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XR600 twin 38mm MIKUNI carburetors (project)

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hi guys!

thats another question:

I found these cool flat mikuni 38mm carburetors from Ducati 600 ss

and my question is:

give it to Honda? there is 5mm bigger bore for each carb ...

my plan is to do intake porting head for connection with these carb ...

Will the engine starts and running at higher speeds with out any choking?

and will sell xr600 carbs and buy Ducati - the same money ...

what do you think?

HONDA.jpg

MIKUNI.jpg

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Those are CV carbs. They are not "flatslide" carbs like an FCR. The CV (Constant Velocity) carb has a butterfly and a slide. A FCR only has a slide and, more importantly it has an accelerator pump. The CV regulated the throttle opening with engine vacuum so you don't have the throttle stumble when you crank open the throttle like you get with a carb like you have from your XR. The CV carbs will be "smoother", but I doubt you'll get any real performance gains from them. You'll replace the stumble you get now from a vacuum drop on rapid throttle opening with the throttle lag of the CV carb. The FCR carb adds an accelerator pump to a "open on demand" carb so you don't have the stumble when the vacuum drops (and fuel stops flowing) because a pump squirts a blast of raw fuel into the throttle bore. FCRs are not emissions friendly due to the blast of raw fuel into the carb... but they rock as far as performance goes.

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You're measuring carbs incorrectly. Carb size is measured directly in the slide area not the intake bell. That said correct measurement will seem even more of an issue when you get it right. Stock XR carbs are 28mm IIRC. Porting the head entails matching the manifold side of the carb not the airbox side. There isn't nearly enough metal in the head to port that big (38mm). And then the fact that oversize valves are only 36/37mm and there's no point making ports that are larger than the valve seats. Dual carb XRs usually end up with 2 something more in the 32/34mm area such as flatslide Mikuni TM34's or XL600 30mm..

Edited by valvesrule

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no worries guys about how i will fit it in to the head... i just wonder to ask you ;

Will the engine starts and running at higher speeds with out any choking?

because, anyway i think this mikuni 38mm which will give the engine few horse power more... and better throttle response

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no worries guys about how i will fit it in to the head... i just wonder to ask you ;

Will the engine starts and running at higher speeds with out any choking?

because, anyway i think this mikuni 38mm which will give the engine few horse power more... and better throttle response

 

PM sent. 

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no worries guys about how i will fit it in to the head... i just wonder to ask you ;

Will the engine starts and running at higher speeds with out any choking?

because, anyway i think this mikuni 38mm which will give the engine few horse power more... and better throttle response

You are going to do a lot of work and end up very disappointed. 

If your choking question is about using the choke at start up, the answer is that the CV carbs will not make a difference. If it refers to being able to breathe better to run at higher rpms/higher load, again the answer is no. The CV carbs are not big enough to make a difference. As Valvesrule pointed out, they probably are not any larger than your stock carbs.

Larger bore carbs do NOT equal higher horsepower. If they did, every bike maker would simply put huge carbs on their bikes. A larger bore carb on an engine that doesnt NEED more air flow will have a LOT worse throttle response and produce no more peak horsepower. Read up a bit on how a carb works and it will start to make sense. 

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You're right, the CV carbs will not make a difference, ok .... but flat CV carbs are easier to tuning than stock ones

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CV carbs are easier to tune because they are about as sophisticated as a bathroom toilet. The complexity that other carbs have is part of what makes them superior. No free lunch.

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You're right, the CV carbs will not make a difference, ok .... but flat CV carbs are easier to tuning than stock ones

CVs have all the same circuits as a regular carb PLUS the vacuum operated slide system. They are more complicated not less. The only advantage of a CV carb is it's ability to compensate for operator error. IE, you whack the throttle wide open and the CV says no you don't. It keeps the vacuum slide shut longer till the engine can actually use the extra air flow. The operator merely opens the butterfly behind the vacuum slide. Throttle response may be better for an inexperienced rider but not as good as a well prepped non CV and an experienced wrist.

Edited by valvesrule
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No, I'm being sarcastic. No, they are not good for XR600. No, I do not recommend it. You are going totally the wrong way with this entire idea. You don't seem to understand that the stock carbs are more than adequate for a motocross engine. Even a roadrace engine can't use what you propose to do. But do what you want even though it's a complete waste of time, effort and money. A CRF450 only has a single 40mm carb and revs twice as high as an XR600 so basically a 40mm is capable of feeding about 900ccs to 7000 rpm. You really need to study up on theory first before pissing away money on any carbs.

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