I think this was mentioned somewhere, but I can't find it. My S40 backfires sometimes when slowing down. I have heard this is normal. I would like to hear other owner's comments concerning this issue.

What is a spacer mod?

Its a common carburator modification (raises the main jet metering rod) that fixes an intermedieate 1/4 to 1/2 throttle lean fuel condition. There is info on doing this, and several other mods somewhere in this site.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       

h20skier wrote on 02/26/10 at 05:23:34:

There is no need to live with backfiring.  
As mentioned it can be caused by a loose exhaut system which allows air into the exhaust pipe, but can also be caused by the carburetor being set "too lean" in the low throttle range which is controlled by the pilot system.  Sometimes a lean midrange carb setting may also contribute which is controlled by the needle in the slide (throtle valve).

Check to make sure that your exhaust is leak free.

Begin by checking the right side of the carb.   On the right side about mid level is a small brass screw inside a small hole and it has a standard screw driver slot.  If the screw is not visible then you will need to remove the tiny brass dome that covers the hole, and usually it has a small dab of paint on it.  To do so, drill a very small hole in the little brass dome...do it carefully so the drill bit does not go through too far and damage the brass screw inside.  Insert a small metal screw into the drilled hole and turn it enough to securely grab the dome, then pull it out with pliers.  The dome shoud come out.

Check the setting on the brass screw.  Turn it to the right (clockwise)  and count the number of turns until it bottoms out.  Do this carefully; no hard turning; so as to not damage the screw or its bed.  Be accurate to within 1/4 turn.  Note the number of turns ( like 1 1/4 turns for example). Then turn the brass screw counter clockwise to 1 1/2 turns.  This will be your starting point.
With the engine very warm...like after at least a 5-10 minute ride,
you can adjust the brass screw (pilot air adjusting screw).
Turning it counterclockwise will richen the fuel/air mix in the low throttle range.  As you turn the brass screw the engine should speed up and sound better/stronger.
Continue turning CC until you get the highest rpm from the engine.  If you turn too much the engine will  begin to decrease rpm.  Find the best setting for highest rpm.
If you turn the screw CC and get to 3 turns out, and the engine is still running faster with no slow down, then you need to install the next larger sized pilot jet, and then repeat the tuning process.  The pilot jet is in the bottom of the carb and sits just off center of the main jet and is down in a deep hole.  It is also brass and has a slotted head.
Remove carefully.
DO NOT GO BEYOND 3 TURNS OUT with the brass screw since this can cause the screw to loosen and fall out while riding.  That would not be good.

** Just a note... working on a motorcycle carb is MUCH EASIER if you use a small impact driver.  You can get a cheap one from Harbor Freight tool stores for about $5-7 or from Sears Craftsman tool stores for a bit more.  Of course, the quality of the latter is much better.
This small impact driver will make the job so much easier and save you all sorts of #*&%$#*# words as the soft metal screws from the stock carb give way and get all bungled up.  This leads to drilling and playing with the "easy out' bits.  yea  ... fun...no thanks.
With a good tight leak free exhaust system, the pilot screw/jet setting is 90% responsible for the backfires.

***
If you wish to do the mod to the plastic white spacer that is on the needle, which richens the midrange of the fuel/air mixture, then you will need to remove the top of the carb.  It is usually much easier to remove the carb from the engine and then remove the top of the carb on a work bench.  You will need space to spread things out, check and clean parts, and make the jetting changes/adjustments that are necessary.
With the carb top removed, you then pull up the rubber diaphram, with slide & needle attached, carefully to insure no damage to it.
Look down inside the slide and you will see 2 tiny screw holding down a little metal plate which holds the needle assembly in position in the bottom of the slide. The tiny screws are difficult to get out undamaged, thus the need for the small impact driver.  Place the slide/diaphram, with needle pointing down, on a good stable surface that has a hole or slotted space so the needle can point down while the base of the slide is supported.  Then you can use the impact driver to remove the tiny screws.
When removed, the plate will come up and expose the needle with tiny washer, white spacer and spring.  Pay attention to the order of the pieces.  
By taking the white plastice spacer, which is basically 5mm thick, you can rub it back and forth on some sand paper or file and reduce the thickness by 1/2, then reinstall the needle assembly into the slide.
Some folks will replace the white spacer with tiny washers instead, which are 1 mm thick each and most will use 2-3 of them.
When complete, then reinstall the slide/diaphram assembly back into the carb body.

So far you have taken care of the low and midrange with the pilot and needle work, now on to the high throttle range and the main jet which controls it.  
If you feel your engine is lacking some power at wide open throttle then install a new main jet that is 1 size larger than what you have now.  The main jet is in the bottom of the carb, is right in the center and easy to access, and can be found by removing the float bowl.  It is a large round head jet.  
Things to consider:
-at wide open throttle the engine should be running smooth and powerful
-if it seems to lack power then try the next larger main jet
-if power improves and the engine is still running strong then try the next larger main jet.
-keep going until you find the main jet that reduces power and smooth running
-then you know that last jet is too large, so go back down to the next smaller jet and this is your best main jet for power in the  3/4-wide open throttle range.


These are the basics of properly tuning the stock carb on the LS650 engine for smoother and powerful running.

I would suggest that if you have not done so already, replace the stock muffler with at least a HD Dyna or Sportster muffler.  It will run better, have more power, and sound like a real motorcycle.
Of course, when changing the muffler, it is necessary to retune/jet the carb.  Change the air intake ?  Retune/jet as needed.


 Some folks prefer to go with a Supertrapp or other aftermarket muffler.  Basically, any "slip on" type muffler made for HD's will work on the LS650.  And remember that a good air-tight seal is necessary for good running.
Now go riding and have fun !

Man, that took a while.

Charon wrote on 02/27/10 at 13:01:27:


Jetting and tuning for maximum EFFICIENCY can result in an exhaust that will be within most limits

Lancer, can you store these responses somewhere? People need that stuff regularly.

  I think some of the reason is because a large carb has to be used with the large (650cc single cyl ) displacement engine. At lower RPMs a lean condition occurs because of lower vacuum. Lancer hit on it slightly when he was recommending carb sizes to use on the engine. I think someone should design a splayed intake and install two smaller carbs that will provide a lot of vacuum at lower RPMs but still big enough to give it the collective volume it needs at higher RPMs. Whatcha think Lancer?? Gene

justin_o_guy2 wrote on 03/01/10 at 11:19:37:


I ALSO POSTED IT IN THE TECH SECTION