Power Republic - Dellorto Carburettor Service For Rotax Max 125

Hi there. In this video I will be taking you through the steps for servicing your Dellorto VHSB 34 carburettor for a Rotax Max 125 go-kart engine.
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[@PowerRepublic]

Jakub. If your not using this carb on a go kart engine like we do, then you could use a lower float level as the engine is not as stressed as when we use them in racing conditions where Wide Open Throttle (WOT) is used predominantly. Common factory settings for the float level is approximately 22.5mm. That should be good float level for a street bike. Also too, you can adjust the idle quality with the air bleed screw (flat blade screw at the front side of carb on the angle). Set this at 3 turns out from closed. Then you should be able to close the throttle slide/idle screw to bring down the idle speed without the bike stalling. Hope this helps. Derek

[@EricBanner571]

Hi, in the last week, I have just about watched every one of your videos and have learnt so much in such a short time. Thank you for your easy to understand tutorials. Just in reference to an earlier question, I am trying to get an older 2009 Rotax max senior 125 Kart going after it's been sitting for a number of years. I have just recently acquired the cart so unsure how it was running previously. I suspect that the Kart was lightly used as there are no wear marks on the chassis underneath. The bore still has cross hatching and the piston is not scored. It has plenty of compression. I changed the spark plug, air filter, a kit through the fuel pump, thoroughly cleaned the carb as per this video, I did not change the needle and seat as there was plenty of spring left and the float was moving up and down freely. I adjusted the mixture screw to 2 turns out, Needle clip 3, adjusted the float to 23.5mm, changed the fuel lines and fuel filter, mixed the rotax oil / fuel ratio to 25ml /litre,. My understanding is that these old carbs run from 158 - 165 jets. A 165 jet was in the carb when cleaning and it also had the restrictor in the exhaust. I have now removed the restrictor. The engine started straight away and sounds OK but but would cut out every now and again like the idle was a little low. The idle at 1800rpm by ear sounded a little low to me. The idle and mixture screw make no difference when adjusting and there it seemed to be more smoke coming from the exhaust the longer it was running like there was unburnt fuel in the exhaust. There was also a little oil leaking from the header / exhaust connection when running. I tried a 152 jet but still can't adjust the mixture or idle screw. If I turn the screws in or out, it makes no difference. It was a cold day in Sydney today so the temperature of the Kart only came up to 25 degrees. Any ideas would be greatly appreciated.

[@thegherkin469]

Great tutorial, really enjoying the channel. Thank you! How often would you recommend disassembling and cleaning the carb? Also wondering if you’d consider doing a video showing what checks and maintenance should be completed to the kart after each race, and at the end of the day? Cheers!

[@PowerRepublic]

Hi Jarren, thanks for the positive vibes! I would clean the fuel out the carb after each use and leave it stored empty. You can take the bowl off and blow it out too and that way it is good to go for your next use. Also too, we will add your suggestion for a maintenance video to the list.

[@tvevol]

Correct me if I'm wrong.....but a higher measurement float setting is actually a lower float level and therefore a leaner float setting.....not a richer setting as mentioned in the video

[@PowerRepublic]

correct my friend. In the video we are upside down which can throw you/me off. The higher the measurement i.e 24mm the richer the float level. The lower the measurement i.e 23mm the leaner the float level. Sorry if that's a little confusing. you can also measure up from the float bowl sealing face too eg 2.5mm, 3mm, 3.5mm using some Allen key or feeler gauges .

[@kristijankopjar4629]

Can you please explain detailed how to check and adjust float height? My tm k9b run superb on the straight but in the corner bogged down completly, soon after I go to the straight the engine go like hell. Can you please advice how to resolve this? On the stan still throttle response is superb, the only problem is in the corners...

[@PowerRepublic]

maybe try to add more float height. then rebalancing the main jet to suit. we have a few video's on this subject too if that helps?

