Hi all,
I had my second test ride on my DT400 twin shock today.
Only did a few miles. Got it up to 70mph.
There is a problem with the mid-range that is spoiling the enjoyment of the bike.
Tickover is fine (1500 rpm) and it accelerates well when you open up the the throttle.
The problem is the mid-range, at a constant speed and coming off the throttle. It’s difficult to describe it and I unfortunately, haven’t got a go-pro to video it.
It burbles/spits/farts, fires/mis-fires and makes the bike jerky/erratic, not smooth.
A brief history of what I’ve done so far…
New 12V ignition system from Rex’s speed shop and am confident there are no electrical/timing issues.
New spark plug B9ES, correct gap.
New air filter, lightly oiled
NOS float valve, correct size
Carburettor cleaned twice in Chinese ultrasonic cleaner.  All jets that I checked were the correct size.
Clip on mid position.
Air screw 2.5 turns out
Piston is 1st oversize and slightly rattley.
New ‘Yamaha’ crankshaft seals.
2-stroke oil pump is working fine.
I split the engine and used the proper sealant to join the cases together and new gaskets/sealant where necessary on the engine/intake etc
What would you check/re-check?
I’m leaning towards getting the carburettor ‘professionally’ cleaned.
All suggestions gratefully received…..
Many thanks


 

It sounds like you've discovered a "feature" of the big bore Yamaha. 

Lots of threads and opinions on the "cruise surge" but not too many concrete answers on a solution.  Some bikes are worse than others.  Service bulletins exist on the RT1.

Here's a magazine comment from the previous RT series...  in the mid 70's magazines tended to refrain from saying anything bad but you can find original DT400 road test comments on the issue. 

 
 
 

Sad Face. 
Thanks. Yes - I had heard about that ‘feature’, but was hoping it was limited to the 360’s (sorry 360’s guys).
I can’t believe Yamaha sold them like that without everyone returning them.
Has anyone resolved this ‘feature’ on a DT400 twin shock? I beg you tell me how as the sun is shining and I need to get out there.
Many thanks……
 

It's essentially a carburetion problem.  The twist is that it is impossible to resolve by rejetting and adjusting the carb.  I do have some good results with my 360 after years of trial and error but I've gone so far from stock it's impossible to say what was the single most effective change.  Between the porting, the modern reed block and the modern carb it's remarkably well behaved for a vintage air cooled two stroke.  Mine will run clean and hold speed on flat ground from the bottom of fifth gear at 30 MPH.  The faster you go the cleaner the carburetion becomes.  At 40 I can roll off the throttle to the extent that the bike is actually slowly losing speed and the engine is still running clean.  As good as it is, it's still not as polite as my RD so I'm going to keep screwing with it.

I am about to borrow a trick from the RD400 where Yamaha milled small cutouts into the bottom of the exhaust skirt to open a wee little window between the crankcase and the exhaust port at TDC.  This relieves some of the crankcase vacuum and improves transfer efficiency at low throttle openings.  That is part of the problem with operating under 1/8 throttle, there's just not enough pressure in the crankcase when the transfers open to properly scavenge the cylinder.  Another thing that definitely helps is proper intake porting.  Auxiliary ports on either side of the main intake port that connect to the crankcase at the lower opening of the transfer ports help generate turbulence and improve fuel atomization.

I also had this on my 400, spoilt the ride, i returned all jets and needle position to stock, by setting the bottom end rich (air screw half turn out) i managed to virtually eradicate the problem. Moving to a 16 front and 40 rear also helped at holding 30mph in top gear

my stock 360 will maintain a smooth steady speed above 30mph.

I do some steady state riding starting around 3 minutes and also at about 10 minutes in.
My suggestion would be, long gearing, pull back some ignition timing, try dropping and raising the needle.

When I first got my bike it had too short gears, too much timing, and it was too rich with the stock main jet, I went down one size.
 

My 400 does the same once it planes off at midrange speed. I’ve read about so many others doing it that I simply learned to live with it. When you hear me coming down the gravel roads it’s constant throttle up and down up and down.

Just sounds like plain old 'too rich in the midrange' like clip needs lifting. Unusual to have to do that though so first thought is needle & needle jet are worn. Jet metering bit goes oval & needle shows wear in extreme cases & can eyball the jet when out of the carb & looking in the right light. Had a YZ125K "83 that was very badly worn & sitting on the start line constantly clearing its throat. New needle & jet [emulsion tube] fixed it. Or you have the idle mixture 'so rich' it's not getting over it further up the scale. I had a race motor that was bad like that but was reed flutter so double the reads & fixed it. Was only a 100cc admitedly & right at the point where the power was going from not much to 'everything' in one hit. So unlikely your problem but just remeniscing.

It's not impossible to fix by tuning...  it's more that's there's no single solution for everyone. 

With the powerdynamo ignition, it seems to lean towards a rich issue as RT325 said. 

I got most of the buck and jerk after rolling out of the throttle on my 75 DT400 by running the idle mixture almost closed on it. it is a lot smoother on decel than running the screw at 1.5 turns out. my RT1B also has this problem but not my RT1M. the RT1M is very smooth at factory settings. all bikes mentioned are running the stock ignition system.

you can try locating a 2.5 slide for a DT400 B or C, stock is 3.0 and the 2.5 will fatten it up. 3rd way is getting a UFO installed on the throttle slide, you will need to lean the pilot jet out and run the pilot jet and air screw. Rich's Taylored Porting installs the UFO.