RB 250 wow or shi?

[lammy 225]

ran 250 for 700 miles no probs
li150 box
18 47 sproc,s
franspeed race pipe
mikuni 35 jetted
350 main
55 needle 2nd clip from top
20 pilot or whatever its called
ran well till hit hill or wind then had to work her
so old clever clogs goes to change gearing and went to 17/46
did 30 mile and pop goes the piston whooops
new piston in and up jetting to
370 main
54 neddle 2nd clip from top
30 pilot
rich on the bottom quart then starts going lean but only to weak milk chocolate colour
tryed 3rd clip down but was a pig to run
so all u men of great knowledge please help? oh ok then pretty please

[drunkmunkey6969]

I've found on the Mikuni 35 & 38 TMX carbs that i've dynoed that the balance between rich/lean bottom-end/top-end can be difficult to achieve.

For example, when dynoing my racer, i found that when the top-end rev range was spot on, i had a massive lean spot in the mid-range. So to iron out the mid-range lean spot, i had to retain the 360 main jet i was using, but go to the leanest needle i could get, so that i could find a main-jet/needle combo that allowed me to run the needle on the middle clip.

On testing other Mikunis, i found that running on the top clip, and sometimes even the 2nd clip, meant that the needle wasnt lifted sufficiently to allow the taper on the needle to flow sufficiently in the mid range. The first part of a Mikuni needle is straight followed by two tapered sections of increasing steepness. You need to make sure that when your powerband is coming on song, that the tapered part of the needle is in play, so that it can feed the fuel through and avoid lean running mid-range. Running a Mikuni on top clip doesnt usually allow this to happen, and teh mid range is still using either the straight or less tapered part of the needle, which can lead to disaster, depending on how the rest of the engine is set up, second clip is an improvement.....but for safety, i personally prefer middle clip if you can live with feeding the throttle on.

It doesnt necessarily feel crisp when accellerating, in fact on the racer i have to 'feed' the throttle on to avoid bogging, but i'd rather do that than have a big lean spot in my mid-range. Imagine hooning down a long straight (or a long road) working your engine to the max.....and then rolling off the throttle into a lean spot!

A Mikuni TM24mm has a needle jet that you can richen up on the mid range (QO being the standard lean item, Q2, Q4, Q6 etc getting richer) TMX 35 & 38 don't have these, and so mid-range can be more difficult to dial in without compramise somewhere. A Dellortto of equivalent size has more adjustments to dial in and doesnt flow as well as a Mikuni.....but in the end, because it has more adjustments you can get it closer to ideal rich/lean running throughout the entire rev range. (In my humble opinion)

So working with your Mikuni on the RB, i would be tempted to experiment with a couple of different set-ups.....maybe drop the pilot a size, increase the needle to a leaner item (56, or 57?....maybe even a 58?), but put the needle on the middle clip, and play with the main jet until its dialled in.

Do some runs and plug chops and see what you think.

Let us know how you get on.

[RinB]

Dan you said Mikuni's are easy to set up

[drunkmunkey6969]

Dan you said Mikuni's are easy to set up

Then i eat my words.

Or.......I suppose it would be more accurate to say......

Mikunis are easier to set up than Dellortos, as the have fewer adjustements to make. However, as they have fewer adjustments, this may lead to a compramise for some applications. For the most part it should not be problematic, although riders of performance motors, that need to dial in 100%, may benefit from the more complicated, and therfore more adjustable/flexible use of a Dellorto carb, although the sacrafice with the Dellorto, will be the flow rate.

Happy now Mr Elerphant memory??

[lammy 225]

cheers drunkmunkey
was thinking those ways myself but dont really wanna hole the piston again
have ordered bigger main then will go leaner needle and put the clip down further should richen top then aswell
by what ure saying.
thanks mate cheers

[J1MS]

Ive never ran a Mikuni on a Lambretta ( YET ) but reading this post is an education for me... and I can see there is alot of understanding of these carbs.....brilliant explaination of jetting then re-jetting and its effects....Top post... :bouncing: :bouncing:

[snowytv]

just make sure there are no air leaks anywhere

[lammy 225]

ok ok so we should take ore care and check things more regulary
went on ride out set carb really rich and as of now nearly back to where
i was b4 holeing 1st piston
.when left ride out to grantham she ran worst and worst then when stopped
saw leak on near side and petrol pipe had sort of expanded on petrol tap
hence sucking in air and leaking small amount fuel lukily had couple small jub clips
and got back to pboro running really rich am now thrashing it around the parkway
annoying cars :baddevil: and the plug seems to be nice choccy brown to black down the threads
does this sound better or am i heading for another piston as its fast he he

[dicspeed]

Now then rocket , i would love to say another piston but dont want to tempt fate. Just enjoy the ride mate and keep one eye on that plug. If anyone can persist to get the right set up its goner be you

Then again you could always fit a much simpler TS1 , it would withstand your powerful right wrist a little more