Good Afternoon,

I recently got my hands on a 2013 s40 and have a few questions for the experts here:

Backfiring: I know this has been discussed at length on these forums and elsewhere, I've tried to read up on as much as I could but decided it would be best just to join the forums and ask --  my bike backfires on deceleration constantly, sometimes small pops, sometimes heavy, wet bangs. How concerned should I be about this? I am aware of the Idle Mix screw -- it looks like the previous owner removed the brass screw and I did try to adjust the screw and the idle speed adjuster knob via serobot's old posts kind of instructing on how to do this, however this led to a whole host of issues where the bike would sometimes cut out if I came to a stop and would have trouble starting unless I opened up the throttle (this still happens unless I set the idle via the knob higher.)

I would like to pull the plug to inspect but my supposition is that the bike runs lean which causes the backfiring -- I try to either keep my throttle slightly open when coming to a stop to reduce backfiring or just keep the idle set high to prevent the bike shutting off or backfiring -- i can tell by ear when it feels like it's about to shut off and will hold the throttle somewhat open while adjusting the screw before I get turned into a hood ornament when at a light.

Also i've noticed that if I keep the enrichener (choke) partially open at that weird mid setting the bike runs GREAT, no backfiring and way better low gear acceleration. What can I infer from this? does this mean i'm running way too lean when it's not partially out?

Sorry this is confusing, i'm just not really sure where to start with this issue or exactly what to ask -- I just know that when the choke is half out, the bike runs so much better than when it's fully pushed in. the backfiring is way less and it has way more power.

Also, the previous owner put some kind of aftermarket muffler on here, I dont know if they made any adjustments to the jets in the carb. I've not pulelled the carb to check and don't know how currently (trying to research this). I was thinking about taking it to a small shop that has a dyno and asking them to tune it based on that, does that seem practical?

if anyone has any suggestions based on this info please let me know.

up next is Tires: it looks like the stock size 140/80's are super hard to find. I want a set of Metzler 888's, I was told they have excellent grip and I'm a year round rider (this is my primary/ only vehicle) in Philadelphia so we have a variety of weather conditions. Does this seem like a good year round tire with decent grip and can I run it at 140/90 in the rear?

Finally the front pully does anyone know of a smaller front pully I could install for more torque? I'm not really interested in better cruising speed, I'm in city traffic all day (I ride all day for work essentially) and simply don't use the interstate -- I just take back roads or smaller highways when needed.

Oh, also does anyone know if there are any owners manuals or shop manuals online somewhere? or like a shop manual?

I'm a shade tree mechanic on the younger side and most of my wrench turning was on crappy hondas before I started riding -- for cars there was a youtube video you could find for basically anything you wanted to do (I replaced my first clutch this way) but for the bike it's hard to find videos. Anyone know how to remove the air box? I want to check the condition of the air filter and replace if needed.

Anyway thank you for your time and consideration,

Apprecaited.

Thank you for your insights.

does the 140/90 have a smaller contact patch than the 130/90?

Regarding the leak, I'll have to check for one. My suspicion is that if there is an exhaust leak it's probably where the previous owner put the aftermarket slip on. out of curiosity what are some of the signs that make you think about an exhaust leak? If it likes running so well with the choke half out should I just keep the idle speed knob high and turn that screw out further?

Also in general with the backfiring am I causing much damage to the engine?

Thanks again

It would be informative to know what the aftermarket muffler is. The muffler can affect performance a lot - especially if it's open (no baffles). Also, muffler other than stock would probably not fit properly as-is and would require some 'encouragement' to seal properly to the header. As Verslagen said, there might be a leak there.

Start by GENTLY shutting the header bolts at the engine. Don't even Try to get them TIGHT so you don't have to come back in A few thousand miles. Use fingers,not a closed hand on a 1/4" ratchet. Thumb on the head,fingers slowly closing on the handle.
If you're a novice,it's okay to go buy bolts and nuts and play with them. Break a few. You can learn what it Feels like when a fastener gets to its Clamping force,and what they feel like just before they give up.
That is a Bad place to twist one off.
Better to have to go back and snug it again.

If it's the breather to the air box, your float needle isn't sealing properly.  

If you have stock petcock that could be the cause of your "lean" condition.

If the previous owner was messing around with the air screw (which does almost nothing BTW) what else was he messing with?  Have you checked the current jetting?

I don't believe these bikes are as lean from the factory as others.  The previous owner of my bike had it jetted extremely rich for a stock bike.  It would hardly go 80 mph and backfired a lot on deceleration.  I went back to stock jetting and it ran 100 times better and the backfiring went away.  Do your own testing.

Surviving Philadelphia wrote on 06/07/22 at 16:48:22:


If it's a paper filter, it's supposed to be dry.
If it's wet... there's your problem.

Surviving Philadelphia wrote on 06/08/22 at 08:33:47:

there are several possibilities.
you said 'oiled' it's possible your oil level is too high.  I had a problem where if I filled the oil to the top line it would blow into the airbox but didn't get on the filter.  If I kept it midway or lower no problem.

I have noticed that if your idle speed is too low, the carb can sometimes spit gas.  And the petcock can leak gas thru the vac line or the carb can flood if the float valve sticks.

Oil shows no signs of gas -- changed the oil/filter last weekend as well, exactly 2 quarts.

regarding the wet air filter -- the filter did not smell of gas, the airbox was completely dry but I did notice some moisture on the filter (it's the stock paper filter), I'm assuming it's oil, I didnt notice any distinct smell nor did I touch the filter directly.

I assumed that the paper filter was intended to be oiled, but someone let me know this is wrong that it's supposed to be totally dry. I will pull it out again today when I have time to let it air dry see if i can figure out what's on it by touch/smell.

The other thing though -- I've been having the idle high (like really high) to keep the bike from stalling at a stop as it does sometimes when idling -- could the very high idle be causing oil mist to get to the filter somehow?

also does anyone have any suggestions for how I can set the idle correctly? like is there a phone app anyone would recommend?

Thanks for your replies everyone