Mikuni problem again

[Mike Hufnagel]

For the most part, jetting is spot on with the exception of a slight hesitation.
 The bike idles fine and does not hesitate while in nuetral when whacking open the throttle. Took it out for a run up the street. Crusing along in 4th gear, opened up the throttle and there was a slight hesitation and it took off like a rocket. Came back in, raised the needle 1 clip from the 3rd to the 4th notch went out again and the hesitaion was much better. Came back in, put a .014" spacer under clip, no change.Then went to the 5th clip, no spacer, took it out again, no change.    
Now I'm thinking I need another needle, one that is just a tad richer. But trying to decipher the code on a mikuni needle is a real pain, even with a diagram.  The needle I'm using is a 6FJ6. I'm thinkingI need a  needle that is a tad richer on the bottom. But which one?

Or Maybe lightly sand the needle itself down, or go with a tad richer needle jet.

Any thoughts?

74 Yamaha TY 250
75 Jackpiner w/ 250 eng.
80 Ktm Mc 400
82 KTM GS250II (import)
82 KTM 495
2) 84 Yam RZ350
98 KTM 300mxc
00 KTM Duke 2
06 KTM 950 Adv.
08 Suzuki DR70Z

[Mike Hufnagel]

PS

Does not hesitate when getting on it, just crusing and wacking it open.

74 Yamaha TY 250
75 Jackpiner w/ 250 eng.
80 Ktm Mc 400
82 KTM GS250II (import)
82 KTM 495
2) 84 Yam RZ350
98 KTM 300mxc
00 KTM Duke 2
06 KTM 950 Adv.
08 Suzuki DR70Z

[joe novak]

Have you considered replacing the main jet with a larger one?

[Mike Hufnagel]

Actually, after I did the first post I thought about that.

 But I don't really have the room (I am on a dead end street)to do a proper plug chop without upsetting the neighbors and having the cops called on me. (usually I run it at about noonish when the neighborhood kids are in school and their parents are at work)


74 Yamaha TY 250
75 Jackpiner w/ 250 eng.
80 Ktm Mc 400
82 KTM GS250II (import)
82 KTM 495
2) 84 Yam RZ350
98 KTM 300mxc
00 KTM Duke 2
06 KTM 950 Adv.
08 Suzuki DR70Z

[tomale]

changing the needle is not a good idea.. more than likely, changing the jet needle is a better way to go.. frankly needle jets are not designed to change jetting but change the power curve on your bike. so if you need to enrichen the mid range, going up one needle jet size should help.

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)
74'250 hare scrambler (project)
74' 1/2 440 maico
78' 440 maico
72' cr125 Husky (project)
74' TM125 suzuki
93' RMx 250 suzuki

[Mike Hufnagel]

I'll give that a try. Found some specs for the Mikuni parts online. Hopefully someone in the Cincy area has one in stock. The one I'll be looking for is a 159-Q6.

What I should have done in the begining, was to restore a stock Jackpiner and not one stuffed with a modified 250 motor. LOL

Jeez, drag racing was so much easier to figure out carb problems. Didn;t have to worry about low end or middle, . Just wfo.
Until I put Lectrons on the bike

74 Yamaha TY 250
75 Jackpiner w/ 250 eng.
80 Ktm Mc 400
82 KTM GS250II (import)
82 KTM 495
2) 84 Yam RZ350
98 KTM 300mxc
00 KTM Duke 2
06 KTM 950 Adv.
08 Suzuki DR70Z

[Ed Chesnut]

You mentioned wanting to richen things "on the bottom" - so I'm wondering, have you got and have you tried a one step richer slide?

When you do your 4th gear acceleration test, if you roll on the throttle (as opposed to a "whack") is the hesitation @ a very low throttle position (say 1/4 throttle?) or farther up (say 1/2 throttle)?

A richer needle jet may be in your future, but it might be good to be sure there isn't a lean spot due to slide cut-away.

Ed

Keep the rubber side down!

[Mike Hufnagel]

I do have a set of very rich slides that I could try, but I do0n't want to risk having to file them down for experimenting as they do go to another bike.

and when I tested it , I did it at 1/4 throttle or less.

74 Yamaha TY 250
75 Jackpiner w/ 250 eng.
80 Ktm Mc 400
82 KTM GS250II (import)
82 KTM 495
2) 84 Yam RZ350
98 KTM 300mxc
00 KTM Duke 2
06 KTM 950 Adv.
08 Suzuki DR70Z

[Ed Chesnut]

If the hesitation is at 1/4 throttle (or less), that is a prime indicator of slide cut-away being too lean (or pilot jet - especially if the "or less" is in play).

Note that you can test with the slides you've got if one of 'em is a step richer (and not several steps richer).

I'm curious, when you were tuning the pilot air screw and idle speed - what setting did you end up with on the pilot air screw?  If you are more than about 1/2 turn away from 1.5 turns out . . . that indicates a different pilot jet would probably improve things.  Example: if you have to "richen up" a pilot jet by setting the pilot air screw at 3/4 of a turn out to get things "right" - then you ought to try a step richer on the pilot jet.

Pilot jet and slide cut-away throttle ranges overlap, so changing one tends to effect the operation of the other a little bit.  It is quite important to get the pilot jet "spot on" before doing a bunch of other fine tuning, because the pilot jet in a Mikuni delivers fuel at ALL throttle settings.  Pilot jet delivers virtually 100% of fuel at idle and a progressively smaller percentage of the fuel at higher and higher throttle settings - but it ALWAYS delivers fuel.  So if you put in a richer pilot jet, there will be a "richening effect" at ALL other throttle settings (which may, or may not, be noticeable).  But it is good to know about this - cuz it can explain why changing a pilot jet changed full throttle operation.

Ed

Keep the rubber side down!

[Mike Hufnagel]

I'm done messing with it (except for main jet)

I couldn't find the slide (even Sudco could not find one), would not get the needle jet for another week, soooooo......... I wound up filing down a 1.5 slide to about a 2.0 slide (filed off about .025" off rhe 1.5). That cured it.  

Did my friends bike, same carb. same motor (although no reeds & stock ignition & pipe) works great, but slightly different specs
Final results:

Mine=                                    Friend
320 (as of now) main jet                 320
42.5 pilot jet                           37
6FJ6 needle (4th clip from top)          same ecept 3rd clip
159-Q5 needle jet                        same
air screw turned out 1.25 turns          1.5
2.0 slide                                2.5

Differences between the 2 motors, mine has a Ace pipe on it, PVL ignition, gem reeds with boost ports opened up, and my carb was bored out.      Now, if there is no rain I get to take it out this weekend and see how it does before Athens.

Thanks to all who responded.  Cheers

Mike

74 Yamaha TY 250
75 Jackpiner w/ 250 eng.
80 Ktm Mc 400
82 KTM GS250II (import)
82 KTM 495
2) 84 Yam RZ350
98 KTM 300mxc
00 KTM Duke 2
06 KTM 950 Adv.
08 Suzuki DR70Z