Cold Cylinder and poor MPG: Coil?

Maintenance, repair, troubleshooting, etc.
Post Reply
User avatar
Giuan
Lookin' Around
Lookin' Around
Posts: 29
Joined: Tue Jun 07, 2016 2:00 pm
Region or State: France
Motorcycles Owned Currently: 1993 Yamaha XJ600

Sun Oct 29, 2017 7:58 am

Hello everyone!
This is basically the coninue of http://ww.xjrider.com/viewtopic.php?f=127&t=8872
After having changed the pilot jet I solved the backfiring issue and the bike was running great and I was doing 38mpg (not great but acceptable).
It only lasted a month or so, then the bike started running rough at low RPM: had to use a lot of clutch to move off and open up the gas with decision. Other symtomps were cold 3rd cylinder, smell of gasoline and a whopping 28mpg :oops:
I thought that the other pilot jets were giving up, so I changed the other 3 and the main jets too.
At this point, I practically rebuilt the carbs (all jets, all gaskets/orings, all needles...) so I am pretty sure it is not a matter of carburation.
I also asked a indie mechanic to check on the valve clearance, but he told me that they were all on specs.
I checked the spark plug and I found it very black and sooty, so I replaced it with a used one I had and the bike ran great. So I started suspecting on the ignition.
I changed the spark plug cap (NGK LD05F, which has 5KOhm of resistence instead of 10 as the stock one, so it delivers more current to the spark plug) and put a brand new spark plug. The cylinder is still cold, but the bike runs fine and the fuel consumption gets closer to the starting 38mpg.
The problem is that this lasts just a week or so, then the spark plug gets fouled again and I go back to square one.
Here's a pic of the spark plug after 60 miles
Bougie.jpg
I am now suspecting on the right coil, which is the one for the spark plugs on cylinder 2 and 3 (the second cylinder's temperature is fine, but its spark plug is dark too).
I measured the R and here's the result:
Primary=3.3 Ohm (the multimeter has an offset of 0.7 when shorting the probes) and it should be [1.92:2.88]Ohm
Secondary= 11.8 kOhm and it should be [9.53:14.28]kOhm
To me it doesn't look tragic, so I don't know if I should replace it or look for another root cause...
Does it seem that I have a fried coil to you? Is it true that these coils don't have replaceable HT leads?
You do not have the required permissions to view the files attached to this post.

User avatar
Ollyrag
Lookin' Around
Lookin' Around
Posts: 42
Joined: Fri Oct 21, 2016 1:48 am
Region or State: Kent, UK
Motorcycles Owned Currently: xj600N 1998
Location: Great Britain
Great Britain

Wed Nov 01, 2017 8:16 am

Swap the coil lead over, see if it does the same on anther cylinder.
It's not how old you are, It's how you are old.

User avatar
Giuan
Lookin' Around
Lookin' Around
Posts: 29
Joined: Tue Jun 07, 2016 2:00 pm
Region or State: France
Motorcycles Owned Currently: 1993 Yamaha XJ600

Sun Nov 05, 2017 12:05 pm

hi Ollyrag! Thanks for the suggestion ;) should I just swap the two terminals to the primary of one coil with those to the other? Won't that mess up the firing sequence of the cylinders?

User avatar
Ollyrag
Lookin' Around
Lookin' Around
Posts: 42
Joined: Fri Oct 21, 2016 1:48 am
Region or State: Kent, UK
Motorcycles Owned Currently: xj600N 1998
Location: Great Britain
Great Britain

Mon Nov 06, 2017 12:31 pm

wasted spark.GIF
Here's a diagram of a wasted spark system (Coil fires plug on compression and exhaust stroke)
A coil = 1 and 3 B coil = 2 and 4
So you can just change the leads to the plugs over.

You hav'nt got a flooding carb on that cylinder have you????
You do not have the required permissions to view the files attached to this post.
It's not how old you are, It's how you are old.

User avatar
Giuan
Lookin' Around
Lookin' Around
Posts: 29
Joined: Tue Jun 07, 2016 2:00 pm
Region or State: France
Motorcycles Owned Currently: 1993 Yamaha XJ600

Tue Nov 07, 2017 4:32 pm

Carbs are fine, all needles are brand new and I also tried to swap the whole jet holder + floats + needle set (3rd and 2nd) to see if the issue moved: it didn't!
Thanks for the picture: I am familiar with the wasted spark concept: I also have a twin cylinder and in that case I have no doubt; with a 4 cylinders it's a bit more difficult:they fire in a sequence (usually 1-2-4-3) so if I swap the plug cap 3 with the 4 the sequence will be 1-2-3-4 and it shouldn't tick over...isn't it?
More than having a weak coil I think that the lead is fried, but as I read in this forum they are not serviceable...The lazy cylinder is the 3rd: what plug cap should I swap it with? Thank you for your interest!

