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Mikuni pumper idle problems?

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With the choke out the bike starts up easily. After about 15-20 sec, I push the choke back in, but the bike does not idle well and eventually dies. After multiple attempts, I raised the idle speed and rev'd her up a bit. The bike is still not idling normally, but I take off and ride till the engine warms up good. After riding about 30 mins, the bike is idling good, but a little high, so I turn down the idle speed to normal. Bike continues to ride fine throughout the day.

A few weeks later, I try to start my bike, but again very difficult to idle after pushing the choke in. This time, being at home, I tried to debug this problem. One thing I notice when I blip the throttle, the revs don't come back down right away, sometimes it comes down, but still revving a bit higher than normal. Blip it a few more times, and it might come back down to normal idle. It has been doing this for quite some time (but got worst over time), but I always thought it was due to old cables. I replaced cables, and since I had it off, also replaced the throttle sleeve. When blipping the throttle everything snaps back OK, nothing hanging.

I tried looking for air leaks by spraying carb cleaner on the intake manifold boot, but didn't notice any change in idle. I'm hope that my carb slide is not worn out. The pumper has been on the bike since 2001.

Any ideas on the problem? I may try messing with the fuel mixture screw next, then probably pull the carb out again to do a once over (btw, I hate pulling this carb out of this bike!).

Anyone else have this problem? If so, how did you resolve it?

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If it idles fine when warm and like crap when cold indicates a lean pilot. If it used to idle fine when cold then I would clean the pilot jet. If this has been happening since day 1 and you eliminated any air leaks then I would richen you pilot circuit. First, I would count how many turns out your fuel screw is, so you can set it back, and I would try turning it out (ccw) 1/2 turn at a time (without exceeding 3 1/2 turns from fully closed). Next, I would bump up your pilot jet 1 size (set the fuel screw back to where it was) and see if it idles better.

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All good suggestions.

To add to them, if the bike is setting a while between rides, drain the float bowl and flush with gas from the tank before ever trying to start it.

What do you consider a "normal idle"? I find 1200 RPM just about right.

If you've have never messed with tuning the fuel screw, you definitely need to do that, but first, as said, make sure the pilot isn't clogged.

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If it idles fine when warm and like crap when cold indicates a lean pilot. If it used to idle fine when cold then I would clean the pilot jet. If this has been happening since day 1 and you eliminated any air leaks then I would richen you pilot circuit. First, I would count how many turns out your fuel screw is, so you can set it back, and I would try turning it out (ccw) 1/2 turn at a time (without exceeding 3 1/2 turns from fully closed). Next, I would bump up your pilot jet 1 size (set the fuel screw back to where it was) and see if it idles better.

A couple of times in the past when the bike would not idle, I would pull the pilot jet and find a blockage (the 15 pilot has a very small orifice). If this was the case, then it doesn't explain why it idles better when warmed up, unless the pilot jet un-blocked itself.

I'll see what the fuel screw it currently set at, and richen it a bit (unless it's too far out from closed).

After the motor rebuilt (overbore of 25cc), I *think* the idle issue I'm experiencing is a bit worst, but it could be coincidence. I can't see how going to 25cc (OEM piston) overbore, can affect the pilot circuit.

I'll keep you guys update... Thanks.

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A cold engine wants a rich mixture for starting and until sufficiently warmed up. Once warmed up it doesn't require as rich a mix to run well. That's how a partially blocked pilot can explain your condition.

For a cold engine the pilot may not be passing enough fuel, but once warmed, just enough.

If you're having constant blocking issues, I'd suggest installing an inline fuel filter of the sintered brass/bronze type.

If this was the case, then it doesn't explain why it idles better when warmed up, unless the pilot jet un-blocked itself.

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Finally got time to take the carb apart yesterday (this time I removed the cylinder intake boot to make it easier to pull the carb out). Unscrewed the pilot jet and couldn't see light through it. Compressed air and carb cleaner fixed that. Went through 2 cans of cleaner on the carb. I also discovered that the needle was not centered when entering the jet. It was kinda skewed to once side. Not sure if this is normal.

Unfortunately, after putting her back together and installing a right angle external fuel filter, she doesn't want to start (not even hinting at it). Verified that the bowl is full of fuel, and plug looks good. I'm hoping it's the old, dark gas in the tank. Finger's crossed...

--Ryan

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I always use a piece of bread wire to clean out jets. Compressed air and carb cleaner alone, just doesn't get all of the brown crud out.

Even though, I'm confused as to why it won't start now. (If it ran before).

