M38 won't start, troubleshooting help needed;

[LoboMike]

Pulled my M38 into the garage about a week ago running, but a bit rich. Today I decided to pull the plugs and replace them with another set cleaned and regapped. M38 wouldn't start, so I put the original plugs back in and no joy.

Ensured it wasn't a fuel issue (I think) by injecting starter fluid directly into the carb. Made no difference. It turns over fine, just no evidence it's even trying to fire up.

So assuming it's ignition I replaced the coil, distrutor cap, rotor, points & condenser and no change . Just beginning to search TM but thought I'd ask for suggestions and/or advice. Thanx in advance!

[OKCM38CDN]

Make sure you ar getting voltage to distributor first then follow it back to the coil. Could have lost the capacitor in the input line... assuming a waterproof distributor.

Hope this helps...

[LoboMike]

I know how to test for current to the distributor, can I assume that testing across the +/- coil terminals with the points closed/jumped will test the capacitor? Not familiar with the capacitor in the circuit (I'll read the TM).

Where in the circuit is the capacitor?

TIA

[OKCM38CDN]

The capacitor is in the connector where the voltage is fed to the distributor. Pull the four small screws holding the connector on the distributor and the capacitor is in there. One end is the center pin of the feed connector, the other connects to the plus side of the coil. This is the readio interference capacitor in the later models. It is the same as the big radio interference module on early M-38s.

Measure from ground (-) to the input lead of the distributor (+), you should read battery voltage. Next; measure the plus (+) side of the coil; should see battery voltage there also. If not then the capacitor is more than likely open.

To replace, pull the four small screws from the input connector and the capacitor should be able to be removed carefully, as the other end feeds up to the coil.

I replaced mine during the distrubutor rebuild...

[LoboMike]

Thanx I'll be working on it tomorrow.

Mike

[LoboMike]

OK here's what I found so far. I have correct voltage out of the ignition switch to the distributor. Voltage appears correct at the positive terminal of the coil, so I'm assuming the capacitor of OK, correct?

Now how to check the coil output? I did replace the coil with a new one, and as previously report points, condensor, rotor and distributor cap are all new.
Where to go now? Any ideas about how to check coil outpu and spark to the plugs (by myself as I am alone doing this).

Thanx again for any help, as the TM generally suggests parts replacement as test for all the above, which I have done. I'm suspecting I'm overlooking something stupid.

Mike

[wesk]

Does the rotor turn when the engine is turned over?

Make sure your points are not grounded on both contacts when they are open.

With the points closed and the ignition switch on open the points manually and this should generate a spark.

[LoboMike]

Well let me start by thanking those who helped. I found two problems with my M38. One was a vacuum leak at the base of the carb. Simply tightened the two bolts to solve that issue. Then I took another set of on-hand plugs and spent an hour cleaning, gapping and installing them. M38 finally started but ran very rough and wouldn't idle. At least now I can put a timing light on it and fine tune the carb setting. Plugs really appear the culprit.

So any suggestions on how to clean carbon fouled plugs? Scraping, etc. does a very marginal job. I hate to buy new plugs because I can't clean perfectly good ones.

FWIW this M38 is 24 volt all "water proof" wiring, using 2245 plugs (even tried old 2243's but no joy).

[wesk]

Mike,
If you live near an airport go out there and talk to a mechanic about cleaning and gapping and spark checking shielded plugs. I have a small one man aircraft maintenance shop (called an FBO) here in northern Wisconsin. I service about 3 aircraft a week. Most are 6 cylinder engines which have 2 plugs per cylinder and two magnetos. So I have become quite good at cleaning and gapping shielded plugs at 12 per engine every 100 hours time in service or each year whichever comes first. We use a vibrator type cleaner to get down inside the electrode area and clean up the carbon. Then we put them in a blast cleaning cabinet then wire wheel the threads clean on both ends and then gap them and finally spark check them in the blaster cabinet which has a special setup so you can apply air pressure while making them spark.

[LoboMike]

First be aware that the carburater on my M38 is a less than two year old rebuilt from a well trusted vendor from this list. After four days of shade tree mechanics following advice I checked to see if the metering rod spring was correct and it was, however the metering diaphragm looked "funny". So I pulled it out and there were a few small pieces of the diaphragm material lying in the carb and the diaphragm is disintegrating where it was in contact with fuel. Shouldn't these last longer than <2 years?

Luckily I had parts from an old carb rebuild that came with my M38. Metering diaphragm didn't look great, but better than the one that came with the rebuilt carb. So made a swap replacement and to my joy the M38 is running great and headed for the parade/show this Saturday.

The amount of carbon build up and the rapidity with which it occurred truly had me in circles. Good side is the M38 is well tuned with new ignition parts. Who'da think such a little thing would have such effect.

Are carb parts, specifically the metering & accelerator diaphragms available for a YS? From whom?

Thanx to all for you help,

Mike

[LoboMike]

For the benefit of the group I posted pix of the "disintegrating metering diaphragm". HTH

http://home.earthlink.net/~lobomikem38/data/

Mike

[wesk]

Mike,
No need to reply. I just copied this over here from the "G".

The old but still NOS kits contain diaphram material that will not hold up to unleaded fuel today. John at Midwest Military and George at AJP have teamed up to provide brand new materials overhaul kits for both YS carbs last year. They are $55 and work very well. Sounds like you have an overhaul done with an NOS kit more than a year ago.
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