Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[jchammer123]

Has anyone heard of (I'm sure many have), or done the Scar Mod on your X?  If so, how much of a difference did it make?  I was searching for instructions and reviews and found some vague answers.  I did find this video and it helped a lot, but I'm wondering if this is all I would need to do, or is there some tuning afterwards that needs to be done?

Here's my setup:  2006 VTX 1300C, K&N AF, Vance & Hines exhaust.  What should I expect to notice if I do this mod?  And what else, if anything, would I need to do? 

Thanks for any input you guys may have!!!!

Here's the video: VTX 1300 "Scar Mod" Reloaded

[Gumbo]

I did it and don't expect a bunch of extra power or anything like that, it did help with responce and backfiring for me.

[jchammer123]

On another site someone recommended that I do the scar mod, pair valve mod, and the decapping the air box.  Sounds like a long weekend to me.

[RedDragon]

I did it and don't expect a bunch of extra power or anything like that, it did help with responce and backfiring for me.

Gumbo, yours was a rejet, not a scar mod. 

[Gumbo]

I did it and don't expect a bunch of extra power or anything like that, it did help with responce and backfiring for me.

Gumbo, yours was a rejet, not a scar mod. 

Yep rejet was done but Jason and I also did scar  yep we did 

[jchammer123]

Gumbo, did you do the pair valve removal?

[Gumbo]

Gumbo, did you do the pair valve removal?

Yes I did -- makes things a lot cleaner if you working on the bike, valves for sure.

[Gumbo]

Gumbo, did you do the pair valve removal?

If you want to do it I will be happy to give you a hand when I get back from Canada next month. 

[xcaliber]

I ordered block off plates from Big Larry and Bones. You can do the desmog without removing the par valves from the heads.

[jchammer123]

Man Gumbo, that would be awesome.  I may try to dive into it between now and then, depending on if I start to grasp all this stuff  .  I definitely like to tinker with things and I'd leave it alone, but, once I learn something can make my bike better, I can't just leave it alone.

[bluestreak]

Here's the video: VTX 1300 "Scar Mod" Reloaded
[/quote]


What this guy is showing you is a way of raising the needle on a CV type carb..
First off let me say beware of people claiming to know what they are doing...........when they can't even use the correct terminology for the part they are talking about.  The item he is altering/raising is the slide needle.  It is not a jet.  He is showing you how to raise the needle in relation to the needle jet which will richen the A/F ratio throughout the 1/8-3/4 throttle position range.  On CV carbs you have to use washers to raise the needle to the height you want because their needles don't have multiple grooves in which to insert a clip like on non-CV carbs.  On non-CV carbs you can raise the needle height just by moving the needle clip to the next higher or lower groove.  Raising the needle richens the mixture and lowering it leans in out.
The process of raising the needle is only one means of altering the fuel curve on a CV carb of which there is about seven.  If you have significantly change the breathing habits of your bike by altering the air cleaner and exhaust, it will take more than just raising the needle to make it run at its best through the entire rpm and throttle range.
Another thing to understand is that the narrator is not telling you how high to raise the needle.  This height change is extremely important. A change of .010"-.040" is typical on CV carbs.  If you need to go higher than that to get the fuel you need to make it run right in that throttle range, you need the next larger size needle jet and/or a different needle profile all together.  On a four stroke it is rare to have to change the needle jet as changing the needle alone is normally suffecient.  The needles profile (taper) determines the fuel curve for the stated throttle position range.  Raising it up higher than its original position just starts the fuel curve sooner relative to throttle position.  Raising just the needle alone above a certain point without changing other fuel circuits such as the pilot jet and main jet, will skew the fuel curve too much in the mid range so as to make it non-linear.  A small height change of up to about .030" is the most you would raise it before the entire curve gets to skewed on a CV carb.  CV carbs are very sensitive to fuel changes.  Non-CV carbs are more tolerate and it's possible to alter their needle as much as .300" before things get out of wack.  

That said, since you probably don’t understand fuel circuits enough to make it run correctly, I would get a jet kit from someone like Factory Pro for your specific mods you stated, rather than experiment.  They have spent the time and have the knowhow to develop the correct fuel curve for your specific mods.  Their jet kit will have the correctly sized pilot and main fuel jets, slide needle and maybe even air jets if they are required.  Raising the needle is just one thing you need to change to get the fuel curve correct.

[VTX-Dan]

On another site someone recommended that I do the scar mod, pair valve mod, and the decapping the air box.  Sounds like a long weekend to me.

I recommend you do the pair valve block off plates first.  This won't give you any power gains, but it will minimize or eliminate any popping or gurgling from your aftermarket pipes during decel. You might also notice a different exhaust tone while riding.

Next, I recommend the scar mod. This is slightly more involved than the pair plates, but should yield some performance gains. I also recommend getting a permanent air/fuel (a/f) mixture screw replacement or an a/f tool to make adjustments to the screw.  This will help fine-tune the carb settings after the scar mod.

Lastly, decapping the stock air box should also yield performance gains, but don't do it at the same time as the scar.  It will be easier to tune the carb if you do one at a time and tune before going to the decap.  Since you already have a more free-flowing exhaust, you may find you will need to rejet when you open up the air box with the decap.

I can help you with this, but I'm in Huntsville.  You may have other offers closer to home.
The pair plates, scar mod, and decap are all easy and inexpensive mods.  A Dremel will help with the decap.  I have one if you don't.  I also have the shims (washers) needed for the scar mod.

