FD620D (Conquest) spark plug help

[sparkola]

Ok I am riding along and everything is fine. See another guy with an Argo and stop to chat for about 15 mins. Go to start up again and Conquest starts and I hear what sounds like a rod knocking( or detonation). Rev it up a little and goes away for a sec and comes back. Limp it back to my truck and pull the rear spark plug. Center electrode is missing. Run to Farm and Fleet and pick up a couple of champion O.E. replacements and replace both( front one looked a little carboned up anyway) Ride for a little maybe 1/2 hour and start bogging again. Load her up on the truck and drive home. Tonight I pull that rear plug aagin and again the plugs center electrode is seperated from the stem and sorta like a little melted ball stuck to the arm electrode. Compression on that cylinder is 210 cold and 220 on the front. Any suggestions?

[sparkola]

Another thing I noticed is that the metal portion of the plug (where the socket grabs) looks dull and the ceramic insulation had some black rings around it that I could scrape off with a fingernail. Does this cylinder sound like its running lean( hot). Any suggestions would help.

[eightwheeler]

very common problem, i have FD Kawasaki in my Argo and 6 wheel JD Gator and they both spit plugs not sure why they just do and the one is rebuilt too??

[sparkola]

The plugs that came in it and I replaced with is Champion CJ14. Is there something better?

[maicolover]

Sounds like its running lean? What color are the plugs. I won't recommend Champion plugs! These engines like new plugs from time to time but Not from being melted or burnt! They normally foul out from running rich and idling around. Newer 620 engines have longer reach plugs BPR2ES in them. But i don't think they will work in older 620'S. So you need to be finding problem before it melts a piston! We run NGK's in ours, you could try Denso, Boosh or Autolites.

[sparkola]

Mine is a 97 and I read somewhere that it's the older shorter plug. Perhaps Rock Doctor can chime in here. Yes it is kind of odd that it would start doing this all of a sudden. Do you have a number for the appropriate short NGK plugs?

[maicolover]

Short NGK plugs are BMR2A for older engines.

[Pophamrt1]

Unless there's been some incorrect plugs installed or other changes made, I'd stick with factory recommended and find the problem instead of throwing different plug values at it. To melt plug tips and discolor the plug shanks the engine is running way, way too lean; too much air and not enough gas. To shatter a plug tip, it's detonating pretty badly, so even more lean when under a hard load or accelerating. Detonation is caused by too little fuel mixed with the air, causing a more flammable vapor that explodes prior to the spark from the plug and unfortunately when the piston is still travelling upward and not on the normal downstroke. The pre-explosion puts a lot of stress and heat on the piston as well as the plug and often ends up in breakage. Different plugs won't change it unless someone has made a radical mistake by adding something really strange. Also, while there are better and worse quality plugs, the correct size and range will all get the job done without failing, much like tires, etc.

To check for vacuum leaks that might be letting more air into the intake tract start the engine and gently spray starter fluid or get a small unlit plumber's propane torch and aim it around the manifold gaskets. If the engine accelerates, you've found a gasket leak where more air is enterring.

If there aren't any gasket leaks, the fuel delivery is suspect: Not enough. To confirm, set in the transmission in neutral, start engine, rev and hold to 1500 rpm or so. Take some cardboard and partially block the air intake. If it accelerates briefly before starting to stall, the mixture of gas/air is too lean. That is, when you cut some air off, the air/gas ratio balances out to the correct 14:1 ratio that it wants and moves the engine faster through its combustion cycles. As a note, but not likley, if it stumbles right away when you cut the air off a bit of air, its running rich. Correct 14:1 mixture will simply very gradually slow down as you choke the air intake more.

If you confirm a lean mixture, check the carb jets for blockage. It's very, very common to find that old gas has turned to varnish and is partially blocking the jets, causing lean running under load at mid to higher rpms in particular. The engine will idle fine and sound good but with some pinging sounds under load. Hope this helps.

[sparkola]

Excellent reply Pophamrt1. Would clogged jets only affect 1 cylinder however? The front one is fine. It is the rear one that is having the problem. If it is the jets in the carb, how do you go about cleaning them? Should I just replace them? Where do I get replacement parts( carb rebuild kit ) for this motor? Both plugs are carboned up pretty good indicating a slightly rich scenario for the most part.

[Pophamrt1]

The carb is a 2 barrel so yes, separate fuel delivery for the front and rear cylinders, including jets. I'd suggest running through the process outlined before starting to replace parts.

The carbon is fouling of the plugs. The FD620 likes to idle a bit rich. But it's not uncommon for there to be multiple problems... float bowl doesn't completely close at idle, so fouled plugs from too much gas there. Then off idle, up in the rpm, jet restrictions, vacuum leaks, etc., which lean it out and cause detonation. As the engines age its normal to get 2-3 or more issues and still be running "pretty good."

Search "FD620" online for parts sources, there are plenty. If the carb is the problem, the jets can be cleaned with a fine wire or needle tip. The best process is to get an overhaul kit with new seals, jets, etc., tear the carb apart, strip and clean it ultrasonically in a tank, then carefully reassemble and adjust to factory specs.

[sparkola]

I yanked the carb off and their is only one opening into the intake manifold. Also there is only one opening out of the bottom of the carb. This would make the carb a single barrel, yes? That is why I am confused as to why this is only happening to one cylinder. Is it possible that it could be an ignition problem?

[sparkola]

Hey eightwheeler. How often do you have to replace the park plugs in your FD620D? You said it was a common problem, to "spit plugs." Do experience exactly what I am experiencing with the center electrode burning up and melting yet the rest of the plug looks fine, ie. porcelain is not craked nor blistered and ground electrode looks just fine. I cleaned the carb ultrasonically and put it back together. Notice that it idles a little slower which makes it easier to get into gears, but haven't had a chance to take it out for a load test. Have you tried running cooler rated plugs?

[gimmegreens336]

Did you ever find resolution to this Spark? I am having similar issues.