Hey everyone,

I'm working on tuning my '75 Yamaha DT400. I have the stock airbox (with a UNI filter), and an aftermarket pipe (Circle F).

(Ignore the tank. It's a 400 )


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.

Thanks in advance,
Travis

You haven’t mentioned what position the clip is on the jet needle.
If it is in one of the lower positions that could be a portion of your problem (lower clip position means richer mixture). If it were me, I’d place it in the middle position and go back to the stock jetting and work from there.
There’s some real pro’s on this site that will chime in......
Brian the biknflyfisher

The clip was in the set in the fourth position from the top, which I believe is as per the service manual. Forgot to mention this, sorry.

I don't have much experience in tuning carbs, so thanks for the advice and I'll be sure to follow it. I asked on a local motorcycle forum, and with these responses 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 causing issues... 30:1, I am using a block-off plate)

Basically a scorched earth battery of tests! After doing all that, I'll fire it up again and see what's what

Impossible to tune over the Internet.

Slow to return to idle is NEVER "rich" in my experience, That always means air leak or very lean in my experience. I really can't say for sure what's going on with your bike though.

One piece of advice I can give is to make sure the ignition and timing are good and that there are no air leaks and that the carb is clean. Also good compression. The basics need to all be good.

Don't forget the air filter. Over-oiling is very common and easy to do. The "sticky" air filter oils simply don't work for me. And there have been several reports on the forum that the Uni replacement is much too restrictive on your specific model.

Not getting a response from air screw adjustment is not a great sign. You can end up chasing your tail with jetting changes when all along it was something else. Something could be wrong someplace else (air leak, timing, compression, filter) or you might just be jumping too many sizes. 55 to 40 to 30 is what you mentioned and those are all big changes.

There are several ways to attack jetting... I like to start with the pilot and getting the idle good. In general, these bikes like to be a little rich on the pilot.

Just my random thoughts without reading fully so will tonight.
Slow to return to idle 'could' be due to far too rich an idle so the idle speed screw has been wound up to keep a rich idle running but causing a slow slowdown from revs. I see you have a trick pipe too so do you have a restrictive muffler on the end. Ok, i'll catch up tonight so take my comments with a pinch of salt as could be 'way off'.

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


First up I removed the exhaust


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 order a genuine Mikuni needle just 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 screw 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 screw 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.

How does it run with no air filter?

Hi Dude,
I’m following your post as I’m facing the same pb as yours with my DT400 75: 4 stroking, loads of smokes, can’t reach 6000rpm normally, struggling when turning the throttle full open...

did you managed to find your problem?
Airleak? I don’t have oil on the ignition case...
Timing? Moving the stator doesn’t do much diff...
Need to change ignition coil and spark cable today
Thanks
Paul