Is Boyesen Bowl on FCR 39 making my DRZ 400s Cut out?

[Rusty Craig]

Setup:

DRZ 400s with 3x3 airbox mod

Old style slant FCR 39 with EMN needle on position 3

MRD Racing exhaust with quiet insert and spark arrestor. 

 

Problem:

If I punch the gas the bike will cut out for a few seconds and then slowly recover. It is especially noticeable when I get on it hard and shift from 1st to 2nd gear. Also really bad if I climb a hill in 1st and then shift to 2nd coming off the hill. It will really cut out when I hit the throttle hard and then recover over the next several seconds.

 

Notes:

- I have done part of the Eddie mod where I zip tied the ap linkage arms together (there is a slight bit of slack so this should reduce/delay the pump shot slightly). 

- I did not grind the diaphragm stop down because I wasn't sure if I should. It has an aftermarket Boyeson pump bowl on it and I think that bowl may already move the diaphragm stop some. It may also make the shot stronger? 

- I moved the needle from 3 to position 1 (more lean) and it helped the throttle bog tremendously (almost doesn't cut out at all now during initial throttle) but the bike still cut out slightly and felt like it wasn't as smooth mid-WOT.

What I'm suspecting is that I am getting too much pump shot initially which is flooding the bike when I hit the throttle. When I made the needle more lean (from 3 to 1) I think it made it better because the overall condition was more lean and the shot didn't flood it as bad.

 

Questions:

- Could the Boyeson pump bowl be causing too much fuel to be shot and causing a rich condition on initial throttle?

- Any tips on how to move forward? The carb is such a huge pain to take off so it makes troubleshooting insanely difficult.

[ohiodrz400sm]

What size are your jets and what is your elevation?

I had the quick shot 1 on a slant and it worked fine. I didn't do the Taffy or "Eddie" mod, just made sure everything was in good working order. I used a YZ400F diaphragm, don't know if it's any different but that's what the shop had in stock.

To me, it just sounds like your jetting is too rich or the needle jet is worn out. The needle jet is the tube the needle fits in and the main jet screws into. How old and what is the origin of the carb? I'd also verify the float level is around 9mm. If it's too high it richens the mixture across-the-board

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[Rusty Craig]

11 minutes ago, ohiodrz400sm said:

What size are your jets and what is your elevation?

I had the quick shot 1 on a slant and it worked fine. I didn't do the Taffy or "Eddie" mod, just made sure everything was in good working order. I used a YZ400F diaphragm, don't know if it's any different but that's what the shop had in stock.

To me, it just sounds like your jetting is too rich or the needle jet is worn out. The needle jet is the tube the needle fits in and the main jet screws into. How old and what is the origin of the carb? I'd also verify the float level is around 9mm. If it's too high it richens the mixture across-the-board

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Jets are:

200 main air jet

160 main jet

45 pilot jet

pilot air jet removed

 

I have pulled the spark plug and it isn't showing a rich condition when clip is at the 3rd position. I had it at position 1 and it looks lean on this plug right (see image)? The carb is probably and older one but it was rebuilt by a reputable mechanic locally. He did a full rebuild and replaced all the rubber that a lot of the fcr rebuilders don't replace because they don't have access. The bike starts easier and has tons of power it's just when you crank the throttle that it will cut out for a few seconds. Maybe I should cut the zip tie on the linkage and see what that does?

[ohiodrz400sm]

You have to cut the threads off and look at the base of the porcelain to read it in a 4 stroke.

Removing the pilot air was bad advice for the FCR-MX. We have since learned from the mistakes of the past. Put a 100 pilot air jet back in.

Ask your mechanic if he replaced the needle jet. It doesn't take much wear to cause an issue.

Slowly open the throttle and listen to the engine. Does it sound rough around 1/4 throttle or like it's struggling? Maybe surging?

Zip ties are cheap, it's worth a try.

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[Rusty Craig]

8 minutes ago, ohiodrz400sm said:

You have to cut the threads off and look at the base of the porcelain to read it in a 4 stroke.

Removing the pilot air was bad advice for the FCR-MX. We have since learned from the mistakes of the past. Put a 100 pilot air jet back in.

Ask your mechanic if he replaced the needle jet. It doesn't take much wear to cause an issue.

Slowly open the throttle and listen to the engine. Does it sound rough around 1/4 throttle or like it's struggling? Maybe surging?

Zip ties are cheap, it's worth a try.

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It's not an FCR MX it's the older slant style. Do they still recommend putting the pilot air back in on those too? When you say slowly open the throttle do you mean when the bike is sitting or when driving down the road? 

[ohiodrz400sm]

I know you have a slant, but you're not supposed to remove the jet for either.

You can rev it either way. The point of doing it very slowly is to minimize the effect of the accelerator pump. It should rev up freely and smoothly. If you find a point that it seems to struggle take note of the throttle position.

 

I used to have a used slant too. It ran too rich on the bottom with a EMN so I switched to an EMP. I later replaced the needle jet and it cleaned up the response and allowed it to run correctly with the EMN. I'm in Ohio around 700 feet.

 

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[Rusty Craig]

Good to know I'll put the air jet back in. I texted the mechanic to see about the needle jet and waiting for a response. I'm at 940 ft here in TN. Will try the throttle tomorrow and see how it goes. 

[Rusty Craig]

@ohiodrz Is the Needle Jet you're referring to the same as the throttle valve? 

[ohiodrz400sm]

@ohiodrz Is the Needle Jet you're referring to the same as the throttle valve? 

Using a YZ400F only because Yamaha parts are usually cheaper. This is from Motosport.com. #17

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