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View Full Version : Can Anyone Figure Out This FR125-SR Problem


Butler Cox
07-26-2011, 01:05 AM
The below has been sliced/diced/dissected and trisected by Rotax techs, savvy top racers, and others, but NOBODY can figure what's wrong...have you any ideas?

Problem: Mounted on a 2010 CRG BadBoy, with any gear ratio combination, this motor will accelerate and run cleanly UNTIL 10,500 - 10,700 RPM. Then it stumbles, sometimes pops a small amount, and just lays down, although it will rev up to 13,100 with certain ratios.

Here's what we've tried, none of which fixes the problem:

1) Every jet from 145 to 172 on any given test day (jet range depending upon conditions).

2) Swapping carbs for ones that run well on similar CRG race carts.

3) Changing plugs (two brands, three differing heat ranges)

4) Swapping *every* part of the entire electrical system for parts that come from well-performing FR125s, from pickup, to wires, coil, ground wires, switches, plug lead...all of it.

5) Swapping PVs for ones from well-running FR125s, cleaning and specing my own two PVs. Sometimes this will make a very slight difference, but no matter how many times we've cleaned, replaced bellows, speced measurements, etc. on the PV, the problem won't go away.

6) Cleaning and replacing fuel pickups, fuel lines, filter in carb,the entire fuel system except the tank which is clean and shows no sign of damage.

7) Cleaning the airbox several times, including several thorough cleaning and very light re-oiling of the filter.

8) Full engine rebuild (at 47 hours). The problem started before the rebuild and continued unabated after break-in following the rebuild.

9) Full cleaning and inspection of reeds which have looked fine, according to the techs and tuners whov'e worked on this problem.

This FR125 was purchased in June, 2010, likely built in February - March 2010. Until the problem started five months ago, it always ran well, provided I hit the tune on any given session.

This FR125 starts easy and pulls well from low RPM...until 10,500 - 10, 700.I keep the water temp between 145 and 155. Have tried a bit hotter to see if it influenced the problem, but it didn't.

The motor has never been overheated. At the 50-hour rebuild all wear parts looked fine and showed no anomalies.

So...what gives with this FR125??? Anybody got any original ideas? Or do I just junk the motor. I got back into kartng after 45 years off and never expected nobody would be able to fix a poor running Rotax. Or is this Rotax just a lemon and I should junk it for a Maxter? The Maxter I've driven sure ran better than this Rotax.

Really hope someone has some constructive suggestions for the issue.

TIA

Bruce Woodrow
07-27-2011, 07:54 AM
Butler,

The problem described sounds very much like a PV that is not opening. First, I will assume that the PV has been assembled with the blade the correct way up and the small pressure signal hole in the PV gasket is aligned with pressure signal port in the barrel. I can only think that the small diameter pressure signal port in the barrel has become blocked by exhaust crud and needs to be cleaned.

You can also prove/disprove the PV as culprit by trying the performance with the PV cover and spring removed, so that the PV stays open. The performance will be very poor below 7,500rpm, but if it revs out past 10,700rpm, you know the PV is to blame.

Bruce

Butler Cox
07-27-2011, 09:08 AM
Bruce,

Thanks for your quick reply and suggestions.

When I can get to the track I'll try your idea of removing the PV cover & spring & see what happens. My gut is telling me that the PV is the culprit, but everyone who's looked at it says there's no crud, leaks, etc. that'd cause the issue. And that it is installed correctly and measures OK. What else in the PV might we be missing?

The signal port bore is clean and has been cleaned often while messing with this problem, so that's been on our watch list as something important to keep an eye on.

Will let you know the results of the cover-off test.

Thanks,
Butler

Larry LaVeque
07-31-2011, 11:38 AM
What about the exhaust pipe? Is it blocked by worn packing material or any other things?

Butler Cox
07-31-2011, 02:06 PM
Bruce:

Results with PV cap off: Low end was great, pulls like a trooper. Upper end goes through the 11,700 - 12,500 "dead - zone boggy like before except perhaps it pulls through slightly better/faster, but still ratty. Signal bore is as clean as can be determined with picks and lights, as is cleaned every 10-15 laps. Valve moves freely. Haven't yet put a camera in there, though.

I also noticed that with the cap off, performance was a bit cleaner at water temps between 140 (F) and 148. Above that to max 153 the dead-zone is wider and boggier.

When the cap is back on evey PRM range is just not as "snappy" no matter how many clicks, even with different springs (factory fresh).

Something else that bothers me and which is why I keep coming back to the PV: It doesn't give me the "extra-gear" kick at 7,500-8,000 *every* time. Sometimes it does, but mostly it doesn't, no matter what click setting. Doubly puzzling is that this occurs with a couple of different speced PVs from race karts that run well.

