[00Dan]

Checking my timing and formed a couple questions.

First, is there a tolerance for the mag sync? Obviously I’m aiming to get them identical, but for say just checking, what’s “close enough” to not bother adjusting?

Second, my impulse coupling seems to be snapping late when I pull the prop through by hand. It’s a 25* lag and I have the mag set at 20* BTDC. When I pull the prop through to release it before checking the timing, it’s snapping at ~10* ATDC rather than the 5* ATDC I’d expect. Is this just due to the slow speed hand pull or is the coupling suspect?

 

[Cumulo]

Checking my timing and formed a couple questions.

First, is there a tolerance for the mag sync? Obviously I’m aiming to get them identical, but for say just checking, what’s “close enough” to not bother adjusting?

Dunno. Whats the book say? As a matter of pride, I like to be spot on to a TDC reference (that is likely +-1 or 2 degrees off).

Second, my impulse coupling seems to be snapping late when I pull the prop through by hand. It’s a 25* lag and I have the mag set at 20* BTDC. When I pull the prop through to release it before checking the timing, it’s snapping at ~10* ATDC rather than the 5* ATDC I’d expect. Is this just due to the slow speed hand pull or is the coupling suspect?

The impulse lag is pretty much baked in by the geometry of parts within the impulse assembly. The stated lag should be evident with a slow pull through. l doubt that the lag is ever specified by dynamic measurement since very slow cranking speed is where the exact lag would be more important.
Two possibilities come to mind: wrong impulse assembly. Maybe a 30 degree assembly. Or worn.

I would want to know. If an impulse comes apart from being worn, it could wreck the engine and put the aircraft on the ground.
If it is a wrong part, but starts OK, I would live with it.

 

[jrs14855]

You need to post exact engine model to be certain. Most of the four cylinder Lycomings are timed at 25 degrees except for some of the angle valve are timed at 20 degrees. 22 degrees will definitely help with high CHT. Properly maintained and inspected Bendix mags are not going to fail if the impulse coupling is properly inspected.

 

[00Dan]

This is a slick mag with a tad over 200 hours on it, on an O-320 with 10:1 pistons. I am intentionally running 20* BTDC timing for detonation margin. The mag is dataplated to have a 25* lag angle on the impulse coupling.

 

[Desert Rat]

As far as tolerance between the two, most certified airplane POH's call for a max differential between the two mags of 50 rpm during the preflight mag check. That's where a timing split between the two would show up.

 

[jrs14855]

300 hours on 0 320 with 10-1 pistons. 22 degrees for controlling CHT until rings were seated. Now 25 degrees. No issues with engine. I have a cowl flap and plenum.Engine runs a bit lean at full throttle. I just haven't gotten around to drilling the jet.

 

[PilotjohnS]

i timed mine within a pens width on the flywheel and see nary a difference in mag drop rpm

 

[00Dan]

Pulled the impulse mag today and inspected the coupling - nothing appeared broken and there was no abnormal wear on either paw. I went ahead and pinned the mag to reinstall it and per the buzz box it fires exactly where I want it to normally.

However, if I back the prop up to reset the impulse coupling, then slowly bump the prop through, the coupling isn‘t releasing until 13* ATDC per my digital level I attached to the crush plate. I do feel resistance starting at approximately TDC commensurate with the paw catching and winding up. I’m now at a bit of a loss as to whether I should try it or pull the mag assuming something is wrong with the coupling.

 

[00Dan]

Pulled the impulse mag today and inspected the coupling - nothing appeared broken and there was no abnormal wear on either paw. I went ahead and pinned the mag to reinstall it and per the buzz box it fires exactly where I want it to normally.

However, if I back the prop up to reset the impulse coupling, then slowly bump the prop through, the coupling isn‘t releasing until 13* ATDC per my digital level I attached to the crush plate. I do feel resistance starting at approximately TDC commensurate with the paw catching and winding up. I’m now at a bit of a loss as to whether I should try it or pull the mag assuming something is wrong with the coupling.

A piece of amplifying data - I checked the base timing of the mag before I pulled it to inspect the coupling and found the timing had not drifted any from when it was first set. Only the snap point of the impulse coupling appears to have drifted.