two-stroke rebuild questions

[typeone]

check out this article for more info on measuring squish: http://www.rb-designs.com/squish-band-test.htm

[Paul_E_D]

Yes, to a large degree setting X also affects squish. Definitely shoot for 0 X dimension, however, sometimes that will still leave too much squish. In that case you can go slightly into negative X.

It does change you port timing a little, but if the didn't ask you for your squish and base gasket numbers, then I doubt they actually changed the timing. Likely they just took material away from the sides. In any case, you will surely need to lower the cylinder to achieve 0 X and a decent squish. This will advance your timing a hair, which is fine on a dirtbike. It will give a little more grunt and a little less overrev. Don't be scared to do this. The improvements from a o X dimension and 1.0mm squish will overcome the effect of slight timing change.

[Paul_E_D]

btw, did you disassemble and clean the powervalve parts? Any scoring on the moving bits and you should just replace them. I'm sure that's somewhat to blame for your poor fuel consumption. The bike is going to feel scary fast with that system working again.

You sent the head? What were they going to do with that? They can take material away to get the desired squish without raising/lowering the jug, but you would have needed to set X to zero and then measured squish for them to know what to do...

setting the X and squish will allow you to actually jet the bike better as you will now be burning the charge more fully and more consistently. There's plenty of good jetting info at KTMtalk.

[Stromper]

Make everything clean and shiny piston crown head and exhuast parts

double check chamfer blending of ports on cylinder walls

I miss my ex300

[peterz85]

Like Paul said. Get your deck height dialed in first. Use your stock base gaskets and see what you have for deck height. Should be spot on. From there dial in your squish by means of skimming the head.

If you raise or lower the ports/transfers via base gaskets, you could wreck your top end or midrange depending on which way you move the cylinder.

I know on my bikes, .5mm change in base gaskets makes a HUGE difference in power delivery.

Pete

[seth399]

is the timing not easy to do on a ktm? My yz only took a few minutes, was also a few thousandths off from the factory.

[Paul_E_D]

Timing and port timing are different. Port timing is complex, but in my experience using base gaskets to tweak deck height and squish doesn't usually move port timing enough to cause problems...

From what I read, the 300s come from the factory with too big a squish gap. A slight negative deck height can help if you just want to get it together and ride without sending the head out for skimming.

[burnham]

Got the parts today, hopefully get it together tomorrow.

[burnham]

OK. Fuck my fucking life. I will probably go to Hatfield McCoy tomorrow and run around all day making motorcycle nioses, because this fucking thing runs like shit.

Put in a new piston, new spark plug, put the bike back together and lit it off. It ran fine on the stand.

I then took the carb off to re-jet. I have a JD jet kit, put in the blue needle and a 172 main. I set the air screw 1.25 turns out. I also adjusted the float height (raised it) by the book that came with the bike. Gas poured out of the carb when I opened the petcock. I then pulled the carb, and lowered the float height. Open petcock, leaks gas. OK third time, ,lower the float height again, and it doesn't leak this time.

I start the bike, turn the choke off, and ride down my driveway. The bike bogs badly as soon as the engine is under load. It races up when I give it throttle, and the RPMs are slow to return to idle speed. (lean) It stalls, and doesn't want to start. I tip the bike onto its side, and no gas comes out of the overflow.


The reeds are fine.
I'm confident it doesn't have an air leak.
I know the petcock flows fuel fine.
It's not consuming anti-freeze.

I'm supposed to leave for Hatfield McCoy in the morning, the trips already been put off one week.

Where should I start? I want to set the float height, but when I do it by the book it's way high.

Thanks guys.

[Kurlon]

I think it's the needle valve, it's taking too much force to seal which results in way too low a fuel level in the bowl. Running problems follow.

Pop that needle out, make sure the tip is clean and soft and the spring loaded plunger on the other end is free. Then clean the crap out of it's seat using something SOFT.

