DavidSS

thanks for rubbing my nose in the dirt.... I misunderstood which hole you suggested i look in (lots of them in this carb...). I was focusing all my efforts on the carb side, not the bowl. I did not know what a "jet in floatbowl for choke is clear " was, (sentence made no sense to me) I thought it was the hole where the choke seated at. O-well, live and learn.

Well I GOT IT FIXED everyone!!! Here is what I found. I took the carb apart again,,, for the 4th time, and checked the float height. Adjusted it to factory spec (again) using calipers. While in there, I noticed a small orifice in the float bowl that i had not before. Reason for this is when I rebuilt the carb and cleaned it, I had my son help me so I gave him the chore of cleaning the bowl, (figured that would be easy and not too delicate for him to work on) so I did not pay much attention to it, as I figured it was just a float bowl!?? However,,, today I noticed this orifice hole. I noticed that it appeared be plugged! So I soaked it in carb cleaner, and then took my air compressor and flew it out good. Then filled the bowl up with rubbing alochol, and put a drinking straw in this hole, and blew threw it. Yep... bubbles came out now. I Put it all back together again.... tried starting the bike, and pow it started right up on the second kick! so.... I am guessing that small orifice was plugged. The bike then after warmed up, was running too rich on the low end, and was loading up. so I had to adjust the air flow screw in about 1 turn. It now idles smooth, and has a ton of mid and top end power now! Whooo hooo! thanks all for the insight and suggestions.

This is a continuation of an earlier blog I had going when I was trying to get the old 79 DT running. Now that I have it running I find that it is still very hard to start when the engine is cold. Carb has been rebuilt and cleaned 3 times! The air screw is set at factory 1.5 turns. here is a link to a video of how it sounds when first starting. Eventually.... about 50 kicks or so, I can get it to start up. http://s913.photobucket.com/albums/ac333/davidscott59/yamaha%20carb/?action=view&current=P1010154.mp4 Anybody have any ideas of where to go from here?

Paul and all else who may have read this..... Finally got the bike running good! the tail pipe was the problem! I cut the pipe open and it was almost 100% occluded! it took me and my son over two hours to remove all the old oil. it was like cleaning a BBQ grill that had not been cleaned in 30 years. We had to literally chip away the crusted on oil. Even used engine cleaner on it to help dissolve it. Then I took it to a muffler shop and had it welded back together. I wish i had taken pictures to show you. It is important to cut the end near the tail on the other side of the bracket. Inside there you will find a chamber with cone shaped tube, and on the there are 6 little holes that spiral out. They were all plugged but one! The bike runs strong now! can rev up into the red over 8000RPM! I took it out and drove around the block. it has zippy power now . Got it home and took the plug out and it is nice and clean with slight brown color so I can assume it is running just right with the fuel. My last challenge now is to figure out how to make it start easier when it is cold! It takes about 20 or so kicks to get it to stay running. Maybe I am just doing it wrong. Maybe I should try it without the choke on and just use throttle, and see if it starts. I will experiment with it... Thanks again for your help!

Paul, I think I figured out the problem. I took off the muffler exhaust (and the spark arrestor insert) and it was drenched with thick gunky goopy oil! The inside was too! So I cleaned it really good with carb cleaner down the hole into the pipe packing material. I got it pretty clean, but not entirely. After it dried out I started the bike. AAAAAHHHH it actually got up to 7000RPM and had abit more power! Drove it around the block and ran a little better but was still a bit rich on the top end!. So I think I will have to replace this muffler exhaust because the packing is probably so gunked up that the bike can't breath! I could cut it open and clean it out but then would have to get it welded back. I did some research and it appears this could be the problem....

I tested the main seals the way you explained Paul. No change in the idle of the engine. I have to assume the seals are not leaking. I took off the muffler end and it seemed to get up to more revs, closer to 7500RPM, but was still a bit rich. Well I think I am going to start a new thread since my bike is running now. But the new issue now is getting the top end to work correctly. I have done some reading that suggests two other possibilities..... 1. The timing is off, meaning the flywheel has shifted due to a broken key. Or 2, the coil is not putting out enough energy at high revs, which could either be a weak coil, or a weak CDI unit Something else I thought of, could this carburetor be the wrong one? Meaning somebody may have replaced it with a similar unit, from another year bike? Perhaps the main jet should be smaller for this carb. I will get a new thread going on this new issue........ stay tuned..

thank you Phil for the encouraging words. I almost decided to just sell it last week! I was so frustrated with it, and my right leg (hip)got twisted out of alignment by kicking over the starter so many times! I've been limping around all week trying to get my back and hip realigned! I'm too old for this abuse!!

