Why won't my #1 cylinder fire?

bajardine

bajardine

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Mar 16, 2006
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Tremonton, UT
We were out riding last weekend and my son was on my old '99 MM700. It was acting really rich and losing power so it seemed. I checked all the plugs and the #1 cylinder was wet with fuel and oil. The other two had a good burn on them. I thought I had a bad plug so we put in a new plug. Took for a spin and it was the same way, wet with fuel/oil. I swapped wires between cylinders 1 & 2 to see if it was the coil. Still wouldn't burn. I swapped coils between cylinders 1 & 2. Still wouldn't burn. I checked for spark and there was spark but if I undestand correctly, all three coils fire at the same time, it's just a matter of where each cylinder is in its cylce as to if it ingnites the compressed air and fuel. It appears that cylinders 2 & 3 are getting their spark at the right time and that there's no spark when #1 is at TDC. What would cause this??
 
If by #1 you mean the one closest to the clutch, 2 things to check, one the PTO crank seal could be starting to go, or the stator could be starting to get weak. Mine started doing that a couple years ago, it turned out that the crank seal was bad and it was sucking all the grease out of the outside PTO bearing.
 
If none of that cures it check copression. If its way low on compression the fuel/air mix is getting blown past piston when it comes up and not much left to burn.
 
I think I ruled out plug caps because when I took the coil and wire from the #1 and moved it to the center cylinder, that cylinder burned just fine. If the stator was getting weak, why would that only affect one cylinder? I would think the whole engine would run weak.

How would you check to see if your crank was out of phase?

On the trail into where we were riding, the washboards were really bad. I wonder if something could have shaken loose. I didn't think about compression. Could a crankcase seal cause the PTO cylinder to not fire at all? I mean when I pulled the plug from that cylinder, it was literally dripping with fuel.
 
bajardine said:
It appears that cylinders 2 & 3 are getting their spark at the right time and that there's no spark when #1 is at TDC. What would cause this??
Click to expand...

when then, is #1 getting it's spark?... do this kind of analysis in a fairly dark place so there's no room for confusion... seeing spark at the right time in daylight is challenging....

I'd actually hazzard a guess that under closer inspection, #1 spark is as it should be since it being out of time but 2 & 3 being correctly timed, is a stretch (mechanically/electronically)...... Report back... tj
 
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Is it possible that you put new plug in #1 with out changeing orientation of coils/cap, test run and fouled new plug, then changed orientation of coils onto a fouled plug? For examples sake lets say cap on #1 has 7 ohm resistance lead to fouling #1 plug, move that cap to #2 and it fired that cylinder, could have ben because that cylinder was already warm! I would put new or known good plug in #1 with cap,coil,wire from #2, put cap,coil,wire from #1 on #2. If #1 still no fire its either stator not juicing #1 at correct time or its a cylinder problem.(compression,stuck ring,carb overfueling) You could spray wd40 or carb clean anything really at crank seal to see if its sucking air. If #2 doesnt fire its in cap,coil or wire. Sounds very confusing but makes sense in my head!
 
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Clean the carbs and make sure the float is not stuck... or the inlet needle is not stuck with grime. And take a look at the reeds as long as you are in there.
 
It's important take your time and try one thing at a time. Narrow the suspect down. If you have no spark, then look at all your electrical one at a time. If it has spark but weak also electrical. if it does have spark but has a fouled plug, then it's a fuel issue, meaning either carb or crank seal. Like I said with the crank seal, even though you would think it would make it lean, when it first starts to go it takes all the grease out of the outer bearing, and can foul the plug. This is what happened to me.

P.S. On the older sleds think 2000 and older the coils are in series, making the #1 coil the last in the line, that's why I said your stator might be getting weak, since the coil on the end of the line would getting the lowest mount of juice.
 
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Wow, these are a lot of things to think about. I'm going to look at running it and spraying some ether around the PTO seal to see if maybe that does anything. Then I'll check compression. I got a PM on how to check the crank phase. I'll check that next. Hopefully I'll figure it out soon before it's too late with the snow starting to melt. My 16 yr old boy is bummed cause that's the sled that he rides and we just got it back into commission about a month ago. We had a crack in the running board just behind where the trailing arm attaches that cracked the heat exchanger. We built a bracket to reinforce the whole area and got it all repaired. We cleaned the carbs since it had been sitting so I'm tempted to rule out any carb related issue. It ran fine for several hours and then progressively got worse.

When we did test it in the field, we put a new plug in #1 and after running it, it was wet, not foulded. I swapped the new plug with the #2 plug and ran it that way. It fired fine and looked good when I took it out to check. Then I put it back in #1 cylinder and did my coil swapping. So now I'll check those things I mentioned and if after all that I don't find anything, I'l suspect the magneto.

Thanks guys for input. I'll let you know what we find. My boy is always there when working on the sled. If there's nothing else he'll learn from me, it'll be how to turn wrenches on his own sled and cars...
 
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