DavidSS Posted June 11, 2011 Author Share Posted June 11, 2011 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... Link to comment Share on other sites More sharing options...
Moderator Airhead Posted June 11, 2011 Moderator Share Posted June 11, 2011 no starter fluid isnt normal behaviour, you may have a lack of suction and this could be bad crank seals... a quick test of the seals without any special tools. Magneto side, remove cover start engine and spray WD40 through slots in flywheel aiming the jet toward the seal, listen for a change in tone or engine speed. Clutch side, place the transmission case breather pipe in a jar with some white spirit in, Start the engine and look for bubbles or for the fluid being drawn up the pipe. I wouldnt start casting doubt over the jet, and definitely do NOT fit one smaller than spec. If you cannot resist then fit a 140. Yor choke is imprinted so if you can find another I would get one they are common to many mikuni carbs of the type. Link to comment Share on other sites More sharing options...
DavidSS Posted June 12, 2011 Author Share Posted June 12, 2011 no starter fluid isnt normal behaviour, you may have a lack of suction and this could be bad crank seals... a quick test of the seals without any special tools. I wouldnt start casting doubt over the jet, and definitely do NOT fit one smaller than spec. If you cannot resist then fit a 140. Yor choke is imprinted so if you can find another I would get one they are common to many mikuni carbs of the type. 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¤t=P1010134.mp4 Link to comment Share on other sites More sharing options...
Moderator Airhead Posted June 12, 2011 Moderator Share Posted June 12, 2011 she's a lovely looking little bike isnt she. Sounds almost restricted, check the exhaust header...some where resricted on later bikes (and later exhausts fitted to early bikes) and it will limit the revs. Link to comment Share on other sites More sharing options...
DavidSS Posted June 12, 2011 Author Share Posted June 12, 2011 she's a lovely looking little bike isnt she. Sounds almost restricted, check the exhaust header...some where resricted on later bikes (and later exhausts fitted to early bikes) and it will limit the revs. 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. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted June 12, 2011 Moderator Share Posted June 12, 2011 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. no way you will see 150psi, you have lots of compression according to your test, thats why I said check the pipe header...I have seen a post from a USA owner who found this in his bike also and would struggle to red-line the rev's. Its easy to check the header...much easier than changing seals or rings without having any evidence to do so...I'll see if I can find the old post of that restriction / revving issue but dont hold your breath and if I remember correct the poster didnt come back to update !! Edit No none of his posts are there any longer so I cant link it. Link to comment Share on other sites More sharing options...
DavidSS Posted June 13, 2011 Author Share Posted June 13, 2011 no way you will see 150psi, you have lots of compression according to your test, thats why I said check the pipe header...I have seen a post from a USA owner who found this in his bike also and would struggle to red-line the rev's. Its easy to check the header...much easier than changing seals or rings without having any evidence to do so...I'll see if I can find the old post of that restriction / revving issue but dont hold your breath and if I remember correct the poster didnt come back to update !! Edit No none of his posts are there any longer so I cant link it. 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? Link to comment Share on other sites More sharing options...
