Rotax jetting

ryan orchard · Posted: Sat Jan 19, 2013 9:25 pm Post subject: Rotax jetting

When looking at the jetting chart it says to run a bigger jet when it's cold out(it's 10 c where we are racing) I would think you should run a leaner jet like 162 or 165 to get heat in the motor, the chart says to run a 175. Am I reading in to this wrong?

Peter Zambos · Posted: Sat Jan 19, 2013 9:50 pm Post subject:

No, that's right. When the air is cold, the air is more dense, so you're effectively increasing the amount of O2 going into the motor. To then achieve the same air to fuel ratio, you need to increase the size of the jet.

It sounds like you used to run an air-cooled motor prior to a Rotax. Very Happy

ryan orchard · Posted: Sat Jan 19, 2013 9:57 pm Post subject:

Ok I get it now, is that chart very accurate?

Justin Martin · Joined: 27 Mar 2012 Posts: 328

Tape your radiator?

Is this a Jr or Sr engine? (power valve, or no powervalve?)

With a SR I have drove on some days that have been in the 30's (3.3-3.4c) and have yet to need more than 4 (4 and a half) strips of ductape to keep temp in the 140 range.

Whoops, PETER beat me to it...

and yes the chart is pretty darn accurate, within a jet usually. Depending on the motor some prefer a one jet down, and others one jet up. Experiment around and see what feels the best. Check your plug regularly so you dont blow up an engine running too lean!

ryan orchard · Posted: Sat Jan 19, 2013 10:14 pm Post subject:

It is a sr engine, I run a 175 jet right now and also have the rad almost taped all the way up. I can only get about 130f for water temp and it takes about 6 laps to get there

Justin Martin · Joined: 27 Mar 2012 Posts: 328

hmm... Interesting. New style radiator or old?

Very recently, we had a race that was 38 degrees on the first practice. with 4.5 strips of ductape (3 strips on the top, 2 on the bottom with one hal way overlapping) I was running 168 (recommended) with P2, and could maintain 135-140. I always Idle to get my temp up to at least 90 before I go out, and it usually is 125+ by 3 laps, and 135-140 by 4-6 laps.

This is the same motor that stays cool during the spring/fall here in AZ. (90-100 ambient temps)

ryan orchard · Posted: Sat Jan 19, 2013 10:41 pm Post subject:

I believe it is the new style

ryan orchard · Posted: Sat Jan 19, 2013 10:43 pm Post subject:

Nope it's the old style

ryan orchard · Posted: Sat Jan 19, 2013 10:47 pm Post subject:

It's the one that came out before the new style

Devin Smith · Joined: 08 Feb 2009 Posts: 87

Try A hotter plug.

Peter Zambos · Posted: Sun Jan 20, 2013 6:55 am Post subject:

bingo

ryan orchard · Posted: Sun Jan 20, 2013 10:44 am Post subject:

Peter Zambos · Posted: Mon Jan 21, 2013 12:43 pm Post subject:

You're missing a letter in there. I'm pretty sure you mean NGK BKR8EIX. Now, I don't have a resource close at hand that lists the proper operating temperatures of each NGK plug, but you really don't need that. Look at your plug, specifically the insulator nose, the part that starts out white. If it looks black, wet, fouled, well, then you need to go to a hotter plug. That number that's in the middle, the 8, that's your temp rating. A hotter plug will have a smaller number.

Once you get that insulator to a nice light brown, like the golden brown of a baked meringue on a pie, you should be at a better engine temp. The threads of the plug should still be black. Now, if your insulator nose starts looking fried and pitted and the threads are dry and brown, you've gone too far. Don't do that. Very Happy

Jim McMahon · Posted: Mon Jan 21, 2013 1:07 pm Post subject:

Do you guys run 7's in a Max? Even an 8 seems very hot to me, but last time I ran a Max was, err, '99?
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ryan orchard · Posted: Mon Jan 21, 2013 1:10 pm Post subject:

Plug looks to be a bit wet, the treads are also wet. I will try a hotter plug and see what happens. Thanks for the advice guys