Idle problem & jet needle question?

[JT2Fast4u]

Hi. I just finished rebuilding the top-end on a 87 Yamaha BW80, & when I went too start it up, the first time, it started right up after a couple of kicks. But It doesn't really want to idle very well. Here's what it does...After warming it, & choke turned off, I set the idle to specs, but after say a minute of idling fine, it will suddenly drop down real, real low, & stall if i don't give it some gas. Then after giving it gas, the idle will be high, then it will jump around between low & high (idle speed).It's like it wont stay at the correct idle speed for longer then a minute or so. It also seems to have just a tiny bit of a hesitation when giving it gas (off idle),But it's barely noticable. Also, with the air screw at the factory spec of 1 1/4 turns out, it runs terrible & won't even idle at all. The only place it runs decent at is just a little less then a 1/2 of a turn out. It has all the stock jetting in it, & I have checked the float height a dozen times already. The only thing I'm not sure about is the jet needle clip position. The spec call for it to be at "4" are they counting from the bottom or the top? One other thing I should mention is that I put in about 1/3 of a gal. of mixed gas (40:1) into the tank, along with about 2/3 gal.straight premium gas. It's an auto oil injection system, I only did that because I wasn't positive that the oil pump was working (just to be safe, thats all). Sorry for the long post, but I wanted to give you guys as much detail about it as I could. Does anyone have any suggestions on what I should do to get this idle problem right? Thanks in advance. JT2Fast4u

 

[bobit490]

anyone messing with carbs. and or jetting would do themselves a huge favor by getting Sudco's "Mikuni Tuning Manual." it's a clear and understandable manual on how your carb. works., how the changes to any give part effects it. any bike shop that vends out of Sudco can get it. it's an awesome book.

 

[nickyd]

What did the reeds look like?

 

[nickyd]

I don't think its the jet needle clip position that is affecting your problem - the needle is connected to the slide and fits itself inside the main jet - the main jet isn't part of the circuit at idle and therefore the needle won't have an impact - as you turn the throttle the slide moves up and in doing so the needle moves up and out of the main jet - the taper on the end determines the amount of fuel that is drawn up into the barrel of the carb. All of this happens from 1/2 throttle and up. Go to Eric Gorr's site and read the 2 stroke tech article about jetting - it may make it a bit clearer (eric-gorr.com). Now lets go back to your problem...if I remember you correctly, we all convinced you that you needed a new top end to get the bike running...now that it is running, you are having some fine tuning issues....assuming you replaced the piston/rings/gaskets, etc the bike should run fine...some exceptions - if you are having problems with the idle you may have an air leak somewhere -- the reason I say this is the wandering idle issue and the fact that you are choking off the air screw to "force" the bike to run better...my first guess - bad ignition side crank seal. I'm not saying this is 100% - just a thought....tell me what the OLD piston looked like? simply worn out? or burned up somewhere (hole in the crown maybe? or a melted ring? Or the top melted into the ring?). If so, you've got an air leak that is letting extra air into the combustion chamber and thereby leaning the fuel/air mixture out...since fuel aids in cooling the cylinder this can cause overheating and a fried piston. I would pull the left cover to expose the flywheel - and with the bike running try to spray some carb cleaner right at the seal...if the idle jumps up, you know you've got it. if not check around for other leaks - between the carb and intake, the base of the cylinder, the head, etc in the same manner). Another possbility would be to start the bike up - get it warm and then ride it a bit....not too long just enough to get it warm and see if the idle catches....if so you may have excess piston to cylinder clearance and until the piston expands after warming up, your seal in the combustion chamber is not sufficient...also as I said before, check the reeds...keep in mind a 2 stroke works from changing pressures - so if you have a leak or bad seal somewhere its not going to run right....there are more things I can tell you but start with these. Good luck

 

[JT2Fast4u]

Sorry guys that I didn't respond to your posts earlier, but I had some other things come up that I had to take care of first.
But anyway, thanks for the tip on the carb. book, bobit490, I'll have too check that out next time I'm at the motorcycle shop. I'm always wanting to learn more on anything that I'm involved in. Thanks again.

