Need advice for jetting an old 2 stroke single

[tricky]

If the right crank seal is leaking, it will suck transmission oil. You say you rebuilt the motor, is it possible you put the seal in wrong way around ?

I don't remember how it went in - but I paid very close attention to all seal orientations, especially with the crank seals because there are oil passages near there. Also, the pressure test I ran showed no leaks, so I think its unlikely. Not impossible, just unlikely........

Is it wet black **** or dry speckles coming out of the exhaust?

If you want to borrow a compression tester, I don’t mind loaning you mine if you’re not too far from Oakville. I don’t mind dropping it off as my mother-in-law is in town until Monday and I need an escape plan .

I'm not sure I'll have time before Monday, unfortunately! Thanks though, I appreciate the offer and wish you all the best in surviving until Monday

As for the exhaust, I do remember a lot of thick wet black stuff. I don't think it was dry

I'm not sure how useful a wide-band lambda sensor would be on a two stroke. A two stroke will always have a lot of air (oxygen) bypassing straight through, EGT will be low, and the oil and carbon are sure to foul the sensor. They're really meant for 4 stroke engines.

Yes - I am committing two atrocities, using it on a 2 stroke and using a tailpipe sniffer. This PDF outlines these problems, and how to (supposedly) mitigate the error, for anyone interested

I would get rid of the old gas in the tank. Compare the old needle to the new one from the kit. Are they the same? Needle position? check the parts numbers. Are the jets made by mikuni? I only trust mikuni jets - not china aftermarket, for accuracy of the drilled holes. Winners circle is a good place to go for jets and needles IMO. Im wondering where you sourced your rebuild kit from? Is it yamaha OEM?

30:1 is a good fuel/oil ratio for an old yamaha 2 stroke. I ran 28:1 in a DT125, IT250, and even in my liquid cooled DT200 and kdx200 - They all were reliable over the years. Never had any fouling problems. Of course they would smoke a bit on start up - but that would clear up in about 30 seconds or so as the motor warmed.

It was a standard issue Keyster carb kit, that's where the new needle is from. I can order a stock needle from another source to be sure, that info is in the service manual. Jets were from Fortnine, they claim to be genuine Mikuni.

I went with 30:1 because I heard the old Yamaha's use a much lower ratio. Also, I have heard anecdotally that the oil tank needs to be filled up every 2-3 tanks, which puts it around 20:1 average, knowing the fuel tank capacity

But - I don't know jack about 2strokes. With a newer oil, can I up that ratio? What is more important - the OEM specified ratio from the 70's or the oil manufacturer suggested ratio on the bottle? Changing premix ratios changes the AFR? A lower ratio means more oil per volume of air, means less gas per volume of air, right?

That is half-burned oil, there is nothing else it could be!

Do you mean half burned oil from the premix, or from the crankcase?

Weak coil spark?

Im surprised its even running with the amount of unburned (oil and gas) sludge described coming out the pipe.

I put on a 12v Powerdynamo system, all new alternator, CDI, ignition coil, everything. They have rave reviews from inside the community. Maybe its a testament to the ignition system that the bike fires at all ?

 

[TK4]

If you choose to run something like Bel-Ray MC-1, 50:1 ratio is plenty safe.

 

[Robbo]

@tricky I wonder if the black **** is a mix of carbon build up in the exhaust combined with a bit of the two-stroke oil. It would make sense if the pipe hasn’t been cleaned out. Hard to confirm without seeing it tbh.

I did find a good amount of carbon in my stock Yamaha RD250 exhaust when I first bought it.

Might be worth removing yours and seeing if there’s a build-up inside.

For the 2-stroke oil, I am still on the pump so no pre-mix for me. I bought all the remaining stock of Redline Synthetic Smokeless two-stroke oil when Cycle World West went out of business. Canadian tire now carries that line.

I would follow the fuel/oil ratio in the service manual.



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[gammaguy21]

I had a similar issue where unburnt fuel was spewing out one of the exhaust.
Had both leaky petcock and bad needle jet passing fuel from the carb and settling in the exhaust.
Pulled the exhaust and dumped a bunch of fuel (Thankfully, before it spilled into the engine).
Fixed the petcock and shut it off when not running the bike. No more spewing.

