View Full Version : 250 ignition probs

01-31-2009, 07:40 PM
ok so i got a 70`S model fl250. it was running real good for awhile, but one day i was just giving it hell... and it stopped lol. it was only getting like 30 psi so i put all new gaskets on it and put it back together.. its getting spark, round 100psi of compression, and gas is getting into the cylinder. if i pull about 10-15 times it will back fire loud as heck but thats it. now i was really thinking it could be the timing. but which one is more likely. the points or the flywheel key?

01-31-2009, 08:36 PM
Once it has backfired you likely sheared the flywheel key. I'd start by checking that.

02-01-2009, 09:00 PM
well today I checked the key. its really short but there is no signs at all that the flywheel spun on it. I then checked the gap on my points.. lol and now it won't spark at all. I willl have that lined back out tomorrow. but then where from there?

02-02-2009, 07:35 AM
Even at 100 psi your compression is still low, and when the compression is low they are very hard to start. Sounds like it might be time for more than "gaskets". Cranking pressure in an unmodified FL250 should be 128 psi give or take.

02-03-2009, 12:22 AM
well I measured the cylinder and piston. piston is 70.5mm and cylinder is 71mm sounds like its time for a new piston huh?

02-03-2009, 09:07 PM
That probably wouldn't be a bad idea. If you are confident in your measurements order the piston and then have the cylinder bored to fit. You'll have to go wiseco so I'd have the machinist who is doing the fitting allow .0045"-.0050" clearance. Anything tighter you'll likely seize.

02-04-2009, 05:47 AM
my measurements are solid.. plus or minus a few inches... but solid lmao.... but no really, im comfortable with them but will the cylinder at 71mm and the piston at 70.5mm couldn't I just go the next size up ( im thinking its .080 over, which would be 70.8? right?) and allow the .2 gab to be my tolerance gap? or am I just completely backwards... fyi I have a lot of experience with the engine mechanics but over boring is one of few I haven't messed with,... thankx

02-04-2009, 06:53 AM
You're measuring in mm, but the oversize is given in imperial. .080" = 2.032mm. You need to up to what is probably a 72mm piston. (I'm guessing that your piston was probably 71mm to start with, but I'm not up on the old Oddy measurements, so I'm just sort of basing this off experience w/overbores in general).

Your best bet is to get the next sized piston and have the jug bored by someone to match the piston. As PilotHawk said, go look at Wiseco and get the next size up from what you have, take that with you to the shop that will do the bore and have them set the clearance at .0045" to .005".

If you're willing to ship the jug out, send it to Bore Tech (http://www.bore-tech.com/) in south western Ohio (just outside Cincinnati). If you supply the jug and the info on the jug, (year, model), Bill can get the correct piston and ring set for you along w/bore the jug to the piston. It will be done correctly if you choose to use Bore Tech (http://www.bore-tech.com/).

02-04-2009, 09:04 AM
well there is a yamaha shop right up the road from my house that im gonna have it bored at.. and order my piston from. they will know whhat they are doing so I don't have to worry to much .. but im guessing that since im getting spark. and gas into the cylinder and I know all of my timing is straight. that the reason its not firing up is simply cuz my piston or cylinder is worn enough that im not getting adequate compression to sustain a ignition .. right? lol its so weird that I can tear down and rebuild my small block v8 and rebuild anything else on a car. with a crap load of computers and such but when it come the most simplest of all modern engines.. im more or less "blonde" about it lmao but its ok. ill stick with my fabricating skills(8 years and haven't found anything to stump me yet)

02-04-2009, 12:36 PM
Sounds like the way to go.

Funny how "new" things can be so intimidating. Hand me a bike or sled engine and I'm all about tearing into it. Automotive stuff? Hell no. That stuff's scary! :rolleyes: