Deceleration Popping

[tbowdre]

Got a FCR MX 39mm from the TT store, jetted per recommendations (removed air jet, 155 main, 45 pilot, fuel screw at 2 turns, EMN needle 3rd clip) for my set up (BBK, OEM refurbished head, hot cams new stage 2, FMF titanium full system) and the bike starts, idles, runs awesome and with no bog.... The new carb replaces an FCR MX 41mm.

 

My issue is the deceleration popping.... I have read about the coast enrichener, or richening the pilot circuit.  I turned the fuel screw out incrementally to 3 turns and the popping is less but still present.  I am wondering if I s should try a 48 pilot jet? is it better to be rich on the fuel screw or the pilot?

 

The popping isn't terrible, not super loud its just annoying... the 41mm on there previously had the coast enrichener still installed

 

Thanks for your thoughts

 

PS:  the 41mm is in the classifieds for 150

[StukaG]

Im interested in this as well. Also, the popping on decel in my situation doesnt sound crisp, it sounds unorganized and fluffy.

[mickeydee]

Popping on decel is normal. Loud bangs that sound like gunshot is not

[tbowdre]

Popping on decel is normal. Loud bangs that sound like gunshot is not

 

Thanks for the reply.... do you think a richer pilot jet will decrease the popping since turning the fuel screw out seems to decrease it?  Any significant reason to not richen up the pilot circuit?  

[mickeydee]

Yes, a richer pilot will get rid of the popping but you'll end up with a bog when cracking the throttle

[jd4x426]

I've got two of these bikes with the fcr carb and both pop a little on heavy deceleration. But run fine otherwise as long as it's not a loud backfire and the spark plug has good color I wouldn't worry to much about it

Edited March 2, 2015 by jd4x426

[tbowdre]

Thank you

[Bluelunatic]

I found the fuel screw setting on the FCR to be really sensitive. I richened mine up and 500 miles later got stranded with a bad spark plug that wouldn't fire. Mine didn't bog, but it was so rich I fouled a plug.

[OhioYJ]

FCR with no pilot air jet = decel popping.

 

FCR with a pilot air jet tuned correctly = no decel popping, without going richer on the pilot jet.

[sdhydro]

TT fcr39mx with open exhaust and no popping= rich pilot circuit.

You can detune like OhioYJ is alwyas talking about but with an open exhaust, ttfcr39mx, there is no way to eliminate the popping because on decel and throttle close causes the exhaust to become lean and since there is no coast enrichener this causes some noise on heavy decel.

[Gsxrstuntdrz]

FCR with no pilot air jet = decel popping.

FCR with a pilot air jet tuned correctly = no decel popping, without going richer on the pilot jet.

TT fcr39mx with open exhaust and no popping= rich pilot circuit.

You can detune like OhioYJ is alwyas talking about but with an open exhaust, ttfcr39mx, there is no way to eliminate the popping because on decel and throttle close causes the exhaust to become lean and since there is no coast enrichener this causes some noise on heavy decel.

Can either of you explain why everyone recommends the pilot air jet be removed for TT FCR's but left in if its not the TT kit? Never really understood that

Just to be on topic I have the MRD and FCR w/ no PAJ and I get popping under decel every time, but it's not excessive. More of a gurgle with a couple pops thrown in than a backfire

[OhioYJ]

TT fcr39mx with open exhaust and no popping= rich pilot circuit.

You can detune like OhioYJ is alwyas talking about but with an open exhaust, ttfcr39mx, there is no way to eliminate the popping because on decel and throttle close causes the exhaust to become lean and since there is no coast enrichener this causes some noise on heavy decel.

 

It's not de-tuning. I will admit my personal bike is a slant FCR, not the MX. No popping on my open Yosh exhaust (with a very much larger than 3x3 air box hole/holes), with a 38 PJ.  I've found similar behavior on the MX carbs I've messed with for buddies. The pilot air jet is controls air into the circuit, so it very much makes since that it will be lean if you remove the jet, and just let a ton of air in.

 

Can either of you explain why everyone recommends the pilot air jet be removed for TT FCR's but left in if its not the TT kit? Never really understood that

 

Actually if you look at any of the FCR "problem" posts, it's starting to be recommended by well known members to put the pilot air jet back in when you have issues.

 

I think the real problem is just like any other forum. Information just gets repeated, right or wrong. Jetting is not a one size fits all, and takes some trial and error. You can be fairly wrong and still have a bike that runs fine. This is compounded by the fact the FCR is so much better than the factory carb, that it's already a huge gain in a de-tuned state. Every one just wants to hear install these jets and go. The fact of the matter is, any recommendation should be considered a starting point.

[sdhydro]

Why do you keep posting about the fcr39mx carbs and how yours doesnt pop on decel cause yours is tuned right of you dont even have the fcr39mx carb. The slant and mx carbs are way different. With the slant you leave the pilot air jet in. The info is misleading to all the mx carb posts you comment on.

[OhioYJ]

Why do you keep posting about the fcr39mx carbs and how yours doesnt pop on decel cause yours is tuned right of you dont even have the fcr39mx carb. The slant and mx carbs are way different. With the slant you leave the pilot air jet in. The info is misleading to all the mx carb posts you comment on.

 

So far from the MX carbs I've messed with, react the same in this regard. I get that the pilot circuit is supposed to be "improved", but it appears to me the idle circuit functions similarly.  As I mentioned it makes sense that if you don't control air into the pilot circuit it will be lean, am I wrong here?

[sdhydro]

It does control air through the empty whole where the pilot air jet used to be. With tt fcr39mx and pilot air jet installed you have no control over the pilot circuit. Adjusting the fuel screw does nothing. Thats why the pilot air jet is removed so you can now control the pilot air circuit using the fuel screw. You don't need an adjustable pilot air screw because thats the point of the fuel screw. With other fcr39mx from suduco or etc i believe the pilot air jet is left in.

[OhioYJ]

It does control air through the empty whole where the pilot air jet used to be. With tt fcr39mx and pilot air jet installed you have no control over the pilot circuit. Adjusting the fuel screw does nothing. Thats why the pilot air jet is removed so you can now control the pilot air circuit using the fuel screw. You don't need an adjustable pilot air screw because thats the point of the fuel screw. With other fcr39mx from suduco or etc i believe the pilot air jet is left in.

 

Other FCR39MX? Mine is the Sudco kit, it's not an MX carb. Interestingly enough adjusting the fuel screw did nothing on my carb either until I went much smaller than a 45 PJ. What you are describing is the exact behavior my carb exhibited.

[GVDRZ400SM]

I was thinking about installing my paj in my fcr mx .. I get a lot of decel popping. And a bad bog / miss off idle.. Will die while riding and pull the clutch in to downshift . Gonna troubleshoot it a lot on friday hopefully.

Anybody have the TT fcr with the paj installed?

Edited March 3, 2015 by GVDRZ400SM

[atarijedi]

I was thinking about installing my paj in my fcr mx .. I get a lot of decel popping. And a bad bog / miss off idle.. Will die while riding and pull the clutch in to downshift . Gonna troubleshoot it a lot on friday hopefully.

Anybody have the TT fcr with the paj installed?

 

I have the TT FCR, and wasn't aware I was supposed to remove the PAJ. I just installed it out of the box. In fact, it might have already been removed, I don't know, I didn't really look (don't know much about carbs in the first place).

 

Bike runs great though, with the exception that I get popping on hard decel (i.e. just letting the throttle go), sometimes like a machine gun, if I soft decel I only get maybe a few pops. Looking through threads now to determine how I can low the amount of popping on hard decel.

[yzf128r]

Check where your exhaust attaches to the cylinder. A small leak will cause popping.

[Thumper\\]

I would almost think 160 main would be the way to go, but I'm slow.....