Plug Chopping

[southwester]

Hello fellow enthusiasts can anybody out there lead me to a resource on checking jetting by plug chopping or describe the process.

The reason I ask is because I bought my first trails bike Back in August and I have had a bit of bother with it.

When I first got the bike a 250 Rev 3 it had a 35 pilot jet a 150 main and needle clip on second notch from the top, it was pinking on throttle chop and leaking fuel, I had carb drilled out to stop the leaking and the shop that did it said that the 35 jet was in there to mask another problem and sent it back with a 30 in there and needle clip in the mid position, the next time I rode it the radiator thermostat packed up and it over heated exhausting coolant.

When I got home I linked the two wires together bypassing thermostat and fan was working so thought all was good, I also drained coolant at this point just to see what it looked like

and on refill squeezed hoses to vent any air out(it looked ok by the way no bits or anything)

I assume at this point the white chalky substance that turned out to be through out the waterways caused a blockage somewhere in the system and the next time I rode it over heated again, the fan was still running.

Some one where I was riding said have a look at the pump impeller and when I took cover off it revealed the inner clutch case was corroded and the impeller area was full of the white substance.

I removed head, cylinder and hoses to find this crap was throughout and the piston and cylinder didn't look that good to me, anyway after posting some pictures up in the Beta thread and then having the shop take a look they said it needed a re plate and a new piston.

Also the new plug I put in when I got carb back was white but the previous plug was the right dry chocolate brown colour.

So when I put it back together I want to check to make sure it's running ok.

Thanks in advance for reading mine and my bikes life story and any replies if you are still awake or aren't off to top yourself!

[zerorev3rev4]

couldnt of been much more wrong with it then bit of a bummer ,, plug or throttle chopping from what i remember is done at full open throttle and then engine killed quickly to give plug colour at full throttle not ideal for a rebuilt engine ,150 main jet sounds right poss on the richer side as some recomend 140 , try putting clip at the bottom position and see if it runs spluttery and rich that way you know it can get more than enough fuel then bring it back to middle and see how it runs , use same idea with idle mixture screw air screw only half out and see if it then runs rich if not go for the bigger idle jet which was fitted, in the ideal world 1 and half turns out +- half would be good also with needle in the middle position would indicate correct main jet all this is ok as long as air filter clean and not over oiled fuel flow has no dirty filters crank seals are good and plug is good (also correct heat range plug fitted) i would get a new stat as you could burn the regulator out running constant ,, the damp cold can also make them run a bit on the richer side as engine will run cooler it is an art to know if a bike is running rich a good indication is smoking and not much power pulling up a steep hill also unburnt fuel dripping from exhaust joints , is there any modifications to take into account

[dadof2]

See my reply #8 on your original post - 2 stroke tuning by Graham Bell

Do as web search and read a few different description of plug chop / check.

It is covered in the bultaco clymer manuals and honda mx manuals (and many more)

[southwester]

zero, don't jinx it lets just say I'm keeping everything crossed!

I have a new stat, what would the symptoms be of leaky crank case seals or bearing seals do you mean.

No mod's that I know of.

I was thinking of splitting cases but not confident to do it myself and the bike shop local to me wanted 8 hrs @

[dadof2]

Briefly carburation setting consist of;

Clean air filter correctly oiled. float height correct, timing correct, make sure cold start enrichment is shutting fully and bike has correct plug.

Mark your throttle twistgrip with marks at 1/4, 1/2, 3/4 throttle opening. You need a good long hill where you can keep the bike pulling steadily at the different throttle openings. Ideally you will be able to maintain the required throttle opening for 2 minutes but 20 seconds will do. Set bike with no tick over, and put clean plug in

Run the bike at the throttle opening for the required time then simultaneously close the throttle and pull the clutch in. remove plug and note colour. Medium to dark chocolate brown indicates mixture correct, paler means weaker, darker means richer.

0 to 1/4 throttle is controlled by idle jet and low speed mixture screw. 1/ to 3/4 mixture controlled by needle jet, full throttle controlled by main jet. There is some overlap between jet effects and it varies from bike to bike

[southwester]

I found this on thumper talk

seems like the most common jetting issue that comes up are pilot circuit related.the following is a sure method to choose the needed changes.

with the bike warm and idleing turn the fuel screw in till the idle drops/misses.then go back out till the idle peaks/smooths.

this should happen between 1 and 2.5 turns on a fcr carb and 1 and 3 turns on a cv.

if you end up at less than 1 turn you need a smaller pilot jet.more than 2.5 (or 3 turns on a cv) you need a bigger pilot jet.

choose the appropriate size and retest.

This post has been promoted to a wiki

[southwester]

Also have a look at this if the powers that be allow (tis for the good of the sport)

http://www.thumpertalk.com/topic/347305-is-your-two-stroke-running-rich-read-here/