I run 30:1 (Amsoil - Interceptor) on all the ones I have runing premix. Smokes on startup - no smoke after a few minutes.

 

[bitzz]

o2 sensors work on 2Ts, they just don't last. I use an EGT which gives you same information.
30:1 premix is fine for a new motor... not synthetic right? DO NOT PUT SYNTH OIL IN A NEW MOTOR, the rings will never seat. I've never blown up a 2T motor with too much oil. While you are trying to make it run, just use premix. Ignore the pump for now. Non synthetic oil, and lots of it is a GOOD thing.

If you haven't replaced the reeds on your 45 year old bike, that would be a good place to start. Hold the reed block up to light, you shouldn't see light shining through. The edges have to seal PERFECTLY, and not be ragged. Nice square edges.
If you are going to pressure test the motor, you have to test it with the reed blocks and the carb spigots installed, otherwise you're just wasting your time.
Where did your VM32 carb come from? There is a big difference between a 2T and a 4T carb. Does it have the right slide, needle valve and emulsion tube? (Keyster kits have non OEM parts. Buy carb parts ONLY from Sudco or Winner's Circle if you want REAL OEM parts).
Yamaha is good at screwing up instructions on seal placement. ANY, EVERY, ALL SEALS are installed with the writing facing OUT(bearings too). Your problem at idle probably isn't main seals, at least not if there is raw gas coming out of the exhaust.

From what you have said it seems to me your problem(s) are:
Reeds
Compression
Timing
or a combination of the above.
... we'll do our best to ignore that pipe with the 24" stinger. It won't change the bottom end much BUT I'm interested to know what it is gonna do around redline.
You keep saying "Sounds like its four stroking." What does that mean?

 

[boyoboy]

^ yup. a lot can be changed over 45 years..any chance the previous owner can shed some light on any changes made?
great thread w lots of info. looking forward to seeing how this issue is resolved. you have some work ahead of you.

curious if you are keeping an eye on the tranny oil level - I would keep a keen eye on its level to see if its going out the tail pipe- but the leak down test says other wise.

the mention by bitzz that all seals and bearings face writing outward is a good 'heads up'

anyone w the same bike care to lend their carb out for an hour or so ! ?

 

[Trials]

... Do you mean half burned oil from the premix, or from the crankcase?

From the pre-mix. That engine should never see clutch/transmission oil getting into the crankcase, it's not a 4-stroke engine, the crankcase on a 2-stroke is used for intake scavenging and never sees oil other then what is mixed in the atomized fuel or pumped in via the oil injection.

You guys never had your 2-stroke exhaust catch fire inside from accumulation of unburnt oil in the exhaust packing? Maybe that is something that only happens regularly on old 2-stroke trials bikes, they spend a lot of time at idle and then you ride it hot for a while and then back to idle again. Must admit I never experienced that on the old MX bikes, but then they didn't have any exhaust packing and I was running them 80:1 on MC-1 and leaded pump fuel was much higher octane back in the day.

 

[tricky]

@tricky I wonder if the black **** is a mix of carbon build up in the exhaust combined with a bit of the two-stroke oil. It would make sense if the pipe hasn’t been cleaned out. Hard to confirm without seeing it tbh.

I did find a good amount of carbon in my stock Yamaha RD250 exhaust when I first bought it.

Might be worth removing yours and seeing if there’s a build-up inside.

I'll have to pull it for the pressure test anyway, so I'll be sure to take a look ?

I had a similar issue where unburnt fuel was spewing out one of the exhaust.
Had both leaky petcock and bad needle jet passing fuel from the carb and settling in the exhaust.
Pulled the exhaust and dumped a bunch of fuel (Thankfully, before it spilled into the engine).
Fixed the petcock and shut it off when not running the bike. No more spewing.

I run 30:1 (Amsoil - Interceptor) on all the ones I have runing premix. Smokes on startup - no smoke after a few minutes.

