Motoplat problems?

[dsklcsw]

Hi everyone,
I have a problem with my 73 250 Hare Scrambler that I hope someone can help me with.  I have only been able to get it started 4-5  times, and when I do, it dies within 10 or 15 seconds.  When it starts, it will only run at high rpms (not less than 3/4 open throttle, and not able to idle)I have had the Motoplat rebuilt by Vintage Dirtbike in Texas and there appears to be a good spark when I kick it through with the plug out and grounded on the head.  Following the rebuild, I did get it started, but it died as described, and I thought it maybe a carb problem.  It has a 34mm mikuni, and I have have swapped carbs with my cz to check if there was an possibility that it was a carb problem, it started twice, but died in the same manner.  Has anyone had a problem similar to this?  It appears that the Motoplat will kind of work at higher rpms but not at lower ones.  I have cleaned all the contact points with the frame, coil, etc. but have not taken the flywheel off and further cleaned under the mounting plate (although I did so before I remounted the ignition).  Can anyone help me?
I am thinking about buying a PVL, but with the internal flywheel, I don't really want the power characterisics of this motor to change.  Does anyone have any advise as to which flywheel weight to add to get the power as close to stock as possible?
Thanks...Dwayne

 

[imported_n/a]

Dwayne,

Why don't you give Vintage Dirt Bike a call and ask them.  They may have some insight.  
I'll be monitoring this, as they rebuilt a Motoplat for me and I haven't installed it yet on my Jackpiner playbike.

Also, PVL ignitions are the best.  If you go that route, you may want to try running without the flywheel weight for a while and see if it works for you that way.  If you want more weight, you can always add it on later.

Glenn

 

[rob w]

Dwayne,
 Make sure your carb is mounted tightly without the possiblity of an air leak. Retorque the cylinder head to make sure that's on tight. Check the gas flow going to the carb to see if that's steady.
Check to see if the timing is set correctly, or check to see if the pilot jet is plugged. That's about all I can think of at the moment. Good luck.
Bob W

 

[Mark Annan]

You may have a bad coil.  I have seen several that display this characteristic with a bad coil.  Plug in another coil to find out.  You may want to have your flywheel remagnetized by Dane at Penton Imports (PVL).  Weak magnets can cause problems (usually a weak spark, especially at low RPM).  

Try using a timing light to check your ignition timing to see if it is jumping around.

Mark

 

[dirtbike]

I think you need to determine if it's the throttle position or rpm's that affects the engine. If for example the engine appears to run at a specific rpm but when you open the throttle some more, it just dies and doesn't continue to run even at the same rpm as before there is likely a carb related or air leak (crank seals, engine halves, cylinder or head) problem. If on the other hand it seems to fail abruptly at, say 1500 rpm's regardless of what you do with the trottle it's likely the ignition.

Check the spark when kicking it over like you did again. Is the spark kinda yellow then you likely have a ignition problem. The spark should be bright blue.
Maybe dumb question but have you tested with other spark plugs? I have had strange things happening that I wouldn't even try to explain with some plugs.

 

[dsklcsw]

Thanks everyone!  When my engine dies, it's like someone turned the ignition switch off, in other words, it just dies.  It will usually not fire again until the next day.  But like I indicated, it will only run for about 10-15 seconds, and will not even hint of running again until the next day, so I ruled out that it was heat related.  I will check if there is a leak in the head, but it's just mind boggling.  The spark is white as far as I can tell.  I did put in a different spark plug, and had the same results.  I have never encountered a problem like this in my 20 years of riding.  If anyone else can think of anything else, please let me know.  I will check all of these things that you all have suggested.  Thanks again!  
Dwayne

 

[Mark Annan]

When it dies have you checked the spark again?  I'll bet you find that it is weak or nonexistent shortly after it dies.  = Bad coil

 

[Kip Kern]

Send it to Dane L. at Penton Imports.  He is the Motoplat man!

 

[dsklcsw]

Hi Everyone,
I am in the process of getting another coil to try this out.  I checked my head, it seemed to be torqued to about 7lbs.  The manual that I have says that this is right.  This seems awfully loose, is this correct?
Thanks...Dwayne