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Post Info TOPIC: Carburettors For Noobs!


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Carburettors For Noobs!
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Ok so I didn't realise how diverse and expansive the carburettor universe is!

I'm reading loads of contradictory information. Loads of information all together that is.


Coming from a performance point of view what is the best carb to buy?


It looks to me that there are so many to choose from. And the biggest carbs don't necessary have the best advantages when tuning. I've just read something about power jets. What are these. From what I've read they're there to lean up the mixture when under full throttle. But Ey? I swear that contradicts everything I've been told about setting up carbs. Why would you want to lean up the mixture when full throttle? Isn't this going to be bad for the engine?

So I was looking to buy a VHSA Dellorto 32mm carb. Since that is what is suppose to be on a 4DL. First problem i'm having is where on earth do you buy this NEW from. EGay had no luck. And then reading up on carbs all together I find other carbs that interest me.

The MIKUNI TM32 carb. Since 32mm was what I was searching for. Then I read that this carb doesn't have this 'Power jet' Which is said to make fine tuning your engine that much easier.


So I look at what carbs have power jets. MIKUNI TMX 30 has this power jet.

5ad9e554-c275-4a98-aeb4-f60847224a65.jpg

Think this one is the one I want ATM. Retailing at 150 + VAT and delivery. Not bad.

Specs

Carburettor MIKUNI TMX 30

flat slider
connection engine: 36mm,

connection filter: 50mm,
main jet 195,

idle mixture jet 40,

Powerjet: 80

Information relating this carb for further reading

"

MIKUNI flat slider carburettors have clearly lower dimensions than the VHSA/VHSB series from DELL'ORTO. This is not only evident when they are installed. The compact and short design ensures an extremely direct response. At the same time, in combination with the enclosed jet system, the voluminous float chamber also guarantees a constant composition of the mix in all ranges, even if extreme centrifugal forces and acceleration forces occur. This makes them the perfect choice for sports Malossi 210cc largeframes and classic smallframe race tuning with engine performances of up to approx. 30 PS.

We stock MIKUNI 24-35mm carburettors in the TM versions (double flat slider, screw-mounted) and TMX (even more compact design, 1 rounded off on the slider side for optimised flow, some with power jets). The power jet which is installed in the TMX 27, 30 and 38 provides the possibility of using a smaller main jet and to enable the range between 3/4 throttle and full throttle to run more leanly in order to get a better torque on straights. So that the mix does not get leaner under full load because of the smaller main jet, this is where the power jet comes in and - depending on the slider position and the load range - injects additional mix into the carburettor. As a rule the carburettors can be tuned via the main jet. In contrast to DELLORTO for instance, MIKUNI not only states the bore, but also the flow-through of the jets. The flow-through of the jets is linear over the entire rev range. This means the individual power ranges of the MIKUNI carburettors can be set and delineated relatively neatly from each other. As a basic guideline, the following applies: MIKUNI jet size corresponds with the DELL'ORTOx2 jet size.

Conclusion: Superb finishing and relatively easy to tune."

I seriously beginning to realise how much of a NOOB I am when It comes to carbs.

I take it the main jet 195 is far too small for the DTR as I'm using 240 main jet. Now are these jets sizes the same for ALL carbs. Or are sizes specific to manufactuer.

How will I know what size Idle Mixture jet, Power Jet to use initially. Where to start from. I know the procedure is to start high and work your way down. But I also realise that a far too rich mixture causes bore wash which is not good.

Some intelligent help and information here would greatly be appreciated. Information on the best/easiest carb to buy would also be helpful. Peoples experiences and views welcome.


Thanks



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Carbs with powerjet allows a 25% smaller main jet. The mainjet and powerjet should add togather a normal mainjet size (for example 195mj + 80pj = 275)

"To summarise various parts of the carb work as follows:
- From cold, closed to 1/4 throttle, choke and idle jet.
- When started, closed to 1/8 throttle idlejet and air screw.
- 1/8 to 1/4 throttle, slide cutaway.
- 1/4 to 3/4 throttle, needlejet (atomiser) needle position and taper of needle.
- 3/4 to full throttle, main jet and power jet if used.
NOTE: The power jet only works when the engine is revving flat. All it does is allows a smaller main jet to be used which weakens off low down carburation. The power jet substitutes the main jet size when the engine is revving high. Both main and power jet should add together to a normal main jet size."
source: http://www.smellofdeath.com/lloydy/jetting.htm


the tmx30 is pretty much plug and play on the dt125 from what I've heard, just need to jet it right.
Just make the intake manifold to 30mm with a flapwheel (It's 28mm standard, right..?) and try make the airbox hole a bit wider with a heatgun or something, else it'll be extra struggle to get it on.

About the mikuni 32mm; There are two different kinds of 32mm flatslide carbs from mikuni, TM32-61 and TM32-1. You can google it and you'll see the differences (e.g. the TM32-61 has an oilfeed pipe, and they are differently jetted). From what I've read, the 32-34mm mikuni carbs will come off very easy with the standard intake manifold and airbox since the carbs' manifolds are so big.

a too large carb will make the low end power bad and a too small carb will make the top end power bad.
Also a too big carb wont give much more if the cylinder ports aren't big enough.
I think 30mm is the most efficient carb to get for the dt125.

Also, stick with the airbox (should atleast remove the snorkle, and perhaps cut off a bit extra if needed), since powerfilter will get wet and dirty all the time.



-- Edited by zander on Thursday 25th of July 2013 09:06:54 AM

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