Mikuni Pilot Jets?

[ChrisK]

I'm revisiting this.

The bike is running fine, but I'm very-much-so over confident in my ability to rip the carbs out, re-jet, and put them back in now so I'm just going to do it.

Current set up: 37.5 pilots, 6th/lowest position on needles, 170 mains. 6 turns out on the mixture screws.
Set up I'll try: 40 pilots, 5th position on needles, 175 mains. 4 turns out on screws.

I feel like the bike has been running slightly rich, lowering the fuel economy. I blame this on the needle jets being in the richest setting. I'm just kind of bored and going to try this set up. If it doesn't work I'll change other variables. Might go 40 pilots, 5th position, and 170 mains. Or maybe 40, 6th, and 170. Maybe try 37.5, 5th position, and 170. I don't know, I'm tired and rambling at this point. Just trying to squeeze as much out of her as I can!

[ChrisK]

Also, I just realized what the original purpose of this thread was... bought the jets in a 4-pack from the local Honda dealer for $12. Mikuni part number N151.067. I think they got them from Parts Unlimited? part #EBC151-40

[ChrisK]

Forgot to report back.

I got it all switched over. So, 40 pilots, 5th needle position, and 175 mains. The thing definitely runs stronger. I think fuel economy will be improved, as it feels like it's not working as hard to go certain speeds. It starts literally at the touch of the button. HOWEVER, when I'm sitting in neutral and I rev it slightly, it feels/sounds sluggish at the very beginning. Then it sounds like when the needles lift up it really takes off. So I need to work out the 'at idle' setup. I believe the screws are around 5 turns out right now, still very odd. My whole setup has just been perplexing compared to everyone else I read about.

[Speeddog]

Does your bike have W heads or V?

[koko64]

With the bigger pilot jets you may want to try 4 turns out on the idle mixture screws.
That may be a rich stumble as you open the throttle.
Be open to trying needle postion #4 if you want to test. At worst you'll go back to #5 if it doesnt work and then you will be sure and fully confident of your settings.
A dyno or drag test times will tell you which main jet is best. Local settings for open an airbox are worth trying. I have run up to DJ 175s (Mikuni 165s) in CV carbs, but I'm down here.

Thats a good question speeddog. What lmpact did you find the different heads have on jetting? I've not tuned W heads.

[ChrisK]

Speeddog - no clue, how do I tell?

koko - The reason I have the screws turned out as far as they are is this: When I twist the lever on the handlebars at idle, the bike starts running better/stronger. So, in my head the logic was, that lever enrichens the mixture, correct? So, if I turn the screws out, I will be richening the mixture, and I won't have to rely on the lever as much. I do like your logic on trying out the fourth needle position, might do that tonight. However, it's raining here right now so I would like to change them on a night when I can immediately test it out. I'm riding the bike to Missouri tomorrow after work (~300 miles) so I don't really have an opportunity other than tonight until next week.

[ChrisK]

Nvm, looks like the forecast has cleared up for the evening, I should be able to try it out tonight.

[Speeddog]

ChrisK:
The heads are stamped on the left side, next to the cam endcaps.

I' ve got no experience on relative tuning of W and V headed bikes.
Just thinking there could be correlation.

[ChrisK]

Okay, I can check just for the record tonight.

Also, while I have you guys here, where can I find a replacement fuel filter? It is the white cylindrical thing underneath the tank near the hinge, correct?

[ducpainter]

Okay, I can check just for the record tonight.

Also, while I have you guys here, where can I find a replacement fuel filter? It is the white cylindrical thing underneath the tank near the hinge, correct?

I'm told Autozone has them.

It is the same as a Renault or some similar obscure Euro car.

[koko64]

Speeddog - no clue, how do I tell?

koko - The reason I have the screws turned out as far as they are is this: When I twist the lever on the handlebars at idle, the bike starts running better/stronger. So, in my head the logic was, that lever enrichens the mixture, correct? So, if I turn the screws out, I will be richening the mixture, and I won't have to rely on the lever as much. I do like your logic on trying out the fourth needle position, might do that tonight. However, it's raining here right now so I would like to change them on a night when I can immediately test it out. I'm riding the bike to Missouri tomorrow after work (~300 miles) so I don't really have an opportunity other than tonight until next week.

