4Skins Posted June 19, 2009 Share Posted June 19, 2009 This bike is 29 years old, I bought it 18 months ago with 2,000km on the clock. It's been running good, the boy has done a further 2,000km on it. Then when on a trip up the highway after about 50km it seized up (while doing about 80 - 90 kmh) So I rebored it and put it back together. I found a mis-fitted gauze in the fuel tap with a build-up of crap behind it. This I summised caused a reduced fuel flow, leaned out hence excessive heat and seizure. I was 100% confident with that theory so sent the boy off to put some miles on it. Next thing, I get a phone call from the roadside, it's seized again - F*%K I said, several times over. He was not experienced enough to know it was about to seize again. So I pulled it down again and found that it was not too badly messed-up. I had my engine recom man clean up the piston and bore and try and suggest what went wrong. He reckoned it had got really hot, probably due to lean-ness. So I've put it back together today and had a short test ride. It seems OK under load (anything upwards of half throttle) but seems flat below that. Feels like it's got a blocked pilot but the carb is all sweet and correct float level. The plug is correct, the exhaust isn't blocked and the air filter is brand new. I'm a bit knackeded as to where to go next. I'm thinking perished crank seal on the magneto side letting in air?? That's a 'split the cases' job isn't it? How can I test that theory? Any further theories or suggestions welcome. Thanks Johnny Wellington NZ Link to comment Share on other sites More sharing options...
Moderator DirtyDT Posted June 19, 2009 Moderator Share Posted June 19, 2009 I would look at the 2 stroke oil, pump and mix first. Is the oil pump working? What is the colour of the plug? It oil getting to the carb and into the engine? All of this can be done without splitting the engine. Link to comment Share on other sites More sharing options...
Moderator Airhead Posted June 19, 2009 Moderator Share Posted June 19, 2009 Hi Johnny, welcome to the forum. The bike looks great for its age, a bit of a sickener this happened twice. have a read of this post Here for and idea how to pressure test your engine. I would also advise a bit of plug chopping to see whats going on here. From experience I find that the inlet manifolds dont seal well against the reed cage due to the in-built o-ring becoming flattened, they need a little extra help using a bead of silicone gasket sealer at this point, also they can split on the underside so have a visual inspaction here. What sort of trim is the bike in, is it totally standard with a good air filter fitted clean and oiled? ...Paul Link to comment Share on other sites More sharing options...
4Skins Posted June 19, 2009 Author Share Posted June 19, 2009 I would look at the 2 stroke oil, pump and mix first. Is the oil pump working? What is the colour of the plug? Is oil getting to the carb and into the engine? All of this can be done without splitting the engine. Yup, the oil pump was the first thing I chacked when I got it going, it's pumping sweet. It's not a lubrication seizure anyhow, it's heat. The plug is no colour and dry. Link to comment Share on other sites More sharing options...
4Skins Posted June 19, 2009 Author Share Posted June 19, 2009 Hi Johnny, welcome to the forum. The bike looks great for its age, a bit of a sickener this happened twice. have a read of this post Here for and idea how to pressure test your engine. I would also advise a bit of plug chopping to see whats going on here. From experience I find that the inlet manifolds dont seal well against the reed cage due to the in-built o-ring becoming flattened, they need a little extra help using a bead of silicone gasket sealer at this point, also they can split on the underside so have a visual inspaction here. What sort of trim is the bike in, is it totally standard with a good air filter fitted clean and oiled? ...Paul The bike is totally standare trim. I might have a look at that intlet/reed block idea, the 'O' ring did look a bit flattenned. I did apply a bit of sealant to the gasket on the other side of it. The boot is not split, it's remarkably still flexible for it's age. Air filter is literally new, and oiled. Thanks for your prompt replys guys. I wish we had these forums available when I was a Yamaha mechanic in the 80s! Link to comment Share on other sites More sharing options...
Moderator Cynic Posted June 19, 2009 Moderator Share Posted June 19, 2009 The bike is totally standare trim. I might have a look at that intlet/reed block idea, the 'O' ring did look a bit flattenned. I did apply a bit of sealant to the gasket on the other side of it. The boot is not split, it's remarkably still flexible for it's age. Air filter is literally new, and oiled. Thanks for your prompt replys guys. I wish we had these forums available when I was a Yamaha mechanic in the 80s! If you get it ticking over nice and even then give behind the mag a zap with WD40 or similar and see if the revs change. The WD is enough to seal a leaky crank seal at least for long enough to tell. It works too made quite a difference to the revs on my bike. Second using a glass of water and again when the bike is ticking over nice dip the crankcase breather in a glass of water. A long stream of little bubbles shows the rh seal or more likely the stupid little o ring has failed. I have used both of these tests and they are effective without too much spanner time to get results. Much as i'd love the credit for these two OG told me about them when i was having similar issues with my engine. Link to comment Share on other sites More sharing options...
TylerP Posted June 20, 2009 Share Posted June 20, 2009 You should be able to gently pry the old seal out if it's bad. Unless it is behind a lip of the case. Two things come to mind here for me. Check the jetting specs in the carb. You may need to go up a size on the main jet if it is seizing up at sustained higher rpm. The other is piston to cylinder clearances. If you made it a tight fit this can be a problem to. Also,try running a cooler range plug. Link to comment Share on other sites More sharing options...
