What causes engines to miss?

[Johnmcc69]

I never get to work on the cool stuff...
 A 2 cylinder (John Deere/B & S riding mower,  seems like it misses at higher RPM,s.)  Is this a carb issue or a ignition issue?

 I know some of you will tell me to look somewhere else, but I've found this forum very helpful, I know a lot of you work on the full size stuff as well.

 Starts a little slower than I like..
 Cleaned & gapped plugs, had intake & "carb" off & fixed an air leak there ( a bad seal/gasket)
 
 Good clean spark off the plugs..getting fuel & I think choke is adjusted properly.

 I apologize for this question, but thought it might apply to the multi cylinder builders out there as well..

 John

[Art K]

John,
You covered all the points that I would have suggested checking, plus. Unless it's something like the coil is bad and gets problematic at higher rpm's.
Art

[crueby]

Old spark plug wires with worn or cracked insulation is a possibility too.

[Captain Jerry]

How old is the engine?  With the newer stuff that have electronic ignition and sealed carbs, there is little to adjust. May not have a coil either.  [size=78%] Are both cylinders missing?  Check the fuel filter, a common source of poor performance.  If your mower has automatic safety devices that shut off the engine if you leave the seat, there may be a box full of old fashioned open relays. A failing relay can cause this.  The ignition switch can also cause poor operation.[/size]



My Yamaha twin would run fine log enough to get me as far from the house as possible and then shut of instantly. No amount of fiddling would get it to restart until it to start until it was damn good and ready.  Sometimes 10 minutes, sometimes next day.  The immediate shutdown symptom made me think it must be electronic.  After removing one of the fuel tanks, I found a big plastic box with lots of wires.  When I took the lid off of the box, I discovered 5060 baby spiders and a 1/2 lb of spider poop along with a gang of relays. By doping out the wiring diagram, I isolated the relay that grounds the ignition when the key is turned off.  I couldn't find a replacement quickly, so I disconnected the grounding wire and wired in a push button so I could shut down when I want to.  Thar was more than a year ago and the push button is still working fine.


Jerry

[Roger B]

Does it just not fire or do you also get pops and bangs? Can you tell if it's just one cylinder missing or both?

A broken valve spring can cause missing at high rpms, but something in the ignition is most likely.

[Admiral_dk]

If it only fires a few pops or runs for short bursts - I will suggest my increasingly mistrust of sparkplugs.

If it runs nicely at lower RPM's but not a higher ones, there are other things to consider and a number off those has already been mentioned.

Best wishes

Per

[cnr6400]

I would recommend checking all wiring connections especially seat safety switch and low oil warning system wiring / contacts. These have caused a lot of trouble on various lawn equipment.

Have you changed the air filter and gas filter? These can also cause trouble. Depending on B&S model the ignition coil / trigger coil system can give trouble.

One very simple diagnostic check that is easy to overlook is to examine the running engine at night in a very dark place. I've found a few high voltage wire issues with arcing along plug wires or at plug caps this way.  Do be careful to keep hands and feet away from moving sharp bits when looking around at night.

[Ian S C]

A while back got two motors, one a Honda 5.5hp horizontal shaft, and the other a clone of the first one, we tried for ages to get a spark, then disconnected the low oil sensor, great spark.
Ended up using the clone on a saw bench for firewood, but had to take the plastic governor out of the Honda and put it ni the clone,Both motors seem to have been made in the Honda factory in China. check all the external wiring and sensors, next change the plugs.
Ian S C

[RJH]

Try new spark plugs

[RayW]

Definitely worth checking the fuel filter. Could be that enough fuel is getting through at lower revs, but you are getting fuel starvation at higher RPM.

[Johnmcc69]

Thank you VERY much gentlemen!
 Those were some great responses & got the wheels turning in my head (& the tractors...)

 The motor was running, just missing at the higher rev's. Great fuel delivery & spark. Fuel filter ok. Re-checked the plugs & closed the gap a bit, was about .04 thou. (I guess it should be around .03), cleaned them up real good. Cleaned up the wires & connections & ran it last night still missing...

 This afternoon, did a little more cleaning, just a good wiping down & tried to start it. Full choke & half throttle, fired up without hesitation. Sounds really good & am very happy how well it starts.

 I guess it was a combination of just crud, makes sense...bad connections & insulating crud...

 Thanks again for replying to my off topic post, very much appreciated!
 
 John

[BillTodd]

To make an engine fire you need two things: an explosive mixture and energy.

Energy can be supplied by either a spark or heat (with enough temperature)
.  Lets assume you have a spark ignition, so a quick test is to make a short spark gap onto the spark plug and listen for missed sparks. If that seems ok , check the plug for oiling or anything that might stop it working.

Explosive mixture is the toughest one to crack , because even if your carb is turned correctly , the mix near the plug might not be quite right to go bang on every stroke .

 We could get into scavenging of the exhaust gases cylinder turbulence (or lack of it) etc. etc. but for most purposes getting the carb tuned in will make the engine run its best.and playing with the exhaust will fix the rest..