Well I’m just have bought a pre evo rotax engine and this conversation here is a great overall help.
Many thanks mates.
Cheers from Greece
Daniel, the colder it is go bigger on the main jet to start with. Use the IW24 Denso spark plug.
40:1 is ok, i personally run more oil to help lengthen engine life, you give up a little power but its barely noticeable. The more important thing is running good oil. My recommendation is to run a 100% synthetic oil that is ESTER based…there are different types of synthetics, the one you’re wanting to get is the ester ones. In your end of the world you’d probably be looking at the motul kart grand prix or something equivalent.
Stamatis, welcome…feel free to ask any questions you may have on the rotax engines I’ll try my best to help.
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Sorry not replied sooner guys, I’ve recieved all the parts for the carb so il have a session tomorrow sorting it out. Got some fresh fuel too as im sure the can I’ve got is about 4 months old saving it for the old strimmer.
Not had chance to get any other oil yet so il only mix a small amount up and use that for the time being, just means more cleaning from what I gather of the powervalve? Would you recommend using 30/1 or more?
New spark plugs are genuine rotax ones however I do have a good iw24 still in the engine just needs a clean up points are good. However Il order a few spares.
Ordered and fitted a smaller fuel tank aswell (being a shorty the 5l tank was just to wide for me reaching pedals comfortably) so now the fuel supply side is completely new/rebuilt.
You will be fine using the remainder of current oil you have. You can switch to the ester based synthetic after if you want, its not absolutely necessary but I use it because its the best trade off you can get in a oil in terms of burn cleanliness and lubrication/protection.
I personally run my rotax at 32:1 however you need to be cognicant of the fact that as you use more oil/fuel you actually lean your air/fuel so theoretically speaking you need to jet the carb a bit richer to keep the same air fuel ratios. The difference between 50:1 and 32:1 is only 1% however and personally I’ve not seen much need for different jetting, this could however be different for you with your atmospheric conditions. If you’re gonna run 32:1 maybe go a little richer on your float setting to 2.75mm vs 3.0mm. Then find optimum jetting for main jet, needle clip and air screw as outlined above.
You can clean up your current plug and set the gap to 0.7mm, get spares because once those plugs foul they need replacing most times
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Finally had a set up session out in the garage, nice warm day circa 16c so fairly warm for us.
Anyway put the carb back on needle set to no2 wound the mix screw out 2 turns, fresh fuel and had a go setting up the idle once warm. Started off with the throttle screw just off the carb slide when no throttle applied, so I knew it should do something, however my problem I think is as I’m manually holding the throttle just enough to idle/keep engine running I don’t think the screw actually moves the slide at all.
I’m going to go back to square one give everything a clean with carb cleaner and air again just to be sure.
Just has me baffled as my mates just picked a rotax up and it idles off the bat and its smooth so god knows.
Daniel, the throttle screw does move the slide but only very slightly, you can see this your self by removing the airbox and looking at the slide as you turn the screw fully in. Note here, make sure the slide is in the correct way…you should see the semi circle cut out facing you if you look from the airbox side into the carb…if its totally blocked off then you have the slide in the wrong way.
I’ve been through this idle thing on rotax seniors for years now and I do not have a definitive answer on why some idle and some don’t. I can say for certain none of our race prepped ones idle here in Trinidad but then I’ve seen more than one that isn’t prepped/maintained the way we do our race ones and they’ve idled! It is very strange.
I am 100% it has nothing to do with dirty carb/blocked passage ways in my case, my carb iis spotless. Infact, the reason mine doesn’t idle off the throttle stop screw alone (as it should if everything were working perfectly) is because it is pig rich with fuel, this can be confirmed by pulling the plug after attempting to get it to idle. In my case its always soaking wet with fuel. Essentially with the throttle stop screw all the way in and utilizing the maximum adjustment possible the engine still isn’t getting enough air to mix with the fuel its seeing (or its getting way too much fuel for the amount of air). The latter could explain why the not looked after ones here idled (maybe there was build up hence leaning the engine out enough to idle?? Who knows)
Basically I’ve given up on that, and unless you’re prepared to go grey over this i suggest you do the same and focus on optimizing the fuel delivery at part throttle and wide open throttle.
One last thing you can try is ask your mate to borrow his carb and stick it on your engine and see if it idles. If it does, then find out his maintenance regime…is the carb religiously cleaned? If not it could very well prove the point I made of partially blocked idle jets/airways allowing the engine to run leaner and hence idle.
Don’t be tempted to force it to “idle” by adjusting the throttle stop on the pedal so that it acts as if you’re manually opening the throttle more with your hand …this will mean its using the progression circuit to “idle” and not the idle circuit as it should. Remember the carb cycles between the different circuits based on throttle position…idle circuit is slide fully closed to about 1/8th open which is around the max adjustment on the throttle stop screw. Forcing it to use the progression circuit may fix the problem at idle but when you’re on track and get off gas and on brake the engine may want to self drive (meaning your on brake wanting to slow down but the engine keeps producing torque and wants to accelerate)…believe me… I’ve tried everything!
As i said before focus on the needle clip and main jet out on track…either set mixture screw to 1.5-2 turns from fully in and leave alone, or manually open the throttle by hand as little as you can by hand then optimize the screw till the engine gives the fastest idle speed…if it doesn’t respond at all when you do this then you’re having to open it too much and its no longer using the idle circuit as previously mentioned
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You sir are a legend 
I would be going grey without a doubt I like to have items spot on and tend to chase problems through until sorted sometimes a pain haha.
It is a grice race engine so could be as you describe I’m fairly certain my mates is a standard rotax engine will have to have a look as that theory does sound interesting. Il take the plug out and have a look one thing I haven’t done since putting the new one in.
Carb slide is definitely the right way it doesn’t go in the other way round (I’ve tried just to be sure).
So yeah il just go off the jetting like you say and alter mix to suit.
Again spot on with your post you deserve a beer or several 
Haha thanks a lot Daniel, I am exactly like you’re describing yourself to be and I have been to the moon and back on that idle thing believe me. If you ever do find out more info on it let me know.
I’ve spoken to guys in the states/canada and they claim all their rotaxes idle there. In the UK I’ve seen both sides, some say they do some don’t. My current engine is from Canada and it was said to idle there but doesn’t here. Maybe its something with the air? The fuel? I really dont know. I’ve gone to the extent of purchasing a brand spanking new carb as i thought maybe my old used one had a problem…that was around £250 to find that I’m in the exact same boat…it made zero difference.
I’ve even reached out to dellorto Europe themself and had a lengthy discussion over email… they told me they believe the fuel pump is simply too strong at idle and is pushing too much fuel…but that doesn’t answer why some idle and some don’t.
There’s one other theory that i have that could possibly hold water and its to do with the alignment of the idle port on the venturi with the idle ports from the idle jet. Remember the two very fine holes I spoke about earlier on the venturi that I said be sure to blast with brake cleaner and compressed air. Basically one is the progression port, the other the idle port. This is where the 12.5 and 8.5 venturi stamping comes from. On the early carbs the progression port was sized 12 and the later ones 8. The .5 is the idle port which is the same on both. From what I’ve heard expert carb builders remove and realign the venturis and this is said to improve overall performance of carbs (this is if your carbs venturi was out of alignment from factory in the first place). Doing this however is some sort of black art thats quite complex and you can mess the carb up if you don’t know what you’re doing hence why I’ve never ventured into it. But then if these idle idle ports/jets were misaligned then it’d point to the problem being lack of fuel and not too much as I’ve already confirmed…you see where this is going…i swear I’ve probably aged quicker by a year or two over this hahaha
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Well it definitely was running rich plug was soaked, and after closer inspection I have fuel in the exhaust.
That’s with it at the 2 turns out base setting and number 2 on the needle, we upped it to number 1 on the needle and no difference. Changed back to no 2 and wound the mix screw out another half turn throttle response was a lot better as was bogging down so to speak lacked grunt beforehand. Wound it back quarter turn started to seam sluggish repeated the opposite way from 2.5 turns out and didn’t sound as smooth so left it at 2.5 turns on clip 2 so far still doesn’t idle but runs lower rpm before cutting out circa 1700rpm (throttle screw still makes no difference).
I’m happy with the pick up and the engine runs smooth, still a tad rich I think next step get it out on the track properly.
Hey Daniel, don’t tune the needle on the stand. The needle won’t do anything for idle, the needle is used to meter fuel from the progression circuit. The needle is track, weather and driver style dependent. On a pre-evo you’d almost always be using P1 and P2, maybe P3 if the track is incredibly grippy and you are on super sticky tires. Yes you read correctly, grip affects jetting due to the G force…in very high grip situations you run the engine overall richer. The needle is tuned by making changes and going out on track and feeling how the engine pulls as your foot applies throttle after braking for a corner say. That transition from no throttle to 3/4 throttle or so…that sweeping movement on the pedal…this is what the needle clip will affect. You’re looking for clean, sharp, crisp response from the engine. Too rich on the needle and you find it feels sluggy and gets better when you snap the throttle open like an on/off switch (hence the driver style thing)…too lean and when you go on throttle the engine feels like it does not respond even though you’re going no throttle to wot…then springs back to life when you’re wot. That pre-evo engine gives the best performance from smoothe, progressive input on the throttle…the carburetor is way oversized for the engine and takes very precise inputs to get the best from it, else the engine bogs and you lose time. This is why the fastest most experience drivers will normally use P1 or P2 and their smoothe inputs on the throttle gets the best from the engine. Don’t go more than 3 turns out on the air screw, you dont want the engine running fuel lean on lift off at high rpm… remember the carb is throttle position based…the idle circuit will be fueling the engime when you lift off at 14000rpm to brake…too lean and you risk seizing the piston.
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Ok makes sense, interesting point on the g force tend to forget that affect on track with modern cars with di, well I shall leave it where it is on p2 as that’s where it was for now then.
I had heard of that issue going too lean and I’ve seen a motor seize due to the very same thing so am very wary of that happening, hence why id rather be slightly rich, which it’s now as mentioned 2.5 turns out as at 2 turns the plug was soaked and fuel was coming into the exhaust and running down the underside, still did it at 2.25 but a lot less still slightly boggy as soon as we went to 2.5 the engine note changed and was as you say quite crisp and matched any movement on the throttle. Il see how it performs on track and let you know. May wind it back to 2.25 until it’s been up to temperature for a bit then see, erring on the side of caution.
All a learning curve very helpful when you have good guidance I’d be stuck in the mid with it else so to speak.
Yep, on the g force. The float height is what i normally tune to deal with this. Changing the float height basically richens or leans the carb across all operating conditions. The float height is sort of analagous to base fuel pressure on a car. Changing it affects the hydrostatic head which in turns affects all the circuits within the carb. So if you want to richen the carb a touch overall you would make the float height a smaller number, say 4mm to 3mm and vice versa if you wanted to lean it overall.
Note on the air mixture screw… winding it out is actually leaning the mixture (more air) while winding it in richens it.
Let us know how it goes out on track and feel free to ask anything else thats on your mind.
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