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83 RM80 - No power band , No high RPM

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My son's 1983 RM80 has been giving me stress the last few days . it was performing great as it should then out of no where it has no high rpm power , will not hit power band . It starts great , it idles fine , part throttle performance is also good , it will perform as it should until it wants to start revving out , then it just lays down . The low end power feels about right , but will not open up . It acts just like a 4 barrel carb that has the secondaries locked shut ( for us old school guys !! ) 

Actions Taken : (In no specific order )

1. Removed carb , disassembled and cleaned out thoroughly , checked all related components , checked float height / drop . all checks out good imo . no debris was found in any passages etc .

2. Cleaned and re-oiled the air filter .

3. Replaced spark plug with new NGK , played with plug gap as a test . regapped plug to OEM spec then reinstalled .

4.The spark plug always seemed to come out looking dry (lack of fuel) Removed fuel tank petcock and checked for a plugged line , but checks out good . Good fuel flow  !

5. Fuel was drained out of tank and carb , replaced with new Premium Shell fuel and proper mix at 25:1 using good quality 2 stroke oil . 

6. Removed cylinder , installed new Wiseco piston & rings , fresh gaskets , honed pot . Piston to skirt clearance was a tad out of spec , but still runs and has good compression .Checked the reed valve while apart , looked fine too , no problems that i could see .

7. Checked for bad / leaking crank seals . No indication of leaks were found . Engine idles smooth , when warmed up is very responsive when you blip the throttle .

 

And , so my problem still exits . 

I'm by no means a 2 stoke tuning wizard , so i'm here to ask for some help .

 

Thanks in advance for any help / input .

 

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Sounds like youve done everything you can fuel wise - what about electrical - cdi, coil, magneto ?

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I was wondering about that as well , but I have no way to test the spark output , nor do I have another coil as a tester etc .

I did however put an old plug into the pigtail , ground it and kick it over , it has spark we know this much , but is it breaking down under high rpm load ?

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I didn't see reeds on your check list. May have broken one. Oh sry see now you did my bad

Edited by Motox367

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Being a 83 check for crank play on right side . Or if you notice lot tranny oil. But could be stator or cool as said above 

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Thanks for the replies !!!

 

 

We have discovered that it has weak spark .

When you kick it over it just has this little faint , almost can't even see it jump the gap !!

I had checked once before last summer cause it was getting hard to start , and i know it had way hotter spark than this year .

Just not sure how to go about diagnosing this ... 

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On 6/4/2017 at 6:46 PM, Motox367 said:

Being a 83 check for crank play on right side . Or if you notice lot tranny oil. But could be stator or cool as said above 

My 1978 RM-125 suffered a stator failure due to excessive play in the outer crankshaft bearings.

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Measure the resistance of the primary and secondary sides of the ignition coil with a DVM set to ohm (resistance) mode.  The primary side is measured by disconnecting the wire from the magneto to the coil and then attaching the meter from this terminal on the coil to ground.  I don't remember what the spec is supposed to be but you should be able to look it up.  The secondary side is measured by disconnecting the spark plug cap from the spark plug and measuring the resistance to ground, just like you did on the primary side.  Again, you should be able to look up the acceptable values for this measurement as well.

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The ironic thing about this type of ignition system is that when everything is working correctly it will all measure a dead short to ground when the engine is not running.  That is because it is all based on a series of coils and the theory that rotating magnetic fields induce current to flow in coils of wire.  The rotating magnets are permanent magnets embedded in the rotor at the end of the crankshaft.  The coils of wire arrayed around the rotor on the stator plate are grounded on one end and produce current once the rotor starts turning.  The rectifier turns this alternating current into a D.C. voltage supply for the CDI and the ignition coil to use.  The ignition coil is actually a transformer, which is a pair of magnetically coupled coils, which amplify the voltage applied to the spark plug from ~12V to ~25000V.  Again these are just coils of wire with one end connected to ground.

Edited by GlennRay
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Any chance a CDI box from a 1986 and up would work on my son's 83 ??

According to OEM part number they are different , but the plug and wires are all the same , even the wire colors are the same ??

 

 

 

 

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I did some checking , and don't think the crank seals / seal are bad , no leaks could be found at this time .

We did find a bad coil lead dropping a lot of spark , so i replaced the coil and retested , the bike starts much better , but still lacks a ton of power .

New Problem !!   The bike now stalls out / shuts off after about 3 mins of run time and can not be restarted , it must be left alone for about 1 hour , then it will start up again no problem.

This is what i'm thinking the CDI box might be NFG ... ( Not Factory Grade )

 

I have no experience diagnosing the Stator assy. Is it possible for that unit to cause  this kind of condition ? 

 

Edited by Mikethe80srocker

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So i will need to check my stator for defects as well i see .

I assume resistance checks hot & Cold  ??  I'm not sure how to go about diagnosing a stator ...

I was hoping it was just going to be a CDI box .

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It sounds like a stator coil, or coils are opening up as the engine temperature increases, thus removing spark and denying the engine of proper functioning.

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