Perc

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If you didn't take out the fuel screw and clean that passage while you were cleaning the rest of the carb, I wonder if with all the cleaner and compressed air you were using you could have back flushed some crap into the fuel screw passage.

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I always use a piece of bread wire to clean out jets. Compressed air and carb cleaner alone, just doesn't get all of the brown crud out.

Even though, I'm confused as to why it won't start now. (If it ran before).

Perc

i doubt that a bread wire is small enough for a 15p jet. i can try next time. thanx for the tip.

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If you didn't take out the fuel screw and clean that passage while you were cleaning the rest of the carb, I wonder if with all the cleaner and compressed air you were using you could have back flushed some crap into the fuel screw passage.

i did pull the fuel screw out, even had to pull out my parts diagram cuz i couldn't remember the order of assembly (screw, spring, washer, o-ring). if the new gas doesn't work, i guess i could always pull the fuel screw assembly out and shoot some cleaner up (not sure if there's enough room to do with the carb in place though). did i mention i hate pulling this pumper out! :bonk:

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I also discovered that the needle was not centered when entering the jet. It was kinda skewed to once side. Not sure if this is normal.

what are your thoughts on this? maybe it's what causing revs to go down slowly after blipping throttle? needle jet not seating properly when throttle is closed?

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what are your thoughts on this? maybe it's what causing revs to go down slowly after blipping throttle? needle jet not seating properly when throttle is closed?

The needle doesn't "seat". If it were meant to, there would be no way for you to be able to adjust its clip position to raise or lower it as desired.

It not being exactly centered is of no concern. As long as it enters the jet without catching or hanging up.

Edited by Trailryder42

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Hi, I am using a mikuni pumper also. When I just started the bike my idling was like 1200. But once the engine is warm up. The idling rose to 1650. Is it normal?

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Emptied the tank, and put in a gallon of new gas. Petcock on, wait till the bowl fills, choke out, kick.... she starts right away. I let her warm up for about 30 secs or so, then pushed the choke in and she immediately dies. Same routine again, but this time after about 30 secs, I push the choke in half way. Wait 30 secs, then push it in all the way, and she dies.

I raised the idle speed a few turns and repeat the process. This time, since the idle was raised, she keeps running with no choke. I slowly then bring the idle speed down, turning CCW 1/8 of turn at a time, blipping the throttle with each change. I get it to a point where she's idling a bit higher than normal and let her idle away and warm up. After awhile, without doing anything, the idle drops to normal idle speed for about 5 secs, sputters and then dies.

So, I'm now back to my original problem...

It seems either the idle speed screw is very sensitive to changes or fuel screw assembly needs replacing (maybe worn o-ring?). It just doesn't make sense that the bike could be idling high (choke off), then after a while, it drops back down on its own to normal idle speed, then simply die. Sometimes, if I move the bike side to side, the idle speed will drop to normal for 5 secs, then die.

I'll try to back out the fuel mixture 1/2 a turn, which will put me at 3 turns out, but I'm not convinced it's a adjustment problem.

Any more ideas?

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Hi, I am using a mikuni pumper also. When I just started the bike my idling was like 1200. But once the engine is warm up. The idling rose to 1650. Is it normal?

In my experience, yes.

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Any more ideas?

Ok, do you know where the pilot "air" jet is? Did you spray out that passage good? Here's a theory. If the pilot air jet is partially blocked, that will create a stronger vacuum signal to the pilot jet, richening the air/fuel mix, possibly too much, and causing it to die out, like what you're experiencing now.

Have you checked to make sure the float level is set right? Supposed to be 19mm. Or if you have nothing to measure it, just make sure the flat surface edge of the float is level with the float bowl mating surface.

If you don't have it yet, use this Mikuni tuning .pdf to help tune and reference your carb.

www.mikuni.com/pdf/hs40_manual.pdf

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Hard to tell from the pic, but the 10-year old o-ring looks a bit flattened.

6546237925_32a03c6343_b.jpg

I guess it doesn't hurt to change it out.

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Ok, do you know where the pilot "air" jet is? Did you spray out that passage good?

Yup, sprayed it out too. From the looks of it, I'm gonna have to pull the carb and clean everything again :bonk: I'm afraid the old gas might have clogged up everything I cleaned up the other day.

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Have you checked to make sure the float level is set right? Supposed to be 19mm. Or if you have nothing to measure it, just make sure the flat surface edge of the float is level with the float bowl mating surface.

I'll check the float level too. Let me see if I have this right... if it's less than 19mm, then there will be more gas than normal in the bowl, right? And if more than 19mm, then less fuel than normal in the bowl.

I'll check it again tomorrow. I should start timing myself on how long it takes me to pull the carb :bonk:

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