The Cessna 120-140 Association Forum

I had noticed occasional slipping when starting but hadn't found a lot written about it. While changing the oil tank gasket we noticed that there was a decent oil leak coming from the starter, also a very common problem that most of us know about. Doing my research, I read the article on the 120 website about seal replacement and then found the wonderful FlyBaby site which has this dedicated page to starter problems:

http://www.bowersflybaby.com/tech/clutch.html#adjust

I was happy to find the paragraph about slipping. Just went out to check the adjustment. Became obvious quickly that I do in fact have a clutch problem not an adjustment problem. The outer and inner shafts are not moving together. Outer engages only about 3/16" while the inner continues the rest of the way.

A quick call to Niagara Air Parts with my credit card in hand and a new clutch with seals and a gasket are on the way! https://niagaraairparts.com/product/pull-clutch/

The outer sleeve of the shaft (I assume this is what you mean) will move until either the gear is fully engaged or the gear teeth touch but are blocked by the cam gear teeth. From there the inner spring compresses allowing the starter switch to eventually make contact. At this point, if the gear teeth did not engage, once things start moving, they will.

Slippage inside the clutch is different from the gears not engaging, which will generally make a nasty grinding sound. (This is what the adjustment is intended to prevent.)

Clutch slippage can sometimes be mitigated by changing the type of oil you use (Shell -> Phillips).

No grinding noise here, but outer sleeve only moving about 3/16". Already using Philips 20W50.

$383 shipped for a new clutch

tonycondon wrote: ↑Thu Jan 07, 2021 12:11 pm outer sleeve only moving about 3/16".

I don't think that has any bearing on the slipping. If you push in on the plunger and move the prop a little the gears should mesh and it will go in about another half inch or so. The point of the adjustment is to ensure the gears mesh before the motor starts turning.

True I was and still have trouble understanding why the engagement length could have anything to do with the clutch slipping. But it was mentioned on the flybaby site and i figured it was a good first step anyway to check the adjustment. I wasn't optimistic. The independent movement of inner/outer shafts was clearly noted on the flybaby site as a symptom of clutch problems.

New clutch will be here tomorrow.

tonycondon wrote: ↑Fri Jan 08, 2021 5:53 pm The independent movement of inner/outer shafts was clearly noted on the flybaby site as a symptom of clutch problems.

Can you point me in that direction? I'm puzzled as to what their point might be. Incorrect adjustment can result in poor engagement and grinding gears, but I don't know what it could tell you about the over running clutch, which is what slips.

The reason the inner and outer shafts move independently is so that you can continue to move the lever to engage the switch even if the gears clash and don't engage. If they don't engage, the outer plunger is blocked. Continuing to move the lever compresses a heavy spring inside the plunger. pressing the drive gear against the cam gear. As soon as the switch closes and the teeth start to move, that spring forces the gears into engagement. (There is no inner shaft, per se, just the spring.)

Here's a picture from my engine electrical lectures:
The return spring is compressed by the pin in the engine case as the gear moves into engagement. The meshing spring is heavier and is compressed by the tip of the lever if the gear cannot move.

New clutch in. Took a single evening with help from my A&P. Took us about three cycles of adjustment to get it right. I'm amazed at the difference. Spins faster!

Pictures posted on the Facebook group since this forum can't handle the size.

It's not a matter of handling the size, it's a matter of who's paying for the storage. With Facebook, the storage is paid for by selling your soul personal information to advertisers. Here it's mostly your dues. I don't believe we derive much revenue from a few Aircraft Spruce ads.

That said, there are plenty of free ways to reduce the size of images. For one, if you have images you want posted, you can email them to me and I'd be glad to post them for you or reduce them and send them back.

I seem to be having almost exactly the same issue. I tried to do an engine run today on my 140 and did not get any prop movement at all. I came home and just found that article mentioned above. I'm pretty sure the linkage has slipped. I might need to break out the credit card for a new one.

Side note: I'm due for an annual and was going to apply for a ferry permit for the 10 min flight to a nearby shop. Did I see its still possible to start it by hand? Obviously will want the starter issue resolved, but if it means getting it started for the ferry flight that might be what I have to do. I also had some avgas leak onto my hand from the primer, so will probably need to look at a new seal there.

-Michael
1946 Cessna 140

Does the starter motor turn, but not the prop?

Are you out of annual now? Getting a ferry permit is almost as complex as an annual; an IA needs to sign the plane off as airworthy for that flight. Why not have whoever is signing off on the permit just do the annual?

Yes, they are easy to hand prop, but find someone with experience to help you with it. More than a few have died trying.