[@mechani-coolworld9154]

I hope you can do a demo also on how to check clearance of piston to cylinder

[@PowerRepublic]

We have a tutorial series of videos, Check them out at our website

[@JakubKlubkoSmejkal]

What is the basic setting for the slide adjust screw? I'm just fitting VHSB 38 carb. on my motorcycle VOR 503 and when I start the bike, the engine seems to be heating up very quickly (much faster and more than usual) on only idle. The idle is quite high compared to previous PHM 38 carb., sometimes it fires into exhaust. Then when I try to lover the idle RPM, by unscrewing the slide screw, the engine starts to lose RPM quickly, so I have to give it a little gas, the RPM rises and then it slowly descends on some constant RPM. So far I couldn't recognize if it's rich or lean from the color of the spark plug, as it's black as it always used to be with previous carb. I’ll check and reset the float level. That’s a good information from the video, thanks for it.

[@terrier53]

Thank you so much for all the videos that you make, we appreciate it. I do have a question, are you placing the vernier at the bottom of the float bowl and then reading the measurement at the piece that you bent?

[@PowerRepublic]

Thanks legend for watching. You can do either , then adjust from there. Do what is easy for you as long as you have a reference point to make adjustments from

[@papaz17]

Hi there. Really enjoy you videos, can you give me an idea of the starting positions of the air/fuel mixing screw and the idle adjustment screw. Regards

[@PowerRepublic]

hey Pap, thanks for watching 1.5 AB screw and the idle screw i normally turn all the way in if i want it to idle

[@adambroc2842]

Hi. I see lads in the Rotax class on the roll up laps have there hand on the motor. I know the club and shut the high speed needle off. Do they do that with the rotaxs too

[@PowerRepublic]

I think it is the arm in front of the radiator to heat up the engine. nothing to do with the jetting

[@coltincraigo5007]

Do you have a video on Dell’Orto carbs for KZ? I don’t think I saw one. Would be nice to have somewhere to reference back for different things like the inner and outer pilot, mains, needle position and so one and when you should be changing it. There’s a lot going on and it’s hard to remember everything.

[@PowerRepublic]

Hi Coltin. great question. I haven't used the KZ enough to make a vid with instructions on the tuning of there carbs.

[@johnlangebraten3448]

@@PowerRepublic HI from Norway PR. Will it come in near future some videos of VHSH 30? Mine is just not responding out of corners and need some rpm to accelerate, sound like to rich (much gas) tuning? The spark plug is pretty wet and the pinion has no black layer on it, as I understand it should.

[@stevethompson9007]

Top video (as usual) and top site for so many of us. Nice one you guys.

[@PowerRepublic]

Thanks Steve, are you new to karting ?

[@stevethompson9007]

@@PowerRepublic No, been involved for 5 years as a 'mechanic' dad whose son is now in seniors BUT learning new stuff all the time!!

[@stevethompson9007]

We're with the Jersey Kart Club in the British Channel Islands so a long way from Oz!

[@PowerRepublic]

@@stevethompson9007 yeah virtually we are racing with you 😉

[@PowerRepublic]

Troy Kubler asked where to measure the float level from. Freeze the video at 1.43sec and you can see the verniers on top of the needle jet (where the main jet screws into) and measure down until the verniers depth gauge touches the float lever. Measure both side and make them the same. 23mm is lean, 23.5mm is good all rounder and 24.0mm is rich. You can run more or less as your set up requires.

[@IggySport1388]

hi, can you explain how to decipher when to use lean or rich on the float height setting? so when for instance would you use the 23mm setting and when the 24mm setting

[@PowerRepublic]

Christopher Ramnauth Sure can. 23 mm is lean and I would use it when it is raining. The engine isn't as 'loaded' off the corners, there is a lot of moisture in the air and the engines run cooler because of water spray cools the radiator better. Then 23.5 is standard starting position. If you put your foot on the throttle and there is a hesitation/no throttle response, then the float level or needle position is too lean. So then I would go 0.5mm richer on the float level (i.e from 23mm increase to 23.5mm or 23.5mm increase to 24.0mm) If you come off a corner and put you foot on the throttle and the engine splutters and hesitates to go (and goes better when you stomp on the accelerator) then the float level is too rich and I would lean it off 0.5mm (i.e from 24.0mm adjust to 23.5mm) I hope this helps. Thanks for the question Derek

[@IggySport1388]

thanks for the swift reply derick, understood on what you said... but how do you decide if to change the float level or move the needle clip position? both from what you're saying can solve the same problem so how do you know if to leave the float level alone and adjust the needle clip richer/leaner or vice versa?

