Rotax extremely high RPM on start! Help!

[Anonymous]

I’m new to karting and especially engine tuning. I’ve got a spare FR125 senior motor that came with the ‘package’ that was sold to me when I bought my kart second-hand. Today my dad and I decided to try the motor to make sure everything was ok. It’s sealed and has a passport from Acceleration Karting. When we start it up, it wants to go to max RPM instantly unless the choke is on even with zero tension on the throttle cable. We can’t seem to figure out why. We’ve tried two carburetors and tore things apart (without breaking the seals of course) and we can’t figure out why this is happening. At first glance, things appear to be normal. It doesn’t seem to add up… it’s not ‘rocket science’ is it? We had to give up for the day, but one thing we didn’t do was have the airbox hooked up to the carb. Could this actually play a role in what’s happening? We will try again with the airbox installed tomorrow. We are really hoping it’s not something complicated that would require breaking the seal. Although we could do the work, I’d rather not void the value of the seal.

[Peter Zambos]

Typically, an engine isn’t going to rev high unless it’s being given enough fuel to do so. Though you’ve tried two carbs, I would think that you’re supplying more fuel than you think. I know that you stated that there is no tension on the throttle cable, but is the slide seated all the way down in the carb? If there’s a hang up anywhere in that assembly (which is pretty easy to achieve if you’re not careful), you could be effectively gassing it more. Since you still have the airbox off, just take a look.
Of course, if you’re still having chronic issues, my Rotax works perfectly and is still for sale. :)

[Ted Hamilton]

Sounds like an air leak to me…

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2014 Praga Dragon / IAME KA-100

[tony zambos]

Hope you’re doing this with a chain on so you can slow the engine down with the brake. Try loosening the clamp holding the carb to the rubber boot and pull the carb. Check the boot for cracks. While the carb is off, find out the size of the main jet.
There are two adjustment screws on the carb, the throttle stop screw and the air screw. Turn them both of them gently in until they stop. Open the throttle stop screw (the top one) a half turn. Open the air screw (next to the fuel line) two turns.
Since you said your were no to karting; is there oil in the gear case and how many onces of oil are you mixing with a gallon of gas? And is there any chance you have a tach to hook up?
One last thought for now, is there a spring above the slid in the carb?

LAD Specialties customer / tony kart / rotax / kt100

[Rodney Ebersole]

Be sure the slide isn’t being put in backwards.

[Anonymous]

Problem solved. Ted was right. It ended up having nothing to do with the carbs, slider, throttle cable, fuel pumps, needle choice, or throttle or air screws. The problem is that when we first started looking at things yesterday we had removed the fuel pump bracket. When we put the bolts back in, they weren’t tightened completely and it was letting in air behind the carb, which would explain why we had the problem when trying three separate carbs. We tightened everything back up the way it should be and the issue went away. Everything looks good now. Thanks for all the feedback and tips.

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