Hello All I'm a relatively new owner and i just had a few questions for the experienced folks.
I have a '46 108 with the franklin 150 and just have a heck of a time getting it started. last summer it would start on the second compression stroke. when the temperature falls below 40 degrees things change and I can crank that sucker until the battery is dead. I feel like ive tried everything to get it going. no prime, lots of prime, preheat. you name it. no sputter. nothing. I would understand hard starts if the temps were in the 20s. but I find it unusual that it has such issue firing in such mild temps. but then again. I'm certainly no expert. any ideas on where to look or what to try?
Since no one will I’ll bite.
Let’s start with the basics, what did your mechanic say? If you don’t have one experienced with Stinsons find one. Not easy not many of us left. If I was there the first thing I would do would be to review the planes history, age of mags carb, overhaul and such. All can point to areas of concern. Then i would ask about how you preheated and details of exactly the coarse of events. Every engine needs fuel, spark and air. Get any one wrong it doesn’t start or run right. Typically you start with the basics, determine you primer is actually working on both sides. I usually pull the nozzles hook them back up and have someone actuate while I observe both should flow smoothly. You also need to check the mags for spark disconnect a lead and pull the prop through. When the impulse snaps you will see a spark. Do this with the plugs removed which you would do when you perform a compression test to see if you have a bad cylinder. Franklins are notorious for induction leaks. I like to pressurize the system and apply bubble leak detector. Cold weather may or may not have anything to do with your current problem. Things fail on their own schedule. Mine starts in the low twenties with a half hour of Red Dragon preheat. I do not prime before I crank only during, seams to be the best method for my girl, hot or cold.
@aliebfriedearthlink-net I did some more diagnosing today with one of the a&ps i work with. And we think we have it narrowed down to a spark related issue. some research has indicated that both slick mags have an impulse coupler but we noticed that we cant hear an audible "snap" when the prop is pulled through about for about half of the compression strokes. and even when it does its more of a light "tick" This could explain why the engine ran normally when there was enough RPM to generate the correct about of voltage for the mags. Should I hear the impulse coupling "snap" every time a a compression stroke occurs?
Hand propping should make an audible snap at EACH compression pulse. When you pull it through and get to the hardest part of the stroke it should snap. Make sure you have grounded the P-lead on both mags before doing this.
I would think it odd to have impulse couplings on both mags, I would think that is a good way to get a backfire. On my 108s the impulse is on the left side only. The 165 are slicks and the 220 are Bendix. I have had a spring break and it was very reluctant to start so to weak spark.
Ok that maybe just not my experience with franks over the years. I do not have my mauals withme so i defer
@rileypowers88gmail-com Yes, 6 times for two revolutions. Check to make sure actually are seeing a spark. Don’t just hope because you hear the impulse you do. Confirm it. Pull at least one plug per cylinder and don’t touch the plug or you will have a very exciting day. This confirms you have spark. If you don’t hear 6 snaps you have at least a mag issue. But wait there could be more, but firsts things first, know you have consistent spark.
@carl no I have only one coupling on each engine. Had the off. Many engines only have one. If set up correctly there is no need for impulse in both. I start my engines on one mag only like many aircraft do, just one more mechanical thing to break. I have never had a hard time starting except when the one spring broke on the slick.
Check the resistance in your spark plugs. Champion plugs have a real problem with failing resistors and that can lead to hard starts.
Bob PicardN6346M Stinson 108-3 Floats/Skis/Wheels
N48923 Taylorcraft L-2B skis/WheelsAnchor Point, Alaska
I had the exact same issue. Went from easy starts to crank for ever with jumper cables and eventually it would fire up. Time to send the mags in for overhaul. The Slick 6399s are prone to breaking impulse coupling springs and they may be hard to find if your mag overhaul shop doesn’t have any. Keep calling around, eventually you’ll find some. This is what solved my starting issue.
@aliebfriedearthlink-net Pulled the top three spark plug wires on the left bank of cylinders. no spark from what the mechanic and I saw. was gonna do the other side but it started to snow. (no hangar obviously) when i head back out there i think we are going to pull the mags and investigate further. Nothing in the logs indicate that these mags have been messed with since the new engine was installed 800 hours ago. so they are probably well overdue for overhaul/ replacement anyway
Sounds like you found the smoking gun. All conventional magnetos need a 500 hr inspection. Points and brushes wear, timing then drifts, parts break. Another big issues on older and high time mags is the magnets degrade. There are a few shops left who have the equipment to recharge them. After installation timing should be rechecked around ten hours. It can drift on initial run in.