If HoBo can ask for help so can I

Some time ago Hobo was stumped on a no go N engine I like others though this is
different as he usually is the one helping. Now Im not a expert like him but I kept
things running for close to 45yrs. 6 years ago my 52 N which had run fine for a long
time no problems.One day would not go,fire gas everything was in order. We worked at
it for about 4 hrs threw no new parts at it had nothing wrong as to go in that
direction. Went through 4 batteries.Anyway it finally took and never had any
problems till today.My buddy said spark was snuffed by cyl pressure.I knew it was
the same thing again. I did replace gas soaked plugs and needle and seat float was
perfect needle was steel put rubber tipped one in. My buddy who helped me the 1st
time saw this proven in a spark plug tester in the 60s and said this was my problem
then. This is something I found on the net----- No spark under compression
I have been riding motorcycles since the late '60s and racing them since the early
'70s and have run into this situation several times,either experiencing it first
hand or seeing it on bikes of friends and I can't understand how it can be.The
situation is when a spark plug will produce a really strong spark while not in the
spark plug hole but evidently is not firing while under compression,when placed in
the head.For years I've heard guys say that this does indeed happen and,as I said,it
seems to be true sometimes,but I don't understand what compression has to do with
the ability of the spark to fire across the plug gap.A buddy just had this happen on
a Ducati GT500.Steve Chancey, Chattanooga,Tn. This guy is not talking Low
Compression as best I can tell. Anybody have any ideas I have good thumb compression
and dont suspect it anyway.Comments or like problems welcome

just makin space

I don't understand what compression has to do with the ability of the spark to fire across the plug gap.

Click to expand...

Theoretically, compression increases resistance so failure to fire is possible but doubtful when working with low compression engines that operate with less than 100 PSI.

Easy enough to prove by installing new spark plugs?

ya low compression is a given, I had a 48 that ran on 35 to 65 comp but my
understanding its pressure in the cyl.and differs from the compression. When I talked
to my buddy explained its not cyl compression.Anyway Im stumped it cranks like a
timing chain in a car thats jumped a bit no fire or pops just cranks like the key is
off. But I got fire with plugs out on the manifold blu/white spark and fuel where I need it.

No spark under compression, remember it well. Back around 1960 every full service gas station or repair shop had a gray colored spark plug cleaner/tester. Machine was about 12 x 18 inches x 18 inches tall, sat on bench.
For our old oil burning $200 clunkers, it was an every 3 month task. Stick the business end of spark plug in the rubber boot of machine & give it a shot of sand blaster. Then screw it into tester position, hook up plug wire, watch spark in the viewer. Add air pressure. When the spark quit, note reading on gauge. I don't remember exact pressure, but the gauge was also marked "good" in green & "bad" in red. If it was in red zone, time for new plugs.
IIRC we put in new plugs at 10,000 miles or 6 months, whichever came first.


Willie

normally happens with a weak coil or mag.neighbors f14 would not start by electric or crank start.pull it 10ft-voila!fire right up.think of a candle flame.no breese-big flame,more breeze?flame starts to gutter until it goes out.

4 batteries? That may have been a clue. 6V/POS GND or a 12V switch over job? If plugs were fouled, then it must be getting fuel but no hurting doing the fuel flow tests. So the next step is spark & electrical. Compression test would be last once the first two passed.

Tim *PloughNman* Daley(MI)

I'm with you willie. Back in the 50s and 60s almost ever repair shop had a sparkplug tester, we had one, as you described, yep 10000 miles was the "life of a plug" And it is true, compression will snuff out spark.

" My buddy said spark was snuffed by cyl pressure."

No, not really. You have a weak spark.


" I can't understand how it can be"

It takes about 17kv to jump a 3/16" gap & 22kv to jump ?? in the open air. Remember, it?s 14psi outside of the engine & about 90psi at a 6:1 compression ratio in the cylinders & compressed air creates electrical resistance, so you really need the 17-22kv to fire the plugs when the engine is running.

Some folks think that checking for spark means pulling a plug wire off & looking for one. Well, it's the distance the spark jumps at the plug that gives you the info you want. A blue spark at the normal plug gap of .025 outside of the engine can easily be non-existent inside the engine at 90 psi. However, using an adjustable gap spark checker opened to .25 will give you the info you need: you have sufficient voltage in the secondary circuit to overcome 90psi and fire.


If your plugs are clean and gapped correctly, the problem isn't your plugs. Nor did your engine gain compression in the past few years.

Find the cause of your weak spark.


The most likely culprits are:

1. A weak battery (charge it & check it per tip # 49. Or get it load
tested.)

2. Dirty grounds/connections (remove the cables & clean the connections)

3. Bad cables or incorrect sized cables (replace them - see tip # 41)

4. Bad ground between the starter & the block. (pull the starter away from the block & clean the mating area - see tip # 36)
75 Tips

You will need a spark that can jump the gap and ignite the fuel under the conditions in the engine. getting a weak spark outside the the engine may not ignite something that is under compression and in basically a turbulent windstorm...

If you ever run across one of those old spark plug testes ask them to fire it up... You put a spark plug in a hole and shoot compressed air across the plug tip its amassing to watch a plug fire under that condition...

BTW that N that was giving me fitzz... He picked it up got down the road 1/4 mile it knocked off. I then put a new coil, points, condenser and set the timing... Sometimes you just got to start over with know good parts... Sometimes you have mutable issues I had my doubts about the coil so stuck one on it I am not gonna chase it all over ell a coil is not gonna break him...

That;s my plan glad at this point you confirmed this un-understandle phenomenon as
if somebody told me this out of the blue I would think he was asking how deep are
your pockets.

blue white jump 1/4 inch? New 437s. Im with HoBo start from scratch at this point I got parts Ill never use anyway.

As I said in my post I done all the trouble shooting, 4 batteries took, ran 6 yrs no problem. You wont believe it unless you experience it. 6V no hei 2 batteries from Ns one 6V Army genny,and the original tractor battery.

Dunno if you saw my post on a Honda generator on tractor talk... The issue ended up being a bad spark plug... How a engine can run for 1 min and then just shut down and the spark plug being the culprit had me going... I had to think about it how many engines do I work on that are a single cylinder well not many... What was unusual this is a common complaint on those engines with no fix posted on the net... I said what the ell I have checked all the basics lets throw a spark plug at it...

To add insult to the injury I got the choke linkage off it became a hard starter...

Its always something but most problems somehow workout except in some cases, There
are 7 Ns within a stones throw away from me parked where they were drug to.