So am I the only one?

I'm having a problem with the left mag on my O-320 RV-9. It will not start normally when cranking on the left mag. When I turn on the right mag it fires right up. (verified only the left has the impulse coupling). I am using independent switches and not a key switch to control the mags. The engine turns over as it should with the lightweight starter, compressions as they should be, fuel etc. all verified as well.

The engine runs well once started and mag checks well at idle, at runup and in flight. My Skyview indicates cranking rpm of 370 but I have no idea how accurate it might be at that low rpm.

I had this problem before when I installed a Catto propeller and solved the problem by changing out the left mag for a spare I had on hand. I subsequently changed back to a sensenich propeller and did not have a re-occurrence of the problem until yesterday after changing out the right mag which had become problematic with mag checks. After changing out the right mag the problem reappeared. What the heck!!!!

During the last year or so I sent the original left mag to the shop for evaluation and repair. The shop could not find anything wrong with it, freshened up the points etc. and returned it.

This morning I installed the original left mag and the darned thing will not start on the left mag and fires instantly when the right mag is switched on. Mag checks are all very good.

Called the mag shop and he told me that my impulse couplings were probably at the low end of the tolerance and were the problem as the starter was spinning up too fast and throwing them out. I find it a little hard to believe that both couplings are bad. One has about 300 hours and the other about 550. So what do the collective minds on here think might be the problem?

PS: Pmags are looking pretty good right now but I really don't want to do away with my mags if a solution is available. I did a search on this problem and couldn't find much. Am I the only one with a starter that spins too fast?????

This may sound like a dumb question, but have you verified that the mag switches are wired to the correct magnetos?

You're sure left and right wires are not swapped?
I'd be careful starting on a non impulse mag. One kick back can ruin your day.
edit: I see someone else beat me to it, so no I don't think it's a dumb idea when it's so easy to check.

Very baffling, but I have not heard of anyone having this problem with a Viking engine

I would guess that you do have an impulse coupling problem. With everything OFF, pull the engine through and listen for the impulse to click as each cylinder reaches TDC. No click=impulse not working. If you hear the click loud and clear the impulse is probably working ok.

Larry

There have been some dodgy point around in the last year or so, and we had exactly this same issue just a week or two back. not only were the points (200hours old) very worn (known problem but no SB's) but the coil was weak.

This coil was 1100 hours old.

it is not common but it can happen.

What we noticed was doing a mag check at idle (very lean mixture) the left mag would not fire nicely. We put it on the test rig and it did not fire all the time. Yet at higher RPM it would work, but not like it does now!

Wiring verified

Wiring and switches have been verified to be correct. What baffles me is that two mags have exactly the same problem. Very strange. Pulling the prop thru confirms the impulse is working as the click is present like it should be. Also I know the mag I removed this morning is hot when the impulse releases.

Use a crummy battery that barely turns the engine over, see if it starts on left. Maybe the mag shop is right, your starter is so fast that the impulse couplings are disconnecting! Seriously....
Pull the #1 plug on the left mag. Re-attach the ignition lead, lay the plug on a good ground. Bring the prop around just short of TDC on #1. Turn on L mag, push prop thru center, see if you can see a spark.

Mags

Is your O-320 a 150hp? Those engines are the 7.5 to 1 compression and the starters can really crank them over.
If so, your starter may be spinning it too fast. I think somewhere around 500-550rpm is where the impulse will not work for 0 degree BTDC.
I have heard of this issue before, although I can't remember what they did to solve the problem.
If it's a Sky-Tec, I would give them a call......
My previous RV-6 has issues with it's left mag and impulse coupling.
However, you said you just had that serviced so it doesn't sound like a magneto problem.

If you are going to pull the prop through with a hot mag be sure at least one spark plug on each cylinder is removed! Otherwise the engine may fire while you have your hands on the prop. Not good.

Bob I already did the pull thru test and yes I know better than to pull the prop thru with a hot mag so plugs came out. All checked out, nice fire with the plugs out, I could even give it a hard spin and the mag fired well. Still will not start on left. Bummer. I guess I should only use old batteries and maybe add some corrosion to my connections. Anybody got an old start solenoid that is arched up? Maybe use one of those too!!!

Doing some checks it is apparent that the left mag will not fire consistently below about 350 rpm the right mag will perform that low. That is why I can start on the right without an impulse coupling. I'm going to send the left mag back to the shop and have them set the e gap more carefully and make sure the rotor magnet is strong. Hopefully that will make it spark at a lower rpm. I'll also buy a new impulse coupling. That should get things working again.
Anybody have a new impulse for sale??

I'm still thinking about ditching the mag and doing the P mag thing anyway whether this fixes the problem or not.

Thanks for the ideas.

Is there any way to adjust (higher) the rpm where the impulse coupling disengages?
You could get a retard mag and Slick start for the left. I'm pretty sure it uses the retard mag as long as the starter is on.