My 380EXC jetting/carb blubbering situation + partial solution

[cujet]

I'm currently riding a 2001 KTM 380EXC with a tag and taller gearing. It's set up so RPM and MPH are nearly married. The problem with the bike has been the awful blubbering situation at part throttle, lower RPM. Such as when cruising down a firetrail at 30-35MPH in Ocala Natl Forest. It's certainly a weird bike. Nothing like the 300EXC it's based on.

I discovered a few oddities. First, the carb is tilted forward (guessing) about 20-30 degrees. Second, the bike misfires and blubbers horribly when braking hard. Simple enough, the carb is pouring gas into the intake upon hard brake application. Lowering the float level to well below spec helps but does not fully solve the issue.

The second issue is the constant misfire under modest load. Not a typical two stroke misfire, but a bit more severe, with alternate pinging and blubbering. Showing the signs of being both too rich and too lean within a few revolutions. This one was quite difficult to troubleshoot. Head squish mods, extended reach plugs, new seals, gaskets, reeds, rings and so on did nothing to help. Then I tried 100LL Avgas. Problem seemed solved. It ran much more cleanly. OK, I'll try race gas, Sunoco 260 from the local pump. Horribly worse. WUT? Ah ha! It's the ethanol. My bike is very intolerant of ethanol in the fuel, as it seems to cause idle and part throttle smoothness issues. So, I purchased some VP T4 (100 octane) and it runs reasonably cleanly without being a blubbering idiot down the trail. But, still not as cleanly as 100LL. A 75/25 mix of T4/100LL seems to work fairly well.

Even with hours of troubleshooting, jetting, modification and tuning, the 380 is still a good bit more unrefined than any 2 stroke I have ever owned. Some of this is likely due to the port timing, the head's odd combustion chamber shape and the tilted carb.

 

[sbest]

I am at the opposite end of the scale with a street driven 2000 KTM 125 EXC but I ran into similar problems when I first started running it on the street. It seems street running is much more fussy about jetting than running on dirt ever thought of being. I also have a 300 KTM and an untried 380 motor to stick in it sometime. I also have a 2003 KTM250 SX in a Yamaha Blaster frame that had the carb tilted. Trust me brother, you can make it work but it was SOOOO much better when I took the effort to level it. I have also done a lot of work with head chamber shapes and squish and agree that the 380 may need some help here. More squish area, deeper more torroidal bowl?

Back to street jetting and my 125. I figured it worked perfect off-road where I mainly worried about getting the main jet right, but discovered on-road the pilot and needle are the crucial jets. Most of the time even with the 125 I am cruising on 1/4 throttle or less. Steady state driving would buck terribly when jetted to seeming perfection, had to go 1 rich on needle and pilot to eliminate the bucking. Running float bowl low creates odd lean spots especially on acceleration. Not recommended but I had to do the same when I had the tilted carb.

Incidentally, I could not always buy good gas at rural locations here, so I shimmed the base gasket up 0.015" or so to run regular 83 octane fuel. The ideal fix would have been to open the chamber up a bit but this was an easy solution and worked for the 125. It softened the low rpm power very slightly. We don't have ethanol adulterated fuel yet, so I cannot comment on that. Hope there is some help in here.

Steve

 

[cujet]

While I like my 380, it has some glaring faults. First, the close ratio 5 speed is not good enough. The bike's top speed is 75MPH and first gear is WAY too tall. That's annoying. Second, the 380 engine really is down on torque, down below the powervalve opening RPM. It's far from being a rip-snorting, arm straightening monster. A well tuned 250 MX bike is just as fast. Coupled with the fact that it just does not run all that cleanly down low. I think the mighty CR5 is much better in the power delivery department.

I like it because it's a street legal, lightweight, 2 stroke. But, I'm not sure it's worth the trouble to fix all of the "designed in" problems.