Howdy Folks,

I have a 75 DT400, the bike has ran great for years after coming out of a long slumber. I was out cruising during a car show on the bike, lots of idling, hot day, all seemed well enough. I suddenly had a single ping noise and ever since the bike will not idle down when warmed up. I can twist carb screws all the way in/out, up/down and they'll barely touch my idle. So far I've tried replacing the reeds as the existing ones had a bit of light shining through and replaced the intake rubber boots to ensure they're full sealing.

The bike has aftermarket J&R exhaust on it and has supposedly had a bit of porting done on it before my father purchased it all the way back in ~1978.

Since this incident the bike is very difficult to start cold, but that may be carb settings. I tried fiddling them the day of the ping, in an effort to get home and had no luck... Naturally in the moment I didn't think to track what my settings were.

I haven't opened the carb up yet and I guess that's my next step unless someone else has something else to check.

The bike sounds a bit lean on warmed up idle to my ear. That's just based on years of riding the thing and having to flip to reserve a few times over.

Any advice is appreciated.

Sounds like an air leak someplace.

Any advice for diagnosing a location or confirmation? I was thinking similar but don't want to jump into a full seal rebuild without some vague confirmation of the issue. I'm also thinking of going to 12V at the same time if I do it.

Leak down test.  Here's a pretty good article.

www.klemmvintage.com/airleaks.htm

I don't go over 5psi as I have seen seals pushed out.  Once I find/correct the pressure leaks then I vacuum test with a MityVac.  Vacuum test fail can only be a seal, which one can be found by packing grease around the seal one at a time or just replace them both (rare to have a leak that doesn't show up under pressure but it can and does happen)

I've had this on 2 of my '73 RT3 360 Enduros. Caused by aftermarket cylinder base gasket that was too thick. The 4 large head bolts loosened causing an air leak and lean condition runaway.
Check all cylinder head bolts, carb mounting bolts, carb top cap and reed valve block.
Crankshaft seals can leak. If the right seal leaks it will pull transmission oil in and make a lot of smoke.
Any air leak after the carb slide will cause this.
Suggest you not run it until you resolve the problem. Piston might seize.
Also recommend using only Yamaha OEM parts.

Maybe something found it's way into the pilot jet. Can you flick the choke on & off & get a rich response briefly with throttle closed. If so just go on & off with choke a few times in a row to keep that richness going to sort of prove s point to the pilot jet blockage--or as said by others--air leak, which should be obvious if base gasket sucked in around transfers. I shouldn't even say the word--ring caught in a port--Ping!!. How's the compression on the kick if you feel it after the decomp has closed so guessing from starting half way down with the lever. Full of joy aren't i.

I've got a hard time believing it's a gasket issue, the bike has been in its current state for over 1000 miles. I could see a gasket failure though, would I have smoke coming out in that case? I should be able to find it by spraying a bit of carb cleaner or starting fluid though right? Do you have any links to OEM parts? I was just going to get a seal kit from fleaBay.

I'm at the cabin now but I'll try the choke trick and get into the carb as well. I need to remove it to do the pressure test anyway. The compression feels fine though while kicking.

Thanks for the advice folks, hoping to confirm the issue next weekend.

 

Just as an aside, with all the messing around I’ve done with jetting lately I’ve begun to get the feel for this bike (76 DT400) and mine pinged like a madman when I first acquired it. Throughout my testing I’ve found that the pinging when holding the revs at around 3000 and just slightly letting off the throttle is lean pilot setting. I richen it up by running the air screw in and then adjusting the idle back up until it quits pinging at the held 3000 rpms. Good gas in the tank as well. Just my experience so far.