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View Full Version : stihl BR420 blower=weak spark


turbofreak1956
05-23-2009, 01:59 PM
Hi gang,my 1st post,I need help with this backpack blower that ran fine for 2 yrs. then this spring became hard to re-start when refueling,then rapidly worse to where the only way to start was R+R plug,+ pull 'till blew gas-out. Found the fuel tank vent plugged,causing flooding+also constant oozing onto floor... Thought that was the end of problem's But I can't start it no matter what.. Found the spark so weak i can hold the plugwire w/ my finger + spin the cord rapidly + still hold onto it. IS There a specification for the airgap@ flywheel? And coil resistance spec,or am i barking up the wrong tree? ARE there common failure's for the 400 blowers? THANKS, Glenn

4.3mudder
05-23-2009, 08:02 PM
When do you have the weak spark, when you first start it when it is cold? Hot? If you can't start after it is hot maybe the coil going out. Weak spark maybe caused form build up of rust on the flywheel magnets and coil. Flywheel and coil gap I used a business card.

turbofreak1956
05-24-2009, 01:35 PM
Thanx for responding 4.3 Mudder; the spark is always weak,i compared it to my tiny stihl chainsaw[12''], big difference,iI won't even try to hold the plug-wire on that w/pulling!! BTW,I removed the coil from blower yesterday+ it was fairly clean,no cracks,wire o.k. I also tottally removed the "KILL" switch wires, still have same weak spark; I tried many different gaps, still, no better, What do you think/// P.S.; this never starts!, thanx, glenn

4.3mudder
05-24-2009, 04:40 PM
There was no rust on the coil/flywheel? Have you tried a new plug? Maybe the coil is going out, I would let Restrorob come in on this one, he might know the deffinite answer.

ed2hess
05-24-2009, 05:29 PM
It might be weak spark but these units flood so easily I would be sure that isn't the problem. What did you do about the gas leaking on the floor...I assume from the carb? Need to clean the needle point/seat to try to address that. I guess you know how to deal with flooding....turn the switch off and pull it about twenty times. Then go back to on and usually it will fire.