Mikuni Question

[Wasp]

I have some issues with my carby settings on the 32mm round slides . The bike starts easy , it screams top end , but around 5 mm lift the the engine (3 x 250 cc ) starts to four stroke like if it was very rich . But as soon as you go another 2-3 mm in lift , the power band comes in and all is clean . I dropped the needle to the lowest clip and  and it improved , but not enough . I think the pilot is OK , but lowest point of the needle is not .Mikuni does not offer different needles for this carb.  Could it be the cut away . At the moment it has 2.5  and no others are available . If I turned down the 2.5 Slide  by 0.5 mm , would that make approximately a 2.0  and lean it off ? I know its a bit of bush mechanics , but am I on the right track ?

[sleepy]

Most likely slide cutaway. You need to go to a bigger cutaway not smaller as it sounds rich. Can be filed but need to be acurate.

[pokey]

Tried retarding the timing a little?

[matcho mick]

whats the needle jet?,go back one size?,

[HeavenVMX]

Just a question really and I am sure you have it covered. Are they new carbies fitted or at least new needles and needle jets.

It sounds a lot like worn needles and needle jets. The throttle opening you are have problems with probably relates to the point were the bike operated often. If you have new needles and needle jets installed disregard this post.

[skypig]

whats the needle jet?,go back one size?,

That's the common wisdom. If you are at one end of the adjustment on the needle jet clip position, changing the needle jet one size should put the needle back on the middle clip. According to Mikuni OZ (Obviously then you have the adjustment back.)

[matcho mick]

eggsakery 
ooh,just looked in my book,they H1,H2's both like O series,yours with 6 to 8 options,so if you have 8's,try 6's?,if you have 6's your'e forked,(pun intended  ),needles same,cutaways 2.5,

[Wasp]

Pilot is 45  Main is 110 .Needle seat is new and needle has 5 clips ( needle in bottom position , fully rised . )When I drop the needle it is starting to surge in mid range (to lean) . Plugs are brown  and fuel consumption yesterday was 21 liters for 160 km !!!. I have used Dellortos all my life and there you have a great arsenal  combinations to cure things . But because those carbies are OEM , parts are limited and not the same as the after market Mikuni . If all fails , I may go to modern half round slide Mikuni if they have one with oil feed in lines for the H2 C . I will try a few things that have been suggested .

[Lozza]

Pilot is 45  Main is 110 .Needle seat is new and needle has 5 clips ( needle in bottom position , fully rised . )When I drop the needle it is starting to surge in mid range (to lean) . Plugs are brown  and fuel consumption yesterday was 21 liters for 160 km !!!. I have used Dellortos all my life and there you have a great arsenal  combinations to cure things . But because those carbies are OEM , parts are limited and not the same as the after market Mikuni . If all fails , I may go to modern half round slide Mikuni if they have one with oil feed in lines for the H2 C . I will try a few things that have been suggested .

Put a  0.5mm shim under each clip failing that put 3x 36mm PHBE's on there and forget

[Wasp]

Lozza , wouldn't lifting the needle more , exaggerate the problem more ?The first 1-5 mm of slide lift , that is where it is too rich , after that it all comes good .

[80-85 husky]

I rekon you need to go down 1 size in the needle jet so you can return the needle to the middle clip position which should put you exactly where you are now but then you can lift the clip 1 slot to lean that blubber out....

[Wasp]

Makes sense , I will give that a try (if they are obtainable ) Thanks

[HeavenVMX]

This may sound strange but are the needle jets fully home in  all the carbies? if one or more is sits even slightly lower than its correct position it will cause richness at small openings.

I know you would have triple checked. Are the carbies exactly synchronised if they don't open off the closed position together it could give the impression of rich as one cylinder tries to accelerate ahead of the others. Just an idea

[Colin Jay]

Have you spoken with Ian Williams ( Ian Williams Tuning, Torrens Rd Brompton)? He has given me some good advise over the years regarding sorting out jetting problems with Mikuni carbs.

[OverTheHill]

Without yet reading all of them & my ideas might be way off--but-- had a realy unusual problem on my bucket racer years 30 odd years ago which i'd ported to the moon & gave rich blubbery type symptoms at about 1/3 throttle exactly where it was "getting on the pipe", couldn't clean it up by needle or slide etc till some smarty came along & said "Reed Flutter" to which i said "yeah rite", said double up the reeds & bugger me it fixed it on the spot. Also do you have an aftermarket pipe fitted which might be too short overall--if so add some into the header [easiest place] which might spread the power & take the sharp kick out of the powerband so to speak, if a 250 has a powerband!!, in saying that a mate had a 250F "79 which came on hard like a lightswitch & someone had been porting it wildly which was hardly necessary on one of those. Similar story on an IT400 motor i had in something 'dirt' & once again it had been got at & had 190 degrees exhaust port duration so had to add 100mm to the header to cure the flat spot--which wasn't blubbery but just flat--good up to that & crazy wild past it but 'hollow right there' where you needed it for dirt. Ignore all this woffle if irrelevant.