When I checked the compression, I did so when the engine was cool. I did the test with 3 different readings, the average being around 115psi. So if 100psi is what you get with "newer fresh" rings, then this bike has very good compression! I wish I had more history on it. I suspect that the owner two guys back, had put a lot of work into restoring it, because the engine is in very clean shape. However, perhaps he made some mistakes and now they are coming forward. Like perhaps leaking seals? I really don't know where to turn anymore for clues. I seem to be running in circles chasing clues. I can do the seal check I guess and eliminate that. How about if I take the exhaust tail piece off, would that make some difference? This bike has a "forest safe" arrester on it. could it be limiting and making it run rich?

I kind of raised my eyebrow to the guy at the bike shop when he said 150psi. But perhaps he was thinking of new bikes or something. I will check the exhaust pipe header since it is an easy check. This may not be the stock pipe. But what if it is restricted. Is it simply an insert that gets pulled out?

Paul, Yes, she is a nice clean bike for being so old. The odometer reads 7800. Not sure if that is a lot for a dirt bike like this? Who ever owned her before me either restored or kept her in real good shape, cuz there is almost no rust, and things like the forks and shock are in good shape. There were no side covers, so I need to buy a set. Now.... about that running rich issue. I spoke with the local bike shop mechanic about it. He suggested that it might be seals too, and also that the rings are bad because he said a 175cc should have closer to 150psi compression not the 115psi I got. I'm not sure why the restriction in the exhaust would be there, I can't imagine it has been running like this by design. Arrrrrgggg! I'm getting tired of this . And I am sure you mates are probably getting tired of helping me! I just want to ride the bike! I was hoping I would not have to rebuild the engine.... but it is sounding like I might have to.

I replaced the main jet today with a new 140 (stock) because the old one looked like it had been enlarged. I will shop around and see if I can buy a new choke assy. And would bad seals be what is causing the high end to be so rich? I took the reeds off today and inspected and here is a photo. It looks good to me. Also, I recorded a small video of the rev up of the engine. Note the sound it has when up around 6000RPM. Hope this works... i'v never tried linking a video before! http://s913.photobucket.com/albums/ac333/davidscott59/yamaha%20carb/?action=view&current=P1010134.mp4

I checked the compression with a gauge. it is reading around 114 to 115psi. Pretty good I would think! So here is a question.... IF the main jet which is supposed to be a 140, were somehow enlarged, would that make it run rich on the top end? maybe my main jet has been tampered with before I bought the bike. It is stamped 140... but is it really? I could try a 130 and see what that does, right? Another thing I noticed is that it is difficult to get it started when cold. I have to spray some starter fluid in the carb for it to get primed. That is not normal behavior either...

Ok, I took out my choke assembly and here is what it looks like Does this look OK? I see that the black seal on the end has some grooves in it. But looks good to me. Here is a link to other photos of the carb after I cleaned it. And the dirty needle valve before I cleaned it. http://www.flickr.com/photos/davidscottphotos/sets/72157626922308430/

Yep airfilter is clean. I think we are getting close here! where does this rubber washer sit?? I did not see one. In less you are referring to the rubber cup that goes on the outside of the choke near the nut?

OK, still working on a compression tester. My neigbor down the way works on cars a lot. He might have one. For the needle. I have tried it at factory default slot 3. And at slot 4. I measured the float height with my calipers, and it was spot on per the # in the book. don't have the book in front of me right now. Now... you got me thinking about that choke again! When I took it out, I did NOT see any rubber seat on it. Where would this be located? At the end? Mine just has this brass plunger with nothing on the end of it. I did not see anything down in the hole where it sits. When I took it out the first time, I found some yellow gunk looking stuff that kind of looked like epoxy glue stuck to the end. I pealed it off. do you have a photo of a good choke plunger you can share? David

Paul, got the needle jet out and cleaned it. It was not realy dirty, just oxidized. Carb is cleaned AGAIN... but the bike still runs just the same. Engine is running rich on the top end after about 5000RPM. I have not done the compression test yet. But I think the issue lies with a rich fuel mixture. I am ready to just try a new carb. Any suggestions of carbs I could use for this model of bike? I was thinking about taking the oil pump out of it and doing the traditional oil/fuel mix.