[@PowerRepublic]

Christopher Ramnauth the float level is not as subtle is the needle. Use the needle for fine tuning. You will have experiment with it as it is driver specific.

[@kevindanahy4665]

I had changed the needle and seat and still have a flooding problem for VHSB 34QS with lean setting on the floats. I checked the floats and they are good. I tried blowing in the fuel line with carb upside down and bowl removed and couldn't get any air through unless I flipped the carb over. Is there a pop-off test pressure that can be used to check the needle valve or is there something else that might be causing the flooding issue?

[@PowerRepublic]

Have you checked for a cracked reed ?

[@paulrenwick6231]

HI, New to karting but have watched all your videos and have learnt so much. I have 2 carbs for my Rotax max. 1 is 1998 and the other is a 2008 model. Is there any difference between them excep for the screw on the bottom of the float blow? Thanks

[@ot77racing27]

@Power Republic When installing the main jet, are you using the number on the jet, or are you pinning the jets and recording the pinned number?

[@PowerRepublic]

if you have the pins, do that as it is more accurate

[@jesperek6402]

Is this the clean that should be done after every second run or so? Or is there a fast way to clean the carburator after every run? I just empty the carburator from fuel after every day on the track, and it seems to work well? But a fast way to clean it wouldn't be wrong!

[@PowerRepublic]

yeah tipping it out is the best way to go about it and the fastest. the full strip down and clean method maybe just after the rain or major rebuilds

[@rm19-racingchannel62]

What carb setting in jets is the best for 3,6 or 5,2 grams floats ? 5,2Gr is best with the K27 needle and the 3,6Gr best with the K98 needle, this is what they told me? Hope you can help me a bit further? Thanks for al the great videos ! Keep up the good work greetings from Holland.

[@PowerRepublic]

we have another rotor tuning video that is a little more in depth. check that out if you haven't already. the standard rotax EVO parts are the ones we use and recommend

[@adamjones9506]

Should you be wearing safety goggles when cleaning your orifices?

[@matthewcarlesso9503]

no be a man

[@downvisor]

Hi. Can you do a video on venturi removal for the Rotax Dellorto?

[@PowerRepublic]

limosinemurders sure can, stay tuned for a Wednesday mid week hitters😎

[@The2fast83]

Hi the pilot jet has 2 parts is the smaller jet needed on Aprilia rs 125

[@PowerRepublic]

Not sure on that one, did it come with one?

[@speed_madness]

Hello! Your channel is awesome! Congratulations! Ive got this carburettor, but im having issues on the high RPM, when the engine gets around 12.000 RPM, it just cuts the fuel, do you think it could be something with the carburettor? Thank you!

[@PowerRepublic]

rotax carby? yeah maybe. otherwise check the float level and a new fuel pump kit if it isn't new.

[@EricBanner571]

Great video. Quick question. if I take the restrictor out of exhaust, what would be the ideal main jet and clip settings for 20 meters above sea level in sydney with the Rotax 125 max? Many thanks

[@PowerRepublic]

125 main jet normally gets the job done and clip 3or 4

[@EricBanner571]

@@PowerRepublic Many thanks. Cheers

[@andyevans116]

What size jet would these carbs use on the 122 engine for an mx 125. Highest to lowest would you say, thankyou

[@PowerRepublic]

For the EVO carb we are in 120-125 main jet range. For the older carb we used 158-165 mostly.

[@BigG-grant]

How tight should the air bleed screw be?

[@leannegray80]

Ggay mate just wondering if there is a larger version of these carbs available lets say about 39mm to suit my honda cr500 dirt bike. And how does it cope with altitude changes i under that they rejetting to suit but how far can you the range change before rejetting thanks mate

[@ot77racing27]

Hi Derek. Does the float height make any difference whether it is for a Senior, Junior or Mini Max? Also, what RON petrol are you using in Australia? (UK either 95 or 98).

[@PowerRepublic]

HBT, thanks for tuning in and sharing the videos. Senior and Junior almost the same except for a leaner main jet on the Junior. Mini Max we lean out from 23-23.5mm max normally cause of the high squish and exhaust restrictor.

