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TT FCR tuning help please

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A few weeks back I bought and installed a Thumper Talk 39mm FCR kit for my DRZ400SM. I played around with jetting and needles and such for about a week and I have it pretty well dialed in. The only thing that is kinda bugging me is that it has a little bit of a surge/burble when I am keeping a constant speed. So basically, somewhere in the range of 1/8th throttle. But as soon as I open the throttle from there, it just goes with no problems.

I started with a 160 main jet, EMN needle on the 3rd clip, removed the pilot air jet, wired the AP arm. That ran fairly well, but I had a decent bog at throttle open. I played around with the AP spring, and fuel screw a bunch, but I couldn't get rid of the bog. So I ended up swapping out to a 155 main jet and the EMR needle on the 3rd clip. That pretty much got rid of the bog, but it was backfiring really bad on decel. I swapped a 48 pilot jet in, and did some tweaking on the fuel screw and AP screw and that has minimized the popping on decel and there is almost no bog at all. But I still have that little bit of surging/burble when at a constant speed.

The bike runs awesome other than the little hiccup. I could totally live with it, but it would be nice to have that smoothed out.

Any suggestions would be great. Here are my specs

3x3

Stock pipe with 5" cut out and all but one baffle removed, and opened stock tip

1500-3000 feet of elevation normally

Wired AP arm

Merge Fuel Screw

Merge AP spring (didn't seem like it was holding the arm tight, so I wired it anyways)

155 Main Jet

48 Pilot Jet

Pilot air jet removed

EMR Needle on 3rd Clip

Thanks!

*edit* I have the EMR needle in it right now!

Edited by Kup

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Not too sure what you have done to that stock pipe...Why don't go to at least a slip on..messing with stock pipe can do odd things to your jetting....

Very unusual that your have gone richer in the pilot circuit and leaner on the needle..

#45 Pj should be sufficient for your set up...don't be concerned about the popping..You are not harming your engine...

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I know that the popping isn't a big deal, but it was still surging with the popping and I don't like riding a machine gun around :thumbsup:

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How many turns on the fuel screw to get peak rpm at idle with your current jetting ?

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I am right at about 1 turn to peak idle with the fuel screw. Maybe a touch more, maybe a touch less. But it does shut off with the fuel screw all the way in. I would have left the pilot jet alone, but it did help with the popping and helped a little with the surging.

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The trick is to determine if it is a lean serge or rich burble. Change something in a known direction and see if it gets better or worse. I'm kind of lost on the no air jet and 48 pilot to compensate. Why not a 45 pilot and add an air jet starting with 100.

Your description as a burble makes me think rich although you have a lean low peed needle. Try to define the throttle position where you get the burble. Your modified pipe may have something to do with the running condition.

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I am totally carb illiterate. I swapped the pilot jet because I knew that would add fuel when the throttle was closed. Maybe I will try to add back in the pilot air jet, and swap the pilot jet back down to a 45.

The surge/burble is happening when I am trying to hold a steady speed, like when I am stuck behind a car going through town, or trying to hold a constant speed on flat ground. Just barely open enough not to have the engine slow me down.

Also, I didn't realize that there isn't usually much adjustment in the AP screw before I started messing with it, and now I have no idea where it started at. But I have tried it all over the place, and it doesn't really seem to change the surge/burble.

But the burble is continuous when I am in that throttle position. Not just when i start to open it. I can make it do it for as long as I want it seems. Does the AP only squirt on throttle opening, or does it squirt as long as you have the throttle barely cracked open?

Thanks for the help guys! Hopefully I'll see how putting the air jet back in and swapping the pilot jet back over changes things tomorrow.

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I'm kind of lost on the no air jet and 48 pilot to compensate. Why not a 45 pilot and add an air jet starting with 100.

Whats the difference ?

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Kup -AP only squirts during throttle movement (and very shortly thereafter). A continuous serge is not the AP.

bill- 45/100 is kind of a basic setting. No air jet is uncalibrated. There is a range of pilot air jets PAJ to pilot fuel jet PFJ settings that will work but you can't just throw in any size PFJ and compensate with the PAJ. The whole point of removing the PAJ was the pilot circuit needed more air. But if you have to increase the PFJ, that shows the low speed circuit was lean with no PAJ. So I would start with known calibrations for both PAJ and PFJ. When 45/no PAJ works, great. But if not, then go back to calibrated parts. Just my thoughts.

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bill- 45/100 is kind of a basic setting. No air jet is uncalibrated. There is a range of pilot air jets PAJ to pilot fuel jet PFJ settings that will work but you can't just throw in any size PFJ and compensate with the PAJ. The whole point of removing the PAJ was the pilot circuit needed more air. But if you have to increase the PFJ, that shows the low speed circuit was lean with no PAJ. So I would start with known calibrations for both PAJ and PFJ. When 45/no PAJ works, great. But if not, then go back to calibrated parts. Just my thoughts.

Noble

Thanks for that, makes sense.

Seems to be a lot of threads about the FCR idle to 1/8 throttle problems.

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l'm running 40/100 at sea level and it's working well. All other settings are the same as yours.

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TT store pilot setup is no alow air jet, 45 pilot jet. Do the following to confim it is right for you:

Fuel Screw/Pilot Jet

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. Do it with the bike fully heated up.

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. Once it is set LEAVE IT ALONE.

Ralph, on the FCR-MX, the bore of the passageway is a calibration. Just the richest you can run. Any PAJ installed will requre a smaller pilot jet to get it right. So no PAJ in the TT store is richer than with a 100 installed. A lot of OEM FCRs do not run a MAJ!

EMN needle too should be good. If there is a running issue at steady throttle, you needl to know EXACTLY where in the total throttle position is it. I use tape on the grip marked in 1/8th increments of total travel and on the throttle housing, here just an indicator. If the burbles happens past the 1/4 throttle mark, it is the clip poistion, below, it is the needle straight diameter. Just be sure that you are not uing the entire diameter of the throttle, only actual travel.

To reset the AP timing, put a .8mm drill bit in the carb mouth under the slide. The slide should be clamping down on it. There should be .010" of a gap at the timing screw. To fine adjust, ride the bike. Ensure your idle speed is correct. A too low idle will make it very hard to get right. If a bog is present during normal riding (do not force a bog like grab a handful from idle, that is abnormal riding), turn the AP timing screw 1/2 turn CCW (keep notes this time) and retest. At most, two turns might be needed.

As far as the popping, you have a butchered exhaust. All bikes pop when coasting, mufflers well, muffle them. You no longer have a muffler.

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If a bog is present during normal riding (do not force a bog like grab a handful from idle, that is abnormal riding), turn the AP timing screw 1/2 turn CCW (keep notes this time) and retest. At most, two turns might be needed.

W1

Just to clarify, is the CCW direction of the AP timing screw making the squirt occur sooner or later ?

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l'm running 40/100 at sea level and it's working well. All other settings are the same as yours.

you do not have a fcr-mx.

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Awesome, thanks guys! I will work on that this weekend and let you know how it goes.

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