October 14, 2013 at 10:59 am #12776
Having and issue with my Rotax FR125 Max engine. Engine only had a couple hrs on it and runs well, except for one issue.
Down the long straight, when running High RPMs 12,000+ I notice if I let go of the throttle the engine speed/rpm does not drop immedialty. In fact, it feels like it continues to raise for a few hundred RPM. I had the track shop look at it and they made some changes to the carb, had no effect. They cleaned everything, changed jetting and replaces some small parts in Carb bowl. They thought I was still burning fuel after I released throttle. Issue does not feel like a linkage problem. Throttle cable, sleeve is new and there is no binding. I’ve not tried to replicate it on the stand.
October 14, 2013 at 10:20 pm #12844
October 15, 2013 at 6:56 am #12855Rodney EbersoleParticipant
Take a look at how your throttle cable is or is not protected from the leg of your racing suit laying against it causing it to stick open.
October 15, 2013 at 8:26 am #12874
Rodney – I’ll check you suggestion on throttle cable routing, but pretty sure that is not the issue.
I had a friend send me the following (BELOW) suggestion, that sounds probable as it does feel like it accelerates rather then just sticks when I let go of the throttle. I will explore it next time I go to the track. It is a new engine install and problem has been there since day one thus I think it is something with my installation.
He says –
<span style=”-webkit-text-size-adjust: auto;”>”Ha! Classic Rotax issue. They’ll sometimes run faster with the slide lower, so that’s why it accelerates when you back off of the throttle. Those motors are over carbureted. Make sure the bottom curved end of the slide does not open past the top of the Venturi. And you can probably run a richer main jet.” </span>
October 20, 2013 at 6:03 pm #13235
Just back from the track and disappointed as the problem still persists. What was done:
1. Track Shop (Authorized Rotax Service Center) rebuilt power-valve and changed bellows.
2. At track I adjusted throttle cable to make sure it was not “Over Opening”. I aligned it so the curved part of throttle slide is flush with the top of the carb at full throttle. I was sure this was the issue.
3. I checked to make sure there was no contact with suit, shoes, leg, arm with throttle cable.
Almost feels like there is excess fuel in the carb and when I let off of the throttle it needs a couple seconds to burn it off.
The other thing I noticed is some clanking/binding from clutch (New style). Could t be possible the clutch is not dis-enguaging and the momentiumn is keep revs up? I’m having the shop inspect the clutch as I am switching bock to a 12T driver fro 11T I am currently running.
October 21, 2013 at 2:09 pm #13299
What Matt & Peter are saying makes sense. I will check that out next time I get to the track.
October 22, 2013 at 8:42 am #13396Matt DixonParticipant
Ambrose just remember that the air density may be different next trip to the track. You should get yourself an AD gauge and take notes on your jetting, performance and the AD%. Before long you will have a good baseline jet set up.
October 24, 2013 at 11:57 am #13692
November 4, 2013 at 7:38 am #14626
Was at the track this weekend (OVRP NY). Still experiencing the OverRev issue at the end the straight. When I let got of the throttle, revs don’t drop right away, in fact it still continues to increase a little.
I adjusted the throttle slide/cable to make sure I am not going “Over the Top” – No change.
I tried main Jets from 152 all the way to 165 – No change.
Power-valve was cleaned and rebuilt – No Change.
Just rebuilt fuel pump
My foot, suit or arm is not interfering with throttle cable.
The throttle cable or pedal is not hanging up. Cable was new and is not binding
The clutch was cleaned as I was thinking maybe it was not disengiaging (New style).
As finial test, as John suggested, I put a good known Carb on and problem persisted. For this test I utilized my current slide and throttle cable assembly – No change.
I made sure the carb was square to both the engine and airbox.
I am driving around the issue for my lifting early, more of a nuisance, but would like to solve this mystery.
Hope to hit the track this weekend, any suggestions?
November 4, 2013 at 10:54 am #14653Mike LydaParticipant
Vacuum leak? Check the carb cap to make sure the rubber gasket is in there..
Have you tried different clip settings on the needle?
November 4, 2013 at 1:44 pm #14667
I just ordered a new carb cap, gasket kit, throttle cable and throttle cable sleeve. I can’t recall if there was a gasket under the carb cap. I know drawing show one exists. It is included in the gasket kit I ordered. That will be the first thing I check.
November 5, 2013 at 10:56 am #14745
November 9, 2013 at 5:50 pm #15159
Was at the Kart Track today…I changed the throttle cable, the fixation kit, cable sleeve, carb cover, cover gasket and sparkplug. Problem still persists. What next?
November 10, 2013 at 6:25 am #15172Richard KirlinParticipant
Had similar problem on my rotax, found out that if the carb is not pushed all the way into the rid valve it causes to lean out when throttle is let off.
Check and make sure that carb is pushed in all the way and clamps are tight.
November 14, 2013 at 2:03 pm #15646
Richard – I will check to make sure carb is pushed in all the way. I know clamps on boht side are tight.
November 14, 2013 at 8:20 pm #15661Mark GrossmanParticipant
certainly sounds like an air leak. If not the more accessible things, carb clamps, etc might want to do a pressure check. Could be bottom end leak or something else less obvious.
November 20, 2013 at 9:32 am #16078
If the weather is OK this weekend, hope to get out for one last time this year and get the “OverRev” issue resolved.
Since issue persisted with another carb, I am thinking Air Leak too and ordered a bunch of parts. I will replace them one by one.
– I got a new air box top
– I got a new airbox socket (Rubber boot between airbox and carb)
– I got new clamp for each side of the carb (Did not realize they are different sizes)
– I got a new reed cage to carb assembly (The rubber boot between the engine and the Carb)
– I got a new gasket for the above
I hope it is not the latter two as I believe that requires me breaking the seal to replace.
October 20, 2013 at 10:42 pm #13245Matt DixonParticipant
Sounds like a lean condition to me.
By backing off throttle you are allowing less air = more fuel and you speed up. Fatten it up and see what happens
edit- I don’t own or race a Rotax but just saying by closing the slide and speeding up to me sounds lean…
October 21, 2013 at 11:33 am #13288Peter ZambosParticipant
Matt’s probably right. That’s the way that many Rotax users utilize to get a feel for their jetting.
Your clutch is not disengaging at high revs when you release the throttle. Your RAVE should have nothing to do with that, too. It doesn’t matter if your carb was “tuned,” you still need to match the air quality of the moment. Under the same air conditions, if you went up one jet size, the issue would most likely be gone.
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