Different Starting Conditions

[davej]

Hi, Have run a GP200 set up for 11000 miles with no starting issues, However recently I have changed the Piston fron a 3 ring to a 2 ring set up. I now have the following:-
1. Starting from cold I need to keep throttle fully open and will start on about 5 kicks.
2. Need to keep choke on longer till engine is hot and runs smooth.
3. Purrs along lovely when cruising.
Then
4 . Stop bike for 20 mins difficult to start in all combinations i.e. i.e. no choke, with choke, choke only, petrol off etc etc bike is firing but not catching on
5. Previously the plug colour on old set up was a light brown colour, Colour now is at ahe other end of scale dark brown (bordering black but not oily)
Thinking wether I have slight size variations with piston affecting the porting but as she runs great when cruising I am sort of dismissing this
Any observations would be appreciated.
Cheers

[Wack]

Did you just change the piston for a like for like size without measuring the bore? If that was ok,ring gap, piston fitted right way round and the inlet skirt was the same length then I'd check the carb for flooding issues although you say you have to keep the choke on longer which goes against the flooding.

[MickYork]

Sounds like the compression might be low........maybe check piston/bore/rings.

Did you check squish and leak test it ?

[coaster]

Is the crown height the same with new piston? they need to be matched to the head ideally or the compression ratio will be affected.

[davej]

Thanks for replies .
1 .004 thou piston gap
2. It was a brand new piston and rings .012 thou ring gap
3. Squish 1.2mm
4. Compression good
5. I did not physically check piston head for any profile difference as the transfer cut outs matched
6. May be unlucky that a carb problem has appeared at the same time

[hullygully]

did you de-glaze the bore prior to re-assembly with the new 2-ringed better piston as the barrel was previously '@ 1' with the 3-ringed old piston?

[davej]

did you de-glaze the bore prior to re-assembly with the new 2-ringed better piston as the barrel was previously '@ 1' with the 3-ringed old piston?

I did, lightly honed the bore