NEV Posted June 13, 2011 Share Posted June 13, 2011 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? Found this in some notes I made for my lads scooters, they're for Peugeot Speedfights, but you'll get the drift. COMPRESSION TESTING: Step one ... remove the spark plug .. connect the gauge to the plug hole ... some screw in ... other have a rubber 'bung' that you hold by hand ... A 'true' reading is testing when the engine is warm (metal expands with heat) .. Holding the throttle wide open ... spin the engine over untill the pointer on the guage will not move any further ... Write the figure down ... Press the button on the gauge to release the pressure (resets the 'clock' to zero).. Then repeat the process a couple of more times to get an 'average ' reading ... Step two ..put a small amount of oil down the sparky hole ... Repeat the ... Hold the throttle wide open ... spin the engine over until the pointer on the gauge will not move any further etc:- three times Noting each of the pressure readings ... If the pressure is quite a bit higher ... the oil may be 'compensating' for a worn bore / piston/ rings )and on a four stroke ...valves not seating right) ... Crankcase compression on a two stoke is a lot more difficult to measure .. this does not actually measure the compression ... but the crankcase .. seals ..piston ...gaskets etc:-ability to maintain it ... A long winded task .. as the inlet/exhaust manifolds have to be 100% sealed ... Like .... if you know your (true) compression ratio ... multiply it by 14.7 (one bar) and that will give you a guide to the PSI (or bar) .. that an engine with a good piston / rings / bore / seals / gaskets should show on the guage ... EG:- Compression ratio = 10-1 .... 10 x 14.7 = 147 psi .. (10bars) SPEEDFIGHT 50 COMPRESSION = 6.6:1 =6.6 X 14.7 = 97.2 SPEEDFIGHT 100 COMPRESSION = 11:1 =11.0 X 14.7 = 161.7 Hope this helps Nev Link to comment Share on other sites More sharing options...
Moderator Airhead Posted June 13, 2011 Moderator Share Posted June 13, 2011 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? no its welded all the way round see the link I posted earlier, I bought some dremel metal cutting disks because they are good quality and very thin and also an arbor for them, If I remember I had to wear them down just a little before I could cut in the right place. I dont think your pipe is aftermarket from the glimpse I got on the video. http://aird-tools.co.uk/shop/dremel-speedclic-metal-cutting-wheels-pack-sc456-p-5760.html Found this in some notes I made for my lads scooters, they're for Peugeot Speedfights, but you'll get the drift. COMPRESSION TESTING: .... if you know your (true) compression ratio ... multiply it by 14.7 (one bar) and that will give you a guide to the PSI (or bar) .. that an engine with a good piston / rings / bore / seals / gaskets should show on the guage ... EG:- Compression ratio = 10-1 .... 10 x 14.7 = 147 psi .. (10bars) SPEEDFIGHT 50 COMPRESSION = 6.6:1 =6.6 X 14.7 = 97.2 SPEEDFIGHT 100 COMPRESSION = 11:1 =11.0 X 14.7 = 161.7 Hope this helps Nev The compression ratio is 6.8:1 for your bike so 6.8 x 14.7 = 100 psi That would be more like it 150 psi indeed Link to comment Share on other sites More sharing options...
philmountains Posted June 13, 2011 Share Posted June 13, 2011 no its welded all the way round see the link I posted earlier, I bought some dremel metal cutting disks because they are good quality and very thin and also an arbor for them, If I remember I had to wear them down just a little before I could cut in the right place. I dont think your pipe is aftermarket from the glimpse I got on the video. http://aird-tools.co.uk/shop/dremel-speedclic-metal-cutting-wheels-pack-sc456-p-5760.html The compression ratio is 6.8:1 for your bike so 6.8 x 14.7 = 100 psi That would be more like it 150 psi indeed Im Not much help Dave sorry,but it realy does look a great DT you have there well worth all the effort your putting into it. All our UK DT,S sufferd being thrashed to near death in the eighties as field bikes. All the best Phil Link to comment Share on other sites More sharing options...
DavidSS Posted June 14, 2011 Author Share Posted June 14, 2011 no its welded all the way round see the link I posted earlier, I bought some dremel metal cutting disks because they are good quality and very thin and also an arbor for them, If I remember I had to wear them down just a little before I could cut in the right place. I dont think your pipe is aftermarket from the glimpse I got on the video. http://aird-tools.co.uk/shop/dremel-speedclic-metal-cutting-wheels-pack-sc456-p-5760.html The compression ratio is 6.8:1 for your bike so 6.8 x 14.7 = 100 psi That would be more like it 150 psi indeed 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? Link to comment Share on other sites More sharing options...
DavidSS Posted June 14, 2011 Author Share Posted June 14, 2011 Im Not much help Dave sorry,but it realy does look a great DT you have there well worth all the effort your putting into it. All our UK DT,S sufferd being thrashed to near death in the eighties as field bikes. All the best Phil 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!! Link to comment Share on other sites More sharing options...
