Rotax 503 - misfire until half -throttle

[techfx]

(Paging Hydromike)

This may be a general 2 stroke question. I've never dug into my Rotax 503 but, I'd like to see what the likely causes are that may be causing a misfire (seems like it's partially firing on the bad cylinder) until over half throttle - then it kicks in (and runs very well!)

I've got a single carb, points/condenser ignition and just modified the exhaust. I've got 2 new spark plugs, fuel was about 6 months old. Never gapped or changed the points/condensers/ wires (never messed with a points ignition sys all together).

there is a tune up kit on Ebay w/ all included: Rotax 377 447 503 Points Plugs Condensers Wires Boots:eBay Motors (item 320378722603 end time Nov-28-09 09:52:29 PST)



Any advice on where to start? I'd like to be able to at least go at a moderate or lower speed without goosing the throttle.

[Amphiman1]

miss fortune

bad plug, wire or a coil? start with easy stuff first.

[cobalt]

Could be:

Bad spark plug cable, and or cap (low vibration may expose a crack in the cable). I would check for dirt or corrosion first.

Could be the carburetor letting in too much gas at first untill the carb's reservoir run's out and has to syphon from the tank which takes more time. At lower speeds the carb has more time to refill it's reservoir from the tank and flood the motor.

I know that another carbs such as the mikuni VM24ss are notorious for this problem.

I seriously doubt that it could be the coil. However it is possible.

Could be something as simple as rust on the stator.


Good luck.

[Coop]

" fuel was about 6 months old."....That is old fuel, you will have a hard time getting anything to run correctly on that. I'm not saying that is your problem, but it most likely is contributating to it. Mobile Oil Co has said that the shelf life of gas is now 12-15 days, meaning that after that, it is starting to deteriorate. Sadly, the gas we are buying at the pumps is probably already at or past that 12-15 day window. 4 cycle engines are more tolerant of expired gas, but 2 cycle engines require fresh, and usually higher octane fuel . I used to work at a Stihl dealership,( until just recently), the #1 problem we were seeing with 2 cycle equiptment not running right was stale fuel. Change the fuel, purge the lines, they would usually start up and run like they should. Stihl issued a bulletin last summer stating that their equiptment should not be expected to run at optimum power and rpm on any fuel that was 30 days old or more. Just some food for thought...

Coop

[hydromike]

Hey Tech....

Sheesh 'Tech, I dunno. My old 503 had this same problem, but I'm running the CDI setup with twin VM34s. She'll still stumble for a few minutes when it's cold... whether that's new plugs, old plugs.... day old gas or couple-a month old gas. I could never explain it, and it's not right that I live with it, but I do. Since you're running a points setup, the sources of our problems may be as different as night and day. I would certainly check your point gap, but since it seems to clean out after half throttle or so, I can't explain it much more. Your fuel delivery seems fine. Fuel quality seems fine. Does it misfire under load, or just if you're bringing the R's up standing still? You're running one Mikuni, right? I plan on being out there in January sometime. If you don't figure it out by then, I'll totally make time to stop by and (at a minimum) stand over it and throw out more ideas...

~m

[PowerFistArgo]

Had the same problem with the single carb 503 that i put on my dune buggy.
I cleaned the carb and the points and it totally fixed the problem. Also you may have too much oil in the gas. Just my 2 cents.

Hope this helps

[techfx]

Thanks, Thanks, Thanks and Thanks!

I was definitely climbing some hills with no problem on full throttle. I don't have a place to dump out the all of fuel (living in the city) so I tried to run down the tank as much as possible before getting new fuel. I also had about 2-3 gallons I dumped in from a sealed container. I will have have new fuel next time (at 50:1 royal purple synthetic). Should I run an octane booster?

The Carb is a VM36 and I haven't cleaned it out yet. I will try that and to check the point gap/ clean the points. I also have some iridium b8res plugs that I didn't get a chance to test out and maybe should have.

It may be a while before I can be back at it with the holidays and other transportation issues. Hydromike, If you're out here and want to do a little riding or meet up in So Cal - I'm down. Getting some pointers would definitely a plus and most of January should be good for me. We could meet up at an ohv area closer to L.A.

[techfx]

7 years later and I'm still tinkering with the carb (jetting) and clutching... Maybe I'll get it to climb a hill with out a 100ft running start in Ashtabula this year. This thread came up in a google search on 503 rotax pilot jets, lol.