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Old 11-29-2012, 06:18 AM   #596
sakurama OP
on an endless build
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Joined: Oct 2004
Location: NYC & PDX
Oddometer: 1,493
So the choke...

Well, there seems to be good news, bad news and no news which isn't necessarily good news. I'll explain.

Last night I made it to the shop after work and Chris and I sat down with his MX-FCR from his KTM 450EXC and my FCR downdraft. The idea was to examine each one to try to understand the choke (which is on the MX) and to see if the same circuitry exists on the downdraft so it could be "enabled". Here's a shot of the top of the MX carb's circuit with the choke plunger removed:

So the FCR actually has two main parts in the body. The main body holds the slide and the second part is the interface between the float bowl and the upper body. If you remove that part you can see the series of gaskets and o-rings that route the gas to the various passages. Here's the top of the lower part of the MX:

So the downdraft carb has the boss for a choke and it has the castings for the passages. The two parts are held together with security torx screws on the MX carb but are epoxied together on the downdraft. We spent a while trying to get the two parts apart and it's apparent that they don't want you to take the two apart.

Even the torch and a lead mallet weren't getting them apart. We could have persisted but there was the worry that if we did get them apart and they had used epoxy to separate the various ports and passages it would have been a nightmare to re-epoxy that all back together. The goal was to see if the passages had been drilled in the downdraft so that you could simply drill the top and bottom ports and then tap and install the choke. Since we couldn't get them apart we don't know but since neither top nor bottom was machined on the downdraft we can assume they weren't. Also, since the carb is epoxied together it is effectively impossible to retrofit the choke for anything close to a reasonable cost.

But it's not all bad news. The good news is that the actual casting has the potential for a choke. The bad news is that unless it was drilled by the Keihin factory before it was epoxied it would a nightmare to try to retrofit. The good news is that the velocity stacks project got Chris to be a Sudco distributer so he has contact with their tech department. He's going to ask if it might be possible to special order downdraft bodies from Japan with the machining operations for a choke made. If that was done he could customize his kit through Sudco to include a choke. So, it's possible. Just not for those of us who already have them.

Once we hear back we'll let you know.

So, that's the choke news.

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