[peterz85]

Rip the carb off the bike and clean that sucker out thoroughly.

My guess is you had some crud in the carb causing it to leak fuel, then by screwing with the floats, you are way off spec.

2T's hate ethanol fuel and the gas goes bad very quickly. Gums up carbs big time.

Take your time, clean the carbs fully, go back to your baseline jetting specs, and tune from there.

How did you make out with your new top end specs, deck height, squish...

Pete

[burnham]

I think it's the needle valve, it's taking too much force to seal which results in way too low a fuel level in the bowl. Running problems follow.

Pop that needle out, make sure the tip is clean and soft and the spring loaded plunger on the other end is free. Then clean the crap out of it's seat using something SOFT.

I'll look at it again. I've pulled everything appart and hosed it down with carb cleaner, there's no crud or anything in the carb that I can see.

Rip the carb off the bike and clean that sucker out thoroughly.

My guess is you had some crud in the carb causing it to leak fuel, then by screwing with the floats, you are way off spec.

2T's hate ethanol fuel and the gas goes bad very quickly. Gums up carbs big time.

Take your time, clean the carbs fully, go back to your baseline jetting specs, and tune from there.

How did you make out with your new top end specs, deck height, squish...

Pete

I did ok with the top end specs. I have the deck height slightly low, which made my squish a little lower than it should be, but I think it's in the ballpark. I was going to run a thinner base gasket but I was afraid I might get some detonation.

The odd thing is I can lay the bike on its side and no fuel comes out of the overflow tube, but when I remove the float bowl it's full of fuel. (I know the overflow tube is not plugged up).

[Paul_E_D]

Whatever you touched last is the problem 90 percent of the time. Something's amiss with the float.

[burnham]

Yeah, it's gotta be the fuel valve. I can't get the float height correct, either the carburetor is dry, or it's pissing out fuel. The rubber part of the needle is kinda hard, but when I put fuel to the carburetor with the bowl off it seems to function fine.


Thanks guys

[gregp]

These guys are most likely correct. You have adjusted the float level way too low now, and there is still crud in the float valve.
I read an article a while ago about similar issues with an R-1. The author mentioned that Yamaha used an O-ring around the female portion of the float valve that was prone to going bad - allowing fuel to leak right by the valve.
Most times, I find that there is a piece of crud stuck in the float valve - sometimes it is stuck right above the valve, and it doesn't flush out well with carb cleaner. I usually need to remove it manually.
Do not forget to set you float level back close to where it was before you lowered it...
I just recently worked on an EX400 quad with a similar problem. The owner and his father insisted that they had cleaned the float valve, but when I asked him if they had trouble getting the float pin out, his response was "Oh, we didn't take that out because we were afraid to break off one of the posts". There is no way they cleaned the float valve if they did not even remove the float! The male and female portions of the float valve must come completely apart, and you must visually check the fuel passages, needle tip, and brass cup. Sometimes I use Scotch-brite and buff up the rubber tip very gently as crud can build up on it.

[burnham]

Ok, unfuck my life, and I'm pretty stupid. Thanks guys.

[tls25rs]

Ok, unfuck my life, and I'm pretty stupid. Thanks guys.

????????

Did you figure it out? Was it something really simple? I hope you got it squared away and are headed down to Hatfield soon. I hope to make a trip to Hatfield myself so be sure to update us on how the trip was when you do go.

Joel

[burnham]

Yeah, too simple. On the needle valve there is a spring that hooks onto the float. I had that on the wrong side of the float, so it was always calling for fuel.

I was handling the carburetor and the float fell off, so I didn't see how it came appart. Just put it back together wrong.

At hatfield now, aside from a little tuning the bike runs like a top. Good times.

[tls25rs]

Sweet! Enjoy your trip!

[burnham]

Just got back from Hatfield McCoy last night, did six days of riding down there. The trails were incredible, and the bike ran better than ever. Thanks again for the help guys.