Lightspeed timing

[ge9a]

I have a Lightspeed Plasma II (right side) with a Slick mag (impulse coupled, left side) on an IO-320. When timing with a strobe light, the manual shows two specs - one with the manifold pressure connected, the other with the manifold pressure disconnected. My question is do I need to plug (block) the connection at the engine side when the manifold pressure line is disconnected or will the engine run smoothly (at idle) with the line disconnected?

As always, thanks in advance for direction/explanation on this.

[gasman]

Just purchase a brass cap at the hardware store and an O ring that just fits inside it and install hand tight. The O ring will make up the difference between 37deg fitting and 45 deg cap, for this application only....

Or you can wait and order one from Spruce.

[rvdave]

The engine will run smoothly, you’re only checking the electronic’s response to no manifold pressure for a change.

[ge9a]

I am thinking the engine will pull in a lot of extra air and run very lean when the manifold pressure line is disconnected. Is this true, or not enough to be concerned about while checking the timing?

[DanH]

Originally Posted by ge9a

I am thinking the engine will pull in a lot of extra air and run very lean when the manifold pressure line is disconnected. Is this true, or not enough to be concerned about while checking the timing?

Typically we tap MP by screwing a restricted fitting into one of the unused cylinder primer ports. The restrictor (1) serves to minimize the intake leak if the MP line becomes disconnected, and (2) helps smooth the oscillating port pressure.

Will the engine run badly with the MP line disconnected? Depends on the size of the restrictor. A large one might make idle a bit lumpy. and would show up in the EGT display as a lean cylinder in cruise. In the context of checking a Lightspeed's timing, no problem. Even if it does run a little rough, it doesn't change the timing.

[Toobuilder]

The need to cap the port off or not depends on the effect on idle speed. If it does not change enough to trigger a timing shift in the Lightspeed, then it does not matter if it runs rough, lean, or otherwise. You are simply looking for the same RPM based timing that you will see in normal ops.

[ge9a]

So when I'm checking the Lightspeed timing, should both specifications (first one with manifold pressure line connected, second one with it disconnected) be checked at the same RPM, say idle ~800 RPM? I'm thinking I should check the timing with the manifold line connected at idle (adjust for ~800 rpm), then when the manifold pressure line is disconnected, I need to leave the throttle adjustment alone and the RPM is the RPM when the manifold pressure line is disconnected. That's why I'm asking if the manifold pressure connection on the engine needs to be plugged/blocked.

Am I over-thinking this?

Greg

[gasman]

Idle rpm is anything smooth and as low as possible. I have my idle set at about 450 at the stop for landing, but manually idle it at about 550-600.

If your idle is set at 800 rpm and you have a FP prop, you may have a hard time landing in a reasonable distance (float).

As you can see, the rpm really doesn't matter for checking the timing with and without the line connected. I would use 600... it may change 25 or so with the line disconnected but the lightspeed doesn't care during this test.

If you are worried about changes, just cap it off. If you don't want to buy a cap, tape a thumb tack over the fitting...... it's only there for 10minutes.

A snip from the manual......

[ge9a]

I have a constant speed prop, FWIW.

I checked my Lightspeed timing with a timing light and with the manifold pressure line connected, it appears to be around 34 to 35 deg. (clearly out of spec relative to what is stated in the manual of 38 to 42 deg). With the manifold pressure line disconnected, the timing is 19 deg, maybe 19.5 deg., so within the specified range 19 to 23 deg. with the manifold pressure line disconnected. Both timing checks were deliberately at ~800 rpm (and at 60’ MSL), which may have been a mistake on my part.

So with the timing needing to be adjusted, and having the (old style) direct crank sensor, how are people adjusting the timing without removing the prop to get to the sensor adjustment screws? Or do I just need to bite the bullet and remove the prop?

[gasman]

The bad news.... you need to remove the prop to adjust the timing!!..

The good news.... your timing W/O mp is fine. And the high numbers with mp is out of the range set by lightspeed...but, it's on the good side. I would be happy with that and just fly on.

Any adjustment to get back into spec will need the brain box sent in.