Need advice for jetting an old 2 stroke single

Hey everyone,

I'm working on tuning my '75 Yamaha DT400 (2 stroke single). Stock jetting is #40 pilot, #160 main on a Mikuni VM32 round slide carb.

I have the stock airbox (with a UNI filter), and an aftermarket pipe.



On the pilot side, I went rich because I am new to this and have no idea how changing the exhaust impacts jetting. Started with 55 pilot, 185 main. Huge amount of four stroking, massive amounts of spooge coming out of the exhaust. Ran like crap, wouldn't idle, no need for a choke. So, way rich. Pilot air screw did nothing

Take two, 45 pilot, 185 main. Still massively rich, spooge everywhere. Pilot air screw was no help

Ok, take 3. Pulled the carb, double checked float height, went to stock jetting (40 pilot, 160 main). Fire it up. Ok, it held an idle now, still rich (I think). Pilot air screw didn't seem to do much

Ended the day at 30 pilot, 160 main. It finally stopped four stroking on idle, and it started feeling and sounding more like it should (I think, based on no experience). My gut is telling me it is still rich on the pilot circuit. Maybe a 25 pilot is needed? Ran a sweep of the pilot air screw from closed to open, didn't find a sweet spot really.

If I blip the throttle from idle to about 1/3, it takes a while to come back down. Sounds like its four stroking. As I blip it, it rolls on good and sounds pretty crisp. Then I shut the throttle and it can hang there for a bit, still firing, then slowly starts to fall while four stroking (or what sounds like four stroking to me). Not sure what this hanging is - seems like fuel is getting in there to sustain the combustion.

My question is - is it common to have to dive this far UNDER stock jetting? I always ran under the assumption that, if you improve airflow, you may have to richen the mixture. My exhaust, I am assuming, improved airflow. So I'm wondering what's going on.

Anyway. I'm still trying to get the pilot circuit right before I move on to the needle and main, just need a little guidance.

Also - the engine was tested for leaks before it went into the frame. It held 7psi for 10 minutes or so. I guess its possible an air leak has developed since then, but it was airtight when the engine went in.

Buy a wideband 02 and stick the probe up the pipe to get the static fueling nailed down. They are not much money now and will save you many hours of guessing.

I once bought an old IT250 for dirt cheap because it ran crappy. really crappy! seller mentioned jetting it w/o success.
I put it back to stock - the only change I found was the needle setting. Main and pilot were still stock. Only change I made was to the needle position to make it all stock. Ran perfect afterwards. yippee !

I would put the jettting back to all stock - including the needle position.
I note you haven't listed the needle position at all. Moving the needle position up or down one slot can make a night and day difference. The needle has a lot of overlap with the pilot and main jets across much of the rev range. Note- the needle's mid point slot isn't always the stock position - look up what the stock needle position should be.

EDIT- Make sure to double check that float level is also at stock spec level.

best luck or...wanna sell your DT for dirt cheap? wink, nice old ride...

If it was mine, I would go back to stock carburetor everything, make sure your bottom end seals are good (sounds like you did that part already), make sure your reed valves close perfect, make sure you don't have any air leaks on the intake manifold or from the original yamalube oil injection system. The bike should at least run like that, modified pipe or not. Adjust the low speed air screw and then the main jet needle position to suit.
I doubt you need to change the size of the original jets, that was a bad assumption.
The 4-stroking sound in my experience was always related to bottom end seals, reed valves or the main jet needle position.
Typically on my Yamaha's I had to move the main jet needle up or down one notch during significant seasonal weather changes.

BTW that looks like a really short expansion chamber on there and the stinger is about twice as long as would seem appropriate.
Expansion chamber is not just more free flowing then the stock exhaust, it is also designed to reflect the exhaust gas pulse back towards the exhaust port at a very specific time, such that it improves scavenging and compression of gasses during the subsequent power stroke.

Hope this helps some.

lol if all else fails I bet boyoboy and myself could get it going pretty quick

... pull the reed cage and hold it up to sunlight, if you can see light getting past the petals then it is not right yet. The pressure test won't prove that the reeds close properly on their own, only that they hold air pressure once it is constantly applied.

If you added a high-flow air filter and an aftermarket expansion pipe you'll need to jet up there's no doubt about it.

Personally I would go up one on the pilot jet and up two or three on the main jet. So if it calls for a 40 pilot go to a 45. If it calls for a 160 main jet go to a 180-190.

Make one change at a time, try and get it to idle properly. Then work on quarter throttle half throttle full throttle.

Did the bike sit? have you properly cleaned the carburetor?

What's the plug look like?
Have you checked timing?

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Question - are you using the stock airbox? correct?
I assumed you are. edit- re read your post and says you are. lid/cover still on air box I assume.

Pull the carb and check all the internals as mentioned.

Needle taper as well as needle position can have an impact so check the numbers on needle, jets, and slide and compare them to stock. Note if the jets and needle are oem or aftermarket.

Not sure if the slide can go in both forwards and backwards on that carb so make sure it’s assembled the correct way and moved freely throughout the full range.

Be careful leaning out that motor. Better to be rich than too lean.
Is the motor working off an oil pump or premix. If premix, what’s your fuel/oil ratio?

I see you’ve done a leak down test. Have you done a compression test?


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Also, what colour is the smoke coming out of the exhaust? Blue? White? Black? Something else?


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GreyGhost said:

Buy a wideband 02 and stick the probe up the pipe to get the static fueling nailed down. They are not much money now and will save you many hours of guessing.

