KT100 Fulcrum Arm spring length

[Patrick Harris]

I purchased a used "blueprinted" can motor but swapped over to pipe for our pipe class here. I checked my popoff on this used engine to find it in the 25 psi range! I couldn't believe it, and it was so high that sometimes it would even stick and go a bit higher!

I pulled the carb completely apart, changed the needle, fulcrum arm, all gaskets, etc, set fulcrum height, and then went to setting popoff. I had to cut 5 coils off of the spring to make it pop down at 12 psi. The length I started with was exactly 5/8" and I ended up with a hair under 1/2" to get it right.

1) Is the factory spring 5/8" long?
2) I am AMAZED that it even ran at 25 psi popoff - am I assuming correctly that this would have leaned my top end flow drastically?
3) Now the kart will crank at 1/3 turn and seems to like very close to factory settings on the needles when running (L 3/4 and H 1/2)

Does anyone know if this is the factory spring?

Thank You,
Patrick

[Walt Gifford]

Pop off don't effect top end, it closes off fuel in the braking zone so you don't load up coming off the turn. 10psi works for most places but long braking zone more pop off pressure. 25psi sounds like it was never blueprinted.

If your track works your engine mostly in the upper rpm range (enduro track) then you can use 1/2H 3/4L but sprint tracks usually need more fuel on the bottom so 1/8H 2L is a good starting point.


Hope that helps,
Gif
_________________
FAA certified jet engine and aircraft technician
Nicholson Speedway class champion 2001
Yamaha KT100 Service Center
40 years karting experience

[Patrick Hubbell]

Get the correct spring. Never cut coils. To shorten one, compress it between your fingers. Wow 25 psi. Never seen one like that.
_________________
Patrick Hubbell

[Patrick Harris]

[Patrick Harris]

[Walt Gifford]

You can bend the cut end flat or just put it on the carb side. So much folk lore has been written about this spring I think they're singing songs about it in certain areas.

[John Mulvihill]

Hi,

That may not have been an OEM spring from the description. Start with a brand new spring.

Removing active coils (those loops that have a gap between them) from a spring, raises the spring rate. That may not be good.

If there are loops that have no gap, removing them changes the pre-load on the spring but not the rate.

Compressing an OEM spring all the way with your fingers REPEATEDLY will shorten it a bit without changing the rate.

Make sense?

John
_________________
I bowl overhand..........

[Walt Gifford]

You can mill the pocket out a little so your in the range on a stock spring without cutting off all the straight coils. Have to bead blast and acid burn it if you think it's going to get teched (yes, karting is that nit picky). Most springs I've had when you leave one flat coil on each end you're still about 12psi.

Gif
_________________
FAA certified jet engine and aircraft technician
Nicholson Speedway class champion 2001
Yamaha KT100 Service Center
40 years karting experience

[Jean Stafford]

The length of the spring has little to do with pressure, other than increase the pop-off pressure as it gets longer. Most pipe motors run much lower than 25# though. I have seen as much as 12#, and as little as 6#.

The fuel pump can put out as much as 6# of pressure, so in order to function as designed, pop-iff must be higher than this number. As the pump diaphragm "bags" pressure output drops. In my opinion, those who advocate 6# have a poorly functioning pump.

Fulcrum height controls the amount of fuel available in the carb. So if you're running lean at top end because of a lack of available fuel, raise the arm a little. Our pipe motors seem to like around 0.052" from the carb body to the arm. Factory says between 0.042" and 0.062". The higher the arm, the higher should be the pop-off, because the diaphragm is pushing down on the arm, a little.

DO NOT CUT THE SPRING OFF. This causes the spring to "****" in the pocket and push the arm to one side or the other.
_________________
Jean Stafford
Stafford's Racing
Dayton, Ohio
ROTAX Repair station,
jean@sgandt.com
Started Karting in 1959
"Some people are like 'Slinkies', worthless, but they bring a smile to your faqce when you push them downstairs."

[patrick slattery]

Where do you get springs that are in the correct range. Everytime I get springs they need to be cut or squished excessively.

Pat

[John Mulvihill]

E.C. Carbs has a variety of springs.

John
_________________
I bowl overhand..........

[al nunley]

So much bull, just get the pop-off right and don’t worry about the rest. It’s not that important anyway.
I had Hartman do a carb and engine for me and the pop-off was higher than my gauge would pump. The metering diaghram is what takes the needle off the seat and the spring pressure has little to do with that.
They come stock with a lot of pressure because the needle has to prevent the flow of gas when your weed eater is on the shelve in the garage.
You need enough to keep the carb from draining fuel into the engine while in the pits. A setting of 10 psi is enough and more does nothing.
_________________
If the data does not support the theory, get a new theory. (Al Nunley)
All else being equal; Compression is the Holy Grail.
45 years, in and around karting

[Steve O'Hara]

Al Nunley said >>A setting of 10 psi is enough and more does nothing.<<

Al forgot to finish his sentence.... he should have added the words "for me" to the end.
Countless other more accomplished racers and tuners disagree and have previously offered numerous examples of cases where a pop-off setting of something other than Al's 10 have made the difference between winning and losing at the highest levels of karting.
Why he chooses to come here and spread misinformation is a mystery to not just me but many other very knowledgable and successful karters.

Pop-off settings make a difference in every pumper type carb used in karting and each carb/application has its own special needs.
Just about any pumper type carb used in karting will run with the pop-off set at 10 psi but the majority will run better with a different setting.
Steve O'Hara

[al nunley]

[Steve O'Hara]

I've done it several time before, feel free to do a search to look it up if you're interested.
Steve O'Hara