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MXA PERFORMANCE GUIDE TO THE 2011 KTM 350SXF

December 23, 2011
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The MXA test crew doesn’t just spin a couple laps and jot down a bike test on the back of an envelope. We spend months testing the latest in motocross iron (and we race the bikes to ensure that they are pushed to the limit). As cruel as it may sound, we go all-out to find the flaws and then, we fix them.

So, if you own a 2011 KTM 350SXF, take a close look at MXA‘s comprehensive performance guide. We give you a complete breakdown of what we did to enhance our 350SXF. It’s all the information you’ll ever need for living with and loving the 350SXF?without any of the ifs, ands or buts.


MAIN POWER RELAY BOX:
KTM’s main power relay box is mounted on the subframe by the right side of the battery box. Unfortunately, its rubber slip-fit mounting tab is aimed downwards, which means that the relay box can slide off its slip-fit tab and fall down between the right side panel and the subframe. When this happens, the relay box rubs against the subframe and wears through the plastic cover on the relay box (our worn one is on the right in the above photo). If it rubs all the way through, it can cause the power to short out. This will first manifest itself as an intermittent cutout, and eventually the bike won’t start. To solve the problem, remove the main relay box from its rubber slip-fit and zip-tie it to the upper subframe rail or put a zip-tie around the relay box until it crimps the rubber flange. This keeps it from rubbing on anything.

0.50 FORK SPRINGS:

We dropped the stock 0.46 kg/mm fork springs, which are way too soft, for the 0.50 springs from the 450SXF or 0.48 springs for a mid-sized rider. With the stiffer springs in place, we were able to juggle the oil height up or down from its standard 365cc to suit track conditions. Since the 350SXF is basically a 450 in size and weight, it needs spring rates closer to a 450 than a 250. The damping in the midstroke seems light and the only real fix for this is a revalve.

CAM CHAIN TENSIONER:
KTM is the only “Big Five” motocross manufacturer to use hydraulic oil pressure to regulate cam chain slap. The downside is that until the oil pressure builds up, there is an irritating ticking that signals that the cam chain is not properly tensioned. Dirt Tricks’ $89.95 automatic KTM cam chain tensioner uses oil pressure, combined with a mechanical ratchet system, to maintain constant pressure on the timing chain (and avoid the possibility of the cam tensioner collapsing during oil pressure fluctuations after a rebuild). Contact them at www.dirttricks.com or (775) 267-6361.


5.7 KG/MM SHOCK SPRING:
The rear suspension was harsh, a little choppy and still seemed prone to G’ing out (in spite of the benefits of the new rising-rate linkage). Our solution was to swap out the stock 5.4 kg/mm shock spring for the 5.7 spring from the 2011 KTM 450SXF. The stiffer spring held the rear higher and allowed us to balance out the chassis in coordination with the stiffer fork springs. At 237-pounds the KTM 350SXF should use the same suspension setup as the 450SXF, since the weight difference between the two bikes is only five pounds.


SHOCK LINKAGE:
MXA has been testing prototype shock linkage arms from 1mm longer to 3mm longer. We liked how much the 3mm longer arms lowered the bike by 18mm, but felt that it made the initial part of the stroke too firm. The 1mm linkage didn’t lower the rear enough to give us the type of balance between the front and rear height. We are currently running a 1.25mm longer shock linkage. This lowers the rear of the bike by 6mm, improves the fore/aft balance, and offers more geometry possibilities. Although we changed the shock linkage, we weren’t doing it for suspension reasons, but instead to improve the handling. On a side note, if you still run the stock shock spring, the longer link arm could make it feel stiff enough to skip the stiffer 5.7 shock spring. Just so you know that this isn’t pie-in-the-sky testing, we conferred with KTM’s test department before we started on our linkage program, and they confirmed that that we running the same tests on their bikes.


RADIATOR VENT HOSE:
This is a minor irritant, but something that bothers us none the less. The stock silicone radiator vent hose vents directly onto the head pipe. Bad idea. When the bike overheated (in back-to-back motos) it spewed out a cloud of steam that almost choked the test riders. We installed a longer blue vent hose that dumped out under the engine.


WATER PUMP COVER:
The water pump cover can leak if the O-ring is not securely seated in the retaining groove. We knocked ours askew in a crash and had a major water leak. We fixed it by removing the cap and putting in a new O-ring. It is important to stretch the new O-ring to ensure that it fits in the groove.


OIL FILLER CAP:
If you wear boots with hinges, they can hook on the black plastic oil filler cap and unscrew it. You have three choices:
   (a)
Clip the winglets off the plastic cap.
   (b)
Switch to an aluminum oil filler cap (shown).
   (c)
Buy new boots.


