The International Cessna 170 Association

I have been having a tuff time starting engine when hot. Always started easy before so this is a new problem. Still warts right up when cold. Plugs and wires are good. Someone said mags might be the problem. I have the slick 6264 mags from 1990 with 900 hours on them. Engine runs perfect when running, about 50 RPM drop either mag is all. Noticed slick doesn't have the 6264 Mag anymore. I think they use a 6364 replacement. Not sure if they have a left and right either as same number? Any ideas or thoughts before I pull mags? Thanks!

Have you had a 500 inspection on your mags? If not have it done, 900 hours and 27 years without an inspection is a long time.

Yes. I believe it was 2010. Thanks for your thoughts!

Basic stuff. You need fuel, oxygen, and fire. Lets assume your mags are good. I'll bet your engine is sucking air as it cranks. Is there enough fuel? To much fuel does not leave enough oxygen. A hot engine environment vaporizes fuel differently than cold. Did you flood the engine. Next time you do a hot start reduce any fuel you might start the engine with. Priming for example but pumping the throttle also pumps fuel into the intake. If less fuel doesn't work try more fuel.

Once you feel adjusting the fuel/oxygen does not solve your problem the only thing left is fire and that comes from the mags. As mag parts age heat effects how or if they work. You could be looking at a weak mag or two.

I have given all the different fuel stuff many tries without any luck. As I have a hard time thinking hot mags can cause this, BUT it does act as if there is no spark. It just wil not fire at all with any fuel throttle combination. So does anyone know if there is a left and right differance in a Slick 6364 mag or is it controlled by the drive gear maybe?

You state "Act as if no Spark"

Can you prove that statement? Investigative diagnosis?

While it is HOT and not starting - Disconnect a wire check for live spark. Report

Be careful as always.

Both mags are the same left and right regardless of mag type on the C-145/0300. The left mag is timed at 28 and the right mag at 26 however.

So you are pretty much suspecting the mags aren't making the required fire.

Either the mags are grounded when you don't expect they should be, ie your mag switch is not working correctly or there is a component in the mag not functioning as it should.

You say you don't understand how heat could effect the operation of the magneto. Simple the mag requires a good properly functioning condenser. This from a wiki page on the subject. I underlined the condenser function in the magneto.

• Working Principle Of Magneto Ignition System
  
  1. The working principle of the this ignition System is similar to the working principle of coil or battery ignition system except that in it magneto is used to produce energy but not the battery. Here are the following scenarios that occur in it.
  2. When engine in the system starts it help magneto to rotate and thereby producing the energy in the form of high voltage.
  3. The one end of the magneto is grounded through contact breaker and ignition capacitor is connected to it parallel.
  4. The contact breaker is regulated by the cam and when the breaker is open, current flows through the condenser and charges it.
  5. As the condenser is acting like a charger now, the primary current flow is reduced thereby reducing the overall magnetic field generated in the system. This increases the voltage in the condenser.
  6. This increased high voltage in the condenser will act as an EMF thereby producing the spark at the right spark plug through the distributor.
  *Note: The term condenser and capacitor are used interchangeably. Capacitor is a newer term for condenser.

And what is a capacitor? From wiki:

• A capacitor is a passive two-terminal electrical component that stores electrical energy in an electric field. The effect of a capacitor is known as capacitance. ... Most capacitors contain at least two electrical conductors often in the form of metallic plates or surfaces separated by a dielectric medium.

How does heat effect the function of a magneto you ask? As a condenser ages heat effects the function of the dielectric medium in the condenser rendering it with less or no capacity to function. When this happens you will have weakor no spark from the mag.

Sounds like you should have your mags removed and inspected and at least have the condensers replaced.

Thanks Bruce for a our you response! I talked with a mag rebuild shop yesterday and they said the 6200 series mags need a inspection and Bearing change at 1000 hours. Also they said a command problem with slick mags is worn contacts resulting in hard starting due to weak spark. I will pull the mags and either have them rebuilt or exchanged for rebuild ones. Might have to wait a few weeks as I leave on vacation. I will let you all know how it works out.

Good deal TW. Of course as the mag shop pointed out, if the mag points are worn out of tolerance, that would also effect the mags ability to produce a strong spark. A corrent point timing allowing the condenser to charge well enough.

Often when discussing issues we tend to pass over the simple stuff as being obvious such as the points need to be in serviceable condition and adjusted correctly.

As you mentioned heat being the difference we jump right to what effect heat has. In this case if the points are at the limit or out and a condenser is not at top notch and the heat pushes past the point the spark produced is strong enough, your engine won't start.

Another common failure of "hot mags" is the coil within the mag. As the mag is heated-up the coil changes physical size (expansion) and coil windings can separate making starting difficult...(but possibly still "run" as the voltage is increased after start by increased RPM.) As the mag cools the windings re-connect and a cold start is easier. In severe cases the coil will fail to operate during ordinary running and is much simpler to diagnose.

Another possibility is your starter system. You did not state which type starter you have... but if you have a converted starter such as one of the lightweights which also involves a key-start rather than the original pull-start.... you can have an ignition problem related to that ignition/start-switch coupled with a failure to properly enable both mags.

Lastly, although much rarer, is the impulse couplings (some of which are subject to a 500 hr inspection AD).... if your impulse couplings are not engaging when hot (rare) then your spark will not only occur too early (usually resulting in back-running and sometimes starter-damage) ...but the spark will also be weak and result in hard-starting.

Thanks to all of you and for all your ideas to help. Aircraft Spruce confirmed that the slick 6364 mag is the direct replacement for my old 6264 mags. They will take my old mags for cores so final price will be $717 each for rebuilt new ones. I Can not get My old ones overhauled for that. I placed order for them today and went out to airplane an pulled my old ones off. Hope my IA will get new ones on before I get back from Alaska visit!

170tw wrote:Thanks to all of you and for all your ideas to help. Aircraft Spruce confirmed that the slick 6364 mag is the direct replacement for my old 6264 mags. They will take my old mags for cores so final price will be $717 each for rebuilt new ones. I Can not get My old ones overhauled for that. I placed order for them today and went out to airplane an pulled my old ones off. Hope my IA will get new ones on before I get back from Alaska visit!

Now's the time to confirm what impulse couplings you have (riveted or pinned) and don't forget to keep/inspect your gears.

Hi everyone!
My IA got the mags on while I was gone to Alaska and I was able to try them out today. They worked perfect! Got it really warmed up and flew it in this very warm WX in the Chicago area. Landed and let it sit in the hot sun much like the days it would not start, but it started right up like the old days. Bottom line, replacing those Mags made all the difference! We are back in business! Thanks for all your ideas and help!