Mikunis won't idle down

Guess I didn't explain what was weird about those 2 things; actually I could see the air restriction acting as a way to richen the mix, but the 115 jets would lean it out, and everyone here is sayin its a lean condition to begin with, sigh. Time to run some more mental simulations. As I see it at present, the issue is getting enough air and fuel thru the idle circuit in the proper mix to allow the engine to idle without the slides being past that 1/4 open position and getting into the needle jet.

Hey I have a question too,I guess this engine has been around a good 10 yrs now , the cams are .380 lift and the guy I bought it from swears they are web cams, does anyone ever remember Web selling such a grind, or maybe have the specs for it?

This applies to Mikuni 29mm smoothbores, but should also work with the RS carb.

Without a side located drain screw, may collect fuel from hole at bottom of bowl.

Some folks adapt a spare 17mm bolt plug by drilling and fitting a nipple to attach the clear tube.

I just use a cone shape rubber plug pressed into the drain, with a hole through the rubber plug, and the clear tube fitted snugly inside the rubber plug hole. Then hold the rubber plug in position during the measurement.

Others report using an inch of 3/8" fuel line (to fit snugly in the bowl drain threads), with a clear 1/4" hose inside the 3/8" hose.

Some minimal leakage won't affect the results, because fuel volume entering the bowl exceeds any leakage.



As shown in the link posted by mark1122, would assure that Part #90 Pilot Air Jet BS30/97 is in position, and is in good condition, and hasn't been reamed out by over zealous cleaning.

The below image is of 29mm smoothbore with the same air jet.



The following post shows closeup of RS carb's Part #90 Pilot Air Jet BS30/97

Good Fortune!

here we go again, see if this makes any sense to any of you.I disassembed the pilot system again today and cleaned it as thoroughly as I know how. In going thru and thinking this thru I realized that i have probably been turning the idle screw and slides a bit more then 1/4 the way open. The engine barely lopes along at 1000 rpm and if I don't try to get it to run smoother or faster, I can crack the throttle open, and it returns and doesn't hover; but there is not enough rpm to let it recover and stay idleing.What i think I may have been doing is in raising the slides , I was getting into the needle and needle jet to get my fuel and air. I expect because of the cams, and the porting the signal strength at idle is not as great as stock, and the pilot air jet is not allowing enough air in to allow a smooth steady idle, without getting into the next circuit or the needle and needle jet. Your thoughts on a larger pilot air jet and matching pilot jet to bring the idle circuit into a usable rpm range please?

Thanks Mark, , no worries , I'm still around, takin my time trying to think this thru; removed inspected and reinstalled /sealed the rubber manifolds; the problem still persists.
Bought a sudco mikuni tuning manual off e bay, should be here tomorrow or Friday. * Yes it idles better at a higher rpm, even 1200, it'll recover till you get up to operating temp, then it won't idle down again. The choke kills it dead right now. 2 things, seems weird to me, but oldkawman recommends buying 115 pilot jets and installing them before ever putting a set of RS carbs on, even with porting and a borekit ( they come stock with 117.5), and from a suzuki forum it was suggested that for smaller displacement engines, RS carbs like restriction aka an airbox.

Guess I didn't explain what was weird about those 2 things; actually I could see the air restriction acting as a way to richen the mix, but the 115 jets would lean it out, and everyone here is sayin its a lean condition to begin with, sigh. Time to run some more mental simulations. As I see it at present, the issue is getting enough air and fuel thru the idle circuit in the proper mix to allow the engine to idle without the slides being past that 1/4 open position and getting into the needle jet.
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This is what u said so far. So lets try to be clear on what the symptoms are.
are the rpms still hanging after u rev it a bit ?
I don’t get what u mean by this
“Yes it idles better at a higher rpm, even 1200, it'll recover till you get up to operating temp, then it won't idle down again. The choke kills it dead right now”
if it gets worse when it warms up it is too rich. The engine naturally gets richer as it warms.

changing pilots it worth the experiment, but until we/you understand the issue it will be a guess which way to go.
my 1200 doesnt like to idle below 1200rpm's. u could try to simulate a richer condition by taping over 1/4, then 1/2 then 3/4 of the pods to see if it helps. if so then bigger pilots would be in order. u could also remove the pods to see if it likes a leaner condition.

check the throttle rod thru the carbs could be slightly bent

This is what u said so far. So lets try to be clear on what the symptoms are.
are the rpms still hanging after u rev it a bit ?
I don’t get what u mean by this
“Yes it idles better at a higher rpm, even 1200, it'll recover till you get up to operating temp, then it won't idle down again. The choke kills it dead right now”
if it gets worse when it warms up it is too rich. The engine naturally gets richer as it warms.

changing pilots it worth the experiment, but until we/you understand the issue it will be a guess which way to go.
my 1200 doesnt like to idle below 1200rpm's. u could try to simulate a richer condition by taping over 1/4, then 1/2 then 3/4 of the pods to see if it helps. if so then bigger pilots would be in order. u could also remove the pods to see if it likes a leaner condition.

