Engine Sputtering Off Corner - How to Diagnose??

Eric Alexander · Joined: 19 Jun 2011 Posts: 121

Me and my friend are experiencing a similar condition with our Rotax Senior engines. We're running at VIR Kart and we're experiencing the engine stuttering when accelerating off the tight 180-degree hairpin corner after hard braking. I've read alot of topics related to this and performed 95% of everything that is usually suspect (RAVE valve, jetting, exhaust packing, battery, etc).

I guess what I here to ask is if there's a way to tell the difference between a hardware-related issue, and the normal Rotax getting on the throttle too qucikly? I was able to minimize it to some degree by easing up on the braking closer to turn-in, and slowly getting back on the gas. Of course I'm feeling I'm leaving some time on the table in doing this. But I did seem to be able to manage the kart bogging down.

So I ask, with a Rotax Senior, just how slowly do you need to roll onto the throttle in a worse-case corner?

Eric Alexander · Joined: 19 Jun 2011 Posts: 121

No takers? So that leads me to believe its a hardware issue because I sure feel like I'm leaving seconds on the table backing off as much as I was.

My float level is set to 3mm. As much as it feels like the engine is having a fuel pick-up problem, my gut is saying to set float level higher. I'm going to make a big change (6mm) and see what happens.

Don Holmboe · Posted: Sun Jul 08, 2012 12:46 pm Post subject:

Too rich.
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Don Holmboe · Posted: Sun Jul 08, 2012 12:47 pm Post subject:

3MM float is high. 6MM is toward the lean side.
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Eric Alexander · Joined: 19 Jun 2011 Posts: 121

Thanks Don. That backs up what I was thinking. Its going to be a few weeks before I make it back to the track, but we're cleaning and reassembling the carbs now. So we'll make adjustments and see what happens.

Chuck Parker · Posted: Mon Jul 09, 2012 6:07 am Post subject:

Have you changed the needle position. Mine did the other weekend and I had to lean the needle by one clip position.

Chuck Parker · Posted: Mon Jul 09, 2012 6:08 am Post subject:

Have you changed the needle position. Mine did the other weekend and I had to lean the needle by one clip position.

Eric Alexander · Joined: 19 Jun 2011 Posts: 121

We did change needle position.

We've been to VIR three times and experiencd this problem each time - regardless of jet and/or needle positions. Both karts are prepared together so we ruled out some hardware things (like wiring harnesses, coil, etc) since both karts experienced the same pick-up issue in the same corners. Both karts this last time had new batteries, new RAVE bellows, and fresh exhaust packing.

At this point the only thing we we haven't adjusted is the float height. And that was because it seemed like a fuel starvation issue and with us running 3mm FH, I didn't think leaning it out would help at the time. But now I'm thinking otherwise.

Adi RocK · Joined: 07 Feb 2009 Posts: 63 Location: Brunei,

hows your air screw setting? sounds like not open up enuff.

John Savage · Posted: Tue Jul 10, 2012 2:16 pm Post subject:

Eric Alexander · Joined: 19 Jun 2011 Posts: 121

You know, I didn't mess with the mixture screw. I'll have to try that next time.

As far as floats - 5.2g 30/30.

John Savage · Posted: Wed Jul 11, 2012 7:25 am Post subject:

Read this and then come back if you have any questions

http://www.karting1.co.uk/jetting-karting.htm
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Richard Thoms · Posted: Wed Jul 11, 2012 11:46 am Post subject:

It is my understanding that a 6mm float height allows fuel level in the bowl to be higher thus giving more fuel before maybe starving but also requiring less carb vacuum to pull the fuel into the venturi (from the higher bowl level).

So with the same jet size a 6mm float height should result in a richer mixture than 3mm float height (again with the same jetting). Therefore when running 6mm float height you would run a smaller jet than with 3mm float height.

Remember it is not float height we are setting but really float depth (because the carb is upside down when you set it!)

Anyway I've convinced myself this is the way it works but I'm at the office now and without having the carb in front of me to play with the float/arm/needle I can't really visualize it. Smile

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John Savage · Posted: Wed Jul 11, 2012 2:16 pm Post subject:

Richard Thoms · Posted: Wed Jul 11, 2012 7:41 pm Post subject:

I stand corrected - I took a look at the carb - and who could argue with John about Rotax carb Laughing

It is now clear to me that a 3mm float height will have ~3mm higher fuel level in the bowl than 6mm float height. This changes everything Surprised

Are you supposed to measure the float height with the fulcrum just resting on the needle or with the needle compressed to closed position? I've always measured in resting position.
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