It's great you are willing to test. Keep an eye on your plugs and fuel consumption. Sometimes its more about feel and response than theoretically good dyno A/F ratio numbers, particularly down low.

The choke/enrichener test is most likely telling you that your pilot jet is too small so that is a good test, but changing the pilot jet usually means needing to readjust the idle mixture screw.
Also a clogged filter leans your jetting, especially when asking for power.

Its easy to fall into the trap of over jetting a CV carb when you dont have an accelerator pump. I usually run a fatter main jet than what the dyno says based on how I tune, but thats me. Like I've said before our fuel has kangaroo poo in it.

Ask DP and SD what they run in CV carbs with an open airbox and US fuel.

[ducpainter]

<snip>

Ask DP and SD what they run in CV carbs with an open airbox and US fuel.

I run 165 DJ mains, needles on the 3rd notch with the spacer underneath, and fuel screws out 4 turns.

My airbox is not fully open...it's swisscheesed...


airboxlid by nh_painter, on Flickr

[Speeddog]

I've only done one 900, oddly enough, the rest 750's.

floats at 14mm
Factory Pro #40 pilot jet
Factory Pro #130 main
needles on 2nd clip position
low speed fuel screw ~2.5 turns
Factory Pro springs
Factory Pro 120k nickel plated emulsion tubes

It was a '95 900SS, IIRC it had a full airbox

[ChrisK]

Thank for the responses guys.

I changed to the 4th needle position last night. IF there is a change, there's less power. It feels so minute that I would need a dyno to truly tell. I'm going to leave it in the 4th slot, theoretically because that provides the best mpg, since I'm going to ride over 1,000 miles in the next 2.5 days.

So a recap:

175 DJ mains
40 pilots
4th needle position - DJ needles
Brand new FP needle jets
DJ springs
5 turns out - mixture screws..... I think.

I'm riding 6 hours to Missouri today after work so I should be able to get a pretty accurate reading on my mpg. I'll report back.

[koko64]

Thank for the responses guys.

I changed to the 4th needle position last night. IF there is a change, there's less power. It feels so minute that I would need a dyno to truly tell. I'm going to leave it in the 4th slot, theoretically because that provides the best mpg, since I'm going to ride over 1,000 miles in the next 2.5 days.

So a recap:

175 DJ mains
40 pilots
4th needle position - DJ needles
Brand new FP needle jets
DJ springs
5 turns out - mixture screws..... I think.

I'm riding 6 hours to Missouri today after work so I should be able to get a pretty accurate reading on my mpg. I'll report back.

The subtle power difference is under what condition? Gentle roll on, crackin' it open, partial throttle?

Something to take into account is that having the needle too rich will help when slamming the throttle open, but will be too rich at steady throttle and gradual winding open of the throttle. That's due to there being no accelerator pump, so you can't have it both ways. You have to "ride the carb" according to it's design characteristics. Winding on a CV carb often gets more response than slamming it open, but you can play around with different slide springs to get the best feel to how you ride. Mind you, the CV carb is more forgiving than a flatslide carb when you pin it.
I used to think that soft springs were better but I have gone back to using the oem springs. Test some spring options if you like to get the response character that suits you.
That goes for any carb. You have to use them in light of their design to get the best out of them, and tune accordingly.

Also, if you fill a dip in the midrange it can make the top end feel weaker even though the top end has not changed. Brad Black has a good article on this on bikeboy.org. Climbing out of a hole in the midrange gives more of a "whoosh" feeling when the top end kicks in, but its slower overall. I tuned some FCR41s for a guy and he liked the midrange hole with the carb too lean on the needle position because it felt so good when the main jet kicked in! I fixed it and gave him a fat midrange as well, but he insisted I put it back.