Lycoming IO-360 start problem.

[Rally]

Hi,

I am having a little bit a of problem that I am helping someone with. The engine is a lycoming IO-360. The fuel spider and fuel control unit were removed and rebuilt by Avstar when the oil pump AD was done recently. Increasingly the aircraft has been hard to start when cold and impossible to start when hot. At one point it was impossible to start when cold also but the problem was narrowed down to a broken wire with the bendix shower of sparks. That has since been fixed and the gap adjusted on the shower of sparks to .018. The mags are slicks, and did have some corroded contacts on the distrubutor and were cleaned up. We are sort of at a loss for what is going on. The engine will normally fire once or twice and backfire when you try to start hot. When could it will do the same but normally catch enough to start, run it for a 15 or so minutes and it can't be restarted. Fuel system? Mags? Shower of spark? The compressions are fine however the engine maybe be slightly rich when running.

Any ideas?
Thanks

 

[avbug]

The engine may be running rich, or is running rich?

Are you experiencing fouled spark plugs, and has anyone pulled a plug when having the starting problems to see? Anybody tested the plugs under pressure in a test cell? Fine wire or massive electrode?

Are you performing flooded starts, or other starting techniques?

 

[Rally]

Massive electrode. One of the plugs is fouling, the others are not so I suspect bad plug or igntion, regardless the start problems wouldn't be limited to one plug. I don't smell gas or anything like its flooded. We are using the normal start procedure. Pump, throttle cracked, mixture forward till fuel flow, mixture lean, then start. I am starting to wonder if it can't be the shower of sparks, but we can't find any troubleshooting manuals or anything for the bendix shower of sparks. The pipes are not black and the rest of the plugs are not sooted.

Mag coils getting weak when they get hot because of shorted windings?

Thanks

 

[avbug]

Are you showing higher than normal fuel flow? This indicates lower than normal fuel flow, as the fuel flow is measured as a pressure differential; resistance or a blocked injector shows as higher fuel flow, rather than what seems like the obvious lower fuel flow.

Are you starting it on one mag or both? Normally starting with shower of sparks on the left mag, to avoid kick back during start, to take advantage of the retarded timing.

If one plug is fouling it may be the plug, it may be something else; you need to determine what's fouling the plug. If you suspect a rich mixture and see glazing on the plug rather than oil fouling, then you've probably got part of the culprit. Is it a bottom plug or a top plug? Swap the plugs top to bottom in the same cylinder, see if the problem continues.

You indicated corrosion in the distributor block; what about corrosion elsewhere? Carbon tracking or other sources of shorts to ground internally mean a weak or no spark at the plug.

With or without shower of sparks the engine should start. Shower of sparks is a starting enhancement, but the engine should start without it.

How much time is on those slicks? How's the timing?

A flooded start procedure involves flooding the engine intentionally and starting it with the mixture off. As the engine is cranked, the mixture is slowly fed in and pushd to rich as the engine fires.

When you said the engine backfires, do you mean it's firing through the carburetor? If it's firing through the exhuast, then it's actually afterfiring. Check your mag timing and your shower of sparks.

 

[Rally]

The mag timing is spot on. Let me say that the engine normally fine cold. However when hot it just pops occasionally and has a LOUD backfire occasionally. Not sure where the backfire is coming from, intake or exhaust. I know it will start without the SOS, as last time we had a problem we just disconnected it and it started so so with just the mags. We haven't tried that again since we thought we found the original problem with the SOS (a broken wire). Can the SOS of sparks working be causing the engine not to fire? When I say we had the SOS disconnected and it started I mean we had both mags online whereas normally its only the left. Makes me wondering if the left mag is crap, IE the coil is shorting and can't handle the extra juice from the SOS barely cold and then when it heats up the short/resistance is worse and it doesn't start at all.