The International Cessna^® 170 Association

OK all you Francklin guys, I got into the middle of a problem with a cylinder replacement (all six) took 18 months to get parts and overhaul the cylinders. the owner run up the aircraft and had 300 + RPM drop....called me and had me check things out....I found several spark leads bad ....fixed and the drop went from 300 to 150....at 1700 RPM, 75 at 1000 RPM, and 300+ at 2000 RPM. The Book says that a 200 drop is OK at 1700.....I don't like the high end drop....I did a compression check and the valves appear to be closing......should I be looking at something else before I start taking the engine apart? Just want to know if I am missing something really simple.(since what I know about Franklin's you could engrave on the head of a pin)......The old adage is go to what was just fixed to find your problem. Since cylinders were the only thing changed I'm looking at valve timing (IE clearances).... .Brad

I wouldn't think you wouldn't treat a Franklin any different than a Continental. Do what you would if it was a Continental.

N9149A wrote:I wouldn't think you would treat a Franklin any different than a Continental. Do what you would if it was a Continental.

Thanks Bruce, that's the way I'm looking at it......It''s just that there is some differances to a Franklin as oposed to a cont. and lyc. I just don't know them all....I'm looking for specifics on the issue that I have found. again, thanks....Brad

Brad besides not being a common engine anymore what differences have you found that might cause a RPM drop? Sounds like ignition (mag) problems to me which would be the same on any brand engine.

The specification (the book says...) of up to 200 RPM difference at 1700 RPM sets the guideline for the engine's ignition performance. The fact that the differential is less at lower RPM and more at higher RPM tells us why they specified 1700 RPM as the yardstick. A mag differential test is a test of the ignition system, not the cam, carb, compression, or anything else. Yours meets the requirement and indicates a healthy dual magneto system, if it meets the RPM criteria AND the engine is smooth on both, or either, magnetos.

As far as magnetos performance goes....You're good to go. Now all you have to do is determine that your engine makes rated static RPM for takeoff according to the TCDS. (See item 111.)

Used to fly a factory brand new Maule with the 220 Franklin in the early 70's. Always had a mag drop like that. Maybe it was because the Franklin got 220 hp out of 350 c.i. Sort of "hot rodded up" if you will compared to the Lyc. and Cont. Also if I remember correctly it had a compression ratio of 10 to 1 which is a couple of points higher than any other aviation engine. Maybe on 1 spark plug firing that cylinder design is more affected than the lower rated hp engines per cubic inch. JMHO

An additional resource is this Yahoo group:

http://groups.yahoo.com/group/franklinengines/

Some very knowledgeable Franklin folks on there...

I'm not claiming to be an expert ( we all know there are plenty of those ) , but in my opinion a 300 rpm drop is normal for a 220 hp Franklin . I have two of them and that is the way they are . As I remember , the Franklin manual says check the mag drop at 1200 and 2200 only , 250 rpm drop allowable with no more than 50 difference . I guess both of mine are right in there with that spec , but to tell the truth , I just check them at 1700 , they usually drop 300 plus , with no difference left or right , then as I Power up on the take-off roll , I look for redline of 2800 rpm and when I see that , I know the Franklin is making all the power I could ever ask for . Andrew

Thanks for everyone's input.......Two hundred sounds to high to me....Maybe I'm just over thinking this (I'm noted to do that). I will go back and use the ideas I have gleaned here. Thanks again....Brad

I'm not familiar with the 220 Franklin, but does it have 14mm spark plugs instead of 18mm? My 150 Franklin did, and it's my recollection that the larger mag drop on the Franklin was due to the smaller plugs. Russ Farris