I have an ultralight with a 503 dual ignition single carb. When the engine is cold it fires up but runs only on the back cylinder for 5-15 seconds then both work.  Vacuume leak at the front seal? After it warms up and I accelerate to 4000 rpm one cylinder(I'll find out tomorrow) EGT shoots up 150 degrees almost instantly. At idle the EGT and head temp are close but it takes the front head 5 min to catch up. There is also a funny rattle, sounds like a ball bearing in a steel can sound at the same time. 

  • Re: 503 wont fire front cylinder when cold

    by » 3 years ago

    Not much experience with these engines, but here is what I have researched while rebuilding mine.  I mounted mine to my work bench and had a fan moving air around it.  Cranked it up (no prop) and allowed it to run a few minutes to warm up.  Then before shutting it down, sprayed carb cleaner around each rubber intake tube right at the cylinder head.  Do one at a time.  Then sprayed right around each carb (I have dual carbs) and the engine picked up some rpm's on the rear carb.  Found a very small crack in it which allowed air get in.  Sealed mine with RTV sealant/adhesive since it was not too bad.  After a cool down period I started it up again and sprayed the rear seal (pusher set up and already removed the "B" box) and nothing changed.  The front seal was a pain.  Dismantled the pull starter and stator, rubbed vaseline around the seal, reassembled and started.  No change.  Since I was just learning about this engine I did what suggestions I read and spent the time on it.  Sprayed the crank case split while running, then spark plug gaskets.  To this point the only leak I found was the rear carb boot.  Since I have dual ignition, I removed the coils and placed them on a block of wood where the plug wires could be swapped around the plugs.  Put both plug wires to one cylinder and the second coil to the other cylinder, started it and wrote down how they were attached and made note of it.  Then put one plug wire from each coil to each cylinder and reapeated.  Did this until each plug wire had been attached to each spark plug and thankfully the engine never missed a beat.  I was checking for possible electrical dead spots.  Also wiggeld the wires leading to the coils from the stator for any possible breaks.  I think the voltage is high so used a tounge depresser to do it.  After doing all of this, I still saved up the money and ordered a complete seal and gasket kit from CPS and opened the engine up, checked every part over, new "O" rings (which may go bad allowing the fuel air mixture flow from one cylinder to another around the crankshaft), resealed it, and reassembled it.  If you have the time and patience, tear down the pistons and use a micrometer on everything, write the findings down, and bump them against the manufacturers specs.  I spent about a week doing this and do not regret it.  Yes it is a lot of work, but you learn so much about your engine this way.  Parts are readily available, unfortunately some, like those ignition triggers, are overpriced in my view.  One last thing, unless you have done so already, replace the fan belt.  I understand that as you are decending, the air flow to the engine is disrupted and the fan is relied on to keep the engine cool.  Well, this may or may not help you, but it is how I tackled my engine even though I actually had no issues and just wanted to make sure I WOULDN'T have issues in the air..... 

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