Sunday 7th Jun 2009
Yesterday I went to see Tim. He had the engine all back together after getting the cam back from Kelford cams.
We put the engine back in the bike. The new carbs fitted in perfectly well. It was quite surprising.
Just as we were starting it up we ran into an electrical problem. It was very difficult to track it down and lead us on a 1 hour goose chase, but Tim discovered that the positive battery wire was not quite making good contact where I had attached it to the battery. D'oh!
The bike seemed to be running quite lean at low revs and Tim later discovered that this was due to the vacuum petrol tap hose (not present on XT but was part of GPX configuration) just loose and letting in fresh air downstream of the LHS carby. This was easily fixed by bunging it up.
The bike now seems to grunt quite well at low revs and low throttle, but the jetting just strangles it anywhere else. I need to make some jetting and needle changes.
Just for the record, the standard Keihin 30mm CV carbies (of a GPX250, Model No. CVK EA24 V592) are jetted accordingly:
Round type main jets 108
Pilot jet 38
Emulsion tube, No Numbers
Needle N16I (Quite a weird needle in my opinion as it tapers of very sharply in the last one fifth of the throttle as the CV carbs open right up.)
Mixture screw,.. I haven't checked it yet but it has been wound in 2 full turns from when I first got it from the wreckers.
Note: The CV carb on the original YDIS setup has a 32mm ID at the intake to the head. The mechanical throttle slide carby on the original YDIS setup has a 30mm ID at the intake to the head. However, both of these carbies have a smaller oval shape to them at the actual throttle slide.
The YDIS mechanical slide carb has:
Round type main jet 108
Pilot Jet 48
Needle 5D90
Emulsion tube V00
The YDIS CV carb has:
Main jet 96 (not 95 or 98,.. I double checked with magnifying glass as 96 is an odd size)
Pilot Jet: there is none
Needle 5Y15
Emulsion tube 90
So I am immediately tempted to try the new GPX250 carbs with:
main jet 108
pilot jet 48
Emulsion tube, not sure
Needle: something similar in profile to the 5D90
I might put the bike on a dyno with an exhaust CO meter to help get the jetting right.
More to come once the jetting is sorted.
We spent about 5 hours on the bike on Saturday (11am - 4pm)
On Sunday we went to the Airshow and worked on bike for 3 hours. (approx 1pm - 4pm)
Yesterday I went to see Tim. He had the engine all back together after getting the cam back from Kelford cams.
We put the engine back in the bike. The new carbs fitted in perfectly well. It was quite surprising.
Just as we were starting it up we ran into an electrical problem. It was very difficult to track it down and lead us on a 1 hour goose chase, but Tim discovered that the positive battery wire was not quite making good contact where I had attached it to the battery. D'oh!
The bike seemed to be running quite lean at low revs and Tim later discovered that this was due to the vacuum petrol tap hose (not present on XT but was part of GPX configuration) just loose and letting in fresh air downstream of the LHS carby. This was easily fixed by bunging it up.
The bike now seems to grunt quite well at low revs and low throttle, but the jetting just strangles it anywhere else. I need to make some jetting and needle changes.
Just for the record, the standard Keihin 30mm CV carbies (of a GPX250, Model No. CVK EA24 V592) are jetted accordingly:
Round type main jets 108
Pilot jet 38
Emulsion tube, No Numbers
Needle N16I (Quite a weird needle in my opinion as it tapers of very sharply in the last one fifth of the throttle as the CV carbs open right up.)
Mixture screw,.. I haven't checked it yet but it has been wound in 2 full turns from when I first got it from the wreckers.
Note: The CV carb on the original YDIS setup has a 32mm ID at the intake to the head. The mechanical throttle slide carby on the original YDIS setup has a 30mm ID at the intake to the head. However, both of these carbies have a smaller oval shape to them at the actual throttle slide.
The YDIS mechanical slide carb has:
Round type main jet 108
Pilot Jet 48
Needle 5D90
Emulsion tube V00
The YDIS CV carb has:
Main jet 96 (not 95 or 98,.. I double checked with magnifying glass as 96 is an odd size)
Pilot Jet: there is none
Needle 5Y15
Emulsion tube 90
So I am immediately tempted to try the new GPX250 carbs with:
main jet 108
pilot jet 48
Emulsion tube, not sure
Needle: something similar in profile to the 5D90
I might put the bike on a dyno with an exhaust CO meter to help get the jetting right.
More to come once the jetting is sorted.
We spent about 5 hours on the bike on Saturday (11am - 4pm)
On Sunday we went to the Airshow and worked on bike for 3 hours. (approx 1pm - 4pm)