nickyd: Thanks for the detailed post you left, you have been a GREAT help to me in all of my posts. THANK YOU...In regards to your post, I haven't even looked at the reed valve yet, I didn't have to take it off during the top-end rebuild (at least that I know of anyway).So I have no idea what condition it is in. On the old piston, both rings were broken (appx 3/4" of each ring was melted into the side of the piston, on the exhaust port side of the piston). The shop I took it to, too have it bored & fitted for a new piston & all, told me that it looked like it was really worn out & that it had gotten pretty hot & that's why the rings broke. He also showed me how the rings got thinner & thinner the closer you got to the ring gap. I have a picture of the old piston, but I don't knoiw how to post it, or even if I can post one here. I'll take off the left cover & spray some carb. cleaner around those areas you spoke of & I'll post my results that I get, as soon as it stops raining here. Also, I havn't ridden the bike much at all, I have only been doing the "break-in" procedure so far, but I did have it running long enough to warm it up thoroughly, & ride it down the street a couple of times, & the idle didn't "catch" after doing that, it was still erractic. But hopefully, the above test will tell me more, & I will also check out the reed valve assembly too, as soon as I can get some new gaskets for it. Thanks, nickyd, I truely appreciate you taking the time to help me out. Thank again... JT2Fast4u

 

[nickyd]

yeah - melted rings - sounds to me like a lean condition caused the overheating. not a problem for the help!!

 

[JT2Fast4u]

Ok, will today I took off the left side cover & sprayed some carb. cleaner around the crank seal & it didn't affect the engine speed at all. I also sprayed around the carb, cylinder & head gaskets & got the same results, nothing... I haven't taken the reed valve out yet thou (waiting on the gaskets for it), so I still don't know the condition of it. But, do you think this could be the problem then? One other thing I should mention is when I was starting the bike, to do the above tests, it wouldn't start at first. The plug wasen't even wet, so I unscrewed the float bowl drain screw, & had a few drops of fuel come out. But after tapping on the float bowl, it then started. I know for sure that the float height is right on the money. I must have checked it & re-checked it at least a dozen times during the past few weeks & I know everything in that carb is spotless too (I've cleaned it at least a half dozen times too). I even purchased another carb from ****, & put the best parts from both of the carb's into making what I thought was one "good" carb. But now I'm really stumped. Does anyone have any other suggestions, or ideas? I sure would appreciate it. Thanks.... JT2Fast4u

 

[nickyd]

well well well.....hmmmmmmm.....I really thought you'd have nailed it with an air leak....have you gotten the chance to look at the reeds? now i'm not even sure that is the problem....And then you had the float needle sticking closed...could that be hanging up and starving the bike's fuel supply? you know when a two stroke runs out of gas, it starts to rev UP real high? check the reeds...then ride the bike...see if the problem still persists. if you don't have any air leaks and your top end is new - the carb is really the only thing left. you do have the air filter cleaned and NOT overly oiled? just thinking anything at this point.

OOO just thought of something else - can you turn the idle up a bit and get it to idle steady?

 

[JT2Fast4u]

Hi nickyd, I havn't had a chance to check the reed valve yet. But I know something is going on with the float. I tried starting it again today, & it didn't want to start. I had to tap on the side of the carb severel times & pretty hard taps too, before it would start. So either my float isn't moving freely in there, or the needle valve & seat is sticking, or something. Also, when i did get it started, it had quite a bit of white smoke coming out of the exhaust, but seemed to clear up, for the most part anyway, after riding it for a few minutes.
About the air filter, it's clean, BUT, it's possible I do have it oiled too much. I didn't have any "filter oil" for it at the time, so I just used some two-stroke mixing oil that I had. Would that make a difference? But I just dipped the filter into the oil & then rung it out really good. So maybe that could be my problem too. Do you think it could be?
About setting the idle up, I believe it will stay up if it's set kind of high, but then when ya shift it into 1st.(it's an automatic) it seems to kind of "jam" into gear pretty hard) I have been trying to get the idle speed to the point of just in-between the "right" speed & a little bit higher, but so far I haven't been able to get it (the idle) to stay inbetween their, it will keep dropping out after about 30 seconds or so. Also, before I tried to start it, I took a compression test on it (just to log it into my maintance schedule) & I was only able to get about 90-95lbs. out of it after kicking it numorous times. But I was wondering if it makes a difference weather the bike is warmed up or cold when taking a compression test? Because I took this test when it was completely cold (was about 50° outside). Shouldn't I get more then that, since I don't even have probably more then 15 minutes on the top-end. Or could it be that the rings haven't seated yet? The engine sounds fine, & I don't hear any unusual noises from it at all. No knocks, pings, rattles, or slapping, etc.). So I have no idea, at this point, what's going on with it.Well, thanks again for all your help, nickyd, I really appreciate it. JT2Fast4u