That's wild! I do shut the petcock off, but yea I'll be sure to check the exhaust like I stated above. Thanks for the heads up

o2 sensors work on 2Ts, they just don't last. I use an EGT which gives you same information.
30:1 premix is fine for a new motor... not synthetic right? DO NOT PUT SYNTH OIL IN A NEW MOTOR, the rings will never seat. I've never blown up a 2T motor with too much oil. While you are trying to make it run, just use premix. Ignore the pump for now. Non synthetic oil, and lots of it is a GOOD thing.

If you haven't replaced the reeds on your 45 year old bike, that would be a good place to start. Hold the reed block up to light, you shouldn't see light shining through. The edges have to seal PERFECTLY, and not be ragged. Nice square edges.
If you are going to pressure test the motor, you have to test it with the reed blocks and the carb spigots installed, otherwise you're just wasting your time.
Where did your VM32 carb come from? There is a big difference between a 2T and a 4T carb. Does it have the right slide, needle valve and emulsion tube? (Keyster kits have non OEM parts. Buy carb parts ONLY from Sudco or Winner's Circle if you want REAL OEM parts).
Yamaha is good at screwing up instructions on seal placement. ANY, EVERY, ALL SEALS are installed with the writing facing OUT(bearings too). Your problem at idle probably isn't main seals, at least not if there is raw gas coming out of the exhaust.

From what you have said it seems to me your problem(s) are:
Reeds
Compression
Timing
or a combination of the above.
... we'll do our best to ignore that pipe with the 24" stinger. It won't change the bottom end much BUT I'm interested to know what it is gonna do around redline.
You keep saying "Sounds like its four stroking." What does that mean?

Thanks Bitzz. Yes, its on dino oil. The carb came with the motor, all internals match the exploded view in the factory service manual. I also own other VM32, they look the same. I remember measuring the bores to make sure they are VM32's and not VM28's from a DT250. But I can do it again as a sanity check.

For pressure testing - my carb spigot was on, but the reeds were out. My understanding was, the test is checking for leaks between crankcase halves, or around the base gasket, decompression valve, head etc etc. I would think the reeds would complicate this, as my pressure gauge and input were on the carb spigot. But I will do another pressure test with the reeds in, so I'll pull the exhaust and carb, leaving everything else in place.

If the bike is idling at 1500, there should be 25 detonations per second. On the 45 pilot, I would hear maybe 5 or so larger detonations per second over some quieter ones, so all together it was very uneven in the volume and frequency. As I leaned it out (now understanding I was chasing my own tail), by the time I got to the 30 pilot it was a lot more consistent, smooth and quiet

^ yup. a lot can be changed over 45 years..any chance the previous owner can shed some light on any changes made?
great thread w lots of info. looking forward to seeing how this issue is resolved. you have some work ahead of you.

curious if you are keeping an eye on the tranny oil level - I would keep a keen eye on its level to see if its going out the tail pipe- but the leak down test says other wise.

the mention by bitzz that all seals and bearings face writing outward is a good 'heads up'

anyone w the same bike care to lend their carb out for an hour or so ! ?

I think we're on our own with this one, boyoboy. It was a bike in a box type situation, probably hadn't been running in a long time. Also, I rebuilt everything on it, and I know whats stock and whats not - if its not running right, its because of something I did, not the PO

From the pre-mix. That engine should never see clutch/transmission oil getting into the crankcase, it's not a 4-stroke engine, the crankcase on a 2-stroke is used for intake scavenging and never sees oil other then what is mixed in the atomized fuel or pumped in via the oil injection.

You guys never had your 2-stroke exhaust catch fire inside from accumulation of unburnt oil in the exhaust packing? Maybe that is something that only happens regularly on old 2-stroke trials bikes, they spend a lot of time at idle and then you ride it hot for a while and then back to idle again. Must admit I never experienced that on the old MX bikes, but then they didn't have any exhaust packing and I was running them 80:1 on MC-1 and leaded pump fuel was much higher octane back in the day.

Right - I was referring to TK4's initial theory that transmission oil was getting drawn into the combustion chamber through a leak. But like we know, thats probably unlikely because of the pressure test. I just needed to clarify what sort of oil you meant

No fires yet, but I'm getting more and more paranoid about it now ?

I may actually have some time to work on the bike today, so I'll try to knock out some of these tests and report back

 

[bitzz]

You guys never had your 2-stroke exhaust catch fire

Most bikes I see are jetted WAY TOO fat for this to happen
On the rich end of things you won't see exhaust gas temps high enough to start a fire... or a decent redline....

