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Mikuni TM40 Tuning Woes

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Symptoms: Smooth throttle application gets smooth engine response. Small, quick blips just off idle up to half throttle also get mostly OK response. Big whacks up to full throttle off idle get a big bog that takes a few seconds to recover from. Big whacks at higher RPMs up to full throttle get good response. 

Jets: 150 Main, 22.5 Slow/Pilot, 2 turns on mixture screw, 40 accelerator pump nozzle, 9DJY4 needle, needle clip at 3.5 position from top (3rd position with nylon shim underneath)

AP Setting: I have the pump set to squirt as soon as possible without hitting the slide. It aims right next to the needle without hitting it. Current settings have the pump finish it's stroke at 2/3 throttle.

Bike: 1998 XR650L (I know, I know, this is the DR page, but you all are more familiar with this carb than the XR guys seem to be!) Bored +2mm over with a ~9.7:1 piston, airbox snorkel out, UNI filter, stock headers with welds ground, old FMF Powercore muffler. I had the OEM carb dialed in great, so it's frustrating not being able to get the pumper to work as well as stock.

On a CV carb, I'd chalk it up as being too lean before vacuum builds to pull fuel, but with this being a pumper I'm inclined to think there is too much fuel squirting in all at once. With the AP the (probably) right size and squirting in the right direction, I'm not sure where to start with dialing it in for that aggressive throttle response we'd like to have.

Thanks!

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On ‎8‎/‎16‎/‎2017 at 7:50 AM, notminimalist said:

Symptoms: Smooth throttle application gets smooth engine response. Small, quick blips just off idle up to half throttle also get mostly OK response. Big whacks up to full throttle off idle get a big bog that takes a few seconds to recover from. Big whacks at higher RPMs up to full throttle get good response. 

Jets: 150 Main, 22.5 Slow/Pilot, 2 turns on mixture screw, 40 accelerator pump nozzle, 9DJY4 needle, needle clip at 3.5 position from top (3rd position with nylon shim underneath)

AP Setting: I have the pump set to squirt as soon as possible without hitting the slide. It aims right next to the needle without hitting it. Current settings have the pump finish it's stroke at 2/3 throttle.

Bike: 1998 XR650L (I know, I know, this is the DR page, but you all are more familiar with this carb than the XR guys seem to be!) Bored +2mm over with a ~9.7:1 piston, airbox snorkel out, UNI filter, stock headers with welds ground, old FMF Powercore muffler. I had the OEM carb dialed in great, so it's frustrating not being able to get the pumper to work as well as stock.

On a CV carb, I'd chalk it up as being too lean before vacuum builds to pull fuel, but with this being a pumper I'm inclined to think there is too much fuel squirting in all at once. With the AP the (probably) right size and squirting in the right direction, I'm not sure where to start with dialing it in for that aggressive throttle response we'd like to have.

Thanks!

Contact ProCycle, they actually sell this exact carb conversion for your bike ~ jetted and ready to install.  https://procycle.us/bikepages/xr650l.html#fuelsystems

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I got a TM40 from Topham.de in germany for my XR600. 

Two jets you have not mentioned : air jet [in mouth of carb, tiny] Topham supplies it with 1.1 air jet [the TM40s sold for Harleys come with to large of a air jet] I have tried a 1.2, forget which one I left in

Needle Jet : Topham supplied it with a Y-6

A #45 starter jet came in it, choke functions as it should

My main jet : 150

pilot jet : 30 

A.P nozzle  40

Bike runs great, I went richer on pilot and main from what they recommended, Ran well with their jetting also

The sight has english translation, they offer a kit for the FMX650, its uses the air cooled 650 

I emailed with Stephen, super informative and glad to answer questions. Quick responses , germany approx. 8hrs ahead 

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