91 Merc 4 kicks then dies - Low primary compression?

[al40]

Motor worked fine then suddenly decided to not start next time I went to use it. It occasionally kicks and runs for under a second before dying. <br /><br />Anyone any ideas on why it won't run? Does this sould electrical or fuel related? Would a damaged coil produce these symptoms? <br /><br />The spark seems fine. I opened up an old plug to 5mm gap and there is a blue/orange spark accross this when plug is well earthed to the motor chassis. Checked stator reistance (82 ohms), Trigger (92 Ohms), Coil primary (0.2 ohms) and Coil secondary (approx 6K Ohms). The primary coil voltage from CDI seems to pulse at approx 100-150V when turning engine over by hand (checked on oscilliscope). <br /><br />I have re-built the carb twice and it seems OK. No visible dirt and washed it out and dried with compressed air. Mixture screw is 1.5 turns out from seated. <br /><br />Also changed the fuel and rinsed out the fuel filter. <br /> <br />Any suggestions, please post!!!!

 

[andy6374]

Re: 91 Merc 4 kicks then dies - Low primary compression?

Check compression and check for a snappy blue spark that can just a 7/16" gap.

 

[al40]

Re: 91 Merc 4 kicks then dies - Low primary compression?

Max gap I could get on a plug was approx 10mm (just less than 7/16"). Spark did jump accross this sometimes blue / sometimes orange. Possibly not as often as it should.<br /><br />Compression is OK. <br /><br />Today the motor started 1st pull, ran for a few seconds then died. I tried spraying cold start into air intake (I know you're not meant to use it on 2 strokes) and it made no difference so I'm convinced it's an ignition issue. It's almost like something gives up as soon as the engine fires...<br /><br />Is the secondary coil resistance reading a good indicator of whether the coil is faulty? Spec is 3000 to 4400 ohms and I was reading 6000 (but resitance will vary with temperature). <br /><br />Al

 

[al40]

Re: 91 Merc 4 kicks then dies - Low primary compression?

Took engine to the repair shop who spent 2-3 hours on it without any real success. Spark is fine, compression is 180PSI, carb is OK.<br /><br />The only thing he suggested was the primary comression (crankcase) was low so it wasn't sucking the fuel properly. <br /><br />Does this sound realistic? Does this just suddenly happen? (ie engine started and ran fine one day then not since then?) I guess if this is the case then the reed valve or drive shaft oil seal could be gone. <br /><br />Engine is obviously sucking on carb as fuel pump is working well, filling up carb bowl quickly. <br /><br />Thoughts?