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Old 08-01-2011, 11:08 AM   #8
Butler Cox
Join Date: Jul 2011
Posts: 7


Thanks for your thorough reply.

1) "Air Leaks - you should do both tests...pressure and vacuum."

2) Air Density - here in AZ we see corrected air density altitude is the winter at 1200' and in the summer at around 4000' ...this really affects jetting and PV operation. We tune to get the PV to open at 7500...based on the air.
Being a bit new at this I'm not sure I understand what you mean. Are you saying that if the jet is spot on for Density Alt (DA) then the PV will open at 7500 using the adjustment setting that worked previously at 7500? Or, is there some way besides the adjuster setting with which you can tweak the PV for DA?

IOW, more than just finding a jet that feels/sounds better for a particular session's DA, you also base your jet decision on how well it allows the PV to open at 7500?

Being at MSL of about 5100 here, we'll see North of 7000 some days.

3) Fuel System - has the fuel pump been rebuilt? Is the gas tank vented properly?
Yes to both. we did discover that the original fuel pump had begun to weep so we rebuilt it, but that had no affect on the problem.

The fuel tank vent is fine. Next time I'm going to drain and flush the tank on the slim chance of condensation in there.

I have an inline fuel filter as well as the one on the carb. I check both daily and the carb filter has always been spotless.

I've also tested without the inline filter to see if perhaps it was influencing flow rate enough to cause the problem, but it isn't. I run the inline because I'm a paranoid 70+ geezer and the extra snippet of performance I might get without it isn't enough to overcome the age/eyesight/reflexes handicap people of my age have when trying to compete in TAG Masters against 30-50 year olds. It'd be nice to have an older class, but since there's hardly any of us, track owners, clubs, and sponsors are not very interested based on my research over he past least at the Rotax TAG level. There is activity at the clone level. So whatever racing I do is against myself.

4) Electrical - we run the the IW-29 or the NGK BR10EGV plugs here in AZ.
Thanks for that. I run the NGKBR 8, 9, 10 EIX as the EIX is what they usually have in stock here. have also tried the Densos. But neither brand produces any noticeable difference to the problem. Usually I run the 9 or 10 heat range except on the days when the ambient temp is below 35-40 (F).

5) Squish - is your motor set to the low or high side of the measurement? this can affect tuning.
Great question! All I can say at the moment is that when the motor was first installed it was with the gasket that was installed at assembly by Rotax.

When it was rebuilt they installed a new one of the same thickness. But we should check the squish measurement.

How far can you go with changing squish and still stay within the RMAX rules? Are you restricted to using only the factory gaskets that ship with the motor? I'll check this out.

5) Gear Ratio - at PKRA (.8 mile with long straight) we run at 12/79 and hit the rev limiter on the straight.
When I was driving right I used to be able to hit the limiter here at GJMS with 12/79. But I can't since this problem started, even with 12/83!

6) Hope this not give up. The Rotax is a great package.
They do need to simplify a few things, though, to make it more user friendly, IMO...such as an easier/faster way to re-install the PV cap spring...and more clearance between the radiator and the carb's float bowl so it doesn't hit the radiator every time you change jets.The airbox needs better design, IMO, too. But all in all it's decent. It'd also be nice if they can design a motor that would produce similar performance and quality without a PV like maybe the TX1 Sonik will become. IMO, there needs to be a simpler, reliable TAG motor for the market that's just below events like SKUSA, Grand Nats, etc...a recreational racing market where one wold get the reliability and decent performance like a Rotax, but with a design and rules that would make it more user friendly for the recreational entrants. Something like what Briggs tried to do way back when, but promoted by companies with savvy. However, it is a tiny market...hard to make the numbers work.

There's no way I'm going to give up on this thing. Since there's no class in which I can race competitively (at least for a podium), I'm trying to learn all I can about the Rotax and how to tune it so maybe someday I can gain a place or two by out-tuning somebody else.

Really appreciate you're hanging in with this thread!


Last edited by Butler Cox; 08-01-2011 at 11:20 AM.
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