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XR 400 FCR Jetting Question

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I'm trying to dail in the jetting on my '96 XR400. It has the HRC kit, oversized header, Megalloy muffler with a spark arrestor. I am running a FCR 39mm carb. The problems I encounter are hard starting when very hot and a hesitation when at low rpm's and I nail it. Any suggestions?

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Hello xrbob.

Sorry I don't have any jetting help for you, but I am about to install a fcr on my 400, and any photos or tips of your installation would be appreciated. Perhaps then we could work through the jetting thing together. I have not seen any jetting specs or even a rough guideline for these carbs on the xr4, so I'll be stabbing in the dark at first.

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Two seperate problems.

First do the pilot jet/fuel screw setup. chances are , the pilot jet is too larger or the choke is not fully closing::

Fuel screw settings in the 'book' are recommended starting points. Every bike is different, as is the temp and altitude. Set the screw according to this method.

Gently turn the screw all the way in. Now back it out two turns. Start the bike and fully warm it up, go for a 10 minute ride. Set the idle to speed to 1,500~1,800 RPM as best you can (I know, without a tach this is tough, just set it to were it idles relatively smoothly). Once warmed, slow the idle to the lowest possible speed.

*** When turning the fuel screw, keep an accurate 'count' of the amount you are turning it and record it in case you have to reset it for some reason. Makes life easier when you can just set it from notes Vs. going through the procedure again.***

Turn the screw in until the idle becomes rough or the bike stalls.

if it stalled, open the screw about 1/4 more turn. Restart it and slowly screw it in till you can just perceive a change.

If the screw can be turned all the way in and the bike still idles perfectly and does not stall, then you need to go down a size in pilot jet.

Now very slowly, open the fuel screw till the idle is smooth. Blip the throttle, let the bike return to an idle, wait say ten seconds. Confirm it is the same smooth idle.

If the screw has to be opened more than 3 turns to get a smooth idle, you need to go up a size in pilot jet.

If you find it does not stall with the larger jet but has to be open more than three turns with the smaller pilot jet, put the larger one in and set the fuel screw at 1/2 turn.

If the idle speed increased, adjust the idle speed knob to return the bike to a real slow idle speed. You must then re-visit the fuel screw. Keep doing this till the fuel screw is opened just enough to provide a nice steady idle at the lowest possible RPM. Once this is done, increase the idle speed to the normal one for your bike, typically about 1,800 rpm, but go by the spec in your manual.

The bog may well be caused by a AP timing being off or too large a leak jet.

O-Ring Mod and AP Tuning

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