complicated tuning with performance carbs.

Could someone give me proper advise and tips on how to tune a 17.5 polini cp or vm 18 mikuni to run right for my tomos a35 airsal 64cc. I've tuned and tuned both carbs. Changing pilot jets raising and lowering needles plus different main jets. I cannot nomatter what I due get the wot to run right with mid and pilot circuit. I'm not embellishing my pathetic tuning skills. I've read up and tried learning with polini cp manual and mikuni tuning basic troubleshooting. I can't figure crap out. But I don't want these Brand New carbs to waste away. I know all engines run different. But someone please tell me your exact pilot and needle clip position and main jet# on your tomos a35 airsal polini cp or vm18 please don't be critical of me I just need the starting point.

Re: complicated tuning with performance carbs.

Have you refered to a tuning manual?

Re: complicated tuning with performance carbs.

This helped me a lot to understand the multi stage carbs like a Polini or Mikuni.

http://www.vintagebikebuilder.com/mikuni-tuning-and-jetting-guide.html

Yes it is describing larger carbs than a vm18, but all of the steps and understanding of them applies.

I will tell you that if you have a vm18 then you will need an idle jet around a 17.5 but, the main jet will vary depending on your setup. The tuning guide will help you know how to decide what size jet you ultimately need based on the plug chop at different throttle positions.

Have fun, it isnt a quick process, but very necessary if you want the performance.

I cant remember if there is a good plug chop image so here is this one.

spark-plug-color-chart_3.jpg

Re: complicated tuning with performance carbs.

Alan Jackson /

get your pilot jet something like then get your main jet one size down from four stroking ay wot.

then ride at 3/4 7/8ths throttle and to get that right choose the right needle tip diameter. moving clips sometimnes isnt enough. if its rich and 4 stroking at 3/4 throttle then fit a thicker tipped needle. and the opposite is true. then when youve got the right needle tip

id try getting the pilot jet bang on with the aid of the pilot screw dellortos screws regulate fuel mikunis regulate air?

then mid range is down to the taper of the needle and clip.

make sure you have a 2 stroke atomiser. if you dont have a few needles to play with you will struggle to get wot and midrange.

im no expert but since i started trying this approach armed with a load of needles it became easy to tune carbs. before this i could never get wot and mid range right

Re: complicated tuning with performance carbs.

The polini is basically bolt on and go. It's very forgiving.

Upjet till it four strokes, then go down one.

Idle jet bogs at take off or mid throttle, go up. (Usually. Sometimes they do the same thing whether rich or lean.) If the screw is out more than 2 turns and your idle is still rising, bigger jet. If it's screwed all the way in and nothing happens, smaller.

Needle isn't do much besides control idle on a polini, and I haven't had to change one yet, all my lips are in the middle.

If nothing works, the carb probably isn't the problem. Air leaks, head gasket leak, plug too cold, etc.

Re: complicated tuning with performance carbs.

Marky Mark /

Hey alan I have 2 different needles for my polinis 17.5 15mm the stock needles 7j18 I have an fj15 should that one clear it up???

Re: complicated tuning with performance carbs.

Alan Jackson /

im sorry mark i have no experience with the polini. all you can do is follow what i said

get main jet right then see how it runs at 3/4 7/8ths throttle and to get that right you might be lucky and find the current needle just needs clipping right - like bairdco says.

ive never been that lucky and just gone straight to needle tip. but im taking phbg might not be the same for polini so apologise if ive led you down the wrong path.

main jet -wot

needle tip 3/4-7/8ths throttle

pilot jet idle to 1/8ths throttle

and finally clip needle or taper to get mid range

there will be much better men than me who tune differently (correctly) but since i started doing it that way its become simple for me to get it running well.

to get it running better than just well for me i'd need a dyno or af sensor

or someone more experienced stood next to me. (edited)

Re: complicated tuning with performance carbs.

one of the finer points of carb tuning i see overlooked is how the various circuits come into play at different operating points. by operating points, i mean RPM, or more specifically, mass intake air flow rates.

for example, most tuning guides/advice tell you that the main jet is working at 75-100% throttle. however, that is not always the case. the main jet is really what clamps the MAX fuel flow rate. at low rpms and WOT, the main jet isn't really restricting fuel flow at all, because you are not delivering the max fuel rate (which occurs at max power). the better definition of where the main jet is important is WOT *and* high rpm.

that means that during low RPM + WOT conditions the needle is dictating how much fuel can be drawn from the atomizer into the throat of the carb. you can verify this by removing the main jet itself--if the needle is correct, the low/midrange will be fine even with the throttle open all the way. as the RPMs climb, the pressure drop through the venturi reaches a point where too much fuel will be sucked up, and it will bog richer and richer with the higher intake air flow rates.

drop the needle a few slots or go to a shallower angle needle and the "too rich" rpm will go up since the orifice size has decreased. the opposite is also true (raised needle or steeper angle, bogs rich @ lower rpm).

i've found the best success in needle choice/position when i start to rich bog right around where the pipe comes up and i've got an oversized (or no) main jet installed. then you clear up peak power and up by reducing the main.

put another way the main jet is a sort of "clamp" on how much fuel can be be pulled into the atomizer no matter how much vacuum there is in the throat. i say clamp but of course it's not a hard knee.

basically WOT tuning has ranges, and the lower range is dictated by the needle and needle jet, and the upper range by the main jet.

just some food for thought.

Re: complicated tuning with performance carbs.

Russell Huddy /

Does your throttle open the carb all the way?

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