I have this 75 DT400 bought non running and had to install a REX Speed ignition as the stator plate was missing under the Flywheel. Got it running. Starts and idles fine but breaks up with throttle. I have a UNI foam wrapped over the original cage for the air filter. Stock pipe. Stock for this is 160 main, 40 pilot and 6F9 needle clip on 4th down.
Since I have the foam filter I went to a 180 main, not good, moved the clip down to the lowest notch and runs worse. Changed to a 165 main with the clip at 4 and runs better but still sucks. The ignition is dead on at 2.9 mm BTDC.
Here's the plug at 3/4 throttle and killed engine.
Moving the clip down makes it richer. I would try position 2 or 3 on the clip.
I am on the verge of trying that. I am thinking that this foam filter may be lees free flowing than I had anticipated.
Thanks for the reply.
Or just break down and buy the stock filter. Then no guessing.
did you take the carb all the way down and clean the jet tube & airjet? also, IF it has a 'remote' main, like on the side of the bowl, take the adapter out of the bottom of the jet tube and install the main jet in the conventional place w a washer. those adapters w o rings are good about leaking over time.
I had the carb completely apart. The main is newer style screwed into the emulsion tube with the washer. I could stand to double check the air jet though. I soak everything in berrymans chem dip and blow everything out. I have a vapor blaster but too lazy to use it. I have to renew the media.
I am still at this thing. I installed a Yamaha air filter and the carb settings (jets etc) and exhaust are stock. The only non stock thing is the REXS Speed shop Ignition. The bike starts easy and will idle fine but breaks up at revs above idle. Is there a possibility that the RSS ignition is not advancing?
I have moved the needle clip down and tried other jetting but no luck.
I had to buy the REX because there was no stator present when I got the bike.
timing won't cause it to break up, but will cause it to go flat. id verify grounds, etc. and, maybe , its just a bad coil. did you get the whole kit w the coil/cap/wire etc or just the plate w the windings?
Whole kit complete, coil, cdi, NGK cap, wire the stator plate has the windings and 12v upgrade. Plug and play setup. I will check all grounds. I ground the paint off of the frame where the ground is at the coil.
When I say breaking up...like missing.
There must be something as the stator plate with the wiring was not on the bike when I got it. The flywheel was there and the Yamaha CDI.
Man I have never had so many issues with 2 strokes as I'm having lately. Usually can clean carbs and have running by the afternoon. I guess they are just getting older and more spent.
OK, removed the genuine Yamaha air filter and the thing runs good now. It doesn't like the restriction. Now trying to figure out what will make it happy with the filter in. The Yamaha manual calls for a 40 pilot and 160 main 4th on the clip. I look on a Yamaha parts site and it shows anywhere from a 150, 170, 180 main and 40 or 50 pilot. It's hot as hell here and sweating my ass off testing.
I guess my next move will be 150 main and see where that leaves me.
is the jet tube and needle stock? i mean, give it whatever it needs, or as little.
a different jet/emulsion tube can have a drastic affect on jetting, as will a different needle.
They're stock size.
Are the jets you are using genuine Mikuni or copies?
All Mikuni
Ended up with a 140 main clip on 3 down (may go to 2nd down) pilot left stock at 40 and 2 1/2 turns out on air screw. This thing idles at 1100 RPM, Rex's Speed Shop ignition. I set it to spec at around 1300. Pulls the front wheel with ease now. Still a bit of breaking between 1/4 and half throttle. After half she rips.
This is with the genuine Yamaha air filter and stock intake tract with the side lid on. No mods there.
Mainly posting as reference for anyone having tuning issues on these. 1975 DT400B