30 mpg, Mikuni 42 : V-Twin Forum: Harley Davidson Forums

Masonman · 10-18-2009, 12:42 AM

Just swapped out the EFI system to a 42 Mikuni, bike runs really good but Ive checked 3 tanks of gas over the past two days and they were 29,29,30 mpg riding 2 up on 70 degree days at 65 mph on the freeway.. The main is a 165, pilot stock 125 and a -97 neddle on the 3rd slot. I now have a Yost tube with a 160 main to try if that will help at 65 -68 mph?? There is slack in the choke knob.. Ive checked it about 10 times now theres maybe 1/16 in. slack in it. You can see my build on my Sig. minus the HPI and PC.. Any help would be great..If theres any to be had that is..ha
Thanks

baggersport · 10-18-2009, 12:41 PM

Try a -98 needle.

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Masonman · 10-18-2009, 01:16 PM

Thanks, I'll give it a try next week and let post the results..

fresnohog · 10-18-2009, 01:42 PM

My build was a 95" with stock heads, SE204's, mikuni 42, Se pipes, and Big Sucker with a K&N filter. The first dyno run was disaster. Way to rich. After the dyno the main went from 165 to 145 with no other changes. MPG, top end and torque improved dramatically. Because of the pipes it was restricted. The dyno tech told me if I went to more open pipes to go to 155. I did ( Vance and Hines ovals) and it ran great. That was on my 2001 RK. Another thing also is you cannot use the Harley enrichener with the MIkuni. It doesn't seat. Have to use the mikuni enrichener.

baggersport · 10-18-2009, 01:52 PM

Cruisin @65mph is a small throttle position (not on the main jet yet), jetting will be on the needle....roll the throttle back when at cruise and see where it's at. The best way (IMO) to tune that carb is to get the main set right 1st, then to the needle....work your way down...not up. The other circuit is fed from the main. The clip position will determine when you get on the main jet....and shouldn't affect cruise. You can run a small main jet and a needle that's too rich.......or a bigger main and a leaner needle.....but you're looking for best power on the main jet.....then the best on the needle.

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Masonman · 10-18-2009, 04:34 PM

Thanks, I did a WOT test and the motor and speed stayed the same when I cracked it back out of WOT. So I just dropped the main to a 162.5 yesterday, left the pilot and needle alone. What do you think about the Yost tube and a 160 and dropping the -97 needle down one notch??

baggersport · 10-18-2009, 04:50 PM

Set your main jet for the best pull and speed through the pull. Dropping the needle will not affect anything other than when the taper starts to show (when you get to the mainjet). The needles all have the same taper....richer/leaner is determined with the straight portion of the needle, rich ones are a smaller diameter (to let more fuel by), leaner are larger diameters.

.....you're checking the main at full throttle......needle at cruise.....clip position and taper falls into the area above cruise but not fully on the main alone.

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Masonman · 10-18-2009, 05:03 PM

Ok, I'll try to get a -98 needle tomorrow, just wanted to do as much that might help out while I had it off. I dont know much about the Yost tubes but the guy that gave me this one last night said he picked up about 3 MPG. He said it atomizes the fuel better than the stock Mikuni jets.. I think they use a Klien type jet..and it has a 160 in it. I know its a bad thing to change 2 things at once. Just wondering what you thought about Yost if you have used them..

baggersport · 10-18-2009, 05:07 PM

With or without the Yost........it's still proper jetting. Good luck, I suspect you're not far from right.

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Masonman · 10-18-2009, 05:17 PM

Yeah it runs good and pulls hard, just trying to lean it out a little at cruise to get a little more MPG. I'll test it all next week and see what happens.. Its 72 and clear in Tx. so its good testing weather!! ha

baggersport · 10-18-2009, 05:39 PM

Try the -98 with the clip in the 2nd position (2nd from the top) with your main in the 160-165 range. That's a start.

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Masonman · 10-18-2009, 05:44 PM

Thats what I'll try first with 162.5 whats your opion on a pilot jet for my set up?? After I try this first and check the milage.. I just think 29-30 MPG stinks and it sounds like its real rich. Has that low muffled sound at idle istead of the sharp popping camy soud it had with teh HPI 48 on it..

Masonman · 10-18-2009, 05:49 PM

just dont know how big a jump to make on the pilot at a time.. is 120 too small??

baggersport · 10-18-2009, 05:51 PM

If you don't have a manual, go to mikuni.com and download yours. Pilot size works with the idle mix....

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Masonman · 10-18-2009, 05:58 PM

Ive been on their site going by the numbers and I cant get the air mix at idle to start any change until about 4-5 turns out or in all but a turn.. they say anything past 2.5 turns out the pilot jet is too large..but its a stock 125? I dont know how small to try on the first go at it without being really lean.. It reacts to the choke knob being pulled out when cold but not as much as I would think it should.. could this be a too rich condition also?