Moderator Airhead Posted June 14, 2011 Moderator Share Posted June 14, 2011 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? Why not it's an easy one to try, it will be noisy and you couldnt ride it like that as it may melt the inner mudguard and the side panel. I dont know what a forest safe arrestor is? Did you chech for a header restriction? Link to comment Share on other sites More sharing options...
DavidSS Posted June 21, 2011 Author Share Posted June 21, 2011 Why not it's an easy one to try, it will be noisy and you couldnt ride it like that as it may melt the inner mudguard and the side panel. I dont know what a forest safe arrestor is? Did you chech for a header restriction? 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.. Link to comment Share on other sites More sharing options...
DavidSS Posted June 24, 2011 Author Share Posted June 24, 2011 Why not it's an easy one to try, it will be noisy and you couldnt ride it like that as it may melt the inner mudguard and the side panel. I dont know what a forest safe arrestor is? Did you chech for a header restriction? 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.... Link to comment Share on other sites More sharing options...
Moderator Airhead Posted June 24, 2011 Moderator Share Posted June 24, 2011 Sounds promising David. I maybe fill it with white spirit or paraffin (kerosene) and leave it a few days, then fill and shake a few times? Link to comment Share on other sites More sharing options...
DavidSS Posted June 27, 2011 Author Share Posted June 27, 2011 Sounds promising David. I maybe fill it with white spirit or paraffin (kerosene) and leave it a few days, then fill and shake a few times? 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! Link to comment Share on other sites More sharing options...
enduroseh Posted February 23, 2013 Share Posted February 23, 2013 Got the problem fixed. 1) Replace the choke plunger assembly: cost $11.14 plus shipping at http://www.jetsrus.com/individual_parts/007_752_su.html The company is JetsRUs Part Number: Starter Plunger Assembly VM16/210A 007-752 Bear in mind that the above part number is for the "short barrel plunger" these are hard to find and this is a really good price. If you have the "Long barrel plunger" these are easy to find. Look at yours real closely and then look at the pictures of the long and short barrel plungers on the internet you can tell which one you have that way. I say it like this because after doing the research myself, the parts people I talked to did not know which one would fit my carb, so I just looked at the pictures very carefully and you can see the difference between the two. Note: Air leaks thru an old choke assy. When you get the new one you will see how much tighter the plunger fits the opening it goes into. 2) Make sure a good plug is covering the little brass tube that is under and just to the right of the choke knob. Go to any auto parts store and get the vacuum line junction plugs, you need the 1/8" size, this fits tightly. Don't trust the original one if you still have it on the bike because it could be old and dried out letting air by, even though it looks like it is still intact. 3) This has to do with the oil pump injection system. If your oil pump injection system is connected and working fine disregard this step - just make sure the oil line from the oil pump to the carb is not cracked or worn or dried out or leaking oil where the oil line connects to the carb. If you have the oil pump disconnected and are mixing the oil and gas in the tank, make sure to put one of those vacuum line junction plugs where the oil line is supposed to connect to the carb. Don't leave the oil hose connected to the carb, you are only sucking in air there. You can buy a package of about six of these plugs for around $2.00 This solved my problem entirely. I was getting a bad case of the revs not dropping when I let off the gas, this was really annoying as you know when you are shifting gears. The carb idles perfectly and the revs drop when I let off the throttle. Plus no more run-away idle. One more crazy or odd note. Before the fix, when I started the bike and the engine was warmed up and the choke knob returned to the "in" position for riding. If I ever, ever, ever soooo slightly pulled the choke knob back out, the revs would race to 5000 rpm, no kidding, I pretty much thought something was up with the choke.at that point. Mine was the original choke from 1979 and it was pretty sloppy inside. Hope this works, Steve 818 270 5957 I live in Sylmar California Link to comment Share on other sites More sharing options...
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