 

[nickyd]

the compression test done warm will give you a better reading for sure.

sounds like your float is 100% sticking - at least we know when starting - and it could be what's making your idle jump - I hate to be captain obvious and send you into the carb for the umpteenth time but you need to go in there and see what's up.....scratch the float needle with your finger nail - does some whitish dry stuff flake off? same thing for the fuel inlet (where the needle it going up into) - push the needle jet up there and see if it stays. If so you've got to clean that out - and add a little carb cleaner to there where you re-assemble - the wetness will aid its movement when the first rush of gas comes in. The float is floating right? I think we confirmed that. your inlets and vents are NOT clogged correct? have you blown compressed air through the inlets on the carb?

The air filter isn't likely to be the cause even with 2 stroke oil on there (I would use motor oil in a pinch - a little thinner and less goopy but if you put it on thin, it should be fine).

Is the float mechanically moving ok? meaning there isn't something in its way? Or its not bent and such? the arms where it gets pinned are not pinched hindering its movement are they?

I'm doubting its the reeds now too - and BTW the reeds sometimes come off with the gasket intact to one side of the assembly - I've taken them on/off many times w/o having to replace the gasket so don't stress over that.

 

[JT2Fast4u]

Well, nickyd, Thank's to you, I think I have finally fixed the problem with my grandson's bike. I spent the better part of the day today tearing into the bike, or should I say the carb. mainly. Everything seemed to look fine with it (the carb.), even on a VERY close examination of it. The only thing I did differently this time around with it was that I used a wooden ruler to check the float height, instead of using my vernier caliper, & I'm glad I did too. Because this is what I found... While checking the float height on one side of the float, I put the ruler on the inner lip of the carb float bowl & checked the height, it was ok. So then, I decided to check the other side of the float, & I couldn't get the ruler too go on the inner lip of the float bowl, like I did on the other side, because it would rub on the float, & that's when I noticed that it was just barely bent to one side (****-eyed), & I also found that the part that the float pin goes thou, on the float itself, was opened-up just slightly more on the one side then the other. So that too let the float hang off to the one side even more. But I guess it was tweaked enough to let the float just barely touch the side of the float bowl, enough to hang it up anyway. So after straightening up all that, I put it back together & on the second kick it fired right up, & after adjusting the air screw & idle speed screws a little bit, I had it idling very good & smooth. Oh yeah, while I had the carb.apart again, I decided to change the pilot jet I had in it, since I had an extra one from the other carb that I purchased. It's one size down from what it was. I went from a # 20 to a # 17.5 & I figured since the problem I was having was between idle & 1/8 throttle that I didn't have anything to lose changing it out. But with this newer pilot jet, it didn't run as well as it did before changing it out (the acceleration part anyways). So now I'll have to take the carb apart again & put the old pilot jet back in (the # 20) & I think all of my problems, with the bike, will be gone, or at least I'm hoping anyways. :o)
Well, I really appreciate you taking the time to help me out, nickyd, & I couldn't have done it without your help, so I just wanted to say THANK YOU VERY MUCH !!! Can I buy you lunch or something? Thanks again... JT2Fast4u

 

[nickyd]

no lunch necessary - the bike is for you grandson, right? As long as he's happy, I'm happy.

 

[flyer1171]

JT2Fast4u I am going through the same thing with a friend's kid's bike. He put a new top end in it and the exact idle symptoms you mentioned are happening here. I dont have the manual for this bike, but I was wondering if you could send the float height measurment and where you measued it from. I did all the air leak checking, carb cleaning also, and was going nuts trying to figure it out.
Thank you very much !!!!! Rick