User avatar
Giuan
Lookin' Around
Lookin' Around
Posts: 29
Joined: Tue Jun 07, 2016 2:00 pm
Region or State: France
Motorcycles Owned Currently: 1993 Yamaha XJ600

Wed Nov 08, 2017 4:16 pm

ok I swapped lead&cap #3 and #2: the third cylinder still idles cold! I'm sure that is not a matter of coil or lead...clueless again!

User avatar
Casper
Almost as Awesome as Murdock
Almost as Awesome as Murdock
Posts: 2355
Joined: Sun Feb 26, 2012 2:16 am
Region or State: Motor City (MI)
Motorcycles Owned Currently: 1994 XJ600, 2006 SV650

Thu Nov 09, 2017 8:31 am

Just thinking out loud:

* Have you check the needles are properly seated?
* Have you checked your float heights?
* Have you done a carb sync?

* Is there oil getting into the cylinder through the valve stem seals?
Men do not quit playing because they grow old; they grow old because they quit playing.

User avatar
radare
Site Founder
Site Founder
Posts: 8445
Joined: Mon Nov 07, 2011 12:42 pm
Region or State: Rocky Mountains
Motorcycles Owned Currently: '92 XJR600, '92 Seca II
Location: Denver, CO
Contact:
United States of America

Thu Nov 09, 2017 9:54 am

9 out of 9 times, a cold cylinder has a clogged (or partially clogged) enrichment or pilot circuit. Pull the pilot jet and make sure its clean. When you have the float bowl off, pass a wire through the enrichment hole, too. Then remove the pilot screw and pass a very, very small wire through it to make sure the hole is clean.

If you have the pilot jet screwed in to far, it'll cause the same symptoms. But trust me, its fuel.

User avatar
Giuan
Lookin' Around
Lookin' Around
Posts: 29
Joined: Tue Jun 07, 2016 2:00 pm
Region or State: France
Motorcycles Owned Currently: 1993 Yamaha XJ600

Sun Nov 12, 2017 8:37 am

Hello guys, thank you both! I really appreciate it.
I've been reading you but I couldn't answer before.
Casper, if for "needle" we're talking about the one attached to the floats: it's brand new and its seat clean to perfection! I swapped the floats+holder+needle set with the 2nd cylinder one and the issue didn't follow.
Float heights are fine, all equal and in specs; I didn't perform the synchronization, I usually do that only if I separate at least one carb from the rack...the bike was running fine and the issue showed up at the sudden!
As for the oil, that's what it scares me: I don't have smoke coming out the mufflers, but every 1000 miles or so I have to top a cup of oil!I've always thought that, being an old bike with high mileage (24y 70k mi), there must be more than one leaky gasket...
The mechanic told me that the valve clearance is fine; the spark plug is sooty but doesn't look oily. If the piston is leaking from the ring, I can try the old trick of using a 10w50 oil, but I won't get someone to overhaul the engine: too expensive, it would condemn the bike to the junkyard's press... :(
I changed the oil last july, Repsol 10w40 full synth and barely run 2000 miles.
Radare, now I agree with you that is carburation, but I don't know what else to try!
This morning I opened up that bowl again: the pilot jet is new, all shiny, as it was the first thing that I changed! Today I tried to clean the pilot and enrichment holes in the carb's body: I used a syringe loaded with thinner and I played with the plunger a lot.

Image

I also passed a wire in those holes, then I changed all 4 spark plugs and mounted 5KOhm plug caps also on the other 3 leads. Unwinding sunday morning, but that cylinder still doesn't pass the 210 F while idling :?
You do not have the required permissions to view the files attached to this post.

User avatar
Ollyrag
Lookin' Around
Lookin' Around
Posts: 42
Joined: Fri Oct 21, 2016 1:48 am
Region or State: Kent, UK
Motorcycles Owned Currently: xj600N 1998
Location: Great Britain
Great Britain

Sun Nov 12, 2017 1:22 pm

As Radare said.


Try synching them at least with a very fine drill / needle at the throttle plate as a starting point.
It's not how old you are, It's how you are old.

Post Reply

Navigation:

  Search

  New Posts

  Unread Posts

  The Wall

  Gallery

  About Us


Call to Action:

  Forum Images

  Ride Relay


Cool Projects:

Predrag's Resto

Renegade's 93