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Fantic 200 Carburettor Size

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Has anyone experimented with carburettor size on a rear kick Fantic 200?

The bike is standard with no modifications. It is used in classic trials but not on the hard line, so doesn't need more top end power. 

I have calculated carburettor velocity for a number of air cooled trials bikes including Bultaco, TY175 and 250, TY Mono, and Gas Gas Aire, and all have the same velocity through the carburettor. 

The Fantic 200 carb would need to be reduced to 22 mm to match the velocity in all of the bikes listed above. 25mm is standard but the bike is not used for trail riding or road use.

Has anyone tried a smaller carb to increase throttle response?

Regards,

Peter

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Velocity....or volume of air? By lengthening the inlet manifold you can gain an increase in low to midrange torque, thats not an increase in torque just manipulating where the torque is. High revving engines have short inlets and big throated carbs, mess too much with what the manufacturer intended and you may spoil the performance for trials work. You say ‘the bike is not used for trail riding or road use...more explanation please. If you look at engine power and torque figures you will see that there are some commonalities where by the torque and power  figures at certain revs are the same, this is due to our earths atmospherics. For smaller engines like a 125cc or 156cc the same rule applies. The velocity is speed and the Venturi creates the increase in air speed as it passes over the main jet (like an aircrafts wing shape) causing the fuel to be drawn out into the airflow. Bigger carb allows more air but the same ‘velocity’, superchargers and turbos force more air volume into the carb or throttle body for FI systems. Gravity unfortunately is the regulator that engine tuners have tried to overcome but the only way is pressure charging in some form.

Smaller carbs ie 22mm would give you better throttle response at low revs but have a restrictive effect at higher revs reducing effectively the power available. Two stroke tuning is not just about the carb size, alter one thing in system and it can affect the rest. The exhaust system affects engine response and its ability to ‘breathe’. Varying the length of the exhaust tail pipe affects the engines abilty to rev hence the way an expansion chamber is designed. Should you try a 22mm carb you will most likely find that the engine is good for trickling around obstacles and gentle climbs but the moment you want that extra ‘zap’ or power to lift the front wheel or climb a significant hill etc you will find the engine lacking in power. Its not by chance that nearly all trials bikes with carbs have very similar carb sizes. What I will say is that you might find that two carbs of the same size but different make will respond to your engine their own different ways due to design and fuel metering. So from idle to quarter throttle the fuel metering may be far better controlled by one make to another, then the transition stage from off idle to more than quarter throttle opening you may find the engine experiences flat spots or hesitation while the fuel relents from the idle to main jet and throttle slide and needle movement....some carbs are really smooth whilst others a rough and cause unpredictable engine surging. Tie all this in with engines of varying states of wear with seals, rings and bearings and you can probably imagine that the previously mentioned faults will exacerbate the situation. Fantic had a pretty good idea of what was best and I don’t think that altering the original carb size will improve the little Frantic Fantic, these bikes do a brilliant job when pitted against the rest. Spot on fuelling and engine set up is what will make your bike useful for the type of trialing that you do 👍

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Section Swept.

Thank you for taking the time to put together a lengthy and informative reply.

My comment that the bike is not used for trail riding or road use is really describing that I only do classic trials with the bike. When new the bike had to be set up as the purchaser could use it for trail riding or commuting requiring higher engine speeds for longer periods. I am only using the bike for easy classic trials with short loops and all sections ridden in first or second gear. 

I have experimented with 2 stroke exhaust design in the past which is a fascinating subject, but I am not proposing a change to the standard exhaust.

Re air volumes between the carb and cylinder there is also no scope to vary this if the existing air box is retained.

I haven't followed your logic that different carburettor sizes pass more air but somehow the velocity over the venturi remains the same. Velocity is proportional to velocity pressure so both must surely rise together?

I realise the smaller carburettor bore will reduce top end, but I never use that anyway. So the question is whether gains can be made to bottom end and mid range by fitting a smaller carb as these are the rev ranges I use.

At this stage it appears no-one has tried it. 

I may try the 22 Mikuni off my TY175 however I will need to make adapters and it will only be luck if the TY jetting suits the Fantic. Has anyone tried a Mikuni on a Fantic?

The beauty of using a Dellorto is all the sizes 22, 24, and 25 have the same body dimensions and spigot sizes, as well as being compatable with the standard jets as a starting point.

Regards,

Peter.   

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In case anyone is following this I am adding a note to complete my reply above.

I have been talking to Dellorto and they confirm the venturi pressure is related to slide diameter not carburettor bore diameter.

The 22, 24, and 25 Dellortos all use the same slide diameter so I now understand Section Swept's original comment that they will all have the same venturi pressure.

I guess this also explains why gains can be achieved using a flat slide carburettor, though I was only looking at the different Dellorto body sizes.

