Impulse Coupler Questions

[Frisbie]

I would like to put an impulse coupler on one of my magnetos.

Our engine is the A-65-8 that's STC'd to the 75 hp conversion and is still running strong with the Bendix-Scintilla SF4R-8 magnetos.

I start on the "Right" mag since it's 29 degrees and the other is 32. Generally starts easy first start of the day but if I need to start after it's warmed up it always much more flipping!

Any info, P/N's or ideas would be appreciated.

[waltermrich]

A fellow at Oshkosh who worked at the booth for Champion spark plugs told me the reason coil magnetos are hard starting after being run for a while is because the windings on the coil when it gets hot expands and increases resistance in them, that's why if you wait 1/2 hour or more they will start after cooling down.

[Frisbie]

I think it is as least 1/2 hr before a restart is within a few blades. I didn't know what or why it was hard to start after flying but I do now

[Scott]

An impulse coupling may not fix the problem.

The reason the A65/75 can be hard to start when hot is associated with two realities:
1. You shut the engine down with the mags, and
2. The stromberg is particularly good at adding fuel vapour when the engine is pulled through slowly.

Bottom line is the engine needs less fuel to start when hot, and there's a high probability there will be an over rich mixture in the cylinders.

So couple of things you can try to find a good hot start procedure.

1. Turn the fuel off prior to shutdown. To some degree the earlier you turn the fuel off, the leaner your hot start mixture will be.

Tempting to open the throttle... don't

2. Don't prime at all, leave the fuel off until
the engine starts.

3. pull the prop through 4 compressions in the opposite direction of rotation. This will add air, through the exhaust, without fuel to the combustion chambers.

Work on variations of 1-3 until you find a formula that works.

S

[Frisbie]

Thank you for your inputs...I use the back flip often in the restart but didn't think about doing it before the "contact". Thanks for the reminders on fuel shut off too.

[3Dreaming]

The old Bendix shoe box mags work well, but hot starts are always an issue. Freshly overhauled with magnets re magnetized work better than ones that have been around for a while. As Scott mentioned fuel mixture also plays a roll. We have been operating them on our family Taylorcraft since the mid 1980's. Cold starts are never a problem, and you can be kind of lazy propping it. Just make sure you have enough force to get it through the compression. My standard hot start procedure starts with the shut down. I will set the RPM to about 1100, then turn off the mags and leave the throttle alone. I set the brakes and tie it down unless I have a qualified pilot at the controls. I may also ask someone to stand at the door and pull the throttle back after it starts, and tell them to simply switch the mags off if something goes wrong. Hot starts with the old style mags require a swift strong pull of the propeller. You can not be lazy. Once you develop the procedure for your airplane you should be able to get it every time. If the engine fires but doesn't stay running or kicks back I always advance the throttle more and keep propping. When the mixture is right it will start. There is also a difference between metal and wood propellers. Wood requires a stronger pull due to having less inertia. Our airplane has a strong engine with a wood propeller and I will routinely have it go through two compressions when propping for a hot start. A metal prop might go through three.

Starting with mags that have impulse couplings requires a different procedure.

[Frisbie]

Thanks for your input. We have a wood prop also and a easy pull through compression is all it takes cold.