Also tried manually pushing shaft in and out while on track: When pushed in performance is logier from 5600-12,000 and motor wouldn't rev as easily through the "dead zone" as with the PV fully out.

Based on previous tests, with a ratio of 12/78-79, which is what puts my lap times where I'd like them (before the issue started), if I return to 12/78-79 (from the 12/83 where I'm testing), it will not pull through this dead-zone at all within the confinement of our straightaway lengths.

Everyone is still stumped. Hopefully we'll get a leak-down on it in a week or so, as well as get it on a dyno and see what these show.

I'll keep updating this THD as we go along, but it'll be a few days before I can get back to the track (Grand Junction Motor Speedway

Butler Cox
07-31-2011, 02:30 PM
Larry,

Thanks for your reply.

Pipe is as clean as we can determine by inserting long rods and probing around. Haven't yet put a camera in there as there isn't one real close by. Packing has been replaced twice.

More significantly, we've tried other pipes from well-performing race karts and they don't influence the issue at all.

Again, when this FR125 was rebuilt (because it was at 47 hours and because we thought perhaps the rebuild would uncover some cause) all gaskets were replaced with factory fresh.

PS--Did I see your rig up here (Grand Junction Motor Speedway) at the SummerNats? or am I thinking of someone else?

Larry LaVeque
07-31-2011, 10:30 PM
Butler:

Team AZ trailer did not make the trip to the SummerNats...but there were some folks from Arizona up there and could have been flying our banner.

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

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.

Fuel System - has the fuel pump been rebuilt? Is the gas tank vented properly?

Electrical - we run the the IW-29 or the NGK BR10EGV plugs here in AZ.

Squish - is your motor set to the low or high side of the measurement? this can affect tuning.

Gear Ratio - at PKRA (.8 mile with long straight) we run at 12/79 and hit the rev limiter on the straight.

Hope this helps...do not give up. The Rotax is a great package.

LL

Butler Cox
08-01-2011, 11:08 AM
Larry,

Thanks for your thorough reply.

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

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 year...at 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 helps...do 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!

BC

Butler Cox
08-14-2011, 12:48 PM
Someone who should know is also at a loss about this, apparently. They suggested a cracked piston. This motor has used two pistons, neither of which shows *any* signs of flaws.

Since the results of the suggestions so far in this thread have not produced a positive result, and since nobody else (dealers, tuners, etc.) has been able to come up with an answer, who can I contact at BRP/Rotax for technical assistance?

Or does BRP just leave it up to dealers/shops to solve something like this?

Sure would like some Rotax help with this or else this FR125 is a damn expensive paper weight for me as well as other FR125s around here with similar issues.

Garry Lobaugh
08-14-2011, 07:30 PM
It looks to me that the forum respondents have been very attentive to your needs.

To answer your question: the local dealer is our first response to the customer. We cannot discect your problem from a long distance or by email. This sounds like major surgery.

Please call me to discuss further.

269 720 9196
Garry

Butler Cox
08-14-2011, 08:37 PM
[QUOTE=Garry Lobaugh;7584]It looks to me that the forum respondents have been very attentive to your needs.

To answer your question: the local dealer is our first response to the customer. We cannot discect your problem from a long distance or by email. This sounds like major surgery.

Please call me to discuss further.

Thanks for your quick response. Yes, the respondents have been responsive, as has the dealer, but nothing on this forum since two weeks ago with any new leads to pursue. I know you cannot dissect a problem like this long distance or by email. I'm just trying to see what to do next when the local dealer or other resources can't seem to fix the problem.

We have been trying another experienced Rotax tech who presently has the motor apart and has been working on this problem for several days. So far he's found nothing anomalous, not even something that looks like it requires major surgery. His findings, plus all the others, mean its not something obvious, or perhaps even common as far as tuning goes.

Bottom line is everyone is puzzled, so I hope maybe you can shed some further light on what to do next when I call. Hope so as we've unable to race this motor all season long since it acts like its about to do something that will require major surgery for sure.

Thanks!
Butler

highwayman9011
06-04-2012, 01:37 PM
Hey, Can anyone lend you a known good cylinder? Had similar problem with my sons jr rotax, couldnt rev past past 13000 , no power, ...changed everything like you. Motor was rebuilt , used to run good before the rebuild. Ended up that cylinder was switched and it was a bad one for whatever reason. Put another cylinder on it and it runs fine.

Garry Lobaugh
06-05-2012, 09:27 AM
Hello Highwayman,
Please call our office to discuss: 269 756 7444 WE have tech's on standby to assist.
Garry