got my gal running superbly, two fine tuning issues:

1. the only glitch is when at an engine speed where engine is between accelerating and braking the performance is a bit on and off...if you know what i mean.

Every seal on engine replaced, no intake leaks...carb clean as a whistle, idle bleed about one turn out. All jetting original and needle at 3rd spot from top. Timing was set with dial indicator before i removed mag to replace seal, but i blued and scribed and reinstalled precisely as removed.

2. When i kick the bike i get a fart out of the decompression valve sometimes, like it fired but didn't start the motor. Bike takes 3 or 4 kicks, i would like it to be 1 or 2. Could have have mis-set the decompression valve?

any advice from those with more experience appreciated

Thanks
Martin

I have 3 '73 RT3s that do what you described. My 2 '73 DT3s run smoother and start easier.

The part throttle issue is certainly fueling. Either lean or rich, can't tell from here. Rich tends to break up and sputter, they call this four stroking. Lean tends to surge, as the engine surges momentarily without any change in throttle position.

I also get the occasional sneeze through the compression release. Sounds exactly like you describe. My 360 seems to be suffering from air leaks and I'm sure the jetting is not quite right. I suspect this may not be as likely to occur with perfect carburetion but it will probably still happen from time to time.

On number one... sounds normal for a 360. They have a low load "surge" that isn't present on the smaller models. Magazines even commented on it back in the day. Yours sounds very mild.... here's a comment on an earlier 360.





On number two... might also be normal and possibly related to the ignition. See DT400b magazine article in tech library ignition/starting comments start at about page 7... yamaha-enduros.com/index.php/articles1/7...ine/viewdocument/121

thanks

what an awesome article, your depth of knowledge is superb

what i described is four stroking as the other gentleman put it, precisely as stated in the article, this bike is a badly behaved beast that is not happy unless accelerating, it really doesn't want purr along at say 50 or 60

So I am very pleased, my bike is in like it was in 1974

You might try lowering the needle one clip position. That will have the greatest effect. Adjusting the air screw may help as well.

I detest four stroking. I've gone to great lengths to cure my piston port 250 of this with some success. To completely resolve it on that engine would require excessively lean jetting which would cause engine damage at higher revs. Reed valves allow you to almost entirely eliminate this irritating condition. The DT360 is capable of running crisply and cleanly at all speeds and throttle positions, some jetting is all that is required.

thanks turbodan
so your take is this 4 stroking is caused by too rich of a mixture?

I ask because I am hesitant to further lean things out under load (lowering needle) because plug is already a light tan, so on the lean/clean side already. Air screw is out around ~1 turn so maybe a bit on the rich side, but that was needed to get to a decent idle.

Bike has reed valves so do you think i have an issue or an opportunity messing with this?

Does this 1974 DT360

This Yamaha "big bore" characteristic has been discussed ad-nauseum for decades. Many skilled tuners have worked on these bikes and struggled with trying to solve it... including Yamaha. All kinds of things have been tried... jetting... slide cutaways... in fact one of Yamaha suggestions on a 360 was to raise the exhaust port slightly. I have yet to see a proven solution that works for everyone... improvements sometimes? Yes. Complete solution? Nope.

My point is that it's apparently not that easy to eliminate on the 72-76 360/400 Yamaha. And it might not all be "jetting".

I'm a big fan of the piston port engine... I don't find them impossible to jet cleanly but again there's a lot of factors involved. For example, high compression ratios tends to require a lot of fuel when the engine RPM is near the torque peak to avoid melt down. Which makes it impossible to jet cleanly above and below that narrow peak torque RPM range. (This is true for both reed and piston port air-cooled two strokes.)

Anyway... if you do a search on the forum there are a lot of threads on this topic that might help. Yamaha revised the jetting on the earlier 360s and like I said even the port timing. By 74 the "issue"... if you want to call it that... was pretty mild. Unless it was causing a rideability issue like a really bad surge, I'd leave it be.

These bikes aren't computer controlled with light-speed feedback loops that adjust timing and fuel mixture in real-time... they have "character"... and I enjoy that about them.

thanks, i can appreciate all of this and the history you shared is fascinating and much appreciated

before i finally acted on my enduro impulse i was into building large displacement, tuned up bug engines with 2 x 2 webers, cams, larger valves, etc. Tuning was a fun challenge and much like this bike in the end i had one operating point that i could not tune away without compromising on the other operating points that are more important to me.

i am going to leave it as is, as it makes riding a bit more interesting and fun

my theory is that a majorly oversquare engine makes filling and emptying a bit more difficult than a more square engine by pushing piston porting to the limits.

The bike is a beast, now that i got a good overview of cost drivers going through one of these end-to-end i am going to find another one to restore!