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Before the engine gets heat in it it will idle better at 1200 rpm+, once you get heat and rev it it hangs up at anywhere from 3 grand to 5 grand. If you let it lope along at 1000rpm and rev it , it will return but many times doesn't recover and dies. When I wrote that I had 120 pilots in the carbs; I've since switched to the stock 117.5. Pretty much the same scenerio. I did finds something out by accident, fired it up and had it running at 1k, and it was fairly decent sounding, reved it and it came right down several times, and recovered , was getting excited till it ran out of gas; I'd forgotten to turn the petcock to prime, so I'm inclined to agree, its probably too rich. I've had the air cleaners both on and off , with no noticeable difference. Unless the carbs are excessively rich , the idle screws seem pretty nonreactive, am considering the possibility that the tips maybe worn enough to warrant replacement, and the o rings as well.As far as changing pilots there are only 115s that go leaner, then I will have to go bigger on the pilot airjets; or maybe drop the float level a mm. I've already gone richer, just made the plugs black.

Took the aircleaners off this mornin and changed out the idle air jet from.8 to a 1.0, effectively leaning it out more.air cleaners still off, first thing I notice is its starts easier, both hot and cold, I leave the idle speed as close to 1 k as possible, and as it warms up it sounds pretty decent at that speed, revs and recovers a few times, no rpm hangup, till it got really warm then only 3 k. It did spit as in miss out the carbs twice that I heard during the warm up process. I am wondering if this is partly happenin cause the bike is sitting still in the garage with no air flow to cool it? Anyway I think I'm gonna order the 115 pilot jets, and play with some other variables over the weekend while I wait for them to come.

wow, this is strange isnt it. hanging is a lean sign but u seem to be better off richer???
i found the same issue with my fuel screws, they didnt seem to make any obvious change in rpm's when i turned them. u need to find a test that u can actually feel or hear a change, so u can move forward. what i did to tune the pilots was cruise at 2000,3000,4000 rpms. try to hold the throttle steady for 10/15sec. until u are not accelerating or decelerating,just cruising. note exactly what rpm's the stutter is. then adjust the fuel screws 1/8th out and retest. i was able to really feel and hear the dif this way.
i wouldn't worry about trying to get it to idle below 1200 . mine is very happy there.

Found this yesterday on RS carbs:

So, from fully closed throttle to 1/8th throttle, you're running on the
idle jet, the fuel air mixture that goes around the throttle slide and
comes through the idle bypass passages and that tiny bit of extra gas
that comes past the pilot mixture screws.

IOW, all the idle mixture doesn't come past the pilot screw, only about
1/4 of the needed fuel comes past the pilot screw to fine tune the
mixture. When a shade tree tuner starts screwing around with idle jets
and he doesn't understand that subtle point, he's going to have an
engine that gooes "toot-toot" at idle, and the RPM hangs up when he
blips the throttle because the idle jet is too big and the pilot screws
are open too far and he's compensated by adjusting the idle RPM higher,
or he's going to have an engine that spits back through the carbs,
pings on acceleration, runs too hot when warmed up, but takes forever
to warm up and is hard to start on a cold morning.

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As I said above, the pilot mixture screw only adjusts a small
percentage of the idle fuel air mixture. The correct number of turns in
or out is going to vary if the previous owner has changed the idle jets
from the as-delivered 17.5 idle jet.

Too big an idle jet/too many turns out makes the exhaust go
"toot-toot", you get black smoke, the idle RPM hangs up when you blip
the throttle. When cold, the idle RPM is far too high, so you have to
fiddle with the choke to keep the engine RPM from racing. If you adjust
the idle mixture so it seems OK while the engine is just warm enough to
idle smoothly, you find that the idle RPM is much too high when the
engine is hot. It drives you crazy, you get drunk and kick the dog.

Too small an idle jet/too few turns out results in an engine that is
hard to start, warms up slowly, spits out the carburetor, pops out the
exhaust, stalls when you roll off the throttle.

it does seem to be confusing to me that it hangs when its rich, when everything ive read says the opposite. my bike idles and cruises great, by adjusting it the way i wrote above.
i also unhooked the accelerator pump while i was tuning just to make sure things didnt get confusing to me.

Carbs can hang when thier slightly too rich, and there is too much idle input - thus more overall mixture, too rich. Happens alot with novice tuners on 2-strokes. There are a few old carbs that actually used enrichener circuits as idle control.

Make sure compression is fantastic.

Fire her up and get her good an hot - few minutes or more...

Then change everything to thier stock settings for the carbs. Stock air jets, pilot jets, needle positions, main jets, and fuel/float heights.

Close the idle slides/butterflys completely (twiat out the idle screw for a gap), and attempt to start the bike. most likely wont even burp - IF IT DOES, your sync is too far off for proper tuning!

Turn the idle in 1/4 turn and attempt to start. Repeat this process until the bike will actually start and hold an idle WITHOUT touching the throttle to help it start.

Check the pipes - is it burning hot on all cylinders - all the pipes should scald water/you at this point while idling - IF NOT - again sync is too bad for proper tuning!

Now - TUNE your mixture screws - dont worry about the rpms. Turn them either way 1/4 turn, or 1/2 turn, whichever helps you to hear a difference in the exhaust note/engine whir, and get them to the highest rpm you can.

This will help you determine if the pilot size is too small/large, or if a carbis having a fuel delivery issue. The proper pilot size (usually), is when you can reach the highest idle point in rpm at about 2 turns out.

A bench sync is basically used JUST to get an engine to run - NOT FOR TUNING. To begin to tune the engine, a proper, damn near perfect vacuum accurate sync is NECESSARY.

A perfect bench sync with a precise measuring tool is often 5 or more points of vacuum off and will cause undesired idle effects.

Lets see how far you get with this...

Good luck!