The International Cessna 170 Association

I can't seem to find on the forums anywhere which mag fires which plugs? I did find that one fires the top and the other fires the bottom, but which is which? The TCDS didn't seem to indicate it either. I am installing the Slick ignition upgrade. I noticed when I removed the old set-up, the harnesses were ran like most other installations I am familiar with. One mag fires the top on one side and the bottom on the oter side. Apparently, that is not correct for this installation, so what is the correct installation?

Any help appreciated!

P.S.

This is a rhetorical question, but, are the fuel lines running to and from the engine driven fuel pump supposed to be copper?

Here's what AMT magazine has to say about Slick ignition wire routing: http://www.amtonline.com/publication/ar ... &pageNum=6

The Slick ignition harness spark plug nuts are permanently labeled for position on the engine, e.g., the nut marked "T1" routes to the top spark plug in cylinder number one.

Frequently, the installing mechanic will use the previous harness as a guide to install the replacement harness and finds that the new harness does not fit or route like the one removed. The common assumption is that the new harness was manufactured incorrectly.

But, are the old and new harnesses identical in the way that they are laid out for routing? Engine and airframe manufacturers have specified different routing schemes to address rough running, cable routing, or other issues. In fact, Lycoming issued Service Instruction 1294 to detail alternate ignition cable routing for their four cylinder engines. Ultimately, Slick manufactures ignition harnesses to the engine manufacturer's specifications, so the engine OEM has determined the best lead lengths and routing for the leads. The bottom line: Route the replacement Slick harness as it is marked for left and right location and spark plug placement.

Also, be careful of using the cylinder numbers embossed on the case of the engine for lead placement. Sometimes the baffling can obscure an embossed number causing the ignition lead to be installed on the wrong cylinder. The TCM O-200 is a classic example of an installation that suffers a disproportionately high number of problems due to the placement of the embossed number on the engine case

This should not be confusing, especially on this engine.

The LEFT magneto fires the LOWER plugs, and is timed at 28 degrees BTDC.

The RIGHT magneto fires the TOP plugs, and is timed at 26 degrees BTDC.

The BRAND of magneto and/or lead makes absolutely NO DIFFERENCE.

The REASON the mags are timed differently on this engine is BECAUSE one mag fires the tops and the other fires the bottoms. Flame propagation is affected by such matters. DO NOT UTILIZE any routing scheme just because the magneto leads are so identified. DISREGARD magneto lead routing information. USE THE ENGINE mfr's specifications.

The above specifications are listed in all kinds of places, including the engine overhaul manual.

When I bought my 170 it was miswired, with each magneto firing the top plugs on one side and the bottom plugs on the other. I corrected that this year, and I think the engine ran smoother after the change. George has pointed out the most likely reason, IMHO.

Best Regards,

John