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  #1  
Old 04-16-2012, 12:37 PM
skidsteer.ca skidsteer.ca is offline
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YFM80 Moto 4 Hard starting Weak spark?

I have a old moto4 80 for the young one. Done the piston and valves on it last year, has 160 lbs compression. Valve lash is correct. Battery is new Interstate

Still hard to start cold.
Starts good hot, but cold it often still needs to be push started, which it generally pops right off in 5 feet.

When it does start cold on the starter, it is always because the motor is spinning and then the instant you let go of the starter button is when it fires. Almost never while you are cranking it.
Once you get it going and ride it for 2 minutes it starts like the day it was new.
Take a propane torch and warm the head and jug a bit and it takes right off on the starter.

Cranking rpm sounds normal, spinning it faster with the battery on a changer at 14.5 volts seems to help

The Yamaha manual say the spark should jump .240" on a plug gap tester, this one makes it about .100"
So the big question...
What is the most common causes of weak spark?
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  #2  
Old 04-16-2012, 12:49 PM
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A weak stator.
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  #3  
Old 04-17-2012, 08:25 AM
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Talking

Resistance on the stator and trigger coil is close to what the book say. (shop was not at 68F) Risistance on the plug wire of the oil was tripple the books value, so start with that I guess.

Both the coil and cdi off my parts bike are nfg , so hope I don't ever need the cdi. Not sure how the parts bike got rode to the big puddle that folded the conecting rod without spark, hmmm.
Ken
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  #4  
Old 04-17-2012, 09:51 AM
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What color is the spark you have? When you take the plug out what color is the electrode (tip)? Starting to sound like a lean condition??
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  #5  
Old 04-17-2012, 10:39 AM
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Had one just like it, adjusted valves and problem was solved!!
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  #6  
Old 04-17-2012, 10:48 AM
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He metioned "Valves lash is correct"
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  #7  
Old 04-17-2012, 02:34 PM
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Spark is blue but seems tiny. Won't jump more then .018" Plug should be gapped at .026"
Plug was rich last I looked, but I stuck a carb kit in it yesterday that fixed a 1/2 throttle miss it had. It was ok at wot, and idled good. But stumbled in the 1/2 to 7/8 throttle range. Leaning the needle made it worse.

Which kinda sucked because I keep the young one down to 1/2 throttle with the safety or "limiting" screw. Sometimes he forgets what the shifter is for and goes full flipper in 1st, they are tough little engines.

I found crude on the outside of the tube the needle on the throttle slide goes into. So cleaned the carb and put in new main and idle jets, new idle speed and mixture screw, needle and needle tube. Raised float level 3/16 to 1/4 to correct setting.
The new coil will be here in a few days, only $20 with plug cap and wire, so can't hurt.
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Old 04-17-2012, 03:34 PM
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thought these were all gone...

i remember driving mine out of the showroom in '84, still have it, ran great year...

the thing about yammi's is they run forever and the oil they use is phenomenal! speaking of which after 28 years i guess an oil change is in order...
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  #9  
Old 04-17-2012, 07:41 PM
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Did you check the O-ring between the intake and the head? Also the carb has a gasket between the top slide cap and the carb body.
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  #10  
Old 04-17-2012, 10:31 PM
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Yes thats where I have been removing the carb from, easier to get at those bolts on the intake to head then carb to intake. Both o rings are in good shape. Idle is nice and stable, it used to have the odd miss at idle, but even that had cleared up b4 the carb kit. Even better now as the idle mixture screw seems to better control the engine.

I'll have to do some cruising with it and see if the plug is coloring up.

Tha Dude, this one had such good fenders, seat etc and was basically complete I decided to rebuild the top end. Water had got in the cylinder and rusted the bore on the lower side. Just enough to ruin compression. Wear on the jug was so slight that the machinist talked me out of boring it. He cleaned it up with a hone and still was within the max spec for a standard piston. Valves were sharp so I lapped new ones in. Guides, cam, rockers, springs al looked perfect.
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Last edited by skidsteer.ca; 04-17-2012 at 10:41 PM.
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Old 04-24-2012, 11:48 PM
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Well the spark will jump a .026" plug gap now (with the new external coil and plug wire) but still won't cross the .240" on the spark tester. Tried the tester on the xr70 honda and it throws spark 1/4" with no trouble. Sat there and idled perfect.
Guess it is time to do the coil under the flywheel and the pulser "trigger" col on the outside. There goes another $60
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31 Ft Wildwood, 22 ft Sledbed, 2400 sq ft heated toy box

Last edited by skidsteer.ca; 04-24-2012 at 11:51 PM.
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  #12  
Old 04-25-2012, 02:12 AM
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Does this bike have electric start.If so this bike may have a tci ignition in it ,which means that it needs 12 volts in order for the ignition to produce the correct voltage.The lower h.p outboards and smaller bikes all ran this type of system.
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  #13  
Old 04-25-2012, 06:41 AM
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Quote:
Originally Posted by pipes View Post
Does this bike have electric start.If so this bike may have a tci ignition in it ,which means that it needs 12 volts in order for the ignition to produce the correct voltage.The lower h.p outboards and smaller bikes all ran this type of system.
It is electric start, However I see no place in the manual about needing 12 volts. Where does the power get applied to? The battery is new Interstate.
Thanks for your help.
Ken
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Old 05-04-2012, 06:00 PM
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Changed the coils under the flywheel, got spark jumping 1/2" on the tester and she fires up just like she is efi now.
Guess you hit the nail on the head with stator
Thanks
Ken
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  #15  
Old 05-04-2012, 09:05 PM
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Quote:
Originally Posted by 3Wheeler View Post
He metioned "Valves lash is correct"
Reaaaaaaally
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