tuning for the mid range

[tomale]

I recently put a mikuni on one of my project bikes.(it is not a Penton) I am getting close to having the jetting right (I think)but I am still needing help in the mid range. It idles good, The main jet is huge (way larger than I think it should) but the plug reads lean. I keep using bigger mains and it does seem to improve but not as much as I think it should.  Since most of the time I ride I am on the needle jet, It accured to me that I could have it right on the top and bottom and still be really lean in the middle. How do I tell if this is really the case?

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)
72'sixday (project bike)

[firstturn]

Thom,
  Is this a new carburetor?  If not have you put a new needle in it?

Ron Carbaugh

[tomale]

Hey Ron, good to hear from you....It is a new carb and all the parts are new....

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)
72'sixday (project bike)

[firstturn]

Thom,
  If you can get a reading just idling around that would tell us it has a good setting for idle.  If you are getting a lean reading at mid range be sure you don't have a air leak somewhere?  Normally, as you know, when you have a air leak it will show up with a rough idle....but this isn't always true.  One other thing is be sure from Mikuni that you have the correct needle to main combination before you go looking for ghost.

Ron Carbaugh

[tomale]

well, the motor is a complete rebuild, all new bearings, seals and gaskets...and the rubber boot between the carb and the intake is new as well. I have checked for leaks.
what makes this difficult is that I have never ridden this bike before I built it so I have no idea what it should be like. As for the correct needle/jet that kind of information seems to be a secret.... no one seems to know what it should be...When I bought the carb I was asked what size of bike it was going to be used on. I further asked what size jets came stock and if I needed to make any changes, I was told that as supplied the carb should be fairly close. The odd thing about jetting is that what works for one bike does not seem to work on another... example, a yz250 uses a 70 pilot and every other bike of that size would never use anything larger than a  50 or 55. ( If I remember right, the pilot on my 250 is a 40...reason.... the a difference in crank case vaccum. the smaller the crank case vaccum the larger the jet, because a smaller crank case vacuum would not draw as much fuel, So comparing the jets from one bike to another may not be of much help...the other thing I did was to ask someone with the same bike what they were running, and everything is close except for the main jet which is 7 sizes larger. How can this be, good question, timing, porting, reed valve... maybe.... or maybe I have the wrong needle jet and way too big of a main but it is not drowning the engine because the needle jet is too small....  thanks Ron for the reply

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)
72'sixday (project bike)

[john durrill]

Tom,
 What needle jet and needle do you have? And whats the air jet size?
 That little guy can run you nuts on finding the right needle , needle jet combo.
 I try and get  richer and leaner needle and needle jets on a new cab so i can tune with out havening to wait.
 I would get 2 needle jets 1 step at a time richer and the same with the needle. Your right about the effect the wrong needle , needle jet combo can have on getting the main right. That has happened to me before.
 I would try also the biggest air jet to start with. Get things close with your other jets and the needle then you can play with the air jet to fine tune the mid range. Start rich and work lean always as you know.
Post the jet and needle #'s and maybe some one can get you closer than you are. Its a pill i know but if you get one really right the bike will work like a dream.
John D.

[Ron]

Thom,
When I'm sorting out the needle/needle jet combo, I always start with leaving the main jet out and take a test ride.
If it starts loading at 1/2 throttle go down on the needle jet.
If it doesn't load up until 3/4 throttle its okay for now. It should definately load up at full throttle with the main jet out.
Now you can sort out the main jet without any restriction from the needle/needle jet combo.
When you've found the right main jet, then you can go back and fine tune the needle/needle jet combo, to clean up part throttle response.
Hope this helps,
RonW

[tomale]

Ron, John, thanks for the reply. the needle jet is a 166 Q2 and the needle is the 6f5 I believe. As I understand... the 166 series uses the 6 series needles. I bought a number of richer needle jets. 166 Q4,6,and 8  I understand the the 166 series has been dicontinued and a new jet replaces it that adds a few more sizes. I may end up using those but that falls under fine tuning.... as for the air jet, I am using a 45. that should be fairly close. Almost everything above a 250 uses a air jet in that range, so I am fairly confindent that I am close. Besides messing with the air mixture screw and the throttle seems to indicate that I am close... It runs clean, idles even and the trottle response it good too...
I can remember as a kid there was a guy around the block that was nick named "tinker" I remember him saying that when he was jetting a new carb he always left the main jet out until he had everything else jetted.  It makes sense now.  At this point, the main jet is a 440. my orginal spec sheet that I was working from called for a 360 main. Now individual bikes will vary but that seems excessive.. I have only jetted a few carbs from scratch so I have never had to worry too much about mid range jetting. Again thanks for the info...I will give it a try....

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)
72'sixday (project bike)

[Ron]

Thom,
The pilot jet is correct when the air mixture screw is adjusted between 3/4 to 2 turns out. The motor should stall at less than 3/4 turns out and start to stumble before 2 turns out. If if keeps building rpms past 2 turns, the pilot jet is to large.
Its a balancing act between the idle screw and the air mixture screw to find a good idle speed.
BTW, more than 2 turns out you may lose the mixture screw. Not enough spring tension.
RonW

[john durrill]

Tom,
I think we are talking about 2 different jets. Most Mikunis have an Air jet. Its located dead center in the bottom of the bell on the air boot side of the carb. If yours has a changeable jet you should see a small brass jet with a slotted head looking down the hole. Largest is 2 and the smallest is a .5 i think. It is used to fine tune the needle, needle jet . does that by adjusting air injected in a the nozzle ,  part that sticks up into the bore of the carb the needle goes in.
I you could start out with a 2. when you get every thing working the best you can then go back and try different sizes to see which one works best. Yours could be a fixed jet , no slot cut in , a pressed in item but most are adjustable by jet changes.
 Did that help any?
 John D.

[tomale]

Yes, this helps, I went out and looked to see and discovered that there was no air jet... It looks like there should be one but there is not... Wow I have learned about a jet that I never knew existed... I checked a spare mikuni and it had one.. a .5
so with this jet, does a larger number mean more air...
now I wonder how long it has been gone, with all the trouble I have had getting this bike running, it makes me wonder....
John your the man!
Thanks again....

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)
72'sixday (project bike)

[tomale]

Ron, thanks, I think I knew that but it is good to be reminded.. sometimes in the middle of a problem you can forget the basics...

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)
72'sixday (project bike)

[john durrill]

Tom,
 That what Sudco told me and it makes sense. Larger # = bigger hole on the jet = more air to the nozzle so that should be leaner. With a .5 you should be as rich as you can go with the air jet circuit.
John D.

[tomale]

OK, now to find a few of the air jets...Earlier this week, I went down to my local dealer and when I told him that I was looking for needle jets to fit the VM carb. He gave me that deer in the head light look... there was a long pause. and then he said.. oh, you mean that old stuff... He went back to the service area and found several boxes of stuff and I went through them.... I do not remember seeing air jets...  but that could have been in a different box.... If I get a chance tommorrow, I will take another look, other wise I am sure that I can order them... thanks again...

Thom Green,Still crazy after all these years!
76' 250 MC5 (orginal owner)74'
250 hare scrambler (project bike)
72'sixday (project bike)