 

[Trials]

Ever see a chimney fire? Is the same thing, there are flames exiting your cylinder exhaust port on every stroke.

induced chimney fire.

 

[boyoboy]

I noticed and quick read your project thread.
You seem to have a lot of spares for your dt400.

Do you have another carb you could clean and try on your bike? just to see? don't change out any parts on it. just see how the dt400 runs with it?
Ditto for reeds?

For now this is where Im thinking lol. I trust your work on the rebuild, given the leak down test results and that it fires.
perseverance pays off.

 

[tricky]

Ever see a chimney fire? Is the same thing, there are flames exiting your cylinder exhaust port on every stroke.

induced chimney fire.

Ahhh I see. I remember reading recommendations on the yamaha enduro forum to start a light fire and chuck your exhaust in it to clean it out. Now I understand

I noticed and quick read your project thread.
You seem to have a lot of spares for your dt400.

Do you have another carb you could clean and try on your bike? just to see? don't change out any parts on it. just see how the dt400 runs with it?
Ditto for reeds?

For now this is where Im thinking lol. I trust your work on the rebuild, given the leak down test results and that it fires.
perseverance pays off.

I could definitely try this, those carbs would need a good cleaning probably, but its definitely doable

So today I got some time to work on the bike. Drained the old fuel.

First up I removed the exhaust, no fluid came pouring out ?

A little build up in the pipe

I then pulled the airbox and carb, rejetted the carb to stock, double checked the float height, all good. I did notice the needle was stamped Y263. The service manual calls for a 6F9. I'm not sure if its the same taper, Y263 seems to be a Keyster specific designation. So I'll follow bitzz's advice and replace that to be sure.

When I rebuilt the motor, I put in some composite reeds because the OEM steels were bent. I used some off-brand company, something that was readily available. So I picked up some Boyesen reeds and installed them today. They looked dead flat against the reed cage when I installed them, no light poking through around the edges.

The old reeds were covered with a film of oil and gas. Not sure if this is considered normal or a problem.

The first **** up I noticed, was I was using the wrong spark plug. I never bothered to get a new one, because I forgot. I had just been using the plug that came with the motor. Whoops. It was a B9HS, but the manual calls for extended reach. I accidentally picked up B8 instead of B9, so **** ups all around when it comes to spark plugs.

I then did a compression test, I reached 115 PSI or so. I could maybe have gotten it higher, but that bike is a bastard to kick with the decompression valve disabled. Also remember this is dead cold and dry.

I then checked my timing, it has held at 2.9 mm BTDC as per the service manual

Lastly, I did my pressure test. Unfortunately, it seems I have sprung a leak through my decompression valve (I am not sure how this effects the compression test results, I assume the real values will be slightly higher)

The leak isn't coming from where the decomp valve attaches to the cylinder, its coming through the body of the decomp valve itself. I took it apart and cleaned it up as best I could, adjusted it and retried. No joy. I believe either the seat of the valve isn't sealing properly, and/or the rubber oring halfway up the valve is buggered and needs to be replaced. Those are my best guesses right now.

Depending on the orientation of the decomp housing, it could lose 1 psi per 30 seconds, or 1 psi in 5 seconds. Just by touching the housing I could hear changes in how much air was coming out. I guess something changed from when we rebuilt the motor on the bench. Maybe a few heat cycles and some actual use just finally did it in. I'll have to fix this before I move on

So next up is fix the leak, retest and make sure it holds pressure. Then some new fuel, and we'll give it another go.

 

[Trials]

... When I rebuilt the motor, I put in some composite reeds because the OEM steels were bent. I used some off-brand company, something that was readily available. So I picked up some Boyesen reeds and installed them today. They looked dead flat against the reed cage when I installed them, no light poking through around the edges.

The old reeds were covered with a film of oil and gas. Not sure if this is considered normal or a problem.

...

Nice, 2 stage reeds, much better.
That curved steel plate is to stop the reeds from opening too much, it is suppose to be in place, I always bent/curved it more so the reeds would open way more then usual, but it probably should be in there to protect them from bending in the wrong places.

