28mm Carb jetting

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fastharry
Posts: 63
Joined: 06 Oct 2011 10:38
Location: leicester
Location: Leicester

28mm Carb jetting

Post by fastharry »

I am about to order a pair of 28mm dellorto's that will be going on my 410 flattrack race bike, can anyone suggest any jetting so i can specify when i order.
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72degrees
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Location: West Midlands

Re: 28mm Carb jetting

Post by 72degrees »

fastharry wrote:I am about to order a pair of 28mm dellorto's that will be going on my 410 flattrack race bike, can anyone suggest any jetting so i can specify when i order.
Insufficient data!

Given my long saga with trying to get mine set up perfectly on two 350s I'm loathe to make a recommendation. They are very sensitive to air filter arrangements and to some degree exhaust system, but in my experience the cam being used makes a big difference too (they run identical ignition setups).

My current settings are:
For the one running an L5 cam and NLM 2:1 : Main Jet 132, Atomiser 262T, X1 needle (leanest position), 40 throttle slide and 45 idle jets.
For the one running a ? cam (reground but a kind of Sport/2+2 hybrid according to timing check) and a race replica 2:1 acquired from harrymuffin: Main Jet 125, Atomiser 262T, X8 needle (richest position), 40 throttle slide and 45 idle jets.

Both still have a bit of a flat spot mid-range but seem OK wide open or just coming off idle on progression. I'd experiment with different slides but the £30 each price rules that out.
fastharry
Posts: 63
Joined: 06 Oct 2011 10:38
Location: leicester
Location: Leicester

Re: 28mm Carb jetting

Post by fastharry »

What made you choose the 262T ? i do not see it listed for other models? i will order both needles and see how it go's. but i suspect it will need the 50 slide, to help the midrange flatspot, so many people ignore the slide, it plays an important role and should be changed if necessary, if you have 2 sets of 40's you could file some of the front edge to get the same height as a 50.
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72degrees
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Joined: 31 Aug 2007 21:24
Location: West Midlands

Re: 28mm Carb jetting

Post by 72degrees »

fastharry wrote:What made you choose the 262T ? i do not see it listed for other models? i will order both needles and see how it go's. but i suspect it will need the 50 slide, to help the midrange flatspot, so many people ignore the slide, it plays an important role and should be changed if necessary, if you have 2 sets of 40's you could file some of the front edge to get the same height as a 50.
That's what they came with when I bought the first pair from Eurocarb over 30 years ago - too long ago to remember but presumably that's what they recommended for a Morini. I have actually got spare pair of slides but can't remember what they are - possibly 50, except that when I tried them many year ago they didn't seem to help even though from the smoke people say it puffs out I suspect my problem is rich mid range . I must dig them out again and check. A different atomiser hole pattern might be better. I keep meaning to get some time at a rolling road / exhaust gas testing place.

On the hill climber mid-range isn't usually much of an issue anyway so long as it responds to a whacked wide open throttle coming out of a hairpin.

Someone else must have tried 28s - perhaps best wait ads see what they run first.

You could do worse than ask Eurocarb what they suggest.
fastharry
Posts: 63
Joined: 06 Oct 2011 10:38
Location: leicester
Location: Leicester

Re: 28mm Carb jetting

Post by fastharry »

Ok thanks for the input, i have to say the best way to iron out the mid range problem is a dyno with someone who knows his stuff, iv'e used them for years, it definitely got me in the ball park. saves hours of guess work.
simonnorthroad
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Location: Bath, UK

Re: 28mm Carb jetting

Post by simonnorthroad »

My 501 engined 350 with L cam had/has the flat-spot and would spit-back if shutting the throttle and revs dropping through the mid-range - OBVIOUSLY lean, right? - spitting-back?

Put it on a dyno and it proved to be (literally) off-the-scale rich at 4000 rpm so it's easy to be fooled by experience and guesswork.. Dropping the X18 needles helped but it's still not perfect and still spits back at midrange on a closing/trailing throttle. Am going to try X6s with a slightly long taper but otherwise identical to 18s
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