Click to expand...

I actually bought one a while back (a PLX wideband kit). Tried to use it yesterday, with a tailpipe sniffer I built (as per the advice here and here). But I don't have the capacity to weld, so I used JB weld. It didn't work at all. On the 55 pilot/160 main, it was reporting 15 AFR, while spewing tons of spooge out of the tailpipe and smoking like a bastard. I figure maybe the JB weld was cracked, and fresh air was getting drawn into the chamber with the O2 sensor. It later broke, so thats a real possibility. I'll have to build a new (better) sniffer.

Before being broken:


After

Trials said:

lol if all else fails I bet boyoboy and myself could get it going pretty quick

... pull the reed cage and hold it up to sunlight, if you can see light getting past the petals then it is not right yet. The pressure test won't prove that the reeds close properly on their own, only that they hold air pressure once it is constantly applied.

Click to expand...

I have no doubt you and boyoboy would have a good laugh watching me scratch my head 20 different ways

I'll pull out the reed cage next time I'm working on the bike, and we'll see whats what. The reeds were out for the pressure test

FilzRD400 said:

If you added a high-flow air filter and an aftermarket expansion pipe you'll need to jet up there's no doubt about it.

Personally I would go up one on the pilot jet and up two or three on the main jet. So if it calls for a 40 pilot go to a 45. If it calls for a 160 main jet go to a 180-190.

Make one change at a time, try and get it to idle properly. Then work on quarter throttle half throttle full throttle.

Did the bike sit? have you properly cleaned the carburetor?

What's the plug look like?
Have you checked timing?

Click to expand...

The engine was rebuilt this year, the bike has been running periodically since the spring. The fuel is a month or so old, maybe time for some fresh stuff.

The carb was rebuilt by me, pulled and cleaned everything by hand, then in the ultrasonic, then with compressed air. New jets, float needle, etc etc. from the rebuild kit. I'm fairly confident in that work.

The plug is black, it looked to me like a mixture of dry and wet fouling. I have ordered some new plugs (stock heat rating for this bike, B8ES), so I can try to swap out the plug here and there to see whats going on.

Timing was set earlier this year, I could double check that though.

boyoboy said:

Question - are you using the stock airbox? correct?
I assumed you are. edit- re read your post and says you are. lid/cover still on air box I assume.

Click to expand...

Yes, everything about the airbox is stock, except the foam filter is a UNI replacement for the OEM filter.

Robbo said:

Pull the carb and check all the internals as mentioned.

Needle taper as well as needle position can have an impact so check the numbers on needle, jets, and slide and compare them to stock. Note if the jets and needle are oem or aftermarket.

Not sure if the slide can go in both forwards and backwards on that carb so make sure it’s assembled the correct way and moved freely throughout the full range.

Be careful leaning out that motor. Better to be rich than too lean.
Is the motor working off an oil pump or premix. If premix, what’s your fuel/oil ratio?

I see you’ve done a leak down test. Have you done a compression test?

Click to expand...

The slide only goes in one way, I'm fairly confident the carb is together correctly. I had it apart yesterday to compare with the OEM service manual, and it all looked good.

Yes - I am worried about going too lean, which is why I'm scratching my head a bit, and asking around on here

I am not running it with the oil pump right now - I have a block off plate installed. The oil pump still has to be rebuilt, and then tested to make sure it works. I'm on premix, 30:1 right now

I haven't done a compression test - I don't have the kit for it. But I can pick one up, and add it to the list of tests to run.

Robbo said:

Also, what colour is the smoke coming out of the exhaust? Blue? White? Black? Something else?

Click to expand...

I tried a lot of jetting yesterday.... I can't remember very clearly. It would give the typical white plum of smoke on startup, then the smoke would die down after 20 seconds or so. I'll pay more attention next time. I put my hand by the exhaust quite regularly, it would catch a bunch of black **** every time. Worse when it was rich, but even on the 35 pilot my hand would be speckled in a few seconds

Thanks for all the replies everyone - I'm starting to piece together what I need to look into
- Pull the exhaust and carb, run another pressure test to rule out a leak
- Double check the timing
- Check the reeds
- Compression test
- Replace the spark plug with a fresh stock B8ES
- Back to stock jetting, #40 pilot and 160 main, needle clip in the stock position, air screw in the stock position (1.5 turns out)
- Fresh gas (Wondering if my premix ratio is all buggered...)

After doing all that, I'll fire it up again and see what's what. I'm a bit busy at the moment - I'll try to do this work by next week, and I'll report back

Your sniffer looks narrow and long. I havent used one, but I think shorter and fatter would give better results. That is a long straw to suck through.

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 ?

tricky said:

... I'm on premix, 30:1 right now ...

Click to expand...

Holy I used to run my TY250 80:1 with MC1
30:1 must produce a pretty good smoke screen.

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 .


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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.

imho If the air filter is properly oiled, I doubt you would need to consider it as higher flow then a brand new stock paper air filter would be,
and the chamber we have no idea about,
but I have put chambers and oiled foam air filters on 2-smokes before with zero carburetor problems.
,,, go with high test pump gas and your favourite brand of full synthetic, some brands will make a lot more goop collect in the exhaust then others, none of them will be as bad as 30:1

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.

tricky said:

... put my hand by the exhaust quite regularly, it would catch a bunch of black **** every time. Worse when it was rich, but even on the 35 pilot my hand would be speckled in a few seconds ...

Click to expand...

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

Weak coil spark?

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