll give it a try and get back to you!

Thank you all...
Bry
__________________
bcripps is offline   Reply With Quote
Old 21-11-2014, 09:51   #36
Moderator
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Albany Ga.
Boat: Island Packet 38
Posts: 17,058
Re: Erratic RPM's - Yam15hp

If the plug wires go to two different components, then they can be switched, not by just moving the high tension wires though, you need to move the whole component over.
The intent is to fault isolate the problem to a particular component.
Sometimes one coil is used, with two high tension leads, what happens then is the plugs are fired when one is at TDC, and the other is at BDC, that is the wasted spark ignition.
A picture of you coil or coils would help.
__________________
a64pilot is online now   Reply With Quote
Old 22-11-2014, 12:05   #37
Registered User

Join Date: Jul 2011
Posts: 134
Re: Erratic RPM's - Yam15hp

It's been four days since I swapped the plugs and I just had them out again to have a look. They look pretty similar but the plug in the lower cylinder has lost that tan color on the insulator and is now black. It is not bad but, I'm hazzarding a guess here, that both plugs are functioning and, as others have suggested, it might be a coil or a sticky piston ring. While I had the cover off, I removed the spark plug cap on the lower cylinder. No evedence of an electrical arc but the spring-like clip is not exactly a snug fit. It just slips over the end of the plug. There is no tension, it doesn't snap on. Just slips on and is held in place with the rubber cap. But no signs of arcing!

The photo here, is of the coils. The plug wires seem to be permantly attached, but haven't fiddled with them yet. It's raining. One plug wire is much shorter than the other. Not sure I can swap the coils and wires and reach the lower cylinder with the shorter wire; but I'll give it a try!

Thanks guys... really appreciate the help.
Regards,
Bry
Attached Thumbnails
Click image for larger version

Name:	DSCI0393.jpg
Views:	74
Size:	403.8 KB
ID:	92171  
__________________
bcripps is offline   Reply With Quote
Old 25-11-2014, 19:43   #38
Registered User
 
impi's Avatar

Join Date: Aug 2010
Location: From Cape Town now New Caledonia
Boat: Lagoon 440
Posts: 952
Images: 8
Send a message via Skype™ to impi
Re: Erratic RPM's - Yam15hp

Quote:
Originally Posted by bcripps View Post
It's been four days since I swapped the plugs and I just had them out again to have a look. They look pretty similar but the plug in the lower cylinder has lost that tan color on the insulator and is now black. It is not bad but, I'm hazzarding a guess here, that both plugs are functioning and, as others have suggested, it might be a coil or a sticky piston ring. While I had the cover off, I removed the spark plug cap on the lower cylinder. No evedence of an electrical arc but the spring-like clip is not exactly a snug fit. It just slips over the end of the plug. There is no tension, it doesn't snap on. Just slips on and is held in place with the rubber cap. But no signs of arcing!

The photo here, is of the coils. The plug wires seem to be permantly attached, but haven't fiddled with them yet. It's raining. One plug wire is much shorter than the other. Not sure I can swap the coils and wires and reach the lower cylinder with the shorter wire; but I'll give it a try!

Thanks guys... really appreciate the help.
Regards,
Bry
Bry ... I would really take a look at that cable ... the cap should fit snugly onto the end of the plug and you should get a good spark from it if you keep the plug grounded with the wire attached.

It may not be a bad buy to replace that wire complete with cap ... can always keep one spare!

Have you checked spark on the other plug compared to the lower plug?
__________________
In our own style and our own time ...
http://cat-impi.blogspot.com
https://www.youtube.com/channel/UCIy...Uhlfkd34f8FrEg
impi is offline   Reply With Quote
Old 26-11-2014, 11:39   #39
Registered User

Join Date: Jul 2011
Posts: 134
Re: Erratic RPM's - Yam15hp

Ok. Here's where I'm at with this thing. I swapped the coils and took the dingy out for a hard run: about 35 minutes with two people in the stern and plowing lots of water. Engine sounded strong. After, removed the plugs... both had cleaned up nicely. Both had tan-colored insulators and were dry. Next, idle engine for 30 minutes. After, plugs were still clean but looked a bit wet. Came into the dock under power... seemed to run well but the test will be tomorrow morning when the engine is cold. Let you know.

Thanx everyone,
Bry
__________________
bcripps is offline   Reply With Quote
Old 26-11-2014, 12:21   #40
Moderator
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Albany Ga.
Boat: Island Packet 38
Posts: 17,058
Re: Erratic RPM's - Yam15hp

Sounds like a loose coil wire as in spark plug boot end.
Used to be and I bet you still can replace just the boot and the spring like wire in it, for like $3 or $4 bucks.
Called I swear if I remember right a "sparky" spark plug boot. The kind you see on old motorcycles, chain saws etc.
__________________
a64pilot is online now   Reply With Quote
Old 26-11-2014, 12:22   #41
Moderator
 
a64pilot's Avatar

Cruisers Forum Supporter

Join Date: Oct 2013
Location: Albany Ga.
Boat: Island Packet 38
Posts: 17,058
Re: Erratic RPM's - Yam15hp

Look for something like this

https://www.brillman.com/store/produ...ition-terminal
__________________
a64pilot is online now   Reply With Quote
Old 27-11-2014, 13:02   #42
Registered User

Join Date: Jul 2011
Posts: 134
Re: Erratic RPM's - Yam15hp

Started it up this morning... still missing!
Further thoughts are appreciated...
Bry
__________________
bcripps is offline   Reply With Quote
Old 03-12-2014, 17:49   #43
Registered User
 
impi's Avatar

Join Date: Aug 2010
Location: From Cape Town now New Caledonia
Boat: Lagoon 440
Posts: 952
Images: 8
Send a message via Skype™ to impi
Re: Erratic RPM's - Yam15hp

Quote:
Originally Posted by bcripps View Post
Started it up this morning... still missing!
Further thoughts are appreciated...
Bry

Just to re-cap since this is a 'cold issue' now ... your fuel mix is 100:1 right?

50:1 is more required for newer engines prefer 100:1.

And your fuel is good quality for 2 stroke - 2 stokes do not like fuels with high ethonol content ... I think it should be less than 10% if memory serves me correctly. I remember in the Bahamas we had to use Fuel stabilising agents. The engine struggled and spluttered if idled too long. In any case, the 2 strokes do not want to idle too long as they foul up ... they like revs.
__________________

__________________
In our own style and our own time ...
http://cat-impi.blogspot.com
https://www.youtube.com/channel/UCIy...Uhlfkd34f8FrEg
impi is offline   Reply With Quote
Reply

Tags
rpm

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Erratic Indications From Depth Sounder Alecadi Marine Electronics 23 31-05-2011 17:42
Erratic Gauges svfinnishline Electrical: Batteries, Generators & Solar 19 28-04-2011 01:11
Erratic RPM Amwasp Engines and Propulsion Systems 8 03-12-2009 21:45
Navman 3100 wind indicator erratic at < 10knts jkalucki Marine Electronics 12 17-02-2009 22:50
Erratic Temperature Reading in Perkins 4.108 jlogan Engines and Propulsion Systems 19 22-09-2008 15:00



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 12:02.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.