EXHAUST PIPE SLIP FIT:
Every aftermarket exhaust system that we tried was good for two horsepower more from mid-on-up on the 350SXF. But it was a major hassle to install the pipes (and when we test exhaust pipes we ride with the stock pipe, ride with the aftermarket pipe, ride with the stock pipe and ride with the aftermarket pipe). When you add in multiple test pipes, you are swapping pipes an unbelievable number of times. Since the stock KTM exhaust is hard to remove and install (because it has no slip-fit in the mid-pipe), we cut our stock KTM pipe in half at the mid-pipe and constructed a slip-fit so that we could remove the stock exhaust quicker. Additionally, it made removing the shock a lot easier (sadly, the stock pipe won’t come off unles syou remove the shock from the bike).


FUEL LINE QUICK RELEASE:
We love that KTM was smart enough to put a push-button quick-release fitting on the fuel lines. Other bikes use crimp fittings that are difficult to remove on a bike with 50 psi of fuel pressure. Be very careful when disconnecting the fuel lines on any EFI bike, because even one small piece of grit could clog the 60-micron injectors. Whenever we remove the gas tank, we plug the fuel lines to keep them clean. For 2012 KTM added inline fuel filters to the fuel line above the qucik release (these little filters will fit in the 2011 model).


REPROGRAM THE IGNITION MAP:
KTM sells a map selector switch that allows owners to access the three maps that are already installed in the ignition. Most test riders preferred the #2 “Aggressive” curve over the “Stock” map or “Slow” map. This is “ignition timing only,” not fuel mapping. The KTM part number for the switch is 7651-1101-0000. The retail price is $49.60. The map selector switch (umschalter motorsteuerung) can be ordered with an optional rubber cover for water proofing.


KTM BACKFIRE SCREEN:
The backfire screen on our 350SXF broke along its upper right hand corner. Under hard engine loads, the damaged wire screen flapped back and forth, which increased the size of the breakage. Eventually, small strands of the stainless steel wire mesh were sucked into the engine. We replaced the stock 350SXF air filter cage with the air filter cage from a 2011 KTM 250SX two-stroke (which doesn’t have a backfire screen). For 2012 KTM 350SXF redesigned the airboot to eliminate this problem?then we sucked the screen out of our 2012 KTM 250SXF air filter cage.


KTM 350SXF GEARING:
We geared it down at least one tooth (from 50 to 51), and several MXA test riders chose to gear it down two teeth (52). With the stock setup, the bike has a hard time making the jump from second to third, and it takes a long straight to get the rpm up to the rev limiter. Gearing it down one tooth helps the second-to-third shift and brings the rev limiter into sight. Gearing it down two teeth makes third gear more usable.


GAS CAP:
We cut off the tangs on the gas cap to disable the locking device (if you do it just right, the gas cap will click off, but you won’t need two hands to open it). It makes life easier (and when we tested Max Nagl’s works KTM, he had a custom molded gas tank that used the old threaded gas cap.


HYDRAULIC CLUTCH CARE:
KTMs with Brembo hydraulic clutches, like the 350SXF, can use DOT 4 (or DOT 5.1) brake fluid in the system. Bikes with Magura hydraulic clutches must use mineral oil. To fill the hydraulic clutch reservoir, you need a special KTM syringe to force brake fluid into the slave cylinder’s bleeder port. The KTM part number is 5032-9050-000.


SPOKES:
The MXA test crew watched the spokes like a hawk in 2011. We had issues with the spokes next to the rim lock coming loose with regularity. Additionally, most of our test bikes seem to have flat spots at the rim locks. Which we attribute to soft rims, not race damage. The 2011 rims are Excels, but they aren’t strong. It should be noted that our test bikes came from the first batch shipped into the USA and, perhaps, KTM has improved rim quality in following shipments. we have not had issues with the 2012 KTM 350SXF wheels.

THROTTLE POSITION SENSOR (TPS):

The vast majority of 2011 KTM 350SXF troubles are related to the Throttle Position Senor (TPS). It is the black and gray electronic device attached to the bottom, left-side of the EFI’s throttle body. Although the KTM TPS is identical to the Kawasaki TPS?and is a Keihin part, not a KTM part, the 350SXF’s unit was prone to fritzing out from water or grit contamination. For 2012, KTM has installed a rubber cover over the TPS unit (and this rubber cover will fit on the 20111 unit…and would be a smart investment). If your bike goes full rich, refuses to start, starts but won’t rev or quits suddenly on the track, suspect the TPS first. Sadly, it is an electronic part and is not easily to fix manually, but instead requires an ohm-meter to determine its condition.

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