[VTX-Dan]

Also...
Get the pair block-off plates with the dimples.  With these, you don't actually remove the valves...just the little breather boxes (not sure of the exact terminology for those) that go o er the valves.  This makes sealing the block-off plates much easier because the valves have a gasket built in.  By leaving the valves intact, you retain that gasket.

It shouldn't take a "long weekend" to do all of these mods.  Maybe a long afternoon.  The scar takes the longest because you have to remove the tank to get to the top of the carb.

Your biggest performance gains will come with a rejet after decapping....if you chose to go that route.

[bluestreak]

On another site someone recommended that I do the scar mod, pair valve mod, and the decapping the air box.  Sounds like a long weekend to me.

I recommend you do the pair valve block off plates first.  This won't give you any power gains, but it will minimize or eliminate any popping or gurgling from your aftermarket pipes during decel. You might also notice a different exhaust tone while riding.

Next, I recommend the scar mod. This is slightly more involved than the pair plates, but should yield some performance gains. I also recommend getting a permanent air/fuel (a/f) mixture screw replacement or an a/f tool to make adjustments to the screw.  This will help fine-tune the carb settings after the scar mod.

Lastly, decapping the stock air box should also yield performance gains, but don't do it at the same time as the scar.  
It will be easier to tune the carb if you do one at a time and tune before going to the decap.   Since you already have a more free-flowing exhaust, you may find you will need to rejet when you open up the air box with the decap.

I can help you with this, but I'm in Huntsville.  You may have other offers closer to home.
The pair plates, scar mod, and decap are all easy and inexpensive mods.  A Dremel will help with the decap.  I have one if you don't.  I also have the shims (washers) needed for the scar mod.

My suggestion is to...........
Do ALL of the air mods first, then rejet it ONCE................Unless you just like the process of taking it apart and jetting.
There are jet kits out there that will get the jetting perfect on the first attempt or do it yourself by asking what is the most common jetting set up from those on the main board that have already done it so many times.

This thread is helpful.
http://www.vtxoa.com/forums/showthread.php?206510-1300-Carburator-Scar-Mod-A-F-Mix-amp-Idle-Adj-Fuel-Pump-bypass

Most of the info stated in this post is useful.  There is something I (and most tuners that understand jetting) totally disagree with.  Always jet from the bottom up.  This avoids confusion. Determine pilot jetting and adjust it FIRST then move on to the larger fuel flowing jets like the needle and then finally the main.  You jet this way because all fuel circuits are additive.  Their contributions overlap and are stacked on top of each other as velocity increases.  If you jet for the mid throttle position using the needle BEFORE you establish a pilot size, once you go back and alter the pilot you have then altered the amount of fuel controlling the mid range.  This is what confuses people so much in the jetting process.  Follow a logical jetting procedure and it MUCH easier to get right.......the first time.  There's nothing wrong with getting in the ball park first by installing what you think may be good jetting choices for your first attempt but you should always adjust and establish the pilots circuit first before moving on to the next fuel circuit which would be the needle.
Another item I read was altering the slide spring.  NEVER do that.  The spring tension is designed and balanced for the slide weight and air velocity.  NEVER alter it.  Do not shorten or elongate the spring.  Contrary to what most people think, shorting a spring makes it stiffer not softer.  The spring weight is of such a tension as to balance the weight of the slide to the velocity of air and this relationship establishes a "lift curve" for the slide which controls the A/F ratio to some extent. Cutting the OEM spring will make it stiffer, offer more resistance to slide lift and lean the A/F curve through out the the entire throttle range.  Cutting the OEM spring will not make the bike respond faster in and of itself.  If the spring was cut and the bike actually responds better or "seems to react quicker to a throttle input change", it was actually running rich to begin with at that throttle setting and needed less fuel and the engine responded accordingly (better) to the lesser fuel. Simply reducing spring tension will not make the slide respond any faster, it actually retards (delays) slide movement. To increase slide response to corrospond quicker to the venturie butterfly, which is controlled by rider input at the throttle twist grip, the slide and its spring tension must both be lightened in concert but going too far in this respect can produce eratictic slide operation at cruise conditions.  This will produce surging under fixed throttle positions.  There will always be a certain amount of response delay between the slide and the butterfly on a CV carb.  That is one of its downsides and can not be totally eliminated.  Altering the slide weight and spring tension can be done correctly to alter the overall CV performance but this technique goes far beyond the needs or experience of the average bike owner or the average air modifications done to an engine.  You may hear guys talk about the fact a jet kit they bought "came with a shorter slide spring" so they assume shorter must be better.  Don't assume the shorter spring is lighter than the OEM just because it is shorter. Don't ever cut or alter the slide spring.  
If you WANT to get confused and have jetting problems..........cut your slide spring.

[jchammer123]

First off, to VTX-Dan, thank you so much for offering your help, that's extremely kind of you!!!  If I decide to go this route I will definitely consider taking you up on that offer.  Huntsville is far, but it gives me a reason to ride, (like I need one, lol).

Second, thanks also to Bluestreak for giving me the good advice!!!  Two questions though, if I decide to go with the jet kit from Factory Pro (which I have heard good things about), then there's no reason to do the scar mod and the decapping of the air box, correct?  Also, I found this site: http://www.factorypro.com/prod_pages/prodh80.html which one exactly would you recommend I go with?