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1990 Honda TRX300FW fouling plugs bad!


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  This thing is kicking my @$$! So I've replaced the carb with a new one that was given to me by the owner. Looks like a Chinese cheap model? I'm not sure. It's a kimboline brand. I can get it to run for about 10-15 minutes and it fouls the plug bad enough to where I have to change it.

 I've adjusted the valves, changed coil( had the wrong manual and thought specs were out), adjusted carb, and am now totally lost. Is it worth rebuilding the old carb and trying it again?

the compression is @ 150 dry and 170 wet. Book calls for 185 +/- 14.2.

any help would be great. I have to get this thing back to owner by oct.1 for hunting season.

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Dream Car Giveaway

Are there oil seals on the  valves?  or intake valve guide so  badly worn it's  letting  letting a  lot of oil  be  drawn  in? Compression sounds  decent , but  what shape  is the oil  control  ring  in ? The only  other  thing  beyond those  items  is easy  to  check... Is the  crank case over filled with  oil  and oil being  drawn  through the  carb from the  crank case   vent?

 Those  are the only  ways  I  can think  of that  it  can  get  oil fouled. . If it  is  just  carbon   fouled  it  has to be running super rich, and  I  can't  see it running well at  all if it  can foul the plug that  fast. Is  the  plug  the recommended one ? Have you tried one with  a  higher  heat  range?


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Is it worth getting a carb rebuild kit for the old carb and try it again? It doesn't run very well. Also could the petcock be faulty letting it dump too much fuel into the carb? I have not tried a plug in a higher heat range. I will try that though

Edited by iG00N
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Petcock wouldn't be the problem  if it was getting too  much  fuel  to the  carb .  The  floats and   float needle valve  control  the fuel  height in the   float bowl.  Most  likely  the old  carb doesn't  need a   rebuild  kit ,  just  a  good  cleaning.  Check  the  setting on the  long  needle  for the  main  jet.  Try  setting  it   down   one  notch (  move the  circlip  that  hold  it  up   one  groove so  it  sits deeper  in the   jet,  reducing  fuel  flow in the  main  jet )

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Tried moving it up one notch then two then three. They are all doing the same thing. I can run for about five minutes and it starts fouling. I noticed when I hold the throttle consistent its hunting. The old carb was set on the very top of the needle. The new one was set in the middle. I need to get some new orings for the old carb and I'm going to try that one.

The pilot circuit(the only jet that's adjustable) is for idle speed more or less right? I adjusted it to the service manual spec.

Edited by iG00N
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Yes . The  pilot  jet is   mainly  for  idle , but it  also   has   some effect  throughout the throttle range.  The service manual  spec is the  starting  point   for  the  adjustment ..    Where  it  will   end  up   is usually  within a  half turn  or so  + /-  from that  initial  setting.

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I think the oem carb should go back in. After a good cleaning and rebuild. I'm wondered if you have a week spark that's not allowing full combustion at the cylinder head.  Maybe take a look there. It seems like you've gone through it all and sounds good. But look elsewhere. Could be a weak spark.  Coil could be on the way out. 

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Is it  just carbon  fouled or is it  oily ?  Have you tried  a hotter  spark  plug?  If  it  is  oily ,  check  the  oil  level  to be  sure it  is  not too  high. Have   you  cleaned  the  air  filter? and  made  sure there  is  no  restriction in  the  air  ducts . . Did you try running it  with the  air cleaner disconnected  from the  carb?...

Beyond  repeating   those places  to  look ,  I  am  at  a  loss to  help  more.  It  doesn't  make sense that it  only runs such a short time  before fouling the  plug.


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At first I thought it could be oil getting by. I took almost a whole quart out of it to get it at the correct fill line. So it was over filled by a quart, but the plug is dry. I also cranked the engine over for one minute and the top of the piston is dry. I have a hotter plug coming in the mail. I have ran it without the filter.

i know it doesn't make sense. It's gotta be running too rich right now for sure because this evening I was messing with the pilot screw and it was burning my eyes.

Edited by iG00N
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Finally got a chance to tear into this thing again. I got a carb rebuild kit and just went ahead and rebuilt it. I needed a couple of orings and the kit wasn't much more for all of it. I also got a 115 main jet. A hotter plug and it seems to be running better. 

  I ran it for about 10-15 minutes just putting around the yard and came back and it idled fine. 

  I took the plug out and it still has some black carbon around the base of the plug but the tip looks pretty good. 

I still need to fine tune the pilot jet.

So thanks for all the help and hopefully it holds up. I'm gonna take it down the road for a good trip and I guess that will tell me if I'm saved haha.

Best regards!

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