FB460V Engine Runs Only With Spark Tester Hooked Between Plug Wire and Spark Plug

[Grass Mower]

New member here. I have a 1988 John Deere SX-95 mower with a Kawasaki FB460V engine, serial no. FB460-225099 that has a curious problem that has me stumped. The engine was surging, and then, over a period of intermittent use (weeks), began running rich, with sooty exhaust and increased fuel use. I cleaned the air cleaner (not much dirt), and running it briefly with the cleaner element off didn't make a difference. I replaced the fuel lines and fuel filter and cleaned the fuel tank. The fuel tank cap has a crack in it, so there's no restriction on venting. Also tried two new spark plugs. I had removed and cleaned the carb and all its parts and was preparing to do it again when it developed a seemingly new problem. I could not start the engine, although it cranked well.

I bought an ignition spark tester and hooked it up between the spark plug wire cap and the (new) spark plug and it fired right up. There is a good spark in the 3/16" gap of the tester. Removing the spark tester and attaching the spark plug wire to the plug, it won't start (cranks well). I've gone back and forth many times and it starts well with the tester in the circuit and not at all with the spark plug wire hooked directly to the spark plug. The cap on the spark plug wire seats well on both of the new plugs I've tried. I measured the resistance between the cap spike (where it screws into the end of the spark plug wire itself) and the center electrode of the attached plug to be close to the 4000 K ohms value stamped on the side of the spark plug cap. So it seems to be that the increased resistance introduced by hooking up the ignition spark tester is somehow compensating for the problem preventing starting.

Could the problem be the ignition coil? I'm sure you all will have plenty of ideas, which I'd love to hear.

 

[bertsmobile1]

Check carefully the spark plug you are using.
Sounds like you are using the wrong plug
Your engine takes a BMR4A
The R meaning a resistor type plug

 

[Grass Mower]

Good thought Bert. I'm using a Champion CJ-8, not a RCJ-8, the latter being hard to find at retailers locally. May have to order it online.

 

[bertsmobile1]

Good thought Bert. I'm using a Champion CJ-8, not a RCJ-8, the latter being hard to find at retailers locally. May have to order it online.

A lot of the chips in the ignition modules are really sensative to plug resistance.
Kawakasi Service manual recommends

Champion-RCJ8

NGK-BMR4a

I use this site for cross checking spark plugs.
Note a lo of plugs do not have a single direct replacement but a series of them so I generally pick the one in the middle of the heat ranges.
http://www.sparkplug-crossreference.com/
In this case you check both recommended plugs and then pick an alternative plug that in in both of the cross refference lists.
Interestingly the above 2 plugs do not cross refference to each other both ways.

 

[Grass Mower]

I looked all over our little town today and finally found a NGK-BMR4a spark plug. No one had the Champion RCJ-8 or any of the three plugs they both cross referenced to. I installed the BMR4a and it started right up. Thanks Bert!

I still have the sooty exhaust, so tomorrow will clean the carb again.

 

[bertsmobile1]

Now buy a couple on line and pop them in the box with the replacement air filter & fuel filter so you will have them when needed.
Believe it or not some times the engine will only run properly on the Champs while other engines will only run properly on the NGK's, but you now have time to try both.
Please go to the effort of correctly gapping the plugs as well.
Modern chips are also a lot more sensative to plug gap than older ones were.
bigger = more resistance ( retards engine )
smaller = less resistance ( advances the engine )

 

[Grass Mower]

Good idea. I'll pick up another NGK when I go by that store again. The plug was preset at 0.028" gap, and I left it there, although my owner's manual calls for 0.030" for the recommended plug, RCJ-8. Will order an RCJ-8 online. Would advancing or retarding affect fuel usage? But I know that that can't explain the sooty exhaust I have. Still have to clean that carb again, maybe get a rebuild kit. Thanks again!

 

[bertsmobile1]

Yes it will but we are talking in the beer glass a year type territory
There is a better than average chance the engine is running a little rich and unless you end up with a build up on the plugs I would not bother about it.
A little rich is a lot better than a little lean particularly on an air cooled engine.

 

[Grass Mower]

Thanks!