GPR Camp: Replica Racers

Needle chart:

Referring to the needle charge, A25 is near enough the widest needle they do in the 'A' dimension.

However from the dyno results, at WOT the top end was starting to get too lean, so my thinking is changing to a narrower needle overall, but with a wider tip (B).

Something like A13 looks like a reasonable candidate, and comes in a needle kit (motoforce or tnt) so is fairly cheap and gives a few options to play around with.
A25 is 2.02mm wide at the top, 1.4mm at the bottom. Tapered section is 20mm long, non tapered 12mm.
A13 is 2mm wide at the top, 1.79mm at the bottom. Tapered section is 22mm long, non tapered 10mm.
A12 is the same as A13 but with the same taper section length as A25, so could try both to dial it in.

What do you think?

mace wrote: ↑

Tue Apr 23, 2019 7:11 am

Referring to the needle charge, A25 is near enough the widest needle they do in the 'A' dimension.

However from the dyno results, at WOT the top end was starting to get too lean, so my thinking is changing to a narrower needle overall, but with a wider tip (B).

Something like A13 looks like a reasonable candidate, and comes in a needle kit (motoforce or tnt) so is fairly cheap and gives a few options to play around with.
A25 is 2.02mm wide at the top, 1.4mm at the bottom. Tapered section is 20mm long, non tapered 12mm.
A13 is 2mm wide at the top, 1.79mm at the bottom. Tapered section is 22mm long, non tapered 10mm.
A12 is the same as A13 but with the same taper section length as A25, so could try both to dial it in.

What do you think?

At WOT the needle should be completely out of the Main Jet and not affect the mixture.

If it was lean at WOT you need to install a richer Main Jet.

If you could isolate the exact throttle position where the mixture is off with the throttle markings on the housing, you could (with the carb off) set the throttle to that position and mark the needle where it enters the Needle Jet.

Then remove the needle and compare your dimensions with what is available to get a good shot at finding the correct diameter and taper.

I’m going back to please give is more info, like the dyno charts, and a video illustrating the issue.

I agree it sounds like you need a larger main jet based on what you are saying, and what Derbi fitted in their bike compared to what you have fitted in your bike. But really before you start messing with those things you really should help us be less blind.

Also, the photo of the carb shows us a very dirty carb. Have you cleaned it? Usually when something is that dirty on the outside, it’s probably not so clean on the inside. A good thorough cleaning wouldn’t be a bad thing to do towards getting it to run properly.

toratora wrote: ↑

Tue Apr 23, 2019 8:39 am

I’m going back to please give is more info, like the dyno charts, and a video illustrating the issue.

I agree it sounds like you need a larger main jet based on what you are saying, and what Derbi fitted in their bike compared to what you have fitted in your bike. But really before you start messing with those things you really should help us be less blind.

Also, the photo of the carb shows us a very dirty carb. Have you cleaned it? Usually when something is that dirty on the outside, it’s probably not so clean on the inside. A good thorough cleaning wouldn’t be a bad thing to do towards getting it to run properly.

Hi Toratora,

I'm not being deliberately obtuse; to get the dyno charts involves an hour and a half drive and asking *really* nicely to get access to their computer when they aren't busy (they're always busy); they are barely in the digital world.
I might be over that direction tomorrow, so if they're willing to let me tinker I'll get the dyno graphs then. Unfortunately none are labelled so they'll only be semi-useful, but I have a rough idea of the configuration.

The carb photo was from way back when. It's been cleaned multiple times since then internally. Externally I've done what I can without a ultrasonic cleaner.

Hey Mace,

I didn’t take it that way. I’m merely stating that before you go crazy buying parts, and what not that we get a good look at the problem. While I love cats chasing one’s tail can become quite fruitless!

They probably have a camera phone. You can ask them to text you a photo of the charts. That’s what I post.

toratora wrote: ↑

Tue Apr 23, 2019 7:39 pm

Hey Mace,

I didn’t take it that way. I’m merely stating that before you go crazy buying parts, and what not that we get a good look at the problem. While I love cats chasing one’s tail can become quite fruitless!

They probably have a camera phone. You can ask them to text you a photo of the charts. That’s what I post.

I popped in today, but there were so many bikes in the workshop it wasn't possible to even get in the dyno booth. They've promised they'll sort me out some printouts soon.