[@kubilay6151]

Thank you so much , I will ask something ; On the jetting guide app , there are 3 methods , 1. Needle Position 2.Air Screw 3. Float Level . Should we modify all these for more performance ? Or Is it possible to do same thing ( I mean Float Level Modification ) by using airscrew .

[@PowerRepublic]

kubilay hs not so much for more performance, just for fine tuning for the conditions. With the EVO carb we mostly use 122 main jet, clip 4/5 (bottom clip settings) and 1.5 Air bleed. Start there. If that works, leave it and focus on kickin ass with your driving.

[@BernieTheEvilF1Ruler]

@@PowerRepublic Is that just for Queensland temps or would it work down south? I'm often on 125 and 4/5 in South Vic

[@kubilay6151]

@@PowerRepublic Thank you so much.

[@tomi4744]

Was this XS or QS carb? Mainly interested on float height and is there any difference between those two models with it.

[@PowerRepublic]

Its n XS Evo carb. mostly set them at 23.5mm - 24.00mm IMO. the older carbs seemed to work best at 23.5mm for us

[@McPhersontronic]

Hi, I have a problem with the second jet, the little one underneath, I can't remove it or tighten it again, is there a problem that it stays loose? thanks and good channel.

[@PowerRepublic]

yeah it should be tight, maybe you could drill it out with a Left handed drill (reverse twist) and see if that gets it out. otherwise leave it there and go get some👊🏻👊🏻👊🏻👊🏻

[@McPhersontronic]

@@PowerRepublic ok, I'll try it there, let's see how it works, I don't dare to drill it. Thanks

[@IggySport1388]

one other question Derek, the ranges you mentioned for the float height....is this standard regardless if you are on 3.6g floats or 5.2g floats ?

[@danielcgomez]

and where are the measurements being taken?

[@Villaagee]

Hello! Question. I just bought me a shifter kart and took it to my local track. As I hit corners on 2-3 gear (depending on the corner) my kart begins to lose throttle to the point where I have to down shift and let the kart pick itself back up. There’s times that I lose all throttle leading to the kart stalling and dying. Any clue why it may be doing that? Thank you!!

[@PowerRepublic]

check the fuel pump and fuel lines, could be simply running dry on fuel. also give yourself a good carb service too if you haven't already. let us know how you go

[@Villaagee]

Power Republic will do! Thank you! And amazing work on the videos!

[@woozhi9218]

What is wrong with my rotax pre evo? choke the carb with my hands and it barely runs only(choke is on)

[@PowerRepublic]

floats upside down, fuel pump is no good, leak in the vacuum line, check needle and seat too or main jet for a blockage. start there and report back

[@woozhi9218]

​@PowerRepublic It seems like something is preventing fuel from entering the float bowl. I tried blowing with my mouth and I could not get air to enter is it normal?

[@BLITZKRIEG1]

could you remake this video and go into more detail as well as proper close up shots of what you are doing? thanks.

[@PowerRepublic]

kzbin.info/www/bejne/fKuxZKmlor2hfJY

[@BLITZKRIEG1]

@@PowerRepublic cheers

[@robbielee415]

I I’m having trouble with my rotax max pre evo. It like it’s getting too much petrol and flooding it self and backfiring . Dose this sound like a carb problem to you .

[@PowerRepublic]

if it is overfilling and flooding, then I would change out the needle and seat, and give the carb a good clean out like the video. also check the floats are not damaged and leaking. hope that helps

[@robbielee415]

Power Republic I will try that and cross the fingers . I’m missing the black pipe that goes from one side of the carb to the other . What is the function of it . And is it nesserey

[@donnywalker4691]

When you measure the float height is it from the outer body of carb with gasket in place

[@PowerRepublic]

Hey Donny, you can as the mreasurements are all relative to your situation., I personally like to measure down without the gasket, but it is not important

[@Judah1994]

Where can I buy a gasket kit for the carburetor

[@PowerRepublic]

King mafia Vlogs where do you live? I can send you one here in Australia

[@aneleace3414]

what happen if i dont put the gasket under the m tube

[@PowerRepublic]

Probably leaks or damage the alloy when you do up the ‘seat’.

[@navdude8201]

Hello I like to ask what references point you use to measure the float hight

[@PowerRepublic]

Hi Nav, the bottom of the needle jet tube

[@navdude8201]

Thank you for quick response

[@navdude8201]

Can u show a picture if u don't mind

[@richmac8438]

what changes are needed to this carb if running on the aprilia rs125?

[@PowerRepublic]

I would start with a bit less float level and then ride your bike around for a bit to see what changes are needed. The difference for a road bike to a go kart (assuming your talking about a 125 road bike that is), is that the go karts have only one gear and very sticky tyres without a differential. This makes the go kart engine work very hard around corners and under acceleration off corners. What this means is that us go-karters racers use richer jet settings than the equivalent motor bike, especially if your not racing your bike and more use in on the street and not the track. Are you having any specific tuning issues with your engine?

[@kochFan1]

Hi, have not fitted to the engine yet...I was aware that the 34vhsb carb on motorcycles was a different configeration to the rotax max variant which goes by the code vhsb34QD the QD code must denote rotax max application?? anyway, I know the carbs may use different floats and needles also?? I have a few of these rotax max carbs lying around and was also dabbling with using them on a yamaha TZ250 v-twin in a social stockman superkart. It would be usual to use 36mm or 38mm mikuni carbs on that engine...but I don't have any, and several karter prefer dellorto carbs on twin engines as they claim to find them easy to tune. Would you think that one could just re-jet rotax max carbs from an older pre-evo engine(VHSB34QD ??) to work ok on a TZ-250? or would I be wasting my time?

[@PowerRepublic]

I haven't done that myself, but at the end of the day its just a carb with holes and jets so I don't see why not. If you have some carbs and a lambda sensor, you should be able to test and tune it either on the road or on a dyno. Maybe they are easier to tune because they are 2mm-4mm smaller than the standard mikuni carbs?

[@themessenger4610]

5 Star, Thank ypu

[@PowerRepublic]

thanks mate, and thanks heaps for tuning in and watching the videos

[@robertorochamartinsrocha7041]

Hi. how to set the floats on a PHbl? thanks... new carburator and over flow constant. Montesa cota 315 r. thanks

[@PowerRepublic]

Hey Roberto, yeah I'm not to familiar with that carb mate, what engine is it off?

[@robertorochamartinsrocha7041]

@@PowerRepublic many thanks for your reply... the carb is on a trial bike. Montesa cota 315 r... i will try to come back, maybe it's the inclination of the carb... i don't know, and i don't know yet... thanks

[@robertorochamartinsrocha7041]

@@PowerRepublic the manual says: toadjust the float level, band the float arm carefully, until the float tips contacts the float arm (float level 18,5mm)... i will try that... thanks

[@PowerRepublic]

@@robertorochamartinsrocha7041 okay mate, give it a go and see. sorry I can't help out more for you on this one

[@robertorochamartinsrocha7041]

@@PowerRepublic thanks mate... i have a mikuni 😂... i will not loose more time on it... thanks

[@lindsayriddell7693]

Is the same carburetor being used on the Rotax DD2?

[@SomeSaltySoviet]

yes they are the same, though they use different settings and main jet sizes

[@pauloagurto7154]

Which is the best setting for a micromax, I use the main jet of 118, and the needle in position 2, however I do not exceed 9300rpms

[@PowerRepublic]

Hi Rodrigo, There is no magic solution for any jet of any carb, it is always a combination of elements (weather, driver, gearing, track, elevation etc) that decide the jetting. 9300 rpm is okay for Mircromax as the exhaust restricts the engine from reving much past 10000 rpm.

[@RacerSONGHARIM]

Hello~! Let me ask you a question. I have recently encountered the Rotax Micro engine. As the weather got warmer, the engine power went down. I would like to change the setting. I want to know the standard of the float level. If it rains with 23.5mm standard, you say 23mm. So if the weather is hot, should it increase to 24mm? Do I need to reduce the humidity as the humidity increases? Should I increase? Is that right? And what settings can help you increase engine torque?