That exhaust looks mighty small for the displacement of the motor but it is what it is, will be interested to hear how it performs once you get her running.

 

[Trials]

...
You keep saying "Sounds like its four stroking." What does that mean?

I know what that means! Instead of going pop-pop-pop-pop
it's going yingada-yingada-yingada-yingada

 

[Trials]

Um, on this type of engine, if your exhaust pipe fills with gasoline :/ your bottom end is likely already over-flowing, the exhaust port is not any lower then the scavenging ports.

and if you are mixing oil with the gasoline at 32:1 a healthy portion of that fuel mix will readily evaporate (the gasoline) but the oil portion will largely remain.

add: Redline 2T synthetic oil advertises a recommended oil ratio of between 50:1 and 100:1 dependent on the application. Yes you won't blow-up a motor with too much oil but you will plug it up very nicely unless you are operating it at full revs all the time.
good read: Dan's Motorcycle Two Stroke "AutoLube" Oil Pumps

 

[tricky]

Nice, 2 stage reeds, much better.
That curved steel plate is to stop the reeds from opening too much, it is suppose to be in place, I always bent/curved it more so the reeds would open way more then usual, but it probably should be in there to protect them from bending in the wrong places.

That exhaust looks mighty small for the displacement of the motor but it is what it is, will be interested to hear how it performs once you get her running.

The instructions from Boyesen said that if the kit comes with retaining plates, don't use the stock reed stop. I thought this was a little weird myself, and considered just using the stock stop instead of that tiny retaining plate, but ended up listening to the instructions. Whats your take, knowing the instructions from Boyesen?

As for the exhaust - my stock one is pretty buggered, it has to be cut open, repaired, repacked, then welded shut. Way outside the scope of what I can do, so I picked this one up from a pipe builder for a half decent price. There's not much in the way of readily available and affordable aftermarket pipes for these, that I'm aware of, so I'll have to make this work for now. One day I'll gain the capacity to fix the stock pipe, or design my own if I ever start to understand how the **** 2 strokes actually work ?

and if you are mixing oil with the gasoline at 32:1 a healthy portion of that fuel mix will readily evaporate (the gasoline) but the oil portion will largely remain.

add: Redline 2T synthetic oil advertises a recommended oil ratio of between 50:1 and 100:1 dependent on the application. Yes you won't blow-up a motor with too much oil but you will plug it up very nicely unless you are operating it at full revs all the time.
good read: Dan's Motorcycle Two Stroke "AutoLube" Oil Pumps

OK - makes perfect sense. I remember reading that article a long time ago. I am keen on rebuilding my oil pump, its just pretty far back in the line right now

I have been doing a bit of research - the orings for the decomp valve are no longer available, I could buy one off of ebay for an exorbitant price, but I would still have to wait for November to get it. I am thinking of measuring the ID, taking a best guess at OD based on the o-ring I have and pictures I can find, and ordering generic high temp orings from McMaster-Carr or something like that. Also, there are other suggestions to help my problem - lap the valve to make sure it seats right, and add a spring in there when the cable connects, to help force the valve to retract all the way

As for the needle, page 8 of the factory service manual says a 6F9 needle clipped at position 4, but page 78 says a 6DH2 needle clipped at position 3. I've asked for clarification on the enduro forum. I think the profiles are very similar. Also, that table is labelled "Cabruretor Specifications."

 

[Trials]

O-rings? Davis & Davis in Napanee has oodles of them to browse through, I found perfect ones for my montesa gas caps there just last week
less then a buck a piece. Needle clip starts in the middle, adjust it up or down seasonally (right about now actually), it should work in the middle, if not well then try adjusting one way, if it improves you are moving in the right direction the main needle will affect mostly open throttle, the low speed needs to be set first.
Run it for a while, listen to the motor and read the plug.


... how do you like the scoring on that Beta cylinder :| it swallowed a sand pit at WOT. That was a 1200$ ouch for my buddy.

I would put the reed stops in and curve them open as far as they can go, to my thinking it makes the reeds bend throughout their length instead of a sharp bend at the bolt plate thingy, those things have to bend a lot and slam shut nice about a zillion times :|