Well I removed the carb again and was able to get the needle jet out to clean it. It actually was not real bad! looked like oxidized but not clogged. I cleaned and polished and made sure the holes were clear. Then also went through and cleaned again the carb and all the holes everywhere! Reassembled and installed. Started the bike up, and NO DIFFERENCE! It is still running rich on the top end after it gets past about 5000RPM. I should clarify,,, that I mentioned earlier that it seemed to run fine in neutral, but this is not true, it is running rich even in neutral. However when it is idling it runs just fine. Very nice actually. At this point I am about to give up on the carb and look into buying a new replacement. Any suggestions on what model I can use on this old bike? And where a good source is to buy them? Could I take a newer Mikuni from a say YZ125 or 250 and use it? Just disconnecting the oil pump and go to manual mixing of the fuel??

paul, Let me clarify..... the only part I could find that looks like your photo is labeled in the book as a "main nozzle" I assume this is what you are calling the needle jet. It is the ONLY piece I was not able to get out to clean on the 1st and 2nd attempts. I only cleaned it with a pipe cleaner down it. I did not realize it has little holes in it! When I tried to push it out, it appears it is flanged on the end that is near the throttle throat side. I was not sure it was supposed to come out so I did not force it. But now I will try a bit harder to get it out. I think this time I will boil the entire carb parts in hot water and then flush them out again with carb cleaner and spray with my air compressor. Geeze! these things are picky! Oh, and yes, the finger compression tester is not calibrated very well. I'll see about getting a real gauge. For the RPMs. I am able to rev it close to redline 7K while in neutral. All beit, it seems to lag some on the rev up, but I am used to motocross bikes that are snappy fast on the throttle. I assume that behavior is a reed issue. It is when I put the bike into motion under load that it seems to not have much power. No matter what gear I am in. I bet that needle jet is gunked up like the one you showed in the photo. I think it is still running rich on high rev.

Yes, he certainly does know his stuff! From little knowledge I have of the Mikuni carbs, I assumed the choke was working. I didn't even realize you could take it apart until he mentioned the "choke circuit" to me in the last thread. But I think it was a combination of carb related issues, not just choke. I still have to figure out how to solve this next mystery of no high end power. So not out of the woods yet...

got it going blackhat! see my reply above for more detail...

I GOT IT!!! it runs now! Thanks all of you for all the help and encouragement! not sure what I did, but it was carb related. I took the choke assembly apart and noticed that it had a strange yellow goopy thing on the end. I thought it might be a damaged O-ring or something, but I took it off. I cleaned it good and re-installed it. I also cleaned out the air screw. So once I got it running I was able to adjust the air screw and idle. It idles smooth at low RPM....very little smoke. however, now I have a new problem! It has no high end power! I took it out for a ride, and when it gets to about 5000RPM under load it starts to run like it has no power, like it has a governed on it! or its running too rich or something. I expected this bike to be much more zippy. What is causing that??? I seem to recall you mentioned something like this happened on your bike in one of the threads I was reading. What did you do to fix that?

oldgitonabke! good news to report.... we have compression! this evening I spoke with me neighbor and he came over and helped me troubleshoot it. We took the plug out. He put his finger over the plug hole and I cranked it over about a dozen times. (checking the compression) He said that his finger should have been real wet after, but it was not. Sooooo, we figured it must be a fuel issue still. put the plug back in. I opened up the air screw a couple more turns. then We took the airfilter out and sprayed starter fluid down in the intake side. put the choke on, and cranked it over a bunch of times (throttle open about 1/4 to half) and it started to fire up! So then I sprayed some more fluid in. And we cranked it some more.... it started again! this time I was able to keep it running for about 30 sec before it died. Sooooo, we are back to a carb issue. As a test. I screwed the air screw back in to factory setting of 1 1/2 turns. Then we repeated the process, and could NOT get the bike to fire up. I then backed the air screw out again, and low and behold we got it to start again momentarily. So tomorrow I take the carb apart AGAIN! should i see if I can get a carb rebuild kit for this and replace that air screw and the jets? I am also going to take the reeds out again and inspect more closely to see if they could be sticky. blackhat250 may have had a good idea.... maybe the float valve is stuck, and that is why it only ran for about 30 sec, until the bowl emptied out. Any other suggestions? I think I am getting close!

Blackhat, the bike never really "ran" with the solvent. It just fired up for about 2 seconds. It must be getting fuel to the cylinder chamber because the plug gets wet. I am still suspicious that it could be electrical related.

I will ask my neighbor. He owns a couple of bikes. He might have a gauge I can borrow..... Ah... the saga goes on! Would it help if I posted photos of some of these parts so you can evaluate? You mentioned the reed valve again. When I took that off, I did not clean it. just looked at it. Maybe I should take the carb off (again!) and look at the reeds to make sure they move. Or would that be a waste of my time?

Additional questions.... when you had this similar issue, did you get spark? Did your gauge lights come on (green neutral and red oil)? because mine do. This bike is acting like part of it is grounded out, but that must be impossible. I have literally, ohmed out every wiring going and coming from the CDI to the coil and the switch, and kill switch. I'm just wondering if someone screwed with the wiring and changed something, as the wiring harness has evidence of been tampered with before I got it.