Moderator Cynic Posted June 20, 2009 Moderator Share Posted June 20, 2009 You should be able to gently pry the old seal out if it's bad. Unless it is behind a lip of the case. Two things come to mind here for me. Check the jetting specs in the carb. You may need to go up a size on the main jet if it is seizing up at sustained higher rpm. The other is piston to cylinder clearances. If you made it a tight fit this can be a problem to. Also,try running a cooler range plug. THERE IS NO WAY IN HELL YOU WILL CHANGE THE CRANK SEAL WITH THE CRANK IN PLACE.............. Link to comment Share on other sites More sharing options...
JimR Posted June 20, 2009 Share Posted June 20, 2009 THERE IS NO WAY IN HELL YOU WILL CHANGE THE CRANK SEAL WITH THE CRANK IN PLACE.............. Hi Cynic, I beg to differ ...... you need the following special tools :- Angle grinder, hammer chizel and an Aly welder !!! Regards Jim Link to comment Share on other sites More sharing options...
Moderator Cynic Posted June 20, 2009 Moderator Share Posted June 20, 2009 Hi Cynic, I beg to differ ...... you need the following special tools :- Angle grinder, hammer chizel and an Aly welder !!! Regards Jim You forgot the special tool, Big kin amma. Link to comment Share on other sites More sharing options...
JimR Posted June 20, 2009 Share Posted June 20, 2009 You forgot the special tool, Big kin amma. oh err where do i get one from ? I guess you just sell them Regards Jim Link to comment Share on other sites More sharing options...
Moderator Cynic Posted June 20, 2009 Moderator Share Posted June 20, 2009 oh err where do i get one from ? I guess you just sell them Regards Jim Same place that does the Fork andls, advertised on telly. Old looking place ask for Ronnie Link to comment Share on other sites More sharing options...
JimR Posted June 21, 2009 Share Posted June 21, 2009 Same place that does the Fork andls, advertised on telly. Old looking place ask for Ronnie Ronnie's on holiday for a fortnight can you think of anywhere else Regards Jim Link to comment Share on other sites More sharing options...
JimR Posted June 21, 2009 Share Posted June 21, 2009 Same place that does the Fork andls, advertised on telly. Old looking place ask for Ronnie Ronnie's on holiday for a fortnight can you think of anywhere else Regards Jim Link to comment Share on other sites More sharing options...
TylerP Posted June 21, 2009 Share Posted June 21, 2009 THERE IS NO WAY IN HELL YOU WILL CHANGE THE CRANK SEAL WITH THE CRANK IN PLACE.............. THAT IS NOT CORRECT..... On my 75 DT175 either side crank seal can be removed without splitting the cases. So,there is a way in hell! Link to comment Share on other sites More sharing options...
4Skins Posted June 21, 2009 Author Share Posted June 21, 2009 Ronnie's on holiday for a fortnight can you think of anywhere else Regards Jim But seriously guys, can anybody RELIABLY tell me whether you can replace the crank seals without stripping the motor. I know you won't find the procedure in the Yamaha workshop manual, but I seem to remember being able to do this. All depends on whether the seal has a lip around it. Johnny Link to comment Share on other sites More sharing options...
yamahait Posted June 22, 2009 Share Posted June 22, 2009 But seriously guys, can anybody RELIABLY tell me whether you can replace the crank seals without stripping the motor. I know you won't find the procedure in the Yamaha workshop manual, but I seem to remember being able to do this. All depends on whether the seal has a lip around it. Johnny hi you can change seals while motor together,drill a couple of small holes in seal,screw a couple of self tappers in and pull out,seals are only a push fit,if they have not been done before they will probably be stuck so will need cases splitting,not that hard a job,about an hour to split cases and change seals. Link to comment Share on other sites More sharing options...
Moderator Cynic Posted June 22, 2009 Moderator Share Posted June 22, 2009 But seriously guys, can anybody RELIABLY tell me whether you can replace the crank seals without stripping the motor. I know you won't find the procedure in the Yamaha workshop manual, but I seem to remember being able to do this. All depends on whether the seal has a lip around it. Johnny Seriously no you cannot change the seals with the crank in place, they are pretty solid with metal inserts and the only way to fit them reliably is to heat the cases up and pop them into place at the same time as the crank bearings and the crank. All in one opperation while the cases are hot and the crank/seals and bearings are cold. Fit the bearings, let them heat through, then fit the seals let them heat through then fit the crank. Sounds easy and it is. There may be other more 'imaginative' ways round the issue but TBH if the crank seals have gone then all the other seals on the motor will be had it or all but. When you consider you will have to stip the motor down to the point of splitting the cases just to be able to get at the seals it seems daft to bodge it. Don't forget these seals have to be in exactly the right spot or they will fail soon enough and then your doing it all again. Also the output shaft bearing WILL be past its best as it lives in oily grinding paste most of the time. And for the cost of the parts, especially with the condition of the bike in the photos 80 odd quid (and thats with genuine parts) on a bottom end rebuild would be money well spent. Link to comment Share on other sites More sharing options...
JimR Posted June 22, 2009 Share Posted June 22, 2009 But seriously guys, can anybody RELIABLY tell me whether you can replace the crank seals without stripping the motor. I know you won't find the procedure in the Yamaha workshop manual, but I seem to remember being able to do this. All depends on whether the seal has a lip around it. Johnny No you can not is the answer ... hence the jest on trying to remove them insitu !! if you remove the seals by distroying them you will also distroy the new seals by fitting them Regards Jim Link to comment Share on other sites More sharing options...
yamahait Posted June 23, 2009 Share Posted June 23, 2009 No you can not is the answer ... hence the jest on trying to remove them insitu !! if you remove the seals by distroying them you will also distroy the new seals by fitting them Regards Jim i have done this on dt and it motors,it is possible. Link to comment Share on other sites More sharing options...
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