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Think you will find that a flat slide carb is great for wot performance and more instant off idle zap, but for trials work a conventional slide carb offers best fuel metering in the first quarter of throttle opening. Its taken many years to perfect fuel injection to get to the stage where low speed throttle control is smooth as with a carb and not erratic. Some tuners throw away the fi and fit carbs for more refined fuel control but then they are after out and out power. Fi can now be so accurate in its fuel metering that its being used on nearly everything thats got to be EU emissions compliant. Note flat slide carbs tend to be affected by wear more quickly due to less surface support area for the actual slide, an additional consideration that offers less friction when snapping open the throttle...why some like them too.

To reiterate, a small carb will improve low down engine response to some extent but it won’t increase the torque, a longer inlet will also improve the low to mid range performance...have a look at variable length inlets and two or more throttle plates to affect air flow into the engine. The plenum chamber in most engines hides either sliding venturi tubes or static long tubes to give plenty of oomph in the low to mid range but a breathless performance at the top end...variable length stops this as does more than one throttle plate/body. Not so easy to accommodate in the confines of a trials bike or most other bikes, but common on many vehicles since the 90’s.  

You could experiment with restrictive collar sizes (not shirt size🤪) this will restrict the volume of air that passes through the carb but not the velocity which will remain constant ( effects of atms pressure).

As an aside are you located high up in NZ as air density will be affected by thinning air (reduced oxygen content) and humidity will play its devious part in messing with carburation. Its a pity we can’t get our heads together in person (Covid safely) as it would be great to experiment. A 25mm and 50mm inlet extension can make a noticeable change to low and mid range but the Fantic may prove to be problematic space wise. The Rotax crank induction engines had reasonably long inlet tracts hence their suitability for trials work the carb being some way back from the actual crankcase. Then look at road race 2t race engines with c/case induction and the carbs are virtually straight onto the crank case mouth with hardly any manifold at all!

Its perverse in some ways, people chop holes and add extra slots and use ‘special’ filter elements to improve engine breathing or fit in the case of Bultaco larger volume air boxes from the 159 model to give the engine something to go at...a reserve of air if you will. Yamaha introduced their additional air box on the IT range to improve mid range engine response but soon lost it to other developments, I’m surprised its not reappeared on 2t trials specials, it seemed to be beneficial...food for thought.

Hope you find achieve the result you are looking for🙂👍

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Section Swept. 

Thanks for the additional reply. I was able to try another Dellorto PBHL25BS carburettor today and this resolved the current issues and confirms the need for a replacement carburettor.

As it seems no-one has experimented with different carb sizes, and I don't want to pay for a carb that doesn't work well, I have decided to purchase the original size.

I like your suggestion of trying different diameter collars though so will look at how to achieve this. You are right that the Fantic offers no scope for extending the inlet tract unless a custom air box is used.

Your mention of the IT range reminds me of the boost bottle I made before these were in vogue. A 75 mm irrigation tube the size of a baked bean tin connected via a hose between the carb and reeds. Not sure it increased response but the bike idled much better and would idle as long as you kept fuel in the tank. I think its best use was psyching out other competitors on the start line rather than any increase in performance.

Thanks again for taking the time to reply. Dellorto (Eurocarb) have also been good, so I have learnt a bit by asking these questions.

Regards,

Peter.

PS All my riding is low altitude so this one variable I can ignore. 

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This topic has been both interesting and thought provoking with some great information. With regard to carb venturi shape and size some I have seen have a sort of keyhole or upside down pear shape for presumably faster airflow at minimal slide openings, might this design offer any advantage for trials use?

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10 hours ago, trialsrfun said:

This topic has been both interesting and thought provoking with some great information. With regard to carb venturi shape and size some I have seen have a sort of keyhole or upside down pear shape for presumably faster airflow at minimal slide openings, might this design offer any advantage for trials use?

Yes you are correct and I think you will find this is peculiar to certain engine design such as generator engines and some commercials. Although keyhole and pear shape I have seen in cold start devices for fuel injected engines. Some throttle plates had a small valve in them to allow air to bleed into the inlet on overrun to reduce emissions supposedly. Others used tiny T shaped cut outs for idle stabilisation, later to be replaced by an electric solenoid valve or AIS air idle stabilisation valve as it was labelled. Do you remember some big four stroke singles  used more than one carb, both were of different choke size to offer more power but the fad never lasted very long...neither did the engines I fear! I want to see some enterprising person try fitting fuel injection to an older 2t trials engine, but I think this will only confirm that the carb has a better ability to control the fuelling from idle to mid throttle over the FI system. Wal Phillips tried with his basic fuel sprayer but his success can be measured by its failure to be reliable or work at all on some engines. Even the first fuel injection of the electronic variety was vert crude by comparison to what it is now, I cut my teeth on mechanical petrol injection and stayed with this development right through to 3D mapping. People would be amazed at just how rough and ready some ECU fuelling maps are even now. All of the shapes used for carb Venturi (choke) that you mention had an effect but invariably to minimise fuel usage and hence emissions at idle and fast idle. Roll on the Euro emission legislation...designers and engineers get more inventive every day!👨🏻‍🎨👨‍🏭🙂  

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