[@PowerRepublic]

Hi there. Thanks for your question. Once you have the correct float level for you and your engine, then you should just be able to adjust the main jet for humidity changes and temperature. So as a general rule when the humidity is high, you would go down jets and when it is dry you go up jets, however, there is a point where this rule no longer applies and the best way to know what that point is is by experimenting in the different weather conditions. Hope this helps.

[@RacerSONGHARIM]

So, for example, on the basis that other conditions are right ... If the humidity is high, the oil level goes down to 23mm => 22.5mm If the humidity is low, the oil level goes up to 23mm => 23.5mm Do you mean you can get the setting under the theory of being right? Or is the height of the oil more sensitive to temperature than humidity? I want to know the standard for determining the height of the oil surface.

[@PowerRepublic]

yes that is correct. once set to your liking you should be able to change the main jet for the humidity variations

[@RacerSONGHARIM]

I did not want the answer, but I appreciate the answer. I will take the standard and test it. Some mechanics say that the main nozzle is standard but the oil level is the most important, so I wanted to know the standard of oil level adjustment according to the weather. Cold days, hot days, humid days and dry days. I will test it first. Thank you very much. Let me ask you one more thing. If there is a lot of fuel in the carburetor What problems do you encounter?

[@shrimpo13]

Hi, I've been having a few issues with my pre evo senior rotax. Kart runs OK for most of a session but every now and then out of slow corners, it will die for a split half/second as I'm accelerating out of the corner. (Gearing is correc for the track) Seems to get slightly worse as the session goes on. Plug looks to be a bit lean with a 160 jet. Needle is clip 4, and air screw is 2 turns out. Any advice? Thanks.

[@PowerRepublic]

Hi shrimpo, rise your float level by 0.50mm and see if that help. if engines get worse as they get hotter then it is normally another indicator that it is a touch lean.

[@shrimpo13]

@@PowerRepublic thanks for the super quick reply! As I'm relatively new to the world of karting and from advice of others, I was lead to believe if it was bogging/dying out of slow corners it would be running too rich down low? Other ways to adjust this, other than float level? Thanks.

[@fotokopirnicamoricahanprin9689]

from where to where you are mesauring floating postion? thanks

[@PowerRepublic]

Hi, I measure from the bottom of the brass needle jet (with the main jet removed) to the top of the float levers.

[@barrypinnell7197]

the overflow hose looks to be one piece connected on each outlet is this normal

[@PowerRepublic]

Hi Barry. Great question. Yes that's how they come from Rotax. Its a breather hose with the split in the centre. If we ran the hoses down to under the carb like they do on Dirt Bikes, we would have oil and fuel on the race track.

[@barrypinnell7197]

thanks

[@warriorwal]

hi Derek. thanks for all your videos, I've learned a lot. I've got a Rotax Max 125 running restricted. I've got an issue, where seemingly based only on throttle input, the engine cuts out. So I will have throttle approx 50% down, it runs smoothly and quite well, then past a particulary point, it basically stops running. Any advice on what this could be? Originally I had an issue where I lost the top end (that last bit of power in the rev range which changes the sound of the motor) - then I took carby apart like in this video, cleaned it, then I had this new issue where it seems based on throttle. Any ideas? Thanks. Wally

[@warriorwal]

just to add, the kart is only 10 hours old. I have not done much servicing on it. I checked the power valve, this seems to be moving fine.

[@warriorwal]

Derek - I've since realised I did not 'unwind' the air bleed screw 4x half turns. Do you think this is what could be causing the issue? I've fixed it now - yet to take to track to test.

[@PowerRepublic]

Hey Wally, yeah I reckon that could be it. especially if you have introduced a new problem. If you are using a Rotax EVO carb and Engine, set the floats at 23.5mm, 1.5 turns open for the air bleed, 120 main jet, and clip 2 or 3 on the needle. That should get you started.

[@warriorwal]

hey Derek - just to follow up on this, we took carby apart, nothing else looked wrong - put it back together and corrected the air screw. went out Sunday, it ran very well. thanks for your replies.

[@ptrsrrll]

Poor Troy, he hasn't been well lately.

[@PowerRepublic]

he's coming back

[@markdarunt]

Safety first, where's your safety gases to stop shit getting in your eyes?

[